Weed Seed Dormancy: The Ecophysiology and Survival Strategies

1. Introduction

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.

2. Importance of seed production and differences between weeds and crops

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 Lupinus and Chenopodium can exist and remain viable for thousands of years. Dormancy is keeping seeds or buds safe until the cause of it is over. It is a significant feature contributing to weed survival rate and helps them avoid herbicides and other weed control measures along with unfavorable environmental conditions. The aforementioned weed characters are very well expressed and demonstrated when weeds also exhibit seed polymorphism, heteromorphy, or heteroblasty. Certain weed species produce different types of seeds per different plants or at different parts of the same plant, different in seed colors, structures, longevity, and more importantly germination capacity and requirements. Species of Chenopodium, Amaranthus, Haloxylon, Xanthium, Rumex, and many others are good examples (Figure 1). The ability of certain weed species to produce seeds of different colors, size, or coat characters in response to certain environmental conditions is very well documented. For instance, seeds of Rumex vesicarius L. are polymorphic (light and dark of various shades) and of a high potential viability [4]. Seeds are enclosed within showy, papery fruiting valves at maturation. Naked seeds exhibit non-deep physiological dormancy and usually require an after-ripening period for several months, after which they can germinate at any time of the year in a light-dark period. Light seeds are nondormant and show excellent germination in constant darkness; dark seeds are inhibited in darkness but not in a light-dark rhythm. The conditional dormancy of dark seeds is due to the pericarp that may restrict oxygen consumption by the embryo, contain chemical inhibitors, and/or impede radicle protrusion. A range of environmental variables is likely to affect the specific germination requirements of particular seed types. However, environmental conditions may induce secondary dormancy, in both light and dark seeds [4]. Cirsium arvense (L.) Scop. ecotypes showed differences in seed germination and in reproduction methods at different temperatures: one ecotype tends to reproduce vegetatively at high temperature (37°C), while the other is reproduced by seeds at low (17°C) temperature [5]. Pre-chilling releases seed dormancy of this species. Creeping thistle seeds did not show any apparent endogenous seasonal dormancy. Environmental conditions in the soil seed bank resulted from variations in germination. Dormancy was completely broken when imbibed seeds were stored for 2 months at 19°C, but at longer storage periods, dormancy was developed. Dormancy breaking at lower temperatures was slower and incomplete. Nitrate, desiccation, and light effects on seed germination were season-related.

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.

3. Factors cause seed death in the soil

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.

4. Types of weed seed dormancy

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:

1. Physiological mechanism of dormancy

2. Ecological or demographical consequences of dormancy

Both are important to understand the evolutionary adaptations that weed seeds have developed in the agricultural environment.

5. Physiology of dormancy in weed seeds

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., Ricinus communis, Lactuca sativa, Hordeum vulgare) and weed species (e.g., Avena fatua) just prior to radical protrusion. Nondormant Xanthium pensylvanicum embryos produce more ethylene than dormant seed. Cytokinins increased in dormant seeds of Rumex obtusifolius and Spergula arvensis during dormancy breaking and chilling periods. On the other hand, ABA application inhibits seed germination. It has been suggested that related or common receptors for dormancy-breaking agents are present within the plasma membrane of the responsive embryonic cells. When triggered, these receptors initiate a signal transduction cascade, perhaps involving synthesis of or sensitization to germination-promoting gibberellins (GAs) that complete germination. Changes in the phosphorylating activity of membrane-associated, Ca²⁺-dependent protein kinases that lead to dormancy or germination have been also proposed.

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, CYP707A2, and stimulating ethylene production. The role of nitric oxide and other nitrogen-containing compounds, such as nitrate, in seed dormancy breakage and germination stimulation has been reported in several species. Both ethylene and nitric oxide have been shown to counteract ABA action in seeds, improving dormancy release and germination [36]. Abscisic acid has been also found to inhibit RNA synthesis. In seeds of Chenopodium album, ABA has been found to inhibit the embryo growth necessary to penetrate the coverings of the seed, although the initial events of embryo expansion are not prevented [37]. ABA deficiency was found to associate with the absence of primary dormancy, while high ABA content could promote seed dormancy. ABA is synthesized in the embryo and endosperm, and the balance between GA and ABA determines dormancy in weed seeds. GAs are known to obviate the requirement of seeds for various environmental cues, promote germination, and counteract the inhibitory effects of ABA, frequently in combination with cytokinins.

