Open access peer-reviewed chapter
By Jamal R. Qasem
Submitted: March 21st 2019Reviewed: June 12th 2019Published: August 20th 2019
DOI: 10.5772/intechopen.88015
Downloaded: 707
This chapter deals with seed dormancy of agricultural weeds, its definitions and types from the physiological and ecological point of view, and physiological and ecological factors inducing dormancy in different weed species. The role of different environmental factors, agricultural practices including herbicides application, selection pressure, and seasonal dormancy, weed density and population regulation, seed phenology, polymorphism, and modifications were emphasized. Factors induce or terminate dormancy and enhance seed germination and dormancy breaking have been mentioned and evaluated in addition to the ecological importance of seed dormancy and herbicide resistance, genetic bases of dormancy, and molecular studies were presented. The role of allelochemicals, stresses, and dormancy and their effects on seed longevity and germination regulation were thoroughly discussed. Dormancy breaking under laboratory conditions, role of plant hormones and other chemicals, and dormancy management in the field were reviewed in addition to information on seed dormancy/longevity and germination stimulants. Seed germination stimulants and inhibitors of parasitic weed and seed dormancy as a weed survival strategy were presented and discussed.
Weeds represent a real persistent problem and can be found everywhere in all agricultural systems. They represent one of the main factors responsible for crop yield reductions, lower yield quantity and quality, and cause severe stresses and shortage in the supply of growth factors as they impair or negate crop yield. Losses caused by weeds exceed the combined losses resulting from insect and plant pathogens [1]. Weeds compete with crop plants for water, light, nutrients, and CO2 under certain conditions. They harbor insect and plant pathogens and negatively affect water resources and the environment. Weeds have different life cycles and are grouped into annuals, biennials, and perennials. While perennials are mainly reproduced vegetatively, seed production is the main regenerative strategy involved in the succession of annual, biennial, and simple perennial weeds through the buildup and persistence of their seeds in the soil seed bank. Knowledge on weed seeds and their lifespan is essential for researchers as well as farmers in designing successful weed control programs. Seeds in the soil represent the passive weed population that remain viable for extended periods of time and able to re-infest agricultural lands in spite of effective weed control measures employed against both the active weed population found above the soil level and seed bank as well through certain control measures such as soil-applied herbicides, tillage, soil solarization, mulching, and flooding. Information on weed biology helps optimize weed management strategies by prediction of weed emergence time and weed infestation level and thus avoid unnecessary weed control input. Integration of knowledge on weed emergence and infestation level and seed dormancy status could be used to improve weed control strategies [2], while integrated approaches that place priority on depleting weed seed banks through interfering with dormancy or germination requirements have a strong potential to enhance weed management aspects of agricultural systems [3].
The main objectives of this study were to review the most recent advances on weed seed dormancy, highlighting the importance of weeds as the main agricultural problem, the importance of weed control and weed ecological and agricultural significance, and their importance in the agricultural system and in food production; emphasize the difficulties in weed control and challenges that the farmers face on what the weed species are possessing, role of seed dormancy in weed persistence and difficulties in weed control, role of genetic and ecological factors and their interactions, and influence of these on seed internal structure and physiology; and understand and introduce the readers to the recent findings on weed seed dormancy breaking and possible management under field conditions.
Weed seed bank is described as a reservoir at which both deposit and withdrawal operations occur. Seed production in terms of numbers is considered as a survival strategy that enables weeds to maintain their genetic lines and exist in the environment. It is important in agriculture since weeds can produce a huge number of individuals for ecological invasion and survival under unfavorable environmental conditions and thus maintain species where other regenerative propagules (e.g. vegetative organs for perennials) fail. Weeds are characterized by their huge number of seeds produced which is much higher than crop plants. These seeds are equipped with different modifications that enable their disperse far distances from mother plants to explore and invade new rich sites in growth factors and thus escape hazards in resource-depleted habitats underneath the parent plants. These modifications facilitate seed dispersal by different agents including water, wind, animals, machines, and packed agricultural materials and by man himself. However, the small-size or dustlike seeds of many noxious weed species do not require specialized agent for dispersal but can far-disperse by wind currents. In general, weed seeds are easily spread and transport from their origin, and some have found their way into the earth’s planetary boundary. In order to maintain species genetic line, high seed production and seed modifications are necessary but not enough for species existence and persistence in a changing climate. Hence these characters must be accompanied with other mechanisms that help weed seeds remain viable and survive, and weeds grow and flourish in their habitats away from hazards including weed control measures. Therefore, seed production and modifications must conjugate with dormancy through which seeds of certain weed species such as
In summary, prolific seed production, seed modifications, and dormancy are the characters jointly considered for a successful efficient weed species. These, however, are all absent in crop seeds hence exposed to different breeding programs resulting in loss of many characters that enable them to survive and tolerate harsh conditions including plant disease, salinity, and drought resistance, and thus most crops lost their seed modifications and dormancy. Cultivated crops are well selected, and their seeds do not possess dormancy [6]. Therefore, the time of emergence and the number of established individuals could be simply determined by environmental factors (mainly temperature and moisture) [7, 8]. In contrast, prediction of weed seed germination and their emergence capacity are not possible because of dormancy. The number of established weed seedlings is strongly dependent on the dormancy level of the seed bank, and the emergence time depends largely on the seasonal dynamic variation in seed bank dormancy [9]. In addition to the mentioned characters, weeds continue producing seeds throughout their life cycle, set seeds at all growth stages, have seeds of different stages on the same plant, and show seed polymorphism.
