Morpho-Anatomical Adaptation against Salinity

Abstract

Plants tolerant of NaCl, implement several adjustments to acclimate to salt stress, such as biochemical, physiological, and morphological modifications. Besides, plants also adjust to saline circumstances by altering their anatomical structure of roots, leaves, and morphological modifications. The leaf and roots are among the essential plant organs and are involved in the transport of water and minerals used for photosynthesis. From a plant physiology perspective, water use efficiency in the quantity of CO2 fixed in photosynthesis compared to the leaf anatomy. In this review, we provide a comparative account of the morphology of the leaf and root under normal and salt stress circumstances. There is little information on the ultrastructure changes elicited in response to salt stress. The analysis expands our knowledge of how salt may impact the leaves and root anatomy.

1. Introduction

Salt stress is among the leading abiotic causes of modifications in many physiological, anatomical, and biochemical processes [1]. Anatomical and morphological changes in leaves under restricted moisture availability play a significant role in salt stress, and they are an indication of the level of tolerance. The epidermis is the external tissue of every plant organ and serves as the initial point of contact with its environment. It is essential to preserve physiologically appropriate circumstances in all plant and environment interactions for normal metabolism [2].

In salt-stressed plants, stem vascular cell thickness was much larger than control treatment; the salinity effect was concentration-dependent. Generally, plants grown in saline solution showed higher thickness in the cuticle, vascular tissues, and vessel than unstressed plants (Figure 1B–C). Furthermore, we observed that, in salt-stressed plants, the number of trichomes was increased from epidermal stem cells. In other words, an increase in salinity level led to more trichomes on the epidermal layer compare with control plants [3]. There are several reports on increased trichomes density under environmental stresses such as drought and salinity [4, 5]. An increase in trichome density may be a mechanism to increase tolerance to salt stress.

Figure 1.

Salt stress effects on rice plant growth under control and at 200 mM NaCl treatment(A): rice leaf anatomy, Oriza sativa; (B–C): light microscopy (cross-section) in control; (C): light microscopy (cross-section) in 200 mM NaCl; cu = cuticle, ms: mesophyll cell, Bul: bulliform cells, cp: parenchyma cells, m: mesophyll, s: stomata, vb: vascular bundles, s: stomata stomata, lv: largr vascular bundle and red line indicates the thickness of parenchyma; and scale bar = 100 μm; (D): scanning electron microscopy of the adaxial surface in control rice leaves; and (E): under stress condition (200 mmol/l), showing epicuticular wax deposition.

The importance of the cuticular layer in regulating a plant’s water status and providing protection from environmental challenges has been recognized for a long time. The cuticular layer in plants restricts non-stomatal water loss and protects plants against damage from biotic and abiotic stress [6]. Due to their role in controlling water loss, specialized epidermis structures have the potential to enhance the drought tolerance and WUE (“water use efficiency”) of critical crops [7, 8, 9, 10]. The cuticular wax on the leaves of the control (non-treated rice leaves) showed less than compared with the treated rice leaves (200 mmol/l NaCl), indicating adaptation against salinity to maintain photosynthesis to control water loss (Figure 1D–E).

Trichomes study has conventionally centered on specialized metabolic processes understanding in glandular trichomes [11, 12, 13]. Our data showed that in salinity tolerance, rice varieties have developed dense trichomes and increased in size with the help of a layer of air trapped in trichomes to reduce the rate of water transpiration as compared to susceptible varieties under differing concentrations of salt (40–160 mmol/l) [3].

The epidermis also includes stomata, which constitute epidermal pores that directly control the exchange of gases and also water status management. They may be found on a single surface (hypostomatic) and both leaf surfaces (amphistomatic) [14]. Stomata regulate water absorption through modifications in the stomatal opening, conductance, along with density. Shortly, plants alter their stomata closure to minimize water loss and a moderate absorption of CO₂ for changing circumstances [15]. Our data indicated that tolerant cultivars of rice closed stomata at the highest concentration (160 mmol/l NaCl) of salt [3]. Plant leaves can rise in stomatal density and a decreased saline area indicating a change to saline stress [16, 17]. Thus, the equilibrium between the density of stomatal and site could lead to stomatal conductance control and evaporation water loss [18, 19], which establishes equilibrium in photosynthesis [20]. Similar behavior was found for other species like “Leptochloafusca” [21] Imperata cylindrical [22], and Triticum aestivum [23].