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 Avena fatua. Gibberellins increase in seeds, require stratification, and also facilitate degradation of food reserves in the endosperm or cotyledons necessary for germination.

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, Galinsoga parviflora Cav., Portulaca oleracea L., Chenopodium album, and Amaranthus spp. breaking dormancy is related to light that exists in promoting and inhibiting forms. Promoting form is favored by red light and inhibiting form by far-red light. Pigment generally initiates germination when in far-red light around 750 nm absorbing form and either prevents or has no effect on germination when in the form of red light around 660 nm. The inactive form (P_r) and red light, 650 nm, resulted in germination promotion, while the active form (P_fr) and far-red light, 750 nm, cause germination inhibition. Leaf canopy suppresses germination through shading effects since it promotes relatively low-red to far-red photon flux ratio, producing relatively low P_fr to P_r ratios in underlying seeds which inhibit seed germination. Great variations do exist between weed species in pattern of development of photoplastic properties.

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.

6. ROS production and sensing in seeds

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.

7. Seed dormancy in response to stresses and herbicides

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 Alopecurus myosuroides Huds. seeds depends on the magnitude and timing of temperature and water availability during the reproductive growth phase. Water availability seems more important during maternal environmental perception and temperature during zygotic environmental perception [43]. Both temperature and soil moisture content are important factors in seed germination induction. Maximum (68–100%) and rapid (2.58 days) seed germination of Calotropis procera (Aiton, W.T. Aiton) occurred at 30°C but declined under water stress with increasing temperature, from 92.5 ± 1.1% at 20°C and 0 MPa to 2.8 ± 1.7% at 40°C and −0.4 MPa, respectively. Seeds were unable to germinate at ambient temperatures ≥40°C but remained quiescent and viable. Planting depth also influenced seedling emergence and water stress inducing a reduction in optimum germination temperature from 30 to 20°C. Short mean germination times increase seedling survival by rapid transition from endosperm resources to photosynthesis, whereas seed quiescence (cf. dormancy) optimizes germination opportunities in a semi-arid environment. Thus, the germination traits are likely promoted seedling survival and its spread [44].

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 Brassica napus L. seed in their potential to exhibit secondary dormancy following environmental stress. A significant number of differences in gene expression between the cultivars were apparent in the transition from full-size embryo to mature seed. Most differences were apparent in the desiccation stage, and some were in genes related to signaling processes and protein biosynthesis. Authors suggested that the propensity of Brassica seeds to manifest secondary dormancy may be determined by changes in gene expression that occur during late seed development [47].

Breaking primary dormancy of achenes in Cirsium arvense only took place during the first stratification month at moderate temperature, which is mainly due to an increase in the average water stress tolerance in seed population. The induction of secondary seed dormancy during after-ripening at all temperature resulted mostly from a substantial loss of the seed’s ability to tolerate water stress [46].

The effects of drought and herbivory on biomass and seed quality in Vaccaria hispanica (Mill.) Rauschert have been also studied by Cici [48]. The maternal water stress suppressed seed mass but stimulated seed dormancy in seeds. Progenies from the maternal stress environment were more persistent than those from the maternal control environment after being exposed to 45°C and 100% RH for 8 days.

Nassella trichotoma Hackel ex Arech. was identified to be a non-photoblastic, with germination percentages being similar under alternating light and dark and complete darkness conditions [49]. With an increase of osmotic potential and salinity, a significant decline in germination was observed. N. trichotoma seed dormancy break can be triggered by favorable alternating temperatures of approximately 25/15°C and ample water availability. Radiant heat has a positive effect on total germination. Osmotic stress and salinity significantly reduced germination, while water appeared as the most important limiting factor in germination. Soil pH is not a limiting factor on this species recruitment. Herbicide-resistant populations of the same weed species have been studied to identify differences in important environmental factors on its seed dormancy. The increase in osmotic potential and salinity caused a significant decline in germination. The pH had no effect on germination. Exposure to a radiant heat of 120°C for 9 min resulted in the lowest germination in the first population (33%) and in the second population (60%). In the burial depth treatment, both populations had the highest emergence of 1 cm depth. However, variation between the two populations was observed for the burial depth of 4 cm. Differences between populations were found in emergence and overall germination [49].

