\r\n\tThis book aims to offer readers a comprehensive understanding of ceramic materials. The detailed guidance provided in this book will help not only the students, researchers and professionals in the field of materials science and allied disciplines but the researchers and professionals in other fields.
\r\n\r\n\tThe book aims to provide the latest developments on the advanced ceramics and their latest applications in a wide variety of fields. The key features of this book intend to provide the reader with an understanding of how ceramics are applied, explores recent characteristics and properties of these materials, taking into account their structures and compositions, and discusses the various processing of ceramics.
\r\n\r\n\tThis book will rapidly become the reference work on the subject of ceramic materials.
",isbn:"978-1-83881-212-6",printIsbn:"978-1-83881-204-1",pdfIsbn:"978-1-83881-213-3",doi:null,price:0,priceEur:0,priceUsd:0,slug:null,numberOfPages:0,isOpenForSubmission:!1,hash:"9adbe58d10d5ca2b61e9ff2b6b138f40",bookSignature:"Dr. Mohsen Mhadhbi",publishedDate:null,coverURL:"https://cdn.intechopen.com/books/images_new/9894.jpg",keywords:"New Ceramics, Traditional Ceramics, Crystallography, Composition and Structure, Processing of Ceramics, Grain Size, Dislocations, Nanoindentation, Corrosion Cracking, YoungÃÂs Modulus, Mechanical Testing, Applications of Ceramics",numberOfDownloads:642,numberOfWosCitations:0,numberOfCrossrefCitations:0,numberOfDimensionsCitations:0,numberOfTotalCitations:0,isAvailableForWebshopOrdering:!0,dateEndFirstStepPublish:"May 29th 2020",dateEndSecondStepPublish:"August 5th 2020",dateEndThirdStepPublish:"October 4th 2020",dateEndFourthStepPublish:"December 23rd 2020",dateEndFifthStepPublish:"February 21st 2021",remainingDaysToSecondStep:"6 months",secondStepPassed:!0,currentStepOfPublishingProcess:5,editedByType:null,kuFlag:!1,biosketch:"Dr. Mohsen Mhadhbi is a reviewer and editorial board member of different scientific publishers and congresses, as well as a member of a number of international associations, to name a few: American Association for Science and Technology, International Association of Advanced Materials (IAAM).",coeditorOneBiosketch:null,coeditorTwoBiosketch:null,coeditorThreeBiosketch:null,coeditorFourBiosketch:null,coeditorFiveBiosketch:null,editors:[{id:"228366",title:"Dr.",name:"Mohsen",middleName:null,surname:"Mhadhbi",slug:"mohsen-mhadhbi",fullName:"Mohsen Mhadhbi",profilePictureURL:"https://mts.intechopen.com/storage/users/228366/images/system/228366.jpeg",biography:"Dr. Mohsen Mhadhbi obtained his Ph.D. degree from the Faculty\nof Sciences of Sfax, Tunisia. He is currently Assistant Professor\nof Chemistry in the National Institute of Research and Physical-chemical Analysis, Tunisia. His research interests include\ninorganic chemistry, material engineering, intermetallics, and\npowder technology. He has published works in national and\ninternational impacted journals and books. He is a teacher in\ninorganic chemistry. He has supervised several researchers in materials science. 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From chapter submission and review to approval and revision, copyediting and design, until final publication, I work closely with authors and editors to ensure a simple and easy publishing process. I maintain constant and effective communication with authors, editors and reviewers, which allows for a level of personal support that enables contributors to fully commit and concentrate on the chapters they are writing, editing, or reviewing. I assist authors in the preparation of their full chapter submissions and track important deadlines and ensure they are met. I help to coordinate internal processes such as linguistic review, and monitor the technical aspects of the process. As an ASM I am also involved in the acquisition of editors. Whether that be identifying an exceptional author and proposing an editorship collaboration, or contacting researchers who would like the opportunity to work with IntechOpen, I establish and help manage author and editor acquisition and contact."}},relatedBooks:[{type:"book",id:"6656",title:"Phase Change Materials and Their Applications",subtitle:null,isOpenForSubmission:!1,hash:"9b257f8386280bdde4633d36124787f2",slug:"phase-change-materials-and-their-applications",bookSignature:"Mohsen Mhadhbi",coverURL:"https://cdn.intechopen.com/books/images_new/6656.jpg",editedByType:"Edited by",editors:[{id:"228366",title:"Dr.",name:"Mohsen",surname:"Mhadhbi",slug:"mohsen-mhadhbi",fullName:"Mohsen Mhadhbi"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"6188",title:"Solidification",subtitle:null,isOpenForSubmission:!1,hash:"0405c42586170a1def7a4b011c5f2b60",slug:"solidification",bookSignature:"Alicia Esther Ares",coverURL:"https://cdn.intechopen.com/books/images_new/6188.jpg",editedByType:"Edited by",editors:[{id:"91095",title:"Dr.",name:"Alicia Esther",surname:"Ares",slug:"alicia-esther-ares",fullName:"Alicia Esther Ares"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"6802",title:"Graphene Oxide",subtitle:"Applications and Opportunities",isOpenForSubmission:!1,hash:"075b313e11be74c55a1f66be5dd56b40",slug:"graphene-oxide-applications-and-opportunities",bookSignature:"Ganesh Kamble",coverURL:"https://cdn.intechopen.com/books/images_new/6802.jpg",editedByType:"Edited by",editors:[{id:"236420",title:"Dr.",name:"Ganesh Shamrao",surname:"Kamble",slug:"ganesh-shamrao-kamble",fullName:"Ganesh Shamrao Kamble"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"6517",title:"Emerging Solar Energy Materials",subtitle:null,isOpenForSubmission:!1,hash:"186936bb201bb186fb04b095aa39d9b8",slug:"emerging-solar-energy-materials",bookSignature:"Sadia Ameen, M. Shaheer Akhtar and Hyung-Shik Shin",coverURL:"https://cdn.intechopen.com/books/images_new/6517.jpg",editedByType:"Edited by",editors:[{id:"52613",title:"Dr.",name:"Sadia",surname:"Ameen",slug:"sadia-ameen",fullName:"Sadia Ameen"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"6320",title:"Advances in Glass Science and Technology",subtitle:null,isOpenForSubmission:!1,hash:"6d0a32a0cf9806bccd04101a8b6e1b95",slug:"advances-in-glass-science-and-technology",bookSignature:"Vincenzo M. 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Churchill, Maja Dutour Sikirić, Božana Čolović and Helga Füredi Milhofer",coverURL:"https://cdn.intechopen.com/books/images_new/8812.jpg",editedByType:"Edited by",editors:[{id:"219335",title:"Dr.",name:"David",surname:"Churchill",slug:"david-churchill",fullName:"David Churchill"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"7960",title:"Assorted Dimensional Reconfigurable Materials",subtitle:null,isOpenForSubmission:!1,hash:"bc49969c3a4e2fc8f65d4722cc4d95a5",slug:"assorted-dimensional-reconfigurable-materials",bookSignature:"Rajendra Sukhjadeorao Dongre and Dilip Rankrishna Peshwe",coverURL:"https://cdn.intechopen.com/books/images_new/7960.jpg",editedByType:"Edited by",editors:[{id:"188286",title:"Associate Prof.",name:"Rajendra",surname:"Dongre",slug:"rajendra-dongre",fullName:"Rajendra Dongre"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"7676",title:"Zeolites",subtitle:"New Challenges",isOpenForSubmission:!1,hash:"4dc664fa55f94b38c13af542041fc3cc",slug:"zeolites-new-challenges",bookSignature:"Karmen Margeta and Anamarija Farkaš",coverURL:"https://cdn.intechopen.com/books/images_new/7676.jpg",editedByType:"Edited by",editors:[{id:"216140",title:"Dr.",name:"Karmen",surname:"Margeta",slug:"karmen-margeta",fullName:"Karmen Margeta"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}}]},chapter:{item:{type:"chapter",id:"6528",title:"Cherenkov Phase Matched Monochromatic Tunable Terahertz Wave Generation",doi:"10.5772/6910",slug:"cherenkov-phase-matched-monochromatic-tunable-terahertz-wave-generation",body:'\n\t\tTerahertz (THz) waves present attractive possibilities in advanced applications including biomedical analysis and stand-off detection for hazardous materials. The development of monochromatic and tunable coherent THz-wave sources is of great interest for use in these applications. Recently, a parametric process based on second-order nonlinearities was used to generate tunable monochromatic coherent THz waves using nonlinear optical crystals (Boyd et al., 1972, Rice et al., 1994, Shi et al., 2002, Tanabe et al. 2003). In general, however, nonlinear optical materials have high absorption coefficients in the THz-wave region, which inhibits efficient THz-wave generation.
\n\t\t\tAvetisyan et al. proposed surface-emitting THz-wave generation using the difference frequency generation (DFG) technique in a periodically poled lithium niobate (PPLN) waveguide to overcome these problems (Avetisyan et al., 2002). A surface-emitted THz wave radiates from the surface of the PPLN and propagates perpendicular to the direction of the pump beam. The absorption loss is minimized because the THz wave is generated from the PPLN surface. Moreover, the phase-matching condition can be designed using PPLN with an appropriate grating period (Sasaki et al., 2002). Surface-emitted THz-wave devices have the potential for high conversion efficiency, and continuous wave THz-wave generation has been successfully demonstrated (Sasaki et al., 2005). Unfortunately, the tuning range of the THz waves is limited to about 100 GHz by the nature of PPLN, and a wide tuning range cannot be realized using the quasi-phase–matching method.
\n\t\t\tWe developed a Cherenkov phase-matching method for monochromatic THz-wave generation using the DFG process with a lithium niobate crystal, which resulted in both high conversion efficiency and wide tunability. Although THz-wave generation by Cherenkov phase matching has been demonstrated using femtosecond pumping pulses (Auston et al., 1984, Kleinman et al., 1984, Hebling et al., 2002, Wahlstrand, 2003, Badrov et al., 2009), producing very high peak power (Yeh et al., 2007), these THz-wave sources are not monochromatic. Our method generates monochromatic and tunable THz waves using a nanosecond pulsed laser source.