Upon the fall of mature seeds on the soil, these may be deeply deposited or remained on the soil surface and thus exposed to different climatic conditions including light and air temperature or to agricultural practices such as tillage, hoeing, or herbicides. Seeds on the soil surface may be inserted in the top soil layer 2–5 cm depth which is applied to seeds of most weed species and could be of great value facilitating rapid germination especially photoblastic species that require light for germination, or small seeds contain small food reserve. In addition, surface-laid seeds are liable to drift by wind currents or water erosion or disperse by different agents to new regions and thus avoid suffering from depleted resources under or in the vicinity of their parent plants giving them a survival value. Other soil-deposited seeds remain in full darkness and may be at different soil depths. These either germinate in full darkness or stay dormant if it requires light until brought to the soil surface by deep tillage. However, the ability of different species to emerge from different soil depths depends on their seed food reserve whether it is sufficient to support the seedling travel along the soil way distant or not. Some may consume all food storage before its emergence above the soil, and thus their growth is arrested during transit and dies. When conditions do not permit seed germination in the soil, seeds remain dormant, viable, and ready to germinate when these permit. The longevity of these seeds depends on the stored food and microbial attack of these in the soil. Other factors cause seed death including enzyme action and oxidation that denatures seed-stored food, protein coagulation, nuclei degeneration, and accumulation of toxic materials. In addition, seeds may be attacked by earthworms that collect weed seeds and move them into their burrows, while soil insects such as carabid beetles are voracious eaters and can consume a large quantity of weed seeds that drop into the soil.
Seed is simply defined as a fertilized egg produced after pollination, but apomixes, autogamy, and agamospermy also exist in certain weed species. The seed has been also defined as a ripened ovule consisting of an embryo and coats [10]. The embryo as the new plant in miniature is very well equipped structurally and physiologically for dispersal, has enough stored food that provide the growing seedling at early stages after emergence and until establishes itself for autotrophic organism or partially or completely dependent upon other plant species in case of some parasitic species (hemi- or holo-heterotrophic).
Dormancy in a general term is a state in which viable seeds, spores, or buds fail to germinate under favorable conditions of moisture, temperature, and oxygen for the seedling growth. It is referred to as an adaptive feature that optimizes the distribution of seed germination over time. It characterizes many weed seed populations; and this hampers efforts in predicting timing and extent of weed emergence. Indeed, the number of established plants of a weed is strongly related to the proportion of the seed bank that has been released from dormancy and the carrying capacity of the environment. Dormancy due to external conditions exerts influences on physiological and biochemical seed internal processes including enzyme activities, food transport to embryo, and metabolism or unknown internal factors or ecophysiological behavior that does not allow germination. Therefore, the causes of this status are due to the seed and its environment. On the other hand, germination involves the resumption of embryo growth and seedling emergence and growth. Germination requires moisture, oxygen, temperature, and maybe light in photoblastic seeds. Therefore, it proceeds whenever seeds are laid on a safe site to meet particular sets of environmental conditions which, presumably, are able to support not only germination itself but also to insure the survival and success of the offspring [11].
Several types of weed seed dormancy have been recognized and described under different terms as primary and secondary, inherent/genetic and environmental, innate, induced and enforced, constitutive and exogenous, and seasonal and opportunistic. Nikolaeva [12] mentioned 15 types of dormancy based on germination inhibitory and stimulatory factors. The primary dormancy (induced during seed maturation) and secondary dormancy (induced naturally or artificially following harvest) are mainly considered by most researchers.
However, based on the mechanism that causes dormancy, the following types are well recognized to occur in weeds:
Physiological mechanism of dormancy
Ecological or demographical consequences of dormancy
Both are important to understand the evolutionary adaptations that weed seeds have developed in the agricultural environment.
This dormancy includes the following three types.
Innate dormancy is also termed as a primary or genetic dormancy. It represents seed conditions when they leave parent plants in a viable state but not germinating although conditions are favorable mainly due to some property of the embryo or the associated endosperm or maternal structures. It is an inherited type that characterizes certain plant genera or families, and since genetically controlled, therefore its length depends on environmental factors. Seeds, however, will not germinate although conditions permit and dormancy period expires. The cause of such a dormancy includes a number of morphological and physiological factors, and these are as follows.