Seed germination of the salt-tolerant species, Salicornia europaea L., that produces dimorphic seeds of high salt tolerance limits has been studied by Orlovsky et al. [50]. Germination of large seeds was found to be 3–4 times higher than of small seeds under control and at 0.5–2% of all salts tested. Germination and plant growth in mixed sulfate-chloride salts were distinctly higher than in pure chloride salts; small seeds exhibited deep innate dormancy but stimulated by 0.5–2% of chloride and sulfate salts. Small seeds develop earlier, are more dormant, and are less salt-tolerant than large seeds. Seed dimorphism made the species more flexible in its response to varying salinity and more adapted to salt and temperature stresses.

GA₃ at concentration of 400 ppm strongly stimulated germination of C. bursa-pastoris at 12/12 h of light/dark and continuous darkness. While KNO₃ at 2 mmol had no effect on germination, long wet pre-chilling enhanced germination. Seed germination occurred at 10–30°C and within a range of pH of 3–11. On the other hand, drought and salt stress strongly inhibited germination, but authors suggested that weed seeds can germinate at high salinity. Sowing depth is critical for germination, and seedling emergence decreased with sowing depth. The rates of C. bursa-pastoris germination and seedling emergence were highest for seeds on the soil surface [38].

8. Dormancy and agricultural practices

9. Seasonal dormancy and shift in population germination time

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 Prosopis farcta (Banks and Sol.) Macbride, it inters into dormancy by the end of fall and winter seasons, drops all leaves, and stay dormant throughout the winter season. The weed breaks bud dormancy by the start of spring and grows vegetatively throughout the spring and summer seasons. The seeds of this weed also will not germinate during dormancy period [54]. The main factors control dormancy induction, or releases in all these species are the temperature and photoperiod.

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 Bromus tectorum L. lose primary dormancy in summer and are poised to germinate rapidly in autumn. If rainfall is inadequate, seeds remain ungerminated and may enter secondary dormancy under winter conditions [56]. Maximum secondary dormancy was achieved in the laboratory after 4 weeks at −1.0 MPa and 5°C. Seeds in the field became increasingly dormant through exposure to temperatures and water potentials in this range. They were released from dormancy through secondary afterripening the following summer. Different genotypes showed contrasting responses to dormancy induction cues in both laboratory and field. The changes in Ψb(50) can be used to characterize secondary dormancy induction and loss in this weed species.

A complex nature of responses may be exhibited by Senecio vulgaris L. in response to intense chemical selection. Seed germination and growth normally occur during the spring season (May–April) at which heavy application of simazine herbicide is practised. The repeated application of the herbicide during this growing period forced the weed to shift its germination time and life cycle to the winter annual where no herbicide application is practised. This shift in weed germination and growth is a strategy adopted by this species to avoid phytotoxicity of the herbicide [57].

10. Seed morphology, polymorphism, and dormancy

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 Spergula arvensis L. weed. The seed coat character is genetically controlled and associated with germination. Three different seed coat forms are found in weed seeds, each controls different levels of seed dormancy; these are the homozygous papillate form bearing about 120 papillae homozygous smooth coated form, and a heterozygous form which bears 60 papillae. The papillate seeds germinate readily at 21°C, while the reverse is true at lower temperatures [58]. This is an example of genetic polymorphism which is the production of seed of divergent morphology and behavior as a result of genetic segregation. Genetically controlled polymorphism distinctly showed different dormancy genotypes.

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 Xanthium spp. Fruits of these species each contain a pair of seeds large and small one deeply dormant upper seed and a lower less dormant seed [59]. Dormancy breaking is different with the result of not less than 12 months of separate germination of the two seeds which could be regarded as an obvious insurance strategy. Dormancy breaking requirements are different for the two seeds.

Placement of Datura stramonium L. seeds on mother plants had a significant effect on seed germination. The middle and lower seeds on the maternal plant had less germination rate and seedling vigor than those in the upper part of the plant [60].

In Avena fatua and Avena ludoviciana Durieu, the grains are born on different parts of the individual spikelet that have different germination requirements. In Amaranthus spp. different seeds on the same plant are produced that are different in colors and seed coat morphology (Figure 2). Chenopodium album also produces brown to black seeds. Brown/black seed production ratio is 3–97%. Black seeds require cold temperature or supply of nitrate for dormancy breaking, while brown seeds are readily germinating at low temperature and are thin-walled. However, brown seeds germinate quickly, and seedlings are killed by winter cold or agricultural operations, but if they survive, they produce very large plants with a higher reproductive output, and then black seeds germinate in the spring season. Brown seeds ripened earlier giving the same ratio of brown to black seeds which is probably environmentally governed. Brown seeds represent highly opportunistic strategy, whereas black seeds are more seasonal and predictive in behavior.

