Weed Seed Dormancy

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Important parameters that influence weed seeds’ germination and seedlings’ emergence can affect the efficacy of false seedbed as weed management practice. These parameters consist of environmental factors such as soil temperature, soil water potential, exposure to light, fluctuating temperatures, nitrates concentration, soil pH and the gaseous environment of the soil. Soil temperature and soil water potential can exert a great influence on composition of the weed flora of a cultivated area. Base soil temperatures and base water potential for germination vary among different weed species and their values can be possibly used to predict which weeds will emerge in a field and also the timing of emergence. Predicting the main flush of weeds in the field could maximize the efficacy of false seedbed technique as weed management practice. Timing, depth and type of tillage are important factors affecting weed emergence and subsequently the efficacy of false seedbed. The importance of shallow tillage as weed control method in the false seedbed technique has been highlighted. Further research is needed to understand and explain all the factors that can affect weed emergence so as to maximize the effectiveness of eco-friendly weed management practices such as false seedbed in different soils and under various climatic conditions. Weed Seed Dormancy In this unit we will learn about seed dormancy as seen by many respected seed biologists, the way most scientists in the seed discipline view this complex area of plant

Key Factors Affecting Weed Seeds’ Germination, Weed Emergence, and Their Possible Role for the Efficacy of False Seedbed Technique as Weed Management Practice

Important parameters that influence weed seeds’ germination and seedlings’ emergence can also affect the efficacy of false seedbed as weed management practice. These parameters consist of environmental factors such as soil temperature, soil water potential, exposure to light, fluctuating temperatures, nitrates concentration, soil pH, and the gaseous environment of the soil. Soil temperature and soil water potential can exert a great influence on composition of the weed flora of a cultivated area. Base soil temperatures and base water potential for germination vary among different weed species and their values can possibly be used to predict which weeds will emerge in a field as well as the timing of emergence. Predicting the main flush of weeds in the field could maximize the efficacy of false seedbed technique as weed management practice. Timing, depth, and type of tillage are important factors affecting weed emergence and, subsequently, the efficacy of false seedbed. The importance of shallow tillage as a weed control method in the false seedbed technique has been highlighted. Further research is needed to understand and explain all the factors that can affect weed emergence so as to maximize the effectiveness of eco-friendly weed management practices such as false seedbed in different soils and under various climatic conditions.

Introduction

Weeds that exist with crops early in the season are less detrimental than weeds that compete with the crop later in the growing season, and this principle has supported the timely use of weed management practices (Wyse, 1992). Either early- or late-emerging weeds produce great proportions of viable seeds that can remain in the soil profiles for a long time period, contributing to the perpetuation and the success of weeds (Cavers and Benoit, 1989). As a result, in most arable crop systems, weed management strategies focus mainly on reducing weed density in the early stages of crop growth (Zimdahl, 1988). However, confining weed management to a narrow temporal window increases the risk of unsatisfactory weed management due to unfavorable weather (Gunsolus and Buhler, 1999). Weed seed banks are the primary source of persistent weed infestations in agricultural fields (Cousens and Mortimer, 1995) and if their deposits are increased, greater herbicide doses are required to control weeds afterwards (Taylor and Hartzler R, 2000). Annual weed species increase their populations via seed production exclusively (Steinmann and Klingebiel, 2004), whereas seed production is also important for the spread of perennials (Blumenthal and Jordan, 2001).

Consequently, it is preferable to focus on depleting the seed stock in the soil through time rather than viewing weeds just as an annual threat to agricultural production (Jones and Medd, 2000). This approach is reinforced not only by ecological (Davis et al., 2003) but also by economic simulation models (Jones and Medd, 2000). False seedbed technique is a method providing weed seed bank depletion. The principle of flushing out germinable weed seeds before crop sowing forms the basis of the false seedbed technique in which soil cultivation may take place days or weeks before cropping (Johnson and Mullinix, 1995). Germination of weed seeds is stimulated through soil cultivation (Caldwell and Mohler, 2001). Irrigation is suggested to provide the adequate soil moisture required for sufficient weed emergence. In the case of false seedbed, emerged weeds are controlled by shallow tillage operations (Merfield, 2013). Control of weeds and crop establishment should be delayed until the main flush of emergence has passed in order to deplete the seedbank in the surface layer of soil and reduce subsequent weed emergence (Bond and Grundy, 2001).