\n\t\tThe Cherenkov phase-matching condition is satisfied when the velocity of the polarization wave inside the nonlinear crystal is greater than the velocity of the radiated wave outside. The radiation angle is determined by the refractive index of the pumping wave in the crystal, nopt, and that of THz-wave in the crystal, nTHz (Sutherland, 2003),
\n\t\t\twhere is a wavelength of the contributing waves in the DFG process (1 – 2 = THz), n1, n2 (n1=n2nopt) and nTHz are refractive index of the crystal at pump waves and THz-wave frequencies, respectively, and Lc is the coherence length of the surface-emitted process (Lc = /k, where k=k1–k2 and k is the wave number). The Cherenkov angle, crystal, is determined by the refractive indices of the pumping wave and the THz-wave in the crystal, so the angle is strongly dependent on the choice of material. THz-frequency waves radiated at Cherenkov angles propagate to the crystal-air interface, and if the angle is greater than a critical angle (determined by the difference in refractive indices at the interface), the THz-frequency wave is totally reflected at the interface. To prevent total internal reflection, a clad material with a lower refractive index than that of the crystal in the THz range and a proper prism shape, is coupled in at the output. Figure 1 shows a schematic of Cherenkov radiation and output coupling of a THz-frequency wave.
\n\t\t\tSchematic of Cherenkov phase-matched monochromatic THz-wave generation.
\n\t\t\t\tFigure 2 shows relation of Cherenkov angle and critical angle of several clad materials. We choose polyethylene, diamond, Si and Ge as clad materials, because these materials have low absorbance and low dispersion character at THz frequency region. A total internal reflection occurs below the curve. For example, lithium niobate (LiNbO3) has 2.2 and 5.2 of refractive index at near infrared and THz-wave region, results in 65 degree of Cherenkov angle in the crystal. On the other hands, critical angle of total internal reflection from the crystal to air, polyethylene, diamond, Si and Ge in a manner are 79, 76, 63, 49 and 40 degrees, respectively. The figure tells that diamond, Si and Ge prevent total internal reflection of Cherenkov radiation for lithium niobate crystal.
\n\t\t\tThe angle in the clad material, clad, is determined by Snell’s law as shown in Fig. 1, using the refractive index of the clad material nclad.
\n\t\t\tCherenkov angle for various nonlinear crystals (pink collared diamonds) and calculated critical angle between a crystal and a clad. Black, aqua, green, blue and red curve represent Air, polyethylene, diamond, Si and Ge as a clad material, respectively. A total internal reflection occurs below the curve.
The radiation angle clad, which is important for practical applications, is determined by the refractive indices of the pumping waves in the crystal and the THz-wave in the clad layer. Equation (2) is mathematically equivalent to a model in which the THz-wave is directly radiated to a clad layer. The equation tells us that nclad should be larger than that of the nonlinear crystal in the pumping wave region. A comparison of the refractive indices of various nonlinear crystals with that of Si (about 3.4 in the THz-region) indicates that Si is an appropriate Cherenkov radiation output coupler for many crystals.
\n\t\t\tThe radiation angle hardly changes during THz-frequency tuning because the silicon has low refractive index dispersion in the THz-wave region and the optical wavelength requires only slight tuning. The change in radiation angle is less than 0.01 for a fixed pumping wavelength. The actual angle change of the THz wave is significantly better than for the THz parametric oscillator (TPO) with a Si prism coupler (Kawase et al., 2001), which has an angle change of about 1.5 in the 0.7–3 THz tuning range.
\n\t\tWe demonstrated the method described above using the experimental setup shown in Fig. 3 (Suizu et al. 2008). The frequency-doubled Nd:YAG laser, which has pulse duration of 15 ns, a pulse energy of 12 mJ when operating at 532 nm, and a repetition rate of 50 Hz, was used as the pump source for a dual-wavelength potassium titanium oxide phosphate (KTP) optical parametric oscillator (OPO). The KTP-OPO, which consists of two KTP crystals with independently controlled angles, is capable of dual-wavelength operation with independent tuning of each wavelength (Ito et al., 2007). The OPO has a tunable range of 1300 to 1600 nm. The maximum output energy of 2 mJ was obtained for a pumping energy of less than 12 mJ. The 5 mol% MgO-doped lithium niobate crystal (MgO:LiNbO3) used in the experiment was cut from a 5 65 6 mm wafer, and the x-surfaces at both ends were mirror-polished. An array of seven Si prism couplers was placed on the y-surface of the MgO:LiNbO3 crystal. The y-surface was also mirror-polished to minimize the coupling gap between the prism base and the crystal surface, and to prevent scattering of the pump beam, which excites a free carrier at the Si prism base. To increase the power density, the pump beam diameter was reduced to 0.3 mm. The polarizations of the pump and THz waves were both parallel to the Z-axis of the crystals. The THz-wave output was measured with a fixed 4 K Si bolometer.
\n\t\t\tExperimental setup for Cherenkov phase-matching monochromatic THz-wave generation with a bulk lithium niobate crystal.
The THz-wave output map for various pumping wavelengths and corresponding THz-wave frequencies is shown in Fig. 4. The magnitude of the map denotes the output voltage of a Si bolometer with a gain of 200. The noise level of the bolometer was about 10 mV and is shown as the blue region in the figure. The regions where over 2 V of output voltage were obtained is red. As seen in the figure, wide tunability in the range 0.2–3.0 THz was obtained by choosing the proper pumping wavelength. Especially for lower frequency below 1.0 THz, this was very efficient compared to our previous TPO systems that used 1470 nm pumping.
\n\t\t\t\tTHz-wave output mapping for various pumping wavelengths and corresponding THz-wave frequencies. The X-axis and Y-axis denote pumping wavelength 1 and THz-wave frequency, respectively. The magnitude of the map values indicates the output voltage of the detector.
\n\t\t\t\t\tFigure 5 (a) shows cross sections of the THz-wave output map of Fig. 4. The highest THz-wave energy obtained was about 800 pJ, using the fact that 1 V 101 pJ/pulse for low repetition rate detection, pulsed heating of the Si device, and an amplifier gain of 200 at the bolometer, and the energy conversion efficiency from the 1 wave (1 mJ/pulse) was about 10–4%. This value is comparable to that obtained with our previous TPO systems, despite the low excitation energy of only 1 mJ. The figures clearly show the strong dependence of THz-wave output energy on the pumping wavelength. In the case of 0.8 THz generation, the output energy had a dip at a pumping wavelength of approximately 1400 nm as shown in Fig. 5(a). We obtained extremely high energy in the low-frequency region below 0.3 THz (millimeter wave region) using 1470 nm pumping. The reason for this is not clear, and the dispersion of pumping waves cannot explain the results; thus, an explanation is left for future research. The important result is that we could obtain a flat output spectrum in the range 0.2–2 THz by choosing proper pumping wavelength, as shown in Fig. 5(b).
\n\t\t\t\tCherenkov phase matching inherently requires a waveguide structure for nonlinear polarization waves in the crystal to suppress phase mismatching in the direction perpendicular to the guiding mode (i.e., normal to the crystal surface). If we reduce the width of the pumping beams in the direction of THz-wave propagation to about one-half of the THz wavelength, (i.e., about 10 m for 3 THz) by taking into account the refractive index of MgO:LiNbO3 in the THz-wave region, no need exists to consider phase matching in that direction (Suizu et al., 2006). In our case, the waist of the pump beams in the MgO:LiNbO3 was about 300 m, which corresponds to about five cycles of THz waves at 1.0 THz, and one cycle of THz waves at 0.2 THz. Although the experimental conditions did not satisfy the requirement for Cherenkov phase matching, we did successfully detect Cherenkov-radiated
\n\t\t\t\tTHz-wave spectra (a) at various pumping wavelength and (b) under choosing proper pumping wavelengths.
THz waves, which originated in the higher absorbance area of the crystal at the THz-wave region. The THz waves generated far from the crystal surface would be attenuated and no significant phase mismatch would occur. This also remains an area for future study.
\n\t\t\t\tBy shaping the pumping beams with a focused cylindrical lens or by adopting the waveguide structure of the crystal, we could neglect phase mismatches and obtain a higher power density of the pumping beams, resulting in higher conversion efficiency.
\n\t\t\tWe demonstrated the Cherenkov-type phase-matching method for monochromatic THz-wave generation via the DFG process using bulk lithium-niobate crystal. We successfully generated monochromatic, widely tunable THz waves in the 0.2- to 3.0-THz range. We obtained efficient energy conversion in the low-frequency region below 0.5 THz and achieved a flat tuning spectrum by varying the pumping wavelength during THz-wave tuning. The highest THz-wave energy was about 800 pJ pulse-1, which was obtained for a broad spectral region in the range of 0.2 to 2.0 THz. However, obtaining high conversion efficiency in the frequency domain above 2 THz was difficult, and the output was almost zero at 3 THz. The output of the THz wave decreased in the high-frequency region due to a phase mismatch incurred by the finite size of the pumping beam diameter. As shown in Fig. 6(b), Cherenkov-type phase matching arises due to a superposition of spherical THz waves from the nonlinear polarization maxima created by pumping lights of two different frequencies in the NLO crystal, and thus, when the finite beam size is taken into account, the phase shift of the wave depends on the distance from the y-surface of the crystal. THz waves generated far from the crystal surface destructively interfere with those generated in the neighbourhood of a crystal surface. The beam diameter of the pumping wave in a lithium-niobate crystal in our previous work was about 300 m, corresponding to about the wavelength of the THz wave at 0.2 THz, and ten cycles of THz waves at 2.0 THz, as the refractive index of lithium niobate is about 5.2. Since the 300-m beam diameter is over 15 times the wavelength of a THz wave above the 3-THz region, a phase mismatch occurred and the THz-wave output decreased. In this experiment, we attempted to improve the THz-wave generation efficiency above 3 THz by optimizing the beam shape of the pumping wave to decrease the beam-diameter dependence effect (Shibuya et al., 2009).
\n\t\t\ta) Ideal Cherenkov-type phase-matching condition; (b) Cherenkov-type phase-matching condition when the beam diameter of the exciting light is considered. In (b), the phase mismatch is caused by the finite size of the beam diameter.
A dual-wavelength potassium titanium oxide phosphate (KTP) optical parametric oscillator (OPO) with a pulse duration of 15 ns, a pulse energy of 1.6 mJ, a 50-Hz repetition rate, and a tunable range of 1300 to 1600 nm was used for a DFG pumping source. The size of the MgO-doped lithium-niobate crystal was 5×65×6 mm3. We used cylindrical lenses to reduce the pump beam diameter. The focal lengths of the cylindrical lenses were 20, 50, 100, and 150 mm, and the beam widths parallel to the crystal’s y-axis were 35, 46, 83, and 127 m (FWHM), respectively. The pump power was adjusted, and the power density on the focus position was made constant at 200 MW cm-2 for all lenses.