In Atriplex heterospermum (Greene) Nels. & Macbr weed, black seeds are produced early in the season followed by large brown seeds to avoid unfavorable conditions. Halogeton sp. behaves similarly at which black seeds are produced in short days, while brown seeds in long days, and the weed shows differences in dormancy-breaking requirements. Species of the Cruciferae family show seed size and color polymorphism such as for Eruca sativa Mill. and Sinapis arvensis.

11. Seed dormancy and herbicide resistance

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 Echinochloa oryzicola Vasing. than for herbicide-susceptible seeds, implying greater dormancy in the latter. Germination rate was often more rapid for herbicide-resistant than for herbicide-susceptible seeds, implying greater dormancy in the latter [61]. Population shift in life cycle has been previously discussed as shown by Senecio vulgaris L. in order to avoid herbicide phytotoxicity. Scursoni et al. [62] have shown that seeds from Avena fatua plants that had survived the application of diclofop-methyl in barley crops had a lower dormancy level. Moreover, an anticipated emergence timing of A. fatua was detected in plots that had been treated with diclofop-methyl the previous year in relation to the emergence timing observed in plots that had not been treated with the herbicide.

12. Factors enhance seed germination and dormancy breaking

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, Rhynchosia capitata (Heyne ex Roth) DC. seeds exhibit physical dormancy that is mainly due to the impermeability of their coat. Mechanical scarification and acid scarification (soaking of seeds in H₂SO₄ for 60 and 80 min and in HCl for 12 and 15 h) were very effective in breaking dormancy and enhancing germination [63]. Dormancy can be also overcome by alternate treatments of wetting and drying aiming to destroy the hard seed coat. In addition, seeds may be frozen and then dissolved, exposed to cold temperature and stratification, or washed by water for removal of inhibitors. Seed germination of Eclipta prostrata (L.) L. was completely inhibited in the dark, but in the dark/light, it was 93% at 20/30°C alternating day/night temperature. Germination was more than 80% at 140°C followed by incubation at 30/20°C for 14 days but declined at higher ratio until zero germination at 200°C. Germination is tolerant to salinity but highly sensitive to water stress. Seeds germinate at pH of 4–10 and are enhanced closure to soil surface but reduced thereafter until no germination obtained from a depth of 0.5 cm [64].

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 Lupinus albus L. and Trifolium pratense L., respectively. In other cases genetic-controlled dormancy may not possibly overcome by physical or mechanical treatments as for seeds of Nicandra physaloides (L.) Gaertn. which possess one pair of iso-chromosomes. The presence or absence of an iso-chromosome determines whether a seed will germinate readily (2n = 20) or is dormant (2n = 19). Echinochloa oryzicola dormancy is possibly relieved through stratification. Stratification temperatures, moisture levels, and durations contributed to its seed dormancy released by decreasing hydrotime required for germination and by eliminating any germination sensitivity to oxygen [61]. Alternating temperatures nearly doubled germination rate in all weed populations of this species. Stratification at Y = 0 MPa increased germination rate compared to stratification at lower water potentials. Maximum germination rate occurred after 2–4 weeks of stratification at 0 MPa; it was suggested that field soil saturation in winter would contribute toward E. oryzicola dormancy release and decrease the time to seedling emergence.

Phleum paniculatum Huds. seeds had a shallow dormancy (20–30 d) when stored at room temperature (25 ± 5°C). Seeds could germinate at constant temperatures between 10 and 25°C. Light was not essential for seed germination, and pH values from 4 to 10 did not inhibit germination. Seeds were moderately adaptable to water potential and NaCl concentration, and germination rates decreased by 50% when water potential was −0.4 MPa or NaCl concentration was 130 mM. Increased soil burial depth decreased the seedling emergence, and no seeds emerged at depth more than 4 cm [66].