Hard seed coat that is impermeable or mechanically resists diffusion of water, oxygen, or both is also termed as physical dormancy [13]. It occurs in some or all species of the angiosperm families including Anacardiaceae, Bixaceae, Biebersteiniaceae, Cannaceae, Cistaceae, Convolvulaceae, Cucurbitaceae, Dipterocarpaceae, Geraniaceae, Lauraceae, Leguminosae, Malvaceae, Nelumbonaceae, Rhamnaceae, Sapindaceae, Sarcolaenaceae, Sphaerosepalaceae, Surianaceae, and others; but it has not yet been reported in gymnosperms [14]. Leguminosae includes approximately 800 genera and 20,000 species that are widely distributed and adapted to different habitats [15, 16] and has a high frequency of physical dormancy [17]. Some examples of these are
Other seeds do not diffuse oxygen for energy and embryo respiration and metabolic processes. Such seeds are said to be “gas-hard,” for example,
In the third type of seeds, both water and oxygen are not diffused through the hard seed coat such as for
Hard seed coat may restrict the diffusion of O2 to enter the seed, prevents the outward release of CO2 and/or inhibitors from the seed or embryo and also embryo protrusion and expansion, and blocks light passage to the embryo. However, seed coat and other structures surrounding the embryo are extremely important for seed survival and germination regulation, since they protect the embryo against external hazards and regulate germination time.
In other cases, seed dormancy in many species is imposed by the structures surrounding the seed. In addition to the seed coat or testa, these also include the pericarp, glumes, palea (hull), and lemma in cereals. The palea, lemma, and pericarp are responsible for coat-imposed dormancy in
In order to enhance seed germination, seed coat must be destroyed mechanically or by microorganisms. However, in legumes the seeds are hard with thick-walled cells of testa surrounding the waxy layer.
In other cases seeds may fail to germinate because of mechanical resistance of the seed coat which can withstand a high pressure of 1000 Psi, such as for seeds of
In these species, the passage prevention or difficulty of water and oxygen inside the seed is not the cause of germination failure but may be enhanced to germinate by partial digestion of seed coat by animals and thus overcome their dormancy otherwise they extinct. Germination of many weed species was greatly improved after it passed through the digestive systems of animals and dropped out with animal feces mainly because their hard coat became lenient by secretions from the digestive system of these animals.
These are allelochemicals that prevent seed germination and cause self-inhibition (autopathy). Chemical inhibitors may be found on seed coat or its associated structures.
The perianth associated with the seed coat of
In this case seeds need to be biologically stimulated. The photoperiodically operated triggers act through modification of the phytochrome system. Seeds of
Chilling or temperature fluctuation may be also important. In two populations of
Germination stimulants may be used under laboratory conditions but have little relevance to field situation. Gibberellins, thiourea, or nitrate ion in the soil solution could increase with soil temperature in the spring and thus could stimulate seed germination of
Embryo dormancy is defined as failure of a mature embryo to germinate or to grow even when isolated from the seed or dispersal unit and exposed to conditions favorable for growth. Embryo dormancy may be also exerted by cotyledons as for
Other weed species exhibit polymorphism and produce morphologically and physiologically different seeds that have different after-ripening periods such as for seeds of
Several mechanisms sometimes may operate together in a single seed to break innate dormancy. For example, seeds of
Sometimes overcoming dormancy may be highly specific and adapted to certain conditions. Seed germination of
Cold temperature may be required for germination of certain species; it could activate hormones and enzymes. Some seeds need exposure to alternating temperature between freezing degrees for several weeks to one or two exposures to high temperature. This temperature fluctuation causes heat shock and activates enzymes and hormones and enhances their mobilization and thus germination induction as the treatment for
Light is another regulatory factor of seed germination for certain weed species having a light requirement (photoplasts) before the start of germination. Light-stimulated germination of seeds is known to involve the phytochrome system. The photoconversion of red light (Pr) to far-red light (Pfr) stimulates germination.
Sensitivity to the period of light and dark may determine the season of germination and growth, the flower initiation, and the end of bud dormancy. However, the value of light in stimulating or inhibiting seed germination may be very well demonstrated on species survival and existence knowing that germination inhibition by high light intensity may be of value in preventing germination and growth of winter weeds during the summer time at which soil surface may be exposed to unfavorable conditions such as drying or rapid seedling desiccation due to high temperature, high light intensity, and long photoperiod during summer and unsuitable for winter weeds. Conversely, germination inhibition of summer weed seeds during winter prevents their possible death by freezing temperature, strong cool wind currents, short photoperiod, and low light intensity which are not in favor of summer weed growth and survival. This kind of inhibition caused by light on the two weed groups of different growth requirements is a good example on the important value of light inhibitory effects for the survival of these species. However, seeds of many summer annuals at low temperatures under moist conditions provoke dormancy release, while high temperatures induce secondary dormancy. The seed dormancy level establishes the range of temperatures under which germination is possible [22].
It is an acquired condition of inability to germinate caused by some experience after ripening. This kind of dormancy is also called secondary dormancy as seeds are ready to germinate but may go into dormancy due to sudden changes in environmental conditions such as in temperature, moisture, and oxygen levels that cause physiological changes in seeds. Seeds, however, will not germinate, and dormancy exists even when conditions changed to favorable. This dormancy may be resulted from seed exposure to excessive light which lead to no germination in darkness, lack of moisture, high CO2 pressure, low O2 pressure, and deep seed burying that will not germinate until they are brought to the soil surface. However, other buried seeds by tillage may not germinate even after they are brought to the soil surface.