Salt tolerant plants exhibit leaves thickening [23, 24, 25] which may contribute to the maintenance of turgor and content of leaf water. Wankhade et al. [26] and Hameed et al. [17] identified an optimistic association between salt stress tolerance and epidermal cells’ thickening. The epidermal thickness increases the water effectiveness of plants and offers more area in which NaCl is effective for the epidermis of the leaf [27]. It can also be important to increase the region of sclerenchyma with rising salinity since it provides organ rigidity and this may be an essential characteristic for salt resistance [16]. The area of the photosynthetic leaf, parenchyma tissues exhibited a progressive reduction in salinity which is likely to influence CO₂ diffusion [18].

The most challenging issues to assess by traditional microscopy methods are rice mesophyll tissues as their cells are lesser and have a greater density of chloroplast [28, 29] comparison to other plants. Thus, precise mesophyll morphology evaluation is essential for assessing photosynthetic capability [30] and maintains the leaf structure against salinity via following ways;

1. Salt exclusion: It inhibits salts from entering the vascular scheme.

2. Salt elimination: Salts glands and hair actively remove salts and therefore maintain the concentration of salt below a specific threshold in the blades.

3. Salt succulence: If the amount of cells storage gradually rises with salt ingestion (since the cells raise water constantly), the salt level may be maintained for long durations reasonably consistent.

4. Salt redistribution: Na⁺& Cl⁻ may easily be transported to phloem to allow redistribution across the plant of higher concentrations in actively transpiring leaves.

Significant modifications were detected in salt stress chloroplast in rice leaf contrast to tolerance variety. These include; (1) Modifications in the chloroplasts number & size, and starch level. (2) Disordered membranes of the chloroplast. (3) Variations in plastoglobuli numbers and sizes. (4) Loss of the disorganization and envelope of thylakoids and grana which directly affect the chlorophyll fluorescence and photosynthetic rate, and reduced the productivity of rice (12). Chloroplast is recognized as an organelle susceptible to environmental stress [31], and its pockets [32] are known to be present in salt. Even though earlier TEM investigations have shown alterations in the chloroplast ultrastructure to salt that influence photosynthetic [32], these structural variations are specifically noticeable in the thylakoids [28] that swell under salt stress [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32]. Therefore, mesophyll conductance decrease was correlated with the olive leaf mesophyll thickening [33]. A substantial reduction in mesophyll cell size was also a notable leaf anatomical characteristic found in A. gomboformis. Comparatively, small cells were found to better withstand turgor pressure than massive ones and more efficiently help to maintain turgor [34]. Thus, the decline in cell size was relatively considered to sustain tissue turgescence in A. gomboformis as a leaf tolerance mechanism.

On the other side, improved porosity by raising the parenchyma intercellular space did not promote the propagation conductivity of stressed plants [35]. The existence of highly vacuolated epidermal cells with poor metabolic seemed to function as a dumping mechanism in preventing mesophyll cells from stress [36]. However, decreasing the diameter of the xylem tube contributed in decreased hydraulic and ionic conductance [37] and therefore reduced photosynthesis and plant development.

The influence of salinity on leaf ultrastructure changed with the plant tolerance to NaCl, as reported in two rice species, (Oryza sativa L.). The sensitive rice species (‘Jaya’) indicated variations in the chloroplast integrity compared to the tolerant species (‘Kogut’). Specifically, the majority of chloroplasts of ‘Jaya’ exhibited indications of damage in reaction to elevated NaCl. In comparison, ‘Korgut’ tolerance chloroplasts variety did not exhibit any salinity impact. This response was associated with a 53% decrease in PN in Jaya, while no substantial variation in PN was found in ‘Korgut’ [3] (Figure 2). The impact of NaCl on the ultrastructure of leaf chloroplast was investigated in two distinct pea varieties with numerous sensitivity concentrations to NaCl, one sensitive to salt (“cv. Challis”) and another relatively [38].

Figure 2.

(A) Transmission electron microscope images of salt-tolerant variety, (a) under control (without treatment) and (b) treated with 160mM Nacl, showing thylakoid system of chloroplasts remained unaffected in control as well as under treatment. However, salt-sensitive variety, transmission electron microscope images of leaf, (c) under control, Chloroplasts had a well-developed system of thylakoids, (d) Thylakoid were damaged with loss of grana stacking under treatment; (B) Gas Exchange measurements; (C) Light reaction of photosynthesis [3].