Cold stratification may be a highly selective treatment and hence did not decrease dormancy for any of Lamium species collected in Sweden, while warm stratification (21°C) of seed batches of all species that germinated when fresh increased dormancy but decreased over time for all tested species. Pretreatment with dry storage was most efficient in reducing dormancy for all annual Lamium. In contrast to cold or warm stratification, dry storage led to germination in darkness for all species [67]. Warm stratification increased germination, both in darkness and at 15/5°C, but did not cause germination of any of the two Papaver aculeatum populations tested at 30/20°C. Cold stratification reduced germination and limited germination to the cooler temperatures. Alternating cold and warm stratifications showed that the species undergoes dormancy cycles [23].

Seed dormancy of Polygonum persicaria L., Chenopodium album, Spergula arvensis, and Sisymbrium officinale (L.) Scop. [68] is regulated by field temperature and not nitrate and light. Aciphylla glacialis F. Muell. ex Benth seeds possess physiological dormancy that is overcome by cold stratification. Seeds have undeveloped embryos at dispersal but grow to germinate after 4–9 weeks at both constants 5°C and 10–5°C (day-night) temperatures. The species exhibits morphophysiological dormancy at which final percentage germination and dormancy status varied significantly among natural populations [69]. In another study Rhie et al. [70] reported that warm stratification (25/15°C) stimulated embryo growth and germinated the seeds of bamboo (Nandina domestica Thunb.), but cold stratification (5°C) was not required. Warm stratification at a constant 20°C speeds seed germination more than a fluctuating temperature at 25/15°C and shortened germination time by 4 months compared with that under natural conditions. Gibberellic acid (GA₃) at 100 and 1000 mg L⁻¹ could substitute for warm stratification and break dormancy in seeds incubated at 15/6°C. In addition seeds may be exposed to light for chemical changes inside the seed or can be treated chemically with germination stimulants such as potassium nitrate, gibberellic acid, cytokinins, and auxins.

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 (P_fr) and inactive (P_r) forms while the physiological mechanism involved in this conversion is poorly understood.

The changes in dormancy and germination of Chenopodium album L., Polygonum persicaria H., P. lapathifolium L. subsp. lapathifolium, Sisymbrium officinale (L.) Scop, and Spergula arvensis L. found regulated by temperature. Soil moisture and nitrate content had no effect. Germination of C. album, S. officinale, and S. arvensis was stimulated by light, nitrate, and desiccation. These factors all increased the width of the range of temperatures over which germination could proceed and therefore affected the expression of dormancy. Germination depended on the one hand on the actual field temperature after exhumation and on the range of the germination temperature, which was determined by the dormancy status of the seeds and soil temperature during burial. When nitrate was added during the test, the germination temperature range became wider, and germination could occur during a longer period of the year [73].

Karimmojeni et al. [74] studied dormancy breaking in seeds of Thlaspi arvense L., Descurainia sophia (L.) Webb ex Prantl., and Malcolmia africana (L.) W.T. Aiton (Brassicaceae) and reported that burying seeds of D. sophia, at a depth of 10 cm for 60 days, resulted in 55% germination and seeds dry-stored at 20°C for 180 days (45%) showed the highest level of germination. In M. africana, the germination percentage reached 95% when seeds buried at a depth of 1 cm were soaked in a GA₃ concentration of 150 ppm. T. arvense had the lowest level of germination compared to the other species. The highest percentage of T. arvense germination was obtained in seeds treated with 150 ppm GA₃. Potassium nitrate partly increased germination of M. africana, which initially was less dormant than those of T. arvense and D. sophia. Light is the most common germination trigger, though many seeds also respond to temperature and moisture fluctuations, increased aeration, and increased release of nitrate and other soluble nutrients that occur in tilled soil [75]. Tillage may end dormancy stage by exposing seeds to temperature and light. In many weed species, dormancy may be ended by treatment with a single factor or a mixture of ecological factors such as exposure to cold or humid conditions or to cold temperature and light.

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 Datura ferox exhibit increased endo-p-mannanase and p-mannosidase activities in the micropylar region of the endosperm after red light stimulation and many hours before the radicle protrudes through it [76]. Sometimes it is necessary washing the inhibitory chemicals. In other cases humid cold period or stratification is required during autumn and late in winter.