This kind of dormancy is maintained in or on the soil or with seeds submerged in water. It is defined as the inability of seeds to germinate because of environmental factors. One or more factors necessary for germination are in a short supply or absent including the lack of moisture, low temperature, lack or low oxygen level, and poor aeration and unfavorable atmosphere. However, percentage of O2 found in the soil depends on soil porosity, depth, presence of microbes, and amount of soil moisture. When the external limitation is removed as seeds are brought to the soil surface by tillage, they germinate. Sometimes this dormancy is due to placement of weed seed deeper than 5 cm in the soil by tillage. It results from the absence of red (r) light under the soil surface. Red light induces germination in seeds by activating their phytochrome system (P)-chromophore blue pigment attached to the protein molecule in the seeds. Far-red (Fr) light deactivates the system and thus induces dormancy in weeds. However, dormancy does not persist when the environment changes.
Both induced and enforced dormancy make the secondary dormancy. The importance of secondary dormancy became clearer as a survival strategy prevents seed germination when seeds are found deep in the soil and seedlings will not be able to emerge from deep soil layers. This kind of dormancy may be regulated through the phytochrome pigments found at low concentrations inside the seeds. These pigments when exposed to a high percentage of Pr/Pfr induce germination. The exposure time may be short enough for parts of the second dormancy. However, seeds from a single weed species may exhibit one or more types of dormancy or all three in succession over a period of time. Primary dormancy is found in the freshly shed seeds at which they will not germinate under any environmental conditions until dormancy is broken. After primary dormancy breaking, the seeds may germinate providing that conditions are favorable. If suitable external factors are not present, then secondary dormancy may develop. Secondary dormancy can be relieved and re-induced during many successive years [31] until conditions for germination become favorable. This phenomenon is called dormancy cycling [32]. However, physiological differences between secondary and primary dormancy are unclear [33].
From the ecological point of view, seed dormancy is also termed as a dispersal by time and is defined as an arrest in the development of seed embryo under external environmental conditions suitable for plant growth (phase more resistant to environmental hazards). It is critical for annuals not perennials. However, two approaches are prevalent, and these are as follows.
Dormancy from an ecological perspective is defined as a seed characteristic that prevents germination, even if suitable germination conditions prevail, not involving embryo or seed morphology or germination mechanisms. This is either as follows:
This kind of dormancy occurred at which favorable factors for germination were found but seeds of certain plant species have winter or summer dormancy. It occurs in an environment where favorable growth conditions are seasonal and dormancy is usually clocked by solar rhythm. This is applied to all annual summer and winter weeds at which day length is important, while temperature may not be so if followed by cold weather. Day length is the best indicator of seasonal changes because it is a rather constant feature of the macro-environment. The disadvantage of seasonal dormancy is that seeds may not be developmentally advanced enough to take advantage of especially good spring or summer conditions. If the environment is not stable (rainfall in the desert, fire, soil disturbance), it may make conditions favorable for seedling growth, but the timing and duration of these events can be rather unpredictable.
Differences were found among populations of
In this kind of dormancy, seeds of certain species are able to take advantage from unpredictable environmental conditions or changes. It occurs when there is only a small seasonal element in the occurrence of favorable conditions; dormancy tends to be both imposed and released by the direct experience of the unfavorable or favorable conditions. For instance, deep tillage brings the seeds to the soil surface and thus would allow successful germination and establishment. Ephemerals in the desert sometimes take an advantage from the sudden rain shower during summer at which they germinate but later they suffer death because of the usual prevailing conditions of drought and high temperature in the desert during that period.
The advantage of seasonal dormancy is its predictable nature, while the advantage of opportunistic dormancy is its responsiveness. The differences between the two types are not exclusive but changed when conditions are changed. However, physiological description of dormancy may be a more valuable approach since the conditions of the embryo are what finally determine seed germination.
Dormancy is an adaptive trait that enables seed germination to coincide with favorable environmental conditions. From the physiology perspectives, gibberellins, ethylene, cytokinins, or abscisic acid (ABA) play an important role in inducing or inhibiting seed dormancy. The low level of ethylene is accumulated at the early stage of germination in seeds of different crops (e.g.,
There is considerable circumstantial evidence that ABA is involved in regulating the induction of dormancy and in maintaining the dormant state. However, there is a paucity of unequivocal evidence that ABA is in fact an important controlling factor in the dormancy of most seeds. Dormancy is induced by abscisic acid during seed development on the mother plant. After seed shed, germination occurs due to reduction in the ABA level of the imbibed seeds because of ABA catabolism through 8-hydroxylation. ABA/gibberellins balance is the main environmental factor responsible for inducing or breaking seed dormancy. However, in different species, ethylene counteracts ABA inhibitory effects and stimulates germination. This effect is very well demonstrated in Brassicaceae seeds, which counteracts ABA effects on endosperm cap weakening, facilitating endosperm rupture and radical emergence. In contrast, ABA limits ethylene biosynthesis and action. Nitric oxide has been proposed to act against ABA inhibitory effects on ethylene and hence is produced rapidly after seed imbibitions and promotes germination by inducing the expression of the ABA 8-hydroxylasegene,
From the above information, it becomes clear that some plant hormones have roles in dormancy induction or breaking and thus inhibit or stimulate seed germination. Ethylene stimulates seed germination of several weeds such as in
Some chemical compounds or secondary metabolites are also known as allelochemicals such as phenolics, unsaturated lactones, short-chain fatty acids, coumarins, and many others have been reported as germination inhibitors present in seeds of many weed species [38, 39]. To enhance germination, leaching and oxidative destruction of these chemicals within the seed are necessary for dormancy termination. However, these allelochemicals may also play a positive role in seed viability and longevity since they prevent microbial attack and maybe destruction of weed seeds by soil pathogens and insects.