\n\t\t\t\tThe obtained THz-wave output spectrum is shown in Fig. 7. The vertical axis is the THz-wave pulse energy calculated from the output voltage of a Si-bolometer detector. The horizontal axis is the THz-wave frequency. THz-wave output spectra were measured by selecting the excitation wavelength in which the maximum output was obtained for each THz-wave frequency. The output in the high-frequency region increased as the focal length of the cylindrical lens decreased. THz-wave generation was confirmed over the 3-THz region with the 20-mm and 50-mm cylindrical lenses. The tunable range for the 20-mm cylindrical lens was about 0.2 to 4 THz. This is the widest tuning range for the previous lithium-niobate crystal-generated THz-wave source. The pumping-wave beam diameter in the lithium-niobate crystal using the 20-mm cylindrical lens was about 35 m, which corresponded to about 1.8-THz wave cycles at 3 THz. The phase mismatch is thought to have decreased as the beam diameter decreased, leading to an output improvement in the high-frequency region. Meanwhile, the conversion efficiency decreased because the pumping-wave beam diameter corresponded to over 2.3-THz wave cycles and the absorption coefficient increased rapidly above 4 THz. The absorption coefficient of the crystal at 4 THz was 425 cm-1. When the pump beam moved 100 m away from the y-surface of the crystal, 98.6% of the output was lost. Additionally, narrowing the beam diameter further was difficult due to diffraction. As the beam diameter narrowed, the confocal length shortened and the conversion efficiency decreased. The low-frequency region generation efficiency was expected to decrease for the 20-mm cylindrical lens case because the confocal length shortened. This problem can be prevented by using a waveguiding structure. By limiting the beam diameter of the pump wave to half of the wavelength using only the waveguide mode for THz-wave generation, the phase mismatch can be neglected and absorption loss reduced. This is because the distance from the y-surface to the pump beam drops to almost zero, causing a higher conversion efficiency and a wider spectrum.
\n\t\t\tTHz-wave output spectra obtained using various cylindrical lenses, as measured by selecting the excitation wavelength in which the maximum output was obtained for each THz-wave frequency.
Here, we show that Cherenkov radiation with waveguide structure is an effective strategy for achieving efficient and extremely wide tunable THz-wave source (Suizu et al., 2009). We fabricated MgO-doped lithium niobate slab waveguide with 3.8 m of thickness and demonstrated difference frequency generation of THz-wave generation with Cherenkov phase matching. Extremely frequency-widened THz-wave generation, from 0.1 to 7.2 THz, without no structural dips successfully obtained. The tuning frequency range of waveguided Cherenkov radiation source was extremely widened compare to that of injection seeded-Terahertz Parametric Generator. The tuning range obtained in this work for THz-wave generation using lithium niobate crystal was the widest value in our knowledge. The highest THz-wave energy obtained was about 3.2 pJ, and the energy conversion efficiency was about 10–5 %. The method can be easily applied for many conventional nonlinear crystals, results in realizing simple, reasonable, compact, high efficient and ultra broad band THz-wave sources.
\n\t\t\tHere, we prepared a slab waveguide of a lithium niobate crystal. A Y-cut 5 mol % MgO-doped lithium niobate crystal on a thick congruent lithium niobate substrate was polished down to 3.8 m. A thin MgO-doped lithium niobate layer worked as an optical slab waveguide, because the refractive indexes of 5 mol % MgO-doped lithium niobate and congruent lithium niobate at 1300 nm are 2.22 and 2.15, respectively. The waveguide device was 5-mm wide and 70-mm long (X-axis direction). Each X-surface facet was mechanically polished to obtain an optical surface. We demonstrated difference-frequency generation using the experimental setup shown in Fig. 8(b). A dual-wavelength potassium titanium oxide phosphate (KTP) optical parametric oscillator (OPO) with a pulse duration of 15 ns, a pulse energy of 1 mJ and a 1300- to 1600-nm tunable range was used as a pumping source. A thin (3.4-m thick) polyethylene terephthalate (PET) film was slipped between the array of Si prism couplers and the Y-surface of the MgO-doped lithium niobate crystal. Directly placing an array of Si prism couplers on the Y-surface of the MgO-doped lithium niobate will inhibit the function of the MgO-doped lithium niobate layer as a waveguide for pumping waves, because the refractive index of Si in the near-infrared region is higher (about 3.5) than that of lithium niobate (about 2.2). A PET, in contrast, has a lower refractive index in that region (about 1.3), so adding a thin PET film does not inhibit the function of the crystal as a waveguide. An array of Si prism couplers on a PET film can work as a coupler for THz-frequency waves, because the PET film is thin compared to the wavelength of a THz-frequency wave. A schematic of the coupling system of the pumping wave and THz-wave emitting system is shown in Fig. 8(a). To couple pumping waves, the pump beam was reduced to few micrometers in the X-axis direction by a 3-mm diameter glass rod lens. The width of the pumping beams in the Z-direction was about 1.9 mm. The waveguide power density was about 53 MW cm-2, estimated from the pump wave pulse energy after waveguide propagation (about 60 J). We did not observe or calculate the waveguide mode of the structure in which a thin MgO-doped lithium niobate layer was sandwiched by a thick congruent lithium niobate layer and a thin PET film. It remains an area of future work to optimize the waveguide structure. The pump wave and THz-frequency wave polarizations were parallel to the crystal’s Z-axis. The THz-wave output was measured with a fixed 4-K Si bolometer.
\n\t\t\t\ta) Schematic of the lithium niobate waveguide device with Si prism array coupler. (b) THz-wave detection experimental setup.
\n\t\t\t\t\tFigure 9 shows a THz-wave spectrum at various wavelength of 1 from 1250 to 1350 nm. The spectrum was obtained by varying 2 at fixed 1. As shown in Fig. 9, high-frequency THz-wave output ranging to about 7.2 THz was confirmed. We were unable to observe THz-wave generation around 7.2 THz due to very strong THz-wave absorption at 7.5 THz by the LO-phonon mode. The THz-wave spectrum does not depend on pumping wavelength because the near-infrared refractive index is almost constant in the 1250- to 1350-nm range.
\n\t\t\t\tTHz-frequency spectrum of waveguided Cherenkov radiation. Black, red, blue and green curves represent pumping wavelengths of 1250, 1300, 1350 nm, respectively.
\n\t\t\t\t\tFigure 10 shows a comparison of normalized tuning spectrum of the waveguided Cherenkov radiation source and injection seeded terahertz parametric generator (is-TPG) (Kawase et al., 2002). Nevertheless each THz source were based on a same nonlinear crystal, MgO-doped lihitum niobate, a tuning frequency range of waveguided Cherenkov radiation source was extremely widened compare to that of is-TPG. We converted the output voltage of the Si bolometer to the actual THz-wave energy, using the fact that 1 V ≈ 20 pJ pulse-1 for low repetition rate detection, pulsed heating of the Si device, and an amplifier gain of 1000 at the bolometer. The highest THz-wave energy obtained was about 28 pJ, and the energy conversion efficiency from the λ1 wave (30 J pulse-1) was about 10–4%. This value is comparable to our previous work on Cherenkov radiation using bulk crystal, despite the low excitation energy of only 30 J. The tuning range obtained in this work for THz-wave generation using lithium niobate crystal was the widest value in our knowledge.
\n\t\t\t\tThe THz-wave emitting angle was absolutely constant, as Si dispersion in this range is almost flat. The device would be work well in an optical rectification process using a femtosecond laser. Such a range, free from structural dips between 0.1 and 7.2 THz, is suitable for ultra-short pulse generation. Also, the surface emission process used here is loss-less, permitting the generation of a continuous, widely-tunable THz-frequency range, and requiring only two easily commercially available diode lasers. Compact, robust and reasonable THz-wave sources can be realized by this method. Although we demonstrated this method using only a lithium niobate crystal, it can be adopted for other nonlinear crystals, such as LiTaO3, GaSe, GaP, ZnSe, ZnTe, ZGP, DAST and so on. By choosing the best clad materials for the nonlinear crystals (in many case Si or Ge), the Cherenkov condition is easily satisfied, and control of crystal angles to satisfy phase-matching conditions, such as birefringence phase-matching, is not required. This method opens the door to simple, reasonable, compact, highly efficient and ultra-broadband THz-wave sources.
\n\t\t\tA comparison of normalized tuning spectrum of the waveguided Cherenkov radiation source under 1250 nm pumping (red curve) and is-TPG (black curve).
We demonstrated a Cherenkov phase matched THz-wave generation with surfing configuration for bulk lithium niobate crystal (Suizu et al., 2009). THz-wave output was enhanced about 50 times by suppressing phase mismatching for THz-wave propagation direction. The suppression was achieved by combining two pumping waves with dual wavelength with finite angle, and THz-frequency was controllable by changing the angle within 2.5 degrees range. Higher frequency THz-wave generation at around 4.0 THz was successfully obtained by the method.
\n\t\t\tWe demonstrated Cherenkov phase matching method for monochromatic THz-wave generation via DFG process using bulk lithium niobate crystal. We successfully generated monochromatic THz-waves with wide tunability in the range 0.2–2.5 THz. The highest THz-wave energy was about 800 pJ/pulse, and this energy could be obtained for the broad spectral region in the range around 0.2–2.0 THz. Although we successfully got wide tunable characteristics of THz-wave generation, conversion efficiency of a THz-wave generation at higher frequency region above 2.0 THz was slightly low. It would be caused by phase mismatch of generated THz-wave in a propagating direction of THz-wave. Beam diameter of pumping waves in a lithium niobate crystal in our previous work was about 300 m, which corresponded to about ten cycles of THz waves at 2.0 THz because the refractive index of lithium niobate is about 5.2. THz-wave generated at far from a crystal surface interfered with that generated at neighborhood of a crystal surface, resulted in denying each other. By reducing the width of beam diameter in the crystal in the direction of THz-wave propagation to about one-half of the THz wavelength, there was no need to consider phase matching in that direction. We observed the effects by condensing a pump beam diameter to a THz-wave propagation direction by cylindrical lenses. Although higher THz-wave around 4.0 THz was successfully generated under tight focusing by the cylindrical lens with 20 mm of focus length, output of THz-wave at lower frequency region was reduced, because tight focusing resulted in reducing interaction length for pumping wave propagating direction.