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 (P_fr) and inactive phytochrome red (P_r) determines whether the seed is dormant or not. The optimum ratio is established after exposure of the seed to white light which converts P_r to P_fr in the embryo. Outer coverings form a filter to high incident light on the seed and modify its effectiveness in converting P_r to P_fr. Chenopodium album seeds with dark seed coats are less responsive to light than their thin, light-coated counterparts. Dark seeds filter out light and reduce the conversion of P_r to P_fr when exposed to light; hence they remain dormant for longer periods than thin light-coated seeds.

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 Amaranthus spp., Capsella bursa-pastoris, and wild Brassica.; and chemical inhibitors such as ABA and allelochemicals. Oxygen of percentages in the soil ranged between 1 and 9%. CO₂ with a percentage between 5 and 15%, temperature. Low oxygen prevents oxidative stress and germination, while high CO₂ induced dormancy of Brassica alba seeds. The after-ripening needs which are irrelevant to the degree of the embryo maturation are also involved in dormancy due to physiological changes that are not very well understood.

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 Avena fatua with less intense primary dormancy than the plants grown under resource-rich conditions, but had no apparent effect on seed vigor [80].

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 ex situ and in situ can significantly improve weed seed germination while lower rates were inefficient. Karrikinolide seems working as a germination stimulant rather than a dormancy-breaking agent [81]. The smoke-derived chemical karrikinolide responses of seeds differed among populations of Brassica tournefortii, but this variation was reduced when seeds developed in a common environment. The KAR1 responses of each population changed when seeds developed in different environments. Different parental environments affected germination responses of the populations differently, showing that parental environment interacts with genetics to determine KAR1 responses. Seeds from droughted plants were 5% more responsive to KAR1 and 5% less dormant than seeds from well-watered plants, but KAR1 responses and dormancy state were not intrinsically; however, the parental environment in which seeds develop is one of the key drivers of the KAR1 responses of seeds [82].

13. Parasitic weeds and seed germination stimulants and inhibitors

13.1 Germination stimulants

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 Vigna sinensis L., orobanchol from Trifolium pratense [87], and strigolactones and orobanchol from sorghum [88, 89]. Strigol was identified from the root exudates of Gossypium hirsutum L. plants and Zea mays L., which all are also produced by Striga host plants [90]. Some sesquiterpene lactones induce seed germination of Orobanche cumana Wallr. [91] better than the synthetic germination stimulant, GR24. However, Rice [39] stated that the stimulatory effect of allelochemicals is rather very limited or even rare and usually associated with low concentration effects of the compounds.

Strigol, alectrol, and sorgolactone are Striga germination stimulants, all produced by the parasite host plants, while different plant species have shown a strong ability to stimulate seed germination of different Orobanche species by more than 90% [90]. Ethylene was reported by Zehar and Fer [92] to induce germination of GR24-conditioned seeds of Orobanche ramosa, and its synthesis is required for parasite germination. However, a large number of synthetic substances were reported to stimulate germination of Striga species, among which are coumarin derivatives, scopoletin, thiourea, allylthiourea, sulfuric acid, sodium hypochlorite, and inositol [93]. Thidiazuron herbicide has been reported to activate ethylene release [94] and thus indirectly enhance Striga seed germination, although it is regarded by the same author as an inhibitor of haustorial development.

Ethylene- and ethephon-releasing compounds were effective in stimulating Striga asiatica (L.) Kuntze [95], while under field conditions, it was highly efficient against Striga hermonthica (Del.) Benth. Other chemicals have been found to stimulate Striga germination in vitro including several growth hormones and kinetins. Thuring et al. [96] were able to synthesize two diastereomers of demethylsorgolactone. Nijmegen-1 was active at low concentration as a suicidal germination agent for both Striga and Orobanche [97]. Yoneyama et al. [98] reported that jasmonates and related compounds elicited seed germination of O. minor and methyl jasmonate was the most active stimulant on several Striga and Orobanche species. The synthetic germination stimulants, strigol analogues (GR compounds), stimulate seed germination of different Striga and Orobanche species. Strigol has been reported for Orobanche stimulation [99], and its analogues GR7 and GR45 stimulated germination of O. minor.

Luque et al. [100] reported that parthenolide and 3,5-dihydroxydehydrocostus lactone significantly increased O. cumana germination and exhibited higher activity than GR24. The positive role of this hormone was further confirmed. The activity of germination stimulants for suicidal germination of Orobanche seeds under field conditions could be substantially enhanced by applying brassinolide (2a, 3a, 22R, and 23R-tetrahydroxy-24Smethyl-B-homo-7-oxa-5-a-cholesten-6-ono) and related compounds to the infested soils [101].