Ecological factors are involved in inducing dormancy or stimulation of seed germination and dormancy breaking. These factors include light, temperature, O2, CO2, and nitrate. Light causes weed seed dormancy. Some weed seeds require light in order to germinate, for example,
However, light, moisture, temperature, and O2 all act physiologically in enhancing or ending dormancy. Moisture or water is required to activate enzymes, compensate for water loss by the embryo through respiration, and dissolve and mobilize food into the embryo. Oxygen is necessary for aerobic respiration to provide energy for embryo growth, while water absorption, hormonal balances, metabolic processes, and germination induction will not proceed but only at certain suitable temperature. All factors, however, are required for biochemical and physiological activities that occur inside the seed including the living embryo.
Reactive oxygen species (ROS) play an important role in seed life cycle. In orthodox seeds, ROS are produced at all stages in seeds active cells as well as in dry tissues during after-ripening and storage. ROS, however, are widely regarded as detrimental to seeds, but recent research results reconsider them as beneficial in seed germination and seedling growth. ROS regulate cellular growth, protect against pathogens, or control the cell redox status. They also act as a positive signal in seed dormancy release by interacting with plant hormones such as in transduction pathways of abscisic acid and gibberellins [40]. Different workers emphasized ROS roles in plant physiology and development under stress conditions mainly drought stress, and thus their production has been long considered as detrimental since it is linked with seed aging or seed desiccation, but they have also a positive important role in seed germination or dormancy release. They are important in metabolic activity during cell division, seed filling, seed survival at shedding, and seed rehydration and germination. ROS have an essential role in plant metabolism, energy production, and enzyme activities necessary to start seed germination and seedling growth. Their sensing and signaling role in seed different stages is evident. ROS is important in cell signaling in the dry state since it could accumulate during dry storage but would become actors of cell regulatory mechanisms only after seed imbibition. Oxygen is important in the guise of reactive oxygen species in further modulating dormancy and relaying environmental signals. Seed dry after-ripening is associated with the accumulation of ROS, resulting in targeted mRNA oxidation and protein carbonylation of transcripts and proteins associated with cell signaling (mRNA) and protein storage [41]. These modifications have been linked to dormancy changes during after-ripening and could underpin a mechanism indicating the passage of time. Recently the possibility of a further role for ROS to inform the seasonal response of the seeds through ultra-weak photon emission (UPE) has been suggested. It was hypothesized that beneath the soil surface the attenuation of light (virtual darkness: low background noise) enables seeds to exploit UPE for transducing key environmental variables in the soil (temperature, humidity, and oxygen) to inform them of seasonal and local temperature patterns.
Seed germination is affected by many environmental factors, such as temperature, salt, light, soil moisture, oxygen concentration, and Ca2+ ions. Dormancy is a status to avoid and resist adverse conditions and must be evolved as a solution to the periodic, as well as nonperiodic, changes in the environment which impair the proper function of the plant during certain periods [42]. It may also prevent germination under apparently normal conditions, if they occur occasionally. In this way, it constitutes an evolutionary safeguard against the uncertainty of the environment. Drought, salinity, alternating temperature, photoperiod, burial depth, nitrates, nitrites and soil pH, artificial seed aging, agricultural practices, control methods, and radiant heat all influence weed seed dormancy.
Studies in controlled environments have already demonstrated that thermal conditions and, to some extent, water availability during seed set and maturation have an impact on the level of dormancy [43]. The level of dormancy in
Velvetleaf seeds germinated over a range of constant temperatures from 10 to 40°C regardless of light conditions, but no germination occurred at temperature below 5°C and beyond 50°C. Seeds germinated at alternating temperature regimes of 15/5–40/30°C, with maximum germination (>90%) at alternating temperatures of 40/30°C. Germination, however, was sensitive to water stress, and only 0.4% of the seeds germinated at the osmotic potential of −0.4 MPa. There was no germination at 0.6 MPa. Germination was also reduced by salinity and alkalinity stresses and did not occur at 150 mM NaCl or 200 mM NaHCO3 concentrations. However, pH values from 5 to 9 had no effect on seed germination. The maximum seedling emergence (78.1–85.6%) occurred at 1–4 cm depth [45].