Aranda-Romero et al. [39] investigated anatomical disturbances generated with chloride salts (NaCl, CaCl₂, KCl) in both tolerant (Cleopatra mandarin) and sensitive (Carrizo citrange) citrus cultivars. Salts in P_N were associated with alterations in leaf anatomy, like the decrease in the thickness of the leaf and decreased mesophyll cell area (Figure 1B–C), enhanced leaf succulence, and decreased intercellular air space, surface, and density of the tissue. The anatomical alterations related to Arbutus unedo leaves in semi-thin sections were found by Navarro et al. [40]. A relationship between control plants and saline plants revealed that the cell size was not significantly changed in the 1st layer of palisade cells. Hernández et al. [38] observed in the same research that the NaCl concentration of salt and salt-tolerant pea plants had a distinct impact. Moreover, Argyranthemum coronopifolium, as demonstrated by Morales et al. [41], is similarly susceptible to high levels of salt. These researchers discovered that the salt in these plants does not cause modifications in the chloroplast’s number, while in palisade & mesophyll cells, chloroplasts have risen significantly owing to a rise in the quantity of starch in palisade parenchyma compared to mesophyll tissues. A smaller formation of sucrose and hexoses and high activity of phosphate – sucrose synthase may explain the development of starch in leaf-chloroplasts in saline condition, which may lead to the triose-phosphate path towards the synthesis of starch or/and enzymes damage included in starch deterioration through modifications in the ionic chloroplast’s composition [41, 42]. The variations in the sizes of a chloroplast, as well as starch accumulation among palisade and mesophyll, parenchyma was explained by various photosynthetic leaf rate values observed by Morales et al. [41].

2. Anatomy of roots

The roots under salinity stress are essential to stress management studies since the root surfaces are initially exposed to environmental stress [43]. The root system anatomy correlates to root efficiency and permits plants to get nutrients and water, thus increasing the degree of replacement for lost plant water [44]. The anatomy system also prevents the salt build-up from roots, so that water from salty soils may continue [45]. Salt stress mainly controls root hair production and growth [46]. Root epidermal development demonstrates flexibility because external stimuli affect epidermal cells and root hair commencement [47]. The roots cross-section of rice species seedlings studied, in absence of stress, a greater roots thickness and well-organized tissue were noted in rice but root thickness decreased in salt condition (Figure 3). Growth of the plasticity root epidermis suggests a role of root hairs in detecting environmental signals that plants adapt to stressful circumstances as a reaction to different environmental conditions [48]. Optimal root systems promote plant development and increase plant output, as roots interface plants with the earth [47]. A plant root system that is increasing thus seems better since it enables it to reach deeper soil layers and get water and nutrients [49]. Moreover, soil environmental variables (temperature fluctuations, salinity, mechanical impedance, lack of O₂) may also have significant effects on the root morphology. Salt stress at a lower concentration induced plentiful root hairs, but progressively less root hair counts were calculated at increased salt levels [48]. Under the stress of salt, the root hair length and the root hair density were below 25 & 40% in comparison with untreated hydroponically cultivated wheat genotypes [36]. Two determinants of the total root surface area are total root duration and branching density – improved during moderate drought stress by comparison with “Silene vulgaris seedlings” [43]. Moreover, the morphology of certain plants indicates their salt sensitivity. Salinity inhibits the development of plants via osmotic & toxic effects and high sodium absorption levels because sodium raises soil tolerance, decreases root growth & root water flow by decreasing hydraulic conductivity [50]. The same behavior was also found in “Euonymus japonica” plants treated with a solution of NaCl and retrieved water comprising various levels of salt, which reduced the entire plant’s root length, most particularly in thin (“Ø ≤ 0.5 mm”) as well as average thickness (“0.5 < Ø≤2.0 mm”) roots [51]. In other varieties of the plant (Portulaca oleracea &Pinus banksiana), Croser et al. [52] & Franco et al. [46] have reported a rise in root diameter as a result of salinity. The higher root density found in plants indicates better tolerance and perhaps greater reserved accumulation [45, 46] that might enhance plant resistance and accelerate the plant’s establishment for farming and landscape objectives, particularly ornamental plants [53]. Root hydraulic conductivity can vary according to the salt concentration of irrigated water used in cultivated fields [54]. The root hydraulic behavior usually expresses the entire root dry weight, regardless of the root architectural function in water absorption capacity. However, the number of fine roots that define the root and the area could vary significantly in any given relatively dry weight value, therefore influencing the degree of water absorption [55]. The detrimental impacts of abiotic stress may be reduced more by salt-tolerant rootstocks than by rootstocks sensitive to salt. Navarro et al. [54] reported lower fruit production and attribute in Carrizo grafting of the Clemenules mandarins (“salt-sensitive rootstock”) in comparison with Cleopatra (“salt-tolerant rootstock”), both exposed to a solution of NaCl (30 mM). According to Penella et al. [56], the output of commercial pepper cultivars during salt irrigation was shown to be raised in salt-tolerant rootstocks. Nassar et al. [57] noted that the widths of vascular bundles and rice stems diameters reduced in NaCl. NaCl treatment in mung bean seedlings was noted by Khan et al. [58] to suppress the development of the vascular system. Gal et al. [59] found that shoot/root ratio was an essential indicator in predicting water consumption, water loss, and hydraulic conductivity in the C3 plants. The roots permeability to water is also measured by other root features, like the number of root hairs, root cortex width, xylem vessels number & diameter, and the suberin deposition in root endodermis and exodermis. This xylem structural modification may influence the capacity of water movement [60]. The xylem’s capability to resist negativity depends on the specific environmental limitations [60]. Many studies show that every plant uses the same basic effectors and regulatory systems for salt tolerance and that the variation in glycophytic & halophytic species is quantitative rather than qualitative [61].