The role of microorganisms (e.g., Streptomyces) has also been implicated in Orobanche seed germination. Christeva and Naumova [102] reported different strains of Streptomyces to induce germination of O. ramosa and concluded that microorganisms may take part in germination processes of this parasite. In another study, Yoneyama et al. [103] reported stimulation of O. minor seed germination by certain fungal metabolites including cotylenins and fusicoccins at concentrations as low as 10⁻⁵ M and up to more than 50%.

Different plant species have been reported to show a strong ability to stimulate seed germination of different Orobanche species by more than 90%, and extracts of hundreds of plant species were tested for possibly stimulating or inhibiting seed germination of different Orobanche spp.; many proved effective and may be considered as trap, cover, and catch species or a source of natural germination stimulants for these parasites [104, 105, 106].

13.2 Germination inhibitors

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 Orobanche cumana seedlings and the effect was more intense from the resistant than in the susceptible cultivars. Coumarins (ayapin and scopoletin) from sunflower plants were implicated as inhibitory allelochemicals to germination; growth and development of O. cumana [91] are both excreted by sunflower roots into the environment and could act as toxic allelochemicals to the parasite [108]. Bar-Nun and Mayer [109] reported the presence of large amounts of oleic and linoleic acids and small amounts of stearic and palmitic acids in Orobanche aegyptiaca (Pers.) Pomel. seeds. The authors expected these acids to interfere with sugar metabolism and thus prevent parasite seed germination during the preconditioning period. Sunflower seeds treated with 40 ppm of benzo(1,2,3)thiadiazole-7-carbothioic acid S-methyl ester (BTH) for 36 h completely prevented infection of O. cumana in root chambers [110]. In pot studies, at considerable inoculums of O. cumana seeds, the total number of parasite shoots was reduced by almost 90% with 60 ppm of BTH. Different Fusarium spp. were reported by Sauerborn [111] to infect Orobanche and Striga seeds and parasite plants, and he emphasized the role of phytotoxins in the process. He added that members of the genus Fusarium produce toxins such as enniatin, fumonisin, fusaric acid, moniliformin, and trichothecenes that possess a broad range of biological activities and metabolic effects. However, some of these compounds have been already proposed and considered to be used as natural herbicides [112]. Later studies showed fusaric acid and 9,10-dehydrofusaric acid to be active at low doses in reducing Striga seed germination [113]. Zehar and Fer [92] found T-2 toxin from Fusarium sp. to be the most active among 14 fungal toxins tested, inhibiting 100% seed germination at 10⁻⁵ M and was active down to 10⁻⁷ M (19% inhibition). Deoxynivalenol [vomitoxin], produced by Fusarium spp., was also very effective, causing 100 and 69% reduction in germination when assayed at 10⁻⁴ and 10⁻⁵ M, respectively. The high activity shown by some fungal toxins suggests that they may have potential as more natural and safe herbicides to suppress S. hermonthica seed germination. The use of toxic secondary metabolites could represent a useful alternative strategy in the management of parasitic weeds, by interfering with the induced germination process, and that fungal culture extracts could be an interesting source of new compounds acting as natural and original herbicides on these parasites.

Ancymidol, uniconazole, and paclobutrazol were reported as strong inhibitors of Orobanche ramosa germination, and CCC, daminozide, and prohexadione at 10⁻⁵ had a similar effect [92]. The inhibitory effect of paclobutrazol and uniconazole could be resulted from the increased level of abscisic acid.

Oxidative metabolism of ABA into phaseic acid and exogenous ABA is a strong inhibitor of O. ramosa germination [89]. Germination of S. asiatica was reduced by silver thiosulphate and COCH, inhibitors of ethylene action, and ACC oxide [114]. Aminoethoxyvinyl glycine reduced cytokinin thidiazuron-induced parasite germination.

14. Genetic studies on weed seed dormancy

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 (Oryza sativa). Multiple loci and epistasis control genetic variation for seed dormancy in the weed. Iso-chromosomes have been also mentioned to determine seed germination and dormancy. However, molecular studies on dormancy genetics are clearly rare, and there is a need for research in this aspect and genetic dormancy differences among and between weed species and their populations and the link of these with environmental conditions.

15. Seed dormancy as a weed survival strategy

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.

16. Conclusions

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.