\n\t\t\t\tIn this study, we propose surfing configuration of Cherenkov type phase matching for THz-wave generation for bulk crystal to suppress a phase mismatching. Interference pattern of pumping waves in the crystal is induced by combining the dual wavelengths beams with finite angle. It provides a same spatial pattern of second order nonlinear polarization in THz-frequency. The interference pattern has not checkerboard one, which is a results of interference of tilted beams with same frequency, because dual wavelength beam courses other spatial interference pattern, corresponding to difference frequency, and the interference pattern is superimposed in checkerboard one.
\n\t\t\t\t\n\t\t\t\t\tFigure 11 shows electric field distribution of (a) pumping waves and (b) excited nonlinear polarization, with 1=1300 nm, 2=1317 nm (here, three waves in DFG interaction has a relation of 1=2-THz, and corresponding THz frequency is 3 THz) and 3.7 degrees of angle between divided pumping beams, . The periods of nonlinear polarization pattern of dual wavelengths beams, A for x-axis and B for y-axis are represented by following equations,
\n\t\t\t\twhere k1=2n1/1 and k2=2n2/2, here n1 and n2 are refractive index of 1 and 2, respectively. We used Sellmeier equation at near-infrared region for a lithium niobate crystal (Jundt, 1997). On the other hands, Cherenkov angle of the crystal, c, is decided by relation of length A and THz-wavelength in the crystal, C=THz/nTHz, here THz and nTHz are THz-wavelength in vacuum and refractive index of the crystal at THz frequency. A phase matching condition for THz-wave propagation direction is satisfied by choosing an appropriate angle of the pump beams for required THz-frequency. The angle is formulated from geometric relation of A, B and C, A2C2=B2C2=A2B2, as shown in Fig.11(c).
\n\t\t\t\tGenerated THz-wave can propagate without influence of phase mismatching in the direction of propagating direction, just like as surf rider on nonlinear polarization waves, as shown in Fig.11 (b). The required angle for frequency tuning was shown in Fig.12 (a) internal and (b) external crystal. Phase matching condition is satisfied by changing the angle for required THz-wave and pumping wave wavelength. And slightly narrow tunability (about 300 GHz at around 3 THz generation) is obtained at a fixed angle, =4.0 degrees.
\n\t\t\t\n\t\t\t\t\tFigure 13 shows the schematic of experimental setup. A pump source for DFG process was same as our previous works, and which has a tunable range of 1250 to 1500 nm, 15 ns of pulse duration and 0.88 mJ of pulse energy. An output of the source with dual wavelength
\n\t\t\t\tNormalized electric field distribution of (a) combined dual wavelength pump beams with finite angle, and (b) exited second order nonlinear polarization of difference frequency. Here, 1=1300 nm, 2=1317 nm and 3 THz of difference frequency with 3.7 degrees of beam angle. (c) Geometric relation of A: excited nonlinear polarization for x-direction, B: interference period of pump beams for y-direction and C: THz-wavelength in the crystal.
Tuning angle (a) internal and (b) external of crystal under 1300, 1400 and 1500 nm of pumping wavelength of 1.
was focused by circular lens (f=500 mm) before divided by half beam splitter, and combined again with finite angle. The spot diameter of the combined beam was 0.45 mm. The 5 mol % MgO-doped lithium niobate crystal (MgO:LiNbO3) used in the experiment was cut from a 5 65 6 mm wafer. The polarizations of the pump and THz waves were both parallel to the Z-axis of the crystals. The THz-wave output was measured with a fixed 4 K Si bolometer.
\n\t\t\tSchematic of experimental setup for Cherenkov phase matching THz-wave generation with surfing configuration.
Input-output properties of THz-wave for pumping energy are shown in Fig.14 at 1.0 THz generation with =2.49 degrees. Circles and triangles denotes THz-wave output signal with combined beams and with single beam by dumping the other beam before entrance to the crystal, respectively. Maximum pumping energy of only 0.44 mJ was achieved at single beam pumping, because a half of whole pumping energy was dumped as shown in Fig.13. The vertical axis is the THz-wave pulse energy calculated from the output voltage of a Si-bolometer detector, a pulse energy of about 101 pJ/pulse corresponded to a Si-bolometer voltage output of 1 V when the repetition rate was less than 200 Hz. As shown in the figure, remarkable enhancement of THz-wave generation with surfing configuration, whose magnetic was about 50 times, was successfully observed. Inset of Fig.14 shows double logarithmic plot of input-output properties. Slope efficiency under combined beams and single beam pumping were almost same values. It means that enhancement factor of about 50 was a result of a suppression of phase miss-matching.
\n\t\t\t\tThe generated THz-waves at different position in the crystal were in-phase each other, and outputted THz-wave was enhanced. Intensity of overlapping in-phase THz-waves in an absorptive media was calculated as shown in Fig.15. A 5 mol % MgO-doped Lithium Niobate crystal at THz-wave frequency region would has about 30 cm-1 of absorption coefficient (Palfalvi et al., 2005). The enhancement effect of in-phase interference would be effective for about 2 mm of traveling distance of THz-wave, this fact leads optimum pumping beam width in y-axis direction is about 1.8 mm. In this study, pumping beam width in y-axis was about 0.45 mm, results in a propagating length of a THz-wave was about 1.2 mm. Higher enhancement above 50 would be obtained with tight focused beam only for z-axis by cylindrical lens.
\n\t\t\t\tInput-output property of THz-wave for pumping energy at 1.0 THz generation with =2.49 degrees. Circles and triangles denotes THz-wave output signal with combined beams and with single beam. Inset shows double logarithmic plot of input-output properties.
Calculated intensity of overlapping in-phase THz-waves in an absorptive media.
\n\t\t\t\t\tFigure 16 shows THz-wave output characteristics under fixed pumping wavelength of 1300 nm and several fixed angle, 2.49, 3.80 and 5.03 degrees. Maximum THz-wave output at each angle was obtained at higher frequency in the bigger angle, . Obtained peaks of THz-wave output were about 1.1, 1.6 and 1.9 THz, respectively. The relation between the angle and the frequency where maximum output was obtained agree well with Equation 4, 1.08, 1.61 and 2.07 THz under 1300 nm pumping respectively. Tuning range for higher frequency region was remarkably improved compare with our previous collinear and not tight focused configuration. THz-wave output at around 4 THz was successfully obtained.
\n\t\t\t\tTHz-wave output spectra under fixed pumping wavelength of 1300 nm and several fixed angle, 2.49, 3.80 and 5.03 degrees.
As described in our previous work, because the linewidth of each pumping wave is about 60 GHz, the source linewidth is about 100 GHz, which is slightly broader than that obtained from sources such as injection-seeded terahertz parametric generator (Kawase et al., 2002) or DAST crystal-based difference-frequency generators (Powers et al., 2005). This occurs because the linewidth of the THz-wave depends on that of the pumping source.
\n\t\t\t\tThe spectrum with =2.49 degrees pumping had two dips at 1.8 and 2.6 THz. It coursed by perfect phase miss-matching of THz-wave propagation. Figure 17 shows calculated nonlinear polarization distributions at (a) 1.8 and (b) 2.6 THz generation with =2.49. THz-wavelength in the crystal at 1.8 THz generation is 32.2 m. Generated THz-wave at point “a” in Fig.17 interferes with that at point “b”, which has a phase difference by compare to that of point “a”, results in destructive interference. Similarly, and adding higher order interference, generated THz-wave at point “c” has destructive interference with that at point “d”. THz-wave generation was observed at around the dips, because perfect phase miss-matching was relaxed at these frequencies. We have not yet completed the analytical solution predicting the frequency due to destructive interference, and it remains an area of future work.
\n\t\t\t\tBroader tuning range would be obtained by controlling the angle within about only 2.5 degrees range. Because lithium niobate is strongly absorbing at THz-frequencies, the beam-crossing position was set near the crystal surface to generate the THz-wave. In this configuration, the pumping beam passing through a Si prism yields an optical carrier excitation in Si that prevents THz-wave transmission, while the interaction length decreases at larger pumping angles, . The interaction lengths,
\n\t\t\t\twhere D is the beam diameter, are 21.4 and 10.7 mm for s of 2.49 and 5.03 , respectively. If we use a shorter lithium niobate crystal, the optical carrier excitation can be avoided, and larger pumping angles can be employed to obtain higher-frequency generation. The method is very simple way to obtain higher frequency and efficient generation of THz-wave, because the method does not require a special device such as slab waveguide structure.
\n\t\t\t\tCalculated nonlinear polarization distributions at (a) 1.8 and (b) 2.6 THz generation with =2.49.
Water scarcity jeopardizes not only originally arid, semi-arid regions but also agricultural areas in which farmers obtain flourishing horticulture based on adequate water resources. Nonetheless, ongoing climate change supposed to amplify the frequency and severity of drought in different regions of the globe [1] can wipe out the so far achievements. Drought is the most devastating stress that remarkably diminishes crop productivity more than any other stress factor [2]. Water constraints provoke stomatal closure with a subsequent reduction of CO2 influx resulting in a decrease in photosynthetic activity and carbon partitioning [3]. Also, water scarcity has a negative influence on nutrient supply, reducing phosphate availability. Severe drought profoundly affects plant physiology, growth, development, and reproduction, and exerts substantial losses in crop yield as well as reduces crop quality. In fact, over the past 35 years, worldwide drought inflicted yield decrease by 40% in maize and 21% in wheat production [4]. Thus, there is an urgent need to develop strategies to make agriculture more resilient and to alleviate the adverse impacts of water scarcity on crop yield. Among these strategies, there has been an increasing interest in beneficial soil microbes including arbuscular mycorrhizal (AM) fungi.
\nNotably, under natural conditions, plants frequently interact with microbes, which directly mediate plant responses to environmental adversities. Some microbe-plant interactions lead to a mitigation of stress-related damages and improvement of plant tolerance to stressful conditions [5]. As a crucial element of soils, microbes are an integral part of the agricultural ecosystem. Arbuscular mycorrhizal fungi (AMF) are ubiquitous soil microorganisms, which can form a symbiotic association with most terrestrial plants. These beneficial microbes have been proved to offer an array of benefits to host plants [6]. During mycorrhization, besides significant improvement of plant nutritional status, AMF can enhance plant performance and tolerance against several stresses, particularly drought stress [7]. The exploitation of AMF is considered as one of the most efficient practices to increase plant tolerance to environmental stresses [8]. Previous studies illustrate the substantial contribution of AM symbiosis to improved stress plant tolerance to water deficit by various mycorrhizal benefits such as strengthened water and nutrient uptake, alterations in host physiology, for example, photosynthesis, osmotic adjustment, phytohormones, and more efficient antioxidative systems [9, 10, 11]. This chapter presents the current knowledge on AMF application to crop production under water deficit. Variable benefits of AMF are also discussed to explain the reason why positive outcomes of AM colonization are not always the case. Finally, challenges of the fungal symbiont application are highlighted for practical use in crop production.