Figure 3.

Effect of salinity on root anatomy (10x, magnification) of oryza sativa L.(A): anatomical structure of rice roots under control (without treatment); and (B):under salinity stress (200 mM NaCl). The different letter in the figures represents; a; epidermis, b; exodermis, c; sclerenchyma layers, d; mesodermis, e; endodermis, f; pericycle, g; pholem, h; metaxylem.

3. AMF (“arbuscular mycorrhizal fungi”)

The inoculation of roots AMF (that is considered an essential bio-ameliorators) for salt soils and facilitating host plants’ strong growth under stressful circumstances through various complicated events of communication between the plants as well as the fungus, leading to enhancing photosynthetic activity and other features linked to gas exchanges [62] and enhanced absorption of water. Several investigations have shown that AMF’s effectiveness imparts development and increase in salinity stress plants [63, 64]. AMF enhances plant nutrition by improving the availability and transport of different nutrients [65]; and also enhances soil quality by affecting its texture and structure, and therefore plant health [66, 67] and reduces Na and Cl uptake, resulting in a growth boost [68]. Good interactions between AMF-soil plants may allow reusing of recovered water, especially when roots develop in saline soil [69]. Mycorrhizal inoculation prominently increased photosynthetic rate with other gas interchange characteristics, the content of chlorophyll, and water usage effectiveness in “Ocimumbasilicum L.” in saline circumstances [70]. AMF-inoculated plants with “allium sativum” exhibited increased development rates, such as the leaf area index, dry & fresh biomass in saline circumstances [71] Wang et al. [72] observed a substantial improvement in fresh & dry weights and N levels of Root & Shoot owing to inoculation with mycorrhizal in mild saline conditions.

4. Conclusion

With a burgeoning population estimated to reach around 1.43 billion by 2030, India requires approximately 311 million tons of cereals and pulses to achieve food security. To meet the future food security target, it is expected to increase food grain production by 2 million tons per annum. To increase food grain production, there is a dire need to expand agricultural land and increase crop productivity. One of the possible solutions to address this problem is the genetic improvement of rice varieties in order to enhance their tolerance to salinity. In this review we have discussed morphology and anatomy review that indicates, high salinity is characterized by an increase in the leaf size, trichome and stomata size, and number, thickening of epidermis, area of vascular bundles, maintained thylakoid structure as adaptive characters for salinity stress; these characters are used as an indicator of the salinity of the soil. In addition, AMF is considered essential bio-ameliorators, which can enhance soil quality and maintain better productivity under salinity. Therefore, understanding different mechanisms enable crops to be sustained in hypersaline conditions; this may eventually contribute in improving rice yield on saline lands.