Bochenek et al. [46] reported differences between the cultivars of
Breaking primary dormancy of achenes in
The effects of drought and herbivory on biomass and seed quality in
Seed germination of the salt-tolerant species,
GA3 at concentration of 400 ppm strongly stimulated germination of
Tillage exposes seeds to light before reburial, allows greater diffusion of oxygen into and carbon dioxide out of the soil, buries residue, and promotes drying of the soil, thereby increasing the amplitude of temperature fluctuations and promoting nitrogen mineralization. These factors are known to terminate dormancy in several species. The effects of burial, however, on germination and longevity and of water stress and temperature on germination and dormancy induction of the weed
Tillage effects on seed dormancy of different weed species are very well demonstrated especially on photoblastic species. Tillage may affect Pfr and Pr ratios and germination induction or dormancy. This, however, is varied for different weed species. Chavarria [11] reported that under conventional tillage,
Weed emergence was also reported as increased following frequent, repeated tillage. Cultivation during daylight serves to increase weed populations. Daytime tillage increased seedling emergence of several winter annuals and doubled that in the night time tillage due to the extreme sensitivity to Pfr in buried seeds of certain weed species.
Tillage modifies soil temperature fluctuations or soil nitrate concentration, and continuous tillage depletes organic matter that leads to a change in soil color and thus modifies soil thermal regime. Tillage changes the position of seeds in the soil, while no-tillage leaves most seeds in the top 10 mm of the soil profile.
Nitrates affect seeds of several species and enhanced seed germination in the field. Nitrates may influence mother plant resulting in increased nitrate level in developing seeds. A strong correlation between nitrate concentration in the seeds and their germination capacity was also found. Nitrate and nitrite concentrations have been shown to stimulate dormancy release in some species although other species are released from dormancy by ammonium. Soluble N can stimulate germination of seeds of many weeds including
Under irrigation and flooding conditions, the soil has low oxygen concentrations. Low oxygen concentration terminates dormancy in seeds of some species including
Thick layer of residue increasingly reduces and delays emergence, decreases temperature, and prevents light penetration [3]. Soil-incorporated crop residues yield allelopathic effects on weed seed germination. Decayed residues can immobilize large amount of N that consequently prevents termination of dormancy in some species. The stimulant effect of certain plant residues is also possible. Many plant-derived smoke components have been found to have a dormancy-breaking effect, and the role of nitric oxide has been identified.
Incorporation of legume cover crop materials and application of chicken manure can promote weed emergence and growth. For example, ammonium released from decomposing
Seeds of certain weed species exhibit seasonal dormancy that allow them escape hazards that occur at a certain period in the year or unsuitable environmental conditions. This phenomenon is clearly demonstrated in annual, biennial, and perennial weeds. In annuals, summer-grown weeds will not germinate during winter since growth factors are not in favor of their germination, growth, and survival; the same is true for winter-grown weeds during the summer season. This is a danger avoidance strategy. In perennial weeds such as the woody spread
Karssen [31] stated that seasonal periodicity in the field emergence of annuals is the combined result of seasonal periodicity in the field temperature and seasonal periodicity in the width of the temperature range suited for germination. Germination in the field is restricted to the period when field temperature (environmental factor) and the temperature range over which germination is possible (degree of dormancy) overlap. So, dormancy is related to the width of the temperature range over which germination can proceed and not to temperature in that range. Dormancy varies on a continuous scale, visualized by continuous changes in the range of environmental factors under which germination can take place [55].
Seeds of the winter annual
A complex nature of responses may be exhibited by
Seed polymorphism is an important factor in innate dormancy. It is a significant factor in spreading the germination of seed from the same plant over time and keeps farmers busy with weed control throughout the whole growing season. This phenomenon is widespread in the Amaranthaceae, Compositae, Chenopodiaceae, Cruciferae, and Gramineae families.
Genetic seed polymorphism is very well demonstrated in
On the other hand, certain weed species show somatic polymorphism which is the production of seeds of different morphologies or behavior on different parts of the same plant. It is not a genetic segregation but a somatic one [58]. Among weeds showing such a phenomenon are
Placement of
In
In
Under conditions of herbicide application, some of these chemicals are absorbed by seeds or dormant buds, while others are not. These result in differences in germination, emergence, and growth patterns of different weed species. However, some herbicides may stimulate seed germination, while others inhibit this process or even kill the seed embryo. Differences also exist in hardness and permeability of the seed coat of different weed species at which species of Chenopodiaceae and Leguminosae are good examples on hard seed coat species. These characters cause differences in germination and growth of seedlings and may confer another cause of herbicide resistance. Avoidance of herbicide toxicity may result from seed interring into dormancy and not further responding to the applied herbicide with no absorption or translocation of the herbicide into the embryo. In addition, herbicide molecules may be deactivated or degraded inside the seed itself by some oxidative enzymes or may bound into certain constituent inside the seed. On the other hand, stimulation of weed seeds to germinate using certain herbicides also exists and allows higher seedling emergence and partitioning of herbicide molecules among individuals of weed species. Division of herbicide molecules among the high number of emerged seedlings would further dilute herbicide inside weed plants. All the above mentioned factors should be considered when herbicide resistance is discussed. These may cause great differences in weed seed germination, seedling growth patterns, and distribution in the field. Seed germination rate was often more rapid for herbicide-resistant
Dormancy is synchronized to the environment by a complex regulatory system. This is directed by the balance between the dormancy-promoting (abscisic acid) and dormancy-releasing (gibberellins) hormones via both hormone levels and sensitivity. Seed dormancy is a survival mechanism underlying the life cycle strategies of plants by controlling the seasonal timing of germination in the natural environment.