\nAMF are obligate root symbionts inhabiting almost all terrestrial ecosystems. They can form a symbiotic association with around 80% of vascular plants and with approximately 90% of agricultural plants [12]. In this mutual association, the fungus receives 10–20% of total photosynthates [13] and lipids [14] from the host plant, whereas the plant is enhanced through uptake of water and mineral nutrient by the mycorrhizal partner [12]. AMF are the most common fungi in soils and represent 9–55% of the soil microbe biomass and 5–36% of the total soil biomass [15]. These fungi play a vital role in agricultural ecosystems, since they can improve plant nutrient, water status, and plant growth [12], enhance survival rate and development of seedlings, crop uniformity, and reproductive capacity [16], decrease the input of P and N fertilizer, and increase resistance or tolerance to environmental adversities [8, 17].
\nCurrently, AMF are classified as a member of phylum Glomeromycota including four orders (Archaeosporales, Diversisporales, Glomerales, and Paraglomerales), with 11 families, 25 genera, and nearly 250 species [18]. However, data based on next-generation sequencing of root samples [19] and recent results [20] suggest that its number may be an order of magnitude higher. Spores of AMF which are the major survival units of AMF have multi-nucleate, heterokaryotic structures [21], and are formed singly, in clusters or sporocarps in the soil, and within root tissue in some mycorrhiza species as well (Figure 1A–C). The development of AM symbiosis starts with signaling taking place before physical contact between the plant and the fungus. Both partners produce molecular signals triggering preparative responses in the other [22]. The mycorrhization process can be divided into distinct steps, consisting of germinating spores, hyphae differentiation, appressorium formation, penetration of the host root, intraradical hyphae formation, intercellular growth along with developed external mycelium (extraradical hyphae), and arbuscule formation, subsequently exchanging nutrients and carbohydrates between the host and fungus [23].
\nTomato roots without (A) and with (B–D) staining showing AM fungal structures. The presence of arbuscular mycorrhiza (AM) structures (arbuscules, vesicles, intraradical hyphae, and spore) was assessed by means of an Olympus BX51 light microscope with Nomarski interference contrast optics, using an objective of 40×. Scale Bar representing 20 μm.
The primary structures of AMF consist of coenocytic hyphae with unlimited growth in the rhizosphere called external hyphae, which penetrate the cortex layer of roots and form different organs. The extraradical hyphae merely in some species of Diversisporales [18] producing auxiliary cells could have functions in reproduction or nutrition and storage [24]. Mycelium outside the roots absorb mineral nutrients and water and subsequently transport them to the host plant via intraradical hyphae (Figure 1C,D) growing inside root cells [6]. Hyphae growing within roots form either the Paris-type or the Arum-type. The Paris-type is featured by intracellular mycelium development to shape coils, whereas the Arum-type is characterized by intercellular hyphae growth forming arbuscules [12] (Figure 1D), thereby establishing the nutrient exchange sites between AMF and the host plant [25]. Vesicles containing high quantity of lipids and glycogen are formed from intraradical hyphae at intercalary position (their terminal) in the root, functioning as nutrient storage, and propagules [23] but not all AMF produce vesicles.
\nAMF species isolates differ in the ability to spread mycelia, the viability, structure, and possibility of anastomosis [26, 27]. Taxonomic variation in mycelium structure among AMF families was also observed [28]. Gigasporaceae are prone to possess vigorous, thickly aggregated mycelium with densities from 6 to 9 m cm−3, while Acaulosporaceae and Glomeraceae show a tendency to maintain thinly dispersed mycelium with densities from 1 to 2 m cm−3.
\nAlthough a majority of plants are responsive to AMF, plant species in families Amaranthaceae, Brassicaceae, Caryophyllaceae, Chenopodiaceae, Cyperaceae, Juncaceae, and Urticaceae are rarely or never colonized by the symbiotic fungus [29]. How AMF evaluate the AM host and nonhost status of plant species is not well known. The current hypothesis proposes that nonmycorrhizal plant species lost orthologs of important putative genes, required for symbioses [30], and/or cannot synthesize or degrade strigolactones, essential signals for symbiosis establishment [31], and/or their root exudates constitute antifungal products [29]. Under certain conditions, some nonhost species develop rudimentary AM phenotypes described by Cosme et al. [30] giving a more in-depth explanation of this question.
\nUtilization of AMF has become an appealing tool for sustainable agriculture due to the positive attributes of mycorrhizal symbiosis. Nevertheless, the opposite or neutral influence of AMF has also been found [32]. The obligate biotrophic life cycle of AMF which relies on photosynthates supplied by a nurturing autotrophic host is the key point; therefore, choosing the right partner (target plant) is crucial. Even though this widespread symbiont is thought to be a generalist due to low host specificity, each AMF species highly varies in the responsiveness to the host plant. Hence, the variable benefits of AM symbiosis exist among mycorrhiza species [10, 33]. The interaction between the host plant and AMF could range from mutualism to parasitism in which colonized plants exhibit a decrease in growth [34] owing to the carbon drainage in the host inflicted by the fungus [35]. Many factors that can affect the AM benefits to target plants include host plant genotypes, AMF species, and environmental conditions. Dissimilar plant responses to different AMF species under environmental adversities have been observed [11, 36]. Fascinatingly, AM benefits for plant fitness augment with adversity, supporting the concept of AM colonization as a ‘health insurance’ for host plants, in which the beneficial effects of AMF become more obvious under stressful environments [36]. Metabolites differentially accumulated in roots colonized by different fungal symbionts (Rhizophagus irregularis, Funneliformis mosseae, and Claroideoglomus etunicatum) under abiotic stresses, which may underlie their enhanced stress tolerance in host plants [36]. Cultivar differences in response to mycorrhizas have been reported in many crops such as tomato [37], pepper [38], wheat [39], maize [40], and some other crops [41]. For chickpea, only three of thirteen varieties with different genotypes and phenotypes were more positively responsive to AM mixed inoculation with Diversispora eburnea, Claroideoglomus etunicatum, and Glomus sp. [42]. More recently, twenty geographically different barrel clover (Medicago truncatula) accessions showed differences in their growth, stomatal conductance (gs), and AM colonization in response to Funneliformis mosseae treatment [43]. Also, root hydraulic conductivity, expression of the mycorrhiza-induced phosphate transporter gene (MtPT4), and five aquaporin genes (MtAQP1, MtPIP1, MtPIP2, MtNIP1, and MtNIP4) vary with mycorrhizal treatment during further analysis of five accessions. In the case of wheat, old accessions have been shown to be more responsive to AMF than new ones [39].
\nSelection and breeding programs generally tend to maximize plant performance and crop yield under high-input production systems, which could cause the loss of genes, phytochemicals, and/or other plant traits which are necessary for the establishment of efficient symbioses. Modern cultivars could absorb phosphate without the AM assistance in soils with high phosphorus availability, decreasing the degree of AM dependence. As a consequence, AMF are less responsive to new lines. Recent research has proved that domestication decreased AM benefits for domesticated crops in exposure to high P supply [44]. However, in maize, which is highly mycorrhizal-dependent, modern breeding programs do not necessarily result in the less mycorrhizal colonization. Replicated field experiments with 225 genotypes consisting of hybrids, inbred lines, and landraces originating from different locations were conducted for two consecutive years to explore the variation in mycorrhizal colonization [40]. The findings showed that AM colonization differed profoundly and continuously among genotypes, with substantially greater values in modern hybrids than old landraces and inbred lines.
\nIt is well known that AMF offer indispensable advantages to the host plant subjected to water shortage, with two major strategies that mycorrhizal plants use to deal with water deficit: drought mitigation and drought tolerance. Drought mitigation strategy is involved in indirect AM benefits and enhanced water uptake through the extensive hyphae network, enabling host plants to suffer less stress than non-AM plants, whereas drought tolerance includes a combination of direct AM benefits that improve plant’s innate ability to cope with the stress (Figure 2).
\nStrategies of mycorrhizal plants to cope with water scarcity, that is, drought mitigation and drought tolerance. Multiple benefits/mechanisms could be simultaneously induced by arbuscular mycorrhizal fungi in the host plant exposed to water deficit. The blue arrows show increase/up-regulation, whereas the orange arrows indicate decrease/down-regulation, relative to control non-mycorrhizal plants. Italic words indicate genes. ABA, abscisic acid; AQP, aquaporin; Car, carotenoids; Chla, chlorophyll a; Chlb, chlorophyll b; Fv/Fm, maximum quantum efficiency of PSII; gs, stomatal conductance; IAA, indole-3-acetic acid; iWUE, intrinsic water use efficiency; JAs, jasmonates; LWP, leaf water potential; MDA, malondialdehyde; MeJA, methyl jasmonate; PN, net photosynthesis rate; ROS, reactive oxygen species; RWC, relative water content; SLs, strigolactones.
An important benefit of AM colonization to the host plant under drought stress is a superior water allocation mediated by the fungal hyphal network, facilitating the colonized root access to water in a lower soil water potential [45]. Indeed, the host root system is extended by widespread extraradical mycelia, enabling colonized roots to reach more water and nutrient pools unavailable to uncolonized roots. Fungal hyphae diameters (3–7 μm) are much smaller than those of fine root hairs (5–20 μm); nevertheless, hyphal densities are ten-hundred times higher than root densities [46]. Hence, the absorption surface of mycorrhizal roots is improved substantially. It is calculated that the rate of water transport from external hyphae to the root ranged from 0.1 [47] to 0.76 μl H2O h−1 per hyphal infection point [48], which is adequate to alter plant water relations [47]. Lettuce plants pretreated by Rhizophagus irregularis, Funneliformis mosseae, Funneliformis coronatum (formerly Glomus coronatum), and Claroideoglomus claroideum (G. claroideum) obtained 3–4.75 ml H2O plant−1 day−1 higher than uncolonized plants, which might be related to the amount of extraradical mycelium and root colonization frequency [45]. Furthermore, AMF contribute approximately 20% to total plant water uptake [49], highlighting the role of the symbiosis in the water status of host plants.
\nThe widespread extraradical mycelia also enhance the absorption of mineral nutrients in soils, which is more critical for host plants under water-stress conditions where nutrient mobility is limited. As soon as external hyphae transport water to the host, mineral nutrients also follow the water flow to the plant from the soil-root interface [50]. AM colonization is well known to improve phosphorus (P) nutrient into the host plants particularly under low-nutrient conditions, increasing stress tolerance in plants. Interestingly, plants possess a symbiotic inorganic phosphate (Pi) uptake pathway, and AM symbiosis has been proved to specifically induce the expression of genes encoding plant Pi transporters to enhance P acquisition, for instance, LjPT4 in Lotus japonicus and MtPT4 in Medicago truncatula [51], recently LbPT3, LbPT4, and LbPT5 in Lycium barbarum [52]. Under water restrictions (moderate and severe), different expressions of five tomato PT genes (LePT1-LePT5) in the absence/presence of Rhizophagus irregularis or F. mosseae were observed [53]. LePT4 was overexpressed in R. irregularis-colonized plants exposed to both water-stress levels, while this upregulation was in F. mosseae-infected plants subjected to severe water stress. A role of PT4 genes in root tips, creating a connection among root branching, Pi-signaling mechanisms, and Pi-perception has been proposed [51]. In addition, on the fungal side, R. irregularis PT gene was up-regulated under moderate drought conditions [53]. Phosphate is taken up by mycorrhizal phosphate transporters and assimilated to polyphosphate translocated toward the plant. This process is facilitated by the activation of fungal aquaporins [54].
\nApart from that, AM colonization enhances the rate of nitrogen (N)-assimilation of plants under drought [55] as a result of the direct uptake of NO3− or NH4+ by fungal hyphae [56]. Several NO3− and NH4+ transporters and metal transporters in AMF [57, 58] while mycorrhiza-inducible ammonium transporters in some plants have been identified [59, 60]; therefore, AMF considerably contribute to the total N uptake of the host. Increased N nutrient could promote protein synthesis and higher levels of compatible osmolytes in stressed AM plants. Other studies also confirmed that inadequacy of necessary macro- and micro-nutrients could be alleviated in mycorrhizal plants under water deficit [61, 62]. Hydraulic conductivity of colonized roots was enhanced to absorb more N, P, and K, leading to a higher protein concentration in host plants under drought stress [63]. Thus, more vigorous uptake of water and nutrients may provide adequate necessary substances for better growth of mycorrhizal plants under such stress.
\nThe negative water potential in dried soils exerts the problem for plants to obtain adequate water amount, a process where aquaporins (AQPs) get involved in [64]. AQPs belonging to the large major intrinsic protein family of transmembrane proteins functioning as water channels are crucial in osmoregulation [64]. On top of that, their regulation of transcellular movement of many molecules such as small alcohols, boron, and osmolytes has been reported [65]. In AMF, the first AQP gene GintAQP1 of Rhizophagus irregularis was cloned, with evidence of a compensatory mechanism between GintAQP1 expressions and the host aquaporins under drought stress [66]. Furthermore, two AQP genes GintAQPF1 and GintAQPF2 present in Rhizophagus irregularis were upregulated under osmotic stress, assisting the fungus survival and contributing to the host plant tolerance to water stress [67, 68]. Upregulation of RiAQPF2 in Rhizophagus irregularis was also found under water deficit [10], suggesting its putative involvement in host plant tolerance in response to drought.
\nOn the plant side, AMF could induce changes in the expression of various AQP genes in the host in order to strengthen root hydraulic conductivity and host tolerance under water-stress conditions in several plants, such as maize [69, 70, 71], tomato [10, 11], black locust [72], trifoliate orange [73], olive [74], and Populus x canadensis plants [75]. AM-induced alterations in expression of plant AQPs could depend on stress duration as the observation in maize plants [69]. Under short-term water deficit, the AM symbiosis upregulated ten AQP genes with diverse aquaporin classes in roots inoculated with Rhizophagus intraradices, stimulating more water uptake in the host [69]. By contrast, under sustained water-stress conditions, AM-mediated downregulation of 6 different AQP genes was found, restricting plant water loss [69]. Intriguingly, drought-sensitive cultivars may gain higher physiological benefit from AM inoculation than drought-tolerant cultivars [71]. Downregulation of genes TIP1;1, TIP2;3, PIP1;1, PIP1;3, PIP1;4, PIP1;6, PIP2;2, and PIP2;4 whereas only upregulation of TIP4;1 were observed in drought-sensitive cultivar colonized by Rhizophagus irregularis, supporting the decrease in water loss in host plants subjected to drought stress [71]. Recent research also revealed a significant shift in the transcriptional regulation profiles with AQP genes as potential targets in mycorrhizal roots, in comparison to non-AM ones during a water stress event, which may influence some key metabolic pathways linked with drought response [76]. In parallel, it has been proposed that during drought stress a controlled mechanism mediated by the presence of arbuscules at cortical cells in roots fine-tuned the gene expression regulation in the host plant [76].
\nIn general, fungal and plant AQPs work together in mycorrhizal plants under water restrictions. The simultaneous induction of both fungal and plant AQP genes together with differential regulation of drought-responsive genes in host plant indicates that AMF mediate colonized plant responses to drought stress.
\nNumerous reports illustrate that AMF could increase photosynthetic activity or protect the photosynthetic apparatus under water stress conditions [77, 78]. In fact, AM colonization considerably influences the stomatal behavior in the leaves of host plants, determining the water vapor efflux, CO2 gas exchange, and thus photosynthetic activity [79]. Stomatal conductance changed by AM inoculation is closely connected to leaf water potential and relative water content in host plants. Under water restrictions, the first response of plants is stomatal closure to limit water loss through transpiration. Additionally, reduction of CO2 uptake and carbon assimilation whereas favoring photorespiration may occur in plants [80]. Upregulation of LeEPFL9 involved in the regulation of stomatal development together with greater stomatal density was found in tomato plants colonized by R. irregularis [10]. Inoculation of Septoglomus deserticola or S. constrictum sustained stomatal opening in host plants under drought conditions, substantially contributing to the carbon assimilation [11]. Improvement of stomatal conductance (gs) in mycorrhizal castor bean [78], black locust [72], and strawberry [81] plants exposed to water stress has been detected.
\nOne of the widely known benefits of mycorrhizal inoculation is the improvement of host water status under drought stress. Leaf water potential (LWP) and relative water content (RWC) of plants were substantially higher in the presence of mycorrhiza [11, 81]. Several studies illustrated a higher water use efficiency or intrinsic water use efficiency in AM plants during water stress [10, 81, 82]. It is believed that photosynthetic activity correlates with chlorophyll content and stomatal conductance, which have been enhanced by AMF. Drought stress changes photosynthetic pigments and damages chloroplasts. Nonetheless, AM inoculation alleviates the damage of these parameters caused by the stress [77]. Rhizophagus irregularis-colonized castor bean plants subjected to water restriction increased contents of chlorophyll a (by 26%), b (30%), carotenoid (by 28.5%), and total chlorophyll (25.5%) in comparison to counterparts of non-AM plants [78]. These increases in AM plants may be attributed to the improved nutrient uptake, particularly N and Mg that are structural components of chlorophyll.
\nMycorrhizal colonization has been found to alleviate the adverse impacts of drought stress on photochemical efficiency and photosystem II (PSII) reaction center [77, 83]. Under water deficit, application of AMF promoted a higher maximum quantum efficiency of PSII (Fv/Fm) [11], greater photosynthetic efficiency [84], transpiration rate, and net photosynthesis rate (PN) [10, 81]. Although mycorrhizal plants usually have higher photosynthetic capacities, environmental factors such as high atmospheric drought or low radiation can decide the beneficial effects of mycorrhiza on photosynthesis [85].
\nPhytohormones not only modulate a plethora of events during plant development but also are essential signaling molecules for interaction between plants and AMF [86]. Changes in plant hormone homeostasis also affect plant tolerance against abiotic stresses [87, 88]. During mycorrhization, changes in levels of several plant hormones have been reported [86], hence may contribute to the improved host plant tolerance to subsequent stresses.
\nAbscisic acid (ABA) is the most fundamental stress hormonal signal, modulating transpiration rate, root hydraulic conductivity, and aquaporin expression [89]. The concentration of ABA is heightened in plant tissues under drought stress to induce stomatal closure for reduction of water loss and activate different stress-responsive genes, increasing plant tolerance to drought [90]. A lower ABA concentration was found in roots and leaves of mycorrhizal plants versus nonmycorrhizal plants under drought stress [9, 10, 91]. Downregulation of SlNCED gene, a critical ABA biosynthetic gene, in Septoglomus constrictum-infected roots under water stress concurred with the greater gs and higher water status of tomato plants, indicating a higher stress tolerance in colonized plants compared to uninoculated plants [11]. Nonetheless, an increase in ABA concentration in trifoliate orange plants colonized by F. mosseae was also observed under drought stress [73]. The reason for this remains poorly understood, which requires further research.
\nThe role of jasmonate (JA) in water uptake and transport, exerting influence on stomatal conductance, root hydraulic conductance, and regulating the expression and abundance of aquaporins in tomato plants has been revealed [91]. Tomato plants defective in JA synthesis altered the AM impacts on the host plant, interfering phytohormones and expression of AM-induced aquaporin genes. The content of JA and its precursors was higher in leaves of Digitaria eriantha plants infected by Rhizophagus irregularis under water deficit, relative to noninfected plants, which could enhance plant tolerance to the stress [92]. Likewise, mycorrhizal inoculation substantially increased methyl jasmonate (MeJA) in trifoliate orange plants exposed to drought stress [93]. Under water-stress conditions, significantly higher expression levels of JA-biosynthetic gene SlLOXD in roots and leaves of colonized tomato plants were detected, supporting plant response to drought stress by triggering a LOXD-mediated pathway [10, 11].
\nStrigolactones (SLs), as phytohormones, not only modulate the coordinated development of plants exposed to nutrient shortages but are also host detection signals for AM establishment in the host plant [94]. Upregulation of the SL-biosynthesis gene SlCCD7 together with a greater content of SLs was found in Rhizophagus irregularis-inoculated tomato roots subjected to water-stress conditions, correlated with the increase in AM colonization rate [9]. The stimulated production of SLs promoting symbiosis establishment as a strategy of plants to cope with drought stress has been proposed.