Weed species differ widely in seed dormancy and longevity, season in which they emerge and grow, depth from which they can emerge, and seed responsiveness to light and other germination stimuli. Weed seeds that remain dormant in the soil often germinate in response to changes in temperature, moisture, oxygen, or light.
Overcoming seed dormancy may be easily achieved under laboratory conditions through a number of practices/treatments including seed hand scarification, rubbing and peel/cortex/extra structure removal, alternate temperature, chemical treatments, and dipping/soaking seed in water (legumes) or could be changed by scarification—with acids or microbes, rubbing on a sandpaper or pricking it with a pin or needle. As an example,
A novel method that overcomes coat-imposed dormancy has been reported by Tiryaki and Topu [65]. In their method, seeds were stored at −80°C for certain period and then treated immediately with hot water of 90°C for 5 s. This approach of freeze-thaw scarification provided 84 and 75% germination compared with 3 and 26% of
Cold stratification may be a highly selective treatment and hence did not decrease dormancy for any of
Seed dormancy of
In the field, many agronomic practices affect weed seed dormancy and germination through their effects on the microenvironmental and edaphic conditions surrounding the seeds in the soil. Light penetration, soil water content, soil fertility, and temperature are modified by tillage, planting, harvesting, and other production practices that enhance or prevent weed seed germination. Changes in these environmental factors may modify indirectly phytohormone concentrations during seed development, which can subsequently affect dormancy status of the mature seed [3, 71].
It has been postulated that temperature is the only factor that directly influences the dormancy state of seeds, although the effects of other factors such as nitrate and light cannot be excluded [72]. Germination is influenced by factors such as temperature, light, nitrate, gaseous environment of the seed, and moisture content. Light appears as the most suited factor to influence in the field to enhance weed control. The behavior of weed seeds, in terms of dormancy characteristics, can be substantially different according to the location of the seed in the soil. Much of this response is related to a phytochrome requirement for germination, specifically, to the interconversions between the active (Pfr) and inactive (Pr) forms while the physiological mechanism involved in this conversion is poorly understood.
The changes in dormancy and germination of
Karimmojeni et al. [74] studied dormancy breaking in seeds of
In certain species cracking of the hard seed coat resulted from its exposure to mechanical pressure through coldness, freezing, scarification, or abrasion or through microbial attack that is necessary for germination. In other species seed coat must be destroyed, modified, or partially dissolved to provide the embryo with the necessary secondary growth factors. Embryos that are constrained by a mechanical barrier, such as the surrounding endosperm, perisperm, or megagametophyte (such as those that exhibit coat-enhanced dormancy), appear to require a weakening of these structures to permit radicle protrusion. This weakening involves partial enzymatic degradation of the cell walls. Seeds of
Certain species such as wild oats will not germinate unless seeds are exposed to warm dry conditions for dormancy breaking. Many weed species require light for seed germination, and this occurs when they get mature or may be stimulated by seed burying in the soil. Therefore, one effect of tillage is through dormant seed exposure for red light even for a short time to enhance germination. Some researchers study the implementation of tillage during the night to prevent stimulation of seed germination of certain weed species. The ratio of the active phytochrome far-red (Pfr) and inactive phytochrome red (Pr) determines whether the seed is dormant or not. The optimum ratio is established after exposure of the seed to white light which converts Pr to Pfr in the embryo. Outer coverings form a filter to high incident light on the seed and modify its effectiveness in converting Pr to Pfr.
In general, the main factors inducing or ending seed dormancy are light including day length, light type, dark period, and photoperiod; immature embryo; impermeable seed coat to water, oxygen, or both as in seeds of
Soil disturbance and light stimulate germination and emergence prior to crop planting in order to remove as many weeds from the soil seed bank as possible. However, weed seed banks can be manipulated [75] by encouraging seed germination or trapping them to germinate, modifying their environment, placing seeds in a position that are not able from or in others where they are much exposed to heat and light, and using certified crop seeds.
A dense, shading plant canopy can also deepen the dormancy in some weed seeds. The dim green light under such a canopy can actually be more effective than continuous darkness in inhibiting the germination of light-responsive seeds [77]. Light quality under such foliage may have rendered weed seeds more dormant [78]. Dense crop canopies may also reduce subsequent weed emergence by reducing seed production or increasing seed mortality and hence provide favorable habitat for seed predators, resulting from reductions in the seed bank and subsequent weed emergence [79]. This dormancy strategy works best for annual weeds whose seeds often show conditional, light-mediated dormancy. Drought and shade were found to reduce reproductive allocation and resulted in seed of
Karrikinolide may be an efficient means of stimulating weed seeds to germinate. These weed seeds would otherwise remain viable in the weed seed bank. Karrikinolide appeared to stimulate a broad spectrum of weed species, including wild turnip, wild radish, wild mustard, wild oat, cape weed, barley grass, and Paterson’s curse. Germination stimulation of weed seeds was dependent on seed dormancy state, with some species (i.e., wild turnip and barley grass) responding differently depending on seed maturity conditions in the maternal environment. The application of karrikinolide at relatively low rates (2 g/ha) to weeds both
Natural chemicals may include seed germination and growth stimulants including those for parasitic weeds. The ability of these chemicals to modify or break down seed dormancy and physiologically activate food transport, and embryo growth and development, or ruling out the over wintering stage of different living organisms, defense mechanisms, parasitic plants attachment and historian invasion to host tissues are examples on their positive effects [83].