\nAuxin is a key regulator in root-hair initiation, growth, and developmental processes [95, 96]. In a recent study, an increased content of indole-3-acetic acid (IAA) which is the dominant naturally occurring auxin was found in mycorrhizal tomato plants exposed to drought [91]. Similarly, stimulation of biosynthesis and transport of IAA in roots of trifoliate orange under water restrictions were demonstrated [97]. Under drought conditions, AM colonization overexpressed PtYUC3 and PtYUC8 involved in IAA biosynthesis, and downregulated auxin efflux carriers (PtPIN1 and PtPIN3), while up-regulated auxin-species influx carriers (PtABCB19 and PtLAX2) in roots, leading to significantly higher IAA accumulation in mycorrhizal roots versus non-AM roots [97]. Together with higher IAA, colonized trifoliate orange plants showed a significant increase in MeJA, nitric oxide, and calmodulin in roots, supporting greater root adaptation of morphology as a crucial strategy for drought adaptation [93].
\nAlthough important roles of phytohormones are irrefragable in plant responses to water stress, little attention has been paid to them in mycorrhizal plants. Previous studies have just revealed changes in concentrations and expression of genes encoding biosynthesis of few hormones in colonized plants during drought stress; thereby, further research is required to understand it.
\nIn response to drought stress, plants accumulate compatible solute compounds or osmolytes functioning for osmotic adjustment to maintain a favorable gradient for water uptake [98]. Osmotic adjustment is essential for water influx, turgor maintenance, sustaining physiological activity in plants such as stomatal opening, photosynthesis, cellular expansion, and growth during the stress [98]. Compatible solutes include a variety of sugars, proline, glycine betaine, polyamines, and organic acids such as oxalate and malate [99]. Interestingly, discrepant observations in osmolyte accumulation have been reported in a wide range of mycorrhizal plants [10, 83, 100, 101].
\nProline, an amino acid, plays a crucial role in osmoregulation and acts as an efficient scavenger of reactive oxygen species (ROS) [102] (discussed in Section 4.1.7). Enhanced drought tolerance with a higher proline concentration in mycorrhizal plants has been shown in many studies [10, 78, 100]; nevertheless, opposite results have also been reported [81, 83]. Inoculation of either F. mosseae or Paraglomus occultum in trifoliate orange plants substantially reduced leaf proline content but improved the host plant growth under water deficit [103]. These results suggest that AMF strongly altered leaf proline metabolism through regulating proline-metabolized enzymes, which is important for osmotic adjustment of the host plants.
\nSugars are osmoprotectants, which contribute up to 50% of osmotic potential in plants [104, 105]. In general, under water stress, the higher accumulation of total soluble sugars offers a defense mechanism in mycorrhizal plants such as watermelon [100] and flax [106]. Concentrations of sucrose, glucose, and fructose were significantly heightened in leaves of mycorrhizal trifoliate orange seedlings exposed to drought, which could function as osmolytes to stabilize and protect structures and macromolecules in plants from the stress, therefore improving host plant tolerance [103]. AMF-mediated increases in leaf sugar metabolism by modulating sugar-metabolized enzymes notably contribute to the osmotic adjustment of colonized plants. However, contrast observations have been shown in olive trees [101] and maize [107] colonized by AMF, which may be due to the fact that host plants suffer less stress. Noticeably, under severe drought inoculation with Rhizophagus clarus significantly reduced soluble sugars in leaves of strawberry plants, but this parameter was remarkably enhanced in roots in response to mild and severe water stress [81]. These changes together with an improved water status and plant biomass suggest different strategies for the enhanced water status triggered by AMF in roots and leaves of strawberry.
\nIn summary, increased accumulation of compatible solutes in AM-inoculated plants in exposure to water deficit is supposed to protect plants from the stress and curtail the plant osmotic potential, whereas the lower osmolyte accumulation in host plants is thought to be due to colonized plants successfully gaining drought mitigation.
\nOne of the consequences of water stress is the overproduction of reactive oxygen species (ROS) such as hydroxyl radicals (˙OH), superoxide radicals (O2˙–), singlet oxygen (1O2), and hydrogen peroxide (H2O2) mainly in chloroplasts and mitochondria. The excessive ROS results in unbalanced cellular homeostasis and then oxidative stress, damaging membrane lipids, proteins, and nucleic acids and even causing the death of cells [108]. To cope with oxidative stress, plants have evolved ROS scavengers in both nonenzymatic and enzymatic defense systems. Nonenzymatic antioxidants comprise phenolic compounds, glutathione, ascorbic acid, alkaloids, carotenoids, and tocopherol [109], which not only play a direct role in ROS removal but also serve as a substrate for the antioxidant enzymes in scavenging ROS [110]. Under water deficit, AMF ameliorate oxidative damage through augmented production of phenolic compounds and secondary metabolites detoxifying ROS in various plants [111, 112, 113]. AM inoculation also significantly increased the concentrations of anthocyanins and carotenoids [106] and ascorbic acid [82, 106] in plants in exposure to water constraints.
\nAnother important ROS scavenger system is enzymatic antioxidants which could be enhanced in mycorrhizal plants including superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), guaiacol peroxidase, ascorbate peroxidase (APX), glutathione reductase (GR), dehydroascorbate reductase (DHAR), and monodehydroascorbate reductase (MDHAR) [110]. The AM symbiosis has been reported to improve plant protection against oxidative stress by decreasing the level of lipid peroxidation (MDA) and H2O2 accumulation by strengthening significantly antioxidative enzymes SOD, POD, and CAT in roots and leaves under mild and severe drought [11, 81]. SOD and CAT are the most important ROS scavenging enzymes among the enzymatic antioxidants. These enzymes together with the cooperative enzymes (GR, MDHAR, DHAR, and APX) in the ascorbate-glutathione (ASA-GSH) cycle play pivotal roles in controlling overproduced ROS to maintain cellular homeostasis [114, 115, 116]. Remarkable increases in SOD, CAT, GR, APX, and MDHAR at transcription and enzymatic level correlated with lower O2˙−, H2O2, and MDA have been revealed in drought-stressed mycorrhizal plants versus counterparts of non-AM plants, improving host protection against oxidative damage [101].
\nHigher nonenzymatic and enzymatic antioxidants in colonized plants help for the rapid and efficient elimination of excess ROS. Nevertheless, discrepant results, no change or decrease in ROS scavengers, have also been demonstrated [70, 117]. Results are not entirely consistent with all reports because of different ages of host plants [118] and/or the specific combination of mycorrhizal strains and plant species, even cultivars [11] (as discussed in Section 3) or successful drought mitigation in colonized plants.
\nThe hyphal network of AMF is believed to improve soil water retention properties in the mycorrhizosphere through its physical, biological, and chemical influences. It has been reported that AMF produce polysaccharides, glomalin, mucilages, and hydrophobins that act to bind soil particles, leading to soil aggregation with enhanced water-holding capacity in soil [119]. Glomalin, a stable glycoprotein, highly persists in the soil, defined as glomalin-related soil protein (GRSP) [120]. The higher amounts of GRSP in the soil, the more enhanced capacity of water retention was found since soil aggregation increased protection of C-rich debris from the decomposition of soil microbes [120, 121]. Indeed, fungal hyphae coated by GRSP sharp a hydrophobic layer into the aggregate surface, hence decreasing water loss within soil aggregates [122]. When the fungal hyphae form branching structures with glomalin, they physically stick micro-aggregates with macro-aggregates [119]. The physical interaction of external hyphae on soil particles forms stable aggregates [123] in general and under water deficit [124]. Moreover, mycorrhizosphere also influences soil aggregation through alterations in the soil microbial food web, habitats for soil microbes, and biological activities in the host rhizosphere, which could result in an enhancement in microaggregate soil structure [125]. Thus, soils possess well-structured property in the presence of AMF, maintaining relatively higher available water than poorly structured soils without mycorrhizal presence under water stress [126]. Notably, in artificial substrates, an enhancement in water retention and water transport within substrates inoculated with AMF was observed under severe drought, suggesting that host plants perceive less stress at the root surface as reducing substrate moisture [127]. Hence, AMF postponed the physiological stress response in host plants.
\nIt is often found that AM symbiosis can improve plant growth in numerous plants, such as lettuce [9], tomato [9, 11], strawberry [81], maize [128], black locust [72], digitgrass (Digitaria eriantha), a source of forage [92], and damask rose [129]. The substantial improvement in the growth of mycorrhizal plants could be a result of a combination of AMF-induced mechanisms of plant tolerance under drought conditions, notably enhanced water and nutrient uptake in host plants [60, 117], and increased photosynthetic activity (as discussed in Section 4.1.3) since plant size closely links with measured physiological parameters [11]. It is important because nutrient supply may improve plant drought tolerance for better plant establishment. The increased plant biomass and nutrient uptake in AM plants could be more pronounced during seedling growth stages and in a longer stress duration. For instance, significant increases in shoot dry weight (by 128–242%) and root dry weight (185–328%) in French lavender (Lavandula dentata) plants treated by either single autochthonous AMF (Septoglomus contrictum, Diversispora aunantia, Archaespora trappei, Glomus versiforme, and Paraglomus occultum) or their mixture were recorded, compared to uninoculated plants after 6 months of growth under drought conditions [60].
\nBesides positive mycorrhizal effects on plant growth, discrepant observations have also been reported. Four tomato recombinant inbred lines (RIL 20, 40, 66 and 100) and one commercial cultivar inoculated with Rhizophagus irregularis showed variable results under water stress [37]. AM application remarkably increased shoot dry weight of RIL 40 and RIL 60 lines under drought conditions while no changes were recognized in plants colonized by other AMF. Similar results were found in soybean using single isolates of Septoglomus constrictum, Glomus sp., Glomus aggregatum, or their mixture [130]. Taken together, the benefits of AMF application under water deficit are dependent on the specific combination of plant genotypes and AM isolates.
\nAnother significant benefit of mycorrhizal inoculation is to increase crop yield in exposure to water constraints compared to nonmycorrhizal plants. An array of observations shows a significantly higher yield, importantly marketable yield in mycorrhizal plants subjected to water scarcity in maize [128], tomato [82], flax [131], cowpea [132], and damask rose [129]. Furthermore, AM symbiosis has shown to accelerate flowering and fruit development [133]. Interestingly, re-inoculation of AMF after transplanting seedlings in the field appears to be necessary to strengthen mycorrhizal benefits. This could be seen in the field investigation which with twice application of AMF considerably heightened the marketable fruit yield (by 51–71%) in plants subjected to 50% water supply regime in comparison with those with mycorrhizal inoculation once at sowing and uninoculated ones [82]. The beneficial effect of AMF application on relative water content and nutritional status in plants as well as enhanced shoot accumulation of photoassimilates through higher photosynthetic activity, and improved stress tolerance in the presence of drought could result in higher productivity in colonized plants. Also, fruits are often the main sink for P; therefore, enhanced P nutrient in host plants promotes higher fruit yield.