Several natural chemicals have been identified as seed germination stimulants of parasitic species [84]. The main groups are the sesquiterpene lactones [85, 86]; some are alectrol from
Strigol, alectrol, and sorgolactone are
Ethylene- and ethephon-releasing compounds were effective in stimulating
Luque et al. [100] reported that parthenolide and 3,5-dihydroxydehydrocostus lactone significantly increased
The role of microorganisms (e.g.,
Different plant species have been reported to show a strong ability to stimulate seed germination of different
Inhibitory allelochemicals may work at different stages of the parasite’s life cycle from germination to growth and development. However, a reverse action may be obtained at low concentration received by the parasite.
Serghini et al. [107] reported that coumarins affect the normal growth and development of
Ancymidol, uniconazole, and paclobutrazol were reported as strong inhibitors of
Oxidative metabolism of ABA into phaseic acid and exogenous ABA is a strong inhibitor of
Seed dormancy is mainly found in wild species in which weeds form the integral part of these species and are facing extreme challenges under field conditions. Dormancy is a strategy of weed survival and persistence that challenge farmers under all conditions. In contrast, crops lack such a trait and always show rapid and uniform seed germination [6] with some exceptions as for cereals that possess a moderate degree of dormancy to resist preharvest sprouting (the germination of seeds after maturation but before harvest in moist environment) that results in substantial yield loss. Dormancy is a genetically complex trait controlled by polygenes, but its effects are influenced by the genetic background and environmental factors [115]. However, genotype-by-environment interactions have been reported for seed dormancy in different species [116, 117]. The growth environment greatly affects both the number and the influence of individual quantitative trait locus (QTL) in a mapping population [118]. Gu et al. [119] suggested the presence of genetically complex networks in the regulation of variation for seed dormancy in natural populations of weedy rice (
Dormancy is a property that enables weed seeds to survive conditions hazardous to plant growth, such as the periods of extreme heat and drought in certain geographical regions or the long cold winters in temperate regions, and allows them to germinate at some later time or in some other place [120, 121]. Similarly, Roberts [122] indicated that seed dormancy mechanisms tend to inhibit seed germinating at the wrong time and in the wrong place. Hence, weed seeds can persist in the soil for many years and germinate after experiencing conditions favorable for seedling survival through maturity [120]. Such a behavior results in the accumulation of large quantities of seeds in the soil, forming either transient or persistent banks which constitute the regenerative strategy developed by many weed species [31, 120]. In further support of this concept, the analysis of the composition of most seed pools has revealed that dormant seeds are only produced in large numbers by species whose growing populations are subject to periodic local extinction, such as in the case of early succession. In addition, species exposed to elimination conditions such as the use of extensive applications of herbicides or implementation of certain other agricultural practices (flooding, soil solarization, deep tillage, and continued disturbance) tend to show tolerance or resistance to such practices and deep seed dormancy as a hazard avoidance strategy. Similar strategy is also expressed well under severe stress conditions such as extreme drought and salinity.
Seed dormancy is of different types and has several definitions; it is a highly complicated phenomenon weakly understood until now in spite of the huge number of publications available. The poor understanding of dormancy is mainly due the complexity of the factors involved including mechanical, physiological, and biochemical that may be also genetically and/or environmentally controlled. Although much is known on dormancy induction and breaking, but the complicated and interrelated issues occur in the seed itself including the seed coat, embryo, cotyledon, endosperm, cell organelles, nuclei, and associated structures that all need much research work on their roles and effects on dormancy. In addition, a similar work is required on the role and influence of the external environmental factors. Weeds are of most concern since possess different types of dormancy and challenges farmers as well as researchers under field conditions. Literature on the causal factors of dormancy are huge and varied including information on the seed coat and its structures, effects of temperature, light and phytochrome system, hormones, synthetic chemicals, enzymes, temperature, O2, CO2, seed internal structures, embryo and its surrounding structures, inhibitors, cell membranes, secondary metabolites and allelochemicals, stresses, agricultural practices and genetics, soil moisture and relative humidity, salinity, soil pH, and many other related factors. These factors and their interactions influence seed dormancy and germination. The interaction between environmental factors and seed factors that determines seed germination time, periodicity, sequences and percentages and the final density of the emerged weed population. However, while researchers all over the world are trying to solve and reveal the secrecy stands behind seed dormancy in order to find solutions for some problems that the farmers facing under field conditions at which seed dormancy of weeds is the main issue, dormancy from the weedness perspectives is a survival strategy that these unwanted plant species adapt themselves to survive and exist free from hazards and insure their generations and genetic lines. Therefore dormancy is a natural phenomenon created through genetics, environment, or their interactions, while research work carried out till now is just to understand this trial and to accommodate ourselves accordingly with. Therefore, seed dormancy is one of the most important adaptive mechanisms in plants, which protects seeds from precocious germination in the presence of the inappropriate conditions for growth continuation.
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