\nAs a result of physiological changes during mycorrhization, both transcriptional and metabolic changes occur in host plants influencing crop quality as well. AM symbiosis not only modulates gene expression in tomato fruit, through a systemic impact, but also changes the phenology of flowering and fruit ripening as well as in the amino acid profile [133]. Under water shortage, AMF treatment has been explored to improve quality attributes including antioxidant compounds, carotenoids and anthocyanins [82, 134], essential oils [135], and alteration in seed quality of flax [131], hence highlighting the potential of using AMF in crop production, producing industrial and oil plants.
\nMicrobial symbionts of plants such as AMF represent a huge, but an unrealized resource for improving yields, especially in the tropics [136]; however, lower benefits to plants than the potential of these microorganisms are often found. To predict real benefits as well as all potentials of the fungal inoculation, implementation of field trials before AMF application on a large scale is indispensable in order to choose suitable inoculum or appropriately tune the best AM combination for target crop production systems. Moreover, various environmental factors influence the success of AMF application into the field.
\nAnother critical issue is whether generic or tuned AM products should be utilized in sustainable crop production. One of the challenges of AMF inoculation under open field conditions is the native populations of AMF in soils, which are able to remarkably compete colonization niche with the introduced symbionts. Despite the fact that commercial AMF inoculants are usually advertised as compatible for a variety of host plants and field-cultivation conditions, the AM-induced benefits for crops are not always as expected [137]. The bridge between research and AM suppliers should be strengthened to recommend appropriate AM inocula for most benefits. Due to the specificity of AMF-host plant interaction as described in several places in the chapter, an attempt to exploit advantageous combinations is necessary. Fine-tuning commercial mycorrhizal products is vital to obtain optimum beneficial effects from mycorrhizal inoculation.
\nEven in some circumstances, the symbiotic effectiveness and adaptability of the indigenous fungi are more dominant than non-native ones [138, 139]; therefore, introduced AMF isolates could be less profitable than native ones [140]. Besides, there is an existence of functional diversity among different AMF species [36]. Remarkable differences in performance even among different geographical isolates belonging to the same mycorrhizal species have been described [141]. In such cases, isolation of indigenous mycorrhizal strains for inoculum production, then large-scale reintroduction of these native fungi in the field could be a feasible solution for a useful AMF application [142]. It is worth mentioning that selection of specific AM taxa for particular crops is the best approach to improve crop growth, and there is no ‘one-size-fits-all’ AMF [143]. In controlled environments, application of a single AMF is more effective than using a mixture of different AM taxa [143].
\nDuring the last decades, several molecular techniques have been used to characterize entire communities of mycorrhiza in soil [144, 145] and AMF inocula [146, 147, 148]. These techniques enable to monitor the introduced fungal symbiont both inside and outside the host during plant growth [149, 150]. Tracing the introduced AMF temporally and spatially could be implemented by high-throughput next-generation sequencing, which possibly verifies whether the introduced fungi favor substantial levels of colonization and explores how the inoculated AMF coexist and interact with the local community of AMF [136]. Advances in molecular techniques can further assist the adjustment or tuning of commercial inoculants to specific AMF combinations with host plants under crop production systems.
\nAnother major limitation of mycorrhizal inoculation in horticulture and agriculture is farmer’s awareness and acceptance and the relatively high cost of it. Furthermore, conventional breeding programs have overlooked plant characteristics facilitating mycorrhizal association, and plant breeders have selected varieties in favor of acquiring nutrients in high-input crop production systems without respect to the AMF role in soil nutrient management [151], resulting in the primary challenge to AMF application. Hence, modern breeding programs should consider AMF as an essential component of breeding traits in new cultivars, particularly those cultivated under environmental adversities such as drought stress in which AMF application has been proved to stimulate higher crop tolerance.
\nAM inoculation can offer multiple advantages to host plants in exposure to water scarcity, which could enable inoculated plants to avoid drought stress or tolerate water deficit better than nonmycorrhizal plants. Indeed, various direct and indirect AM-induced mechanisms in mycorrhizal plants could contribute to drought mitigation or tolerance. More importantly, improved crop yield and quality attribute in colonized plants under drought stress highlight the importance of AMF application in crop production as one of the promising practices under water constraints. However, variable plant responses to AMF and the discussed major challenges hinder possible fruitful outcomes of AM inoculation. Identification of the most appropriate combination of fungal inoculants and a given variety, cultivar, or accession grown under water scarcity, and understanding environmental factors deciding the positive results of the inoculation are crucial determinants for successful AMF application. Compatible combination of AMF with other beneficial microbes such as plant growth-promoting bacteria and/or Trichoderma offering synergistic effects on plant tolerance to stressful environments including drought stress is also a bright perspective [38, 106]. Besides that, further research is necessary to shed light on the specific functions of genes mediated by mycorrhiza, which could explore the exact AM-triggered mechanisms of plant adaptation under water deficit. Studies on quantitative trait loci (QTL) involved in mycorrhizal plant responses to drought stress are needed for breeding programs to create new cultivars with a combination of drought-tolerant traits and AM benefits.
\nThis work was supported by 1783-3/2018/FEKUTSTRAT Program awarded by the Ministry of Human Capacities and by the National Research Development and Innovation Office (2017-1.3.1-VKE-2017-00022).
\nWe declare that we do not have any conflict of interest.
Edited by Jan Oxholm Gordeladze, ISBN 978-953-51-3020-8, Print ISBN 978-953-51-3019-2, 336 pages,
\nPublisher: IntechOpen
\nChapters published March 22, 2017 under CC BY 3.0 license
\nDOI: 10.5772/61430
\nEdited Volume
This book serves as a comprehensive survey of the impact of vitamin K2 on cellular functions and organ systems, indicating that vitamin K2 plays an important role in the differentiation/preservation of various cell phenotypes and as a stimulator and/or mediator of interorgan cross talk. Vitamin K2 binds to the transcription factor SXR/PXR, thus acting like a hormone (very much in the same manner as vitamin A and vitamin D). Therefore, vitamin K2 affects a multitude of organ systems, and it is reckoned to be one positive factor in bringing about "longevity" to the human body, e.g., supporting the functions/health of different organ systems, as well as correcting the functioning or even "curing" ailments striking several organs in our body.
\\n\\nChapter 1 Introductory Chapter: Vitamin K2 by Jan Oxholm Gordeladze
\\n\\nChapter 2 Vitamin K, SXR, and GGCX by Kotaro Azuma and Satoshi Inoue
\\n\\nChapter 3 Vitamin K2 Rich Food Products by Muhammad Yasin, Masood Sadiq Butt and Aurang Zeb
\\n\\nChapter 4 Menaquinones, Bacteria, and Foods: Vitamin K2 in the Diet by Barbara Walther and Magali Chollet
\\n\\nChapter 5 The Impact of Vitamin K2 on Energy Metabolism by Mona Møller, Serena Tonstad, Tone Bathen and Jan Oxholm Gordeladze
\\n\\nChapter 6 Vitamin K2 and Bone Health by Niels Erik Frandsen and Jan Oxholm Gordeladze
\\n\\nChapter 7 Vitamin K2 and its Impact on Tooth Epigenetics by Jan Oxholm Gordeladze, Maria A. Landin, Gaute Floer Johnsen, Håvard Jostein Haugen and Harald Osmundsen
\\n\\nChapter 8 Anti-Inflammatory Actions of Vitamin K by Stephen J. Hodges, Andrew A. Pitsillides, Lars M. Ytrebø and Robin Soper
\\n\\nChapter 9 Vitamin K2: Implications for Cardiovascular Health in the Context of Plant-Based Diets, with Applications for Prostate Health by Michael S. Donaldson
\\n\\nChapter 11 Vitamin K2 Facilitating Inter-Organ Cross-Talk by Jan O. Gordeladze, Håvard J. Haugen, Gaute Floer Johnsen and Mona Møller
\\n\\nChapter 13 Medicinal Chemistry of Vitamin K Derivatives and Metabolites by Shinya Fujii and Hiroyuki Kagechika
\\n"}]'},components:[{type:"htmlEditorComponent",content:'This book serves as a comprehensive survey of the impact of vitamin K2 on cellular functions and organ systems, indicating that vitamin K2 plays an important role in the differentiation/preservation of various cell phenotypes and as a stimulator and/or mediator of interorgan cross talk. Vitamin K2 binds to the transcription factor SXR/PXR, thus acting like a hormone (very much in the same manner as vitamin A and vitamin D). Therefore, vitamin K2 affects a multitude of organ systems, and it is reckoned to be one positive factor in bringing about "longevity" to the human body, e.g., supporting the functions/health of different organ systems, as well as correcting the functioning or even "curing" ailments striking several organs in our body.
\n\nChapter 1 Introductory Chapter: Vitamin K2 by Jan Oxholm Gordeladze
\n\nChapter 2 Vitamin K, SXR, and GGCX by Kotaro Azuma and Satoshi Inoue
\n\nChapter 3 Vitamin K2 Rich Food Products by Muhammad Yasin, Masood Sadiq Butt and Aurang Zeb
\n\nChapter 4 Menaquinones, Bacteria, and Foods: Vitamin K2 in the Diet by Barbara Walther and Magali Chollet
\n\nChapter 5 The Impact of Vitamin K2 on Energy Metabolism by Mona Møller, Serena Tonstad, Tone Bathen and Jan Oxholm Gordeladze
\n\nChapter 6 Vitamin K2 and Bone Health by Niels Erik Frandsen and Jan Oxholm Gordeladze
\n\nChapter 7 Vitamin K2 and its Impact on Tooth Epigenetics by Jan Oxholm Gordeladze, Maria A. Landin, Gaute Floer Johnsen, Håvard Jostein Haugen and Harald Osmundsen
\n\nChapter 8 Anti-Inflammatory Actions of Vitamin K by Stephen J. Hodges, Andrew A. Pitsillides, Lars M. Ytrebø and Robin Soper
\n\nChapter 9 Vitamin K2: Implications for Cardiovascular Health in the Context of Plant-Based Diets, with Applications for Prostate Health by Michael S. Donaldson
\n\nChapter 11 Vitamin K2 Facilitating Inter-Organ Cross-Talk by Jan O. Gordeladze, Håvard J. Haugen, Gaute Floer Johnsen and Mona Møller
\n\nChapter 13 Medicinal Chemistry of Vitamin K Derivatives and Metabolites by Shinya Fujii and Hiroyuki Kagechika
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