False seedbed technique aims to reduce weed seed bank by exploiting seed germination biology. Thus, the efficacy of such management practices is directly associated with all the factors affecting germination of weed seeds and seedling emergence. Soil temperature, diurnal temperature variation, soil moisture, light, nitrates concentration in the soil, and the gaseous environment of the soil can regulate seed germination and weed emergence (Merfield, 2013). Except for the case of environmental factors, tillage is the most effective way to promote weed seed germination because the soil disturbance associated with tillage offers several cues to seedbank residents such as elevated and greater diurnal temperature, exposure to light, oxygen, and release of nitrates in the soil environment (Mohler, 2001). The aim of this review paper is to give prominence to the significance of environmental factors and tillage for weed seed germination and seedlings emergence and, therefore, for the efficacy of false seedbed technique as weed management practice.

The Impact of Soil Temperature and Water Potential on Weed Seed Germination and their Roles for Predicting Weed Emergence

The longevity of weed seeds into the soil profiles is attributed to the phenomenon of dormancy that prevents seed germination even when the environmental conditions are ideal (Benech-Arnold et al., 2000). Dormancy is distinguished into two types: primary and secondary dormancy (Karssen, 1982). The end of primary dormancy is sequenced by the establishment of secondary dormancy and this sequence has been defined as dormancy cycling (Baskin and Baskin, 1998). In adapted weed species, dormancy is alleviated during the season preceding the period with favorable conditions for seedling development and plant growth, while dormancy induction takes place in the period preceding the season with environmental conditions unsuitable for plant survival (Benech-Arnold et al., 2000). Furthermore, seeds from summer annual species are released from dormancy by low winter temperatures. High summer temperatures may induce entrance of the same seeds into dormancy again, which is referred to as secondary dormancy. On the contrary, seeds from winter annuals are released from dormancy by high summer temperatures whereas low winter temperatures induce their entrance into secondary dormancy (Forcella et al., 2000). Relatively dry seeds lose dormancy at a rate which is temperature-dependent. In hydrated seeds, high temperatures reinforce or induce dormancy whereas low temperatures between −1 and 15°C may stimulate germination (Roberts, 1988).

Timing of weed emergence is dependent on the timing and rate of seed germination, which is dependent not only on soil temperature and but also on moisture potential (Gardarin et al., 2010). Of the many environmental factors that regulate seed behavior under field conditions, soil temperature has a primary influence on seed dormancy and germination, affecting both the capacity for germination by regulating dormancy and the rate or speed of germination in non-dormant seeds (Bouwmeester and Karssen, 1992). It has been recognized since at least 1860 that three cardinal temperatures (minimum, optimum, and maximum) describe the range of T over which seeds of a particular species can germinate (Bewley and Black, 1994). The minimum or base temperature (Tb) is the lowest T at which germination can occur, the optimum temperature (To) is the T at which germination is most rapid, and the maximum or ceiling temperature (Tc) is the highest T at which seeds can germinate. Seed germination rates also vary with increasing temperature as it increases in the suboptimal range and decreases above the optimum temperature (Alvarado and Bradford, 2002).

To account for the effect of temperature on the progress of germination, the concept of thermal time has been developed (Garcia-Huidobro et al., 1982). The application of thermal time theory to germination is based on the observation that for some species there is a temperature range over which the germination rate for a particular fraction of the seed population is linearly related to temperature. The base temperature Tb is estimated as the x-intercept of a linear regression of the germination rate with temperature (Gummerson, 1986). Once seeds have lost dormancy, their rate of germination shows a positive linear relationship between the base temperature and the optimum temperature and a negative linear relationship between the optimal temperature and the ceiling temperature (Roberts, 1988). For the case of the summer annual Polygonum aviculare (L.), Kruk and Benech–Arnold (1998) demonstrated that low winter temperatures alleviate dormancy, producing a widening of the thermal range permissive for germination as a consequence of a progressive decrease of the lower limit temperature for germination of the population (Tb). In contrast, high summer temperatures reinforce dormancy, which results in a narrowing of the thermal range permissive for germination through an increase of Tb.

Germination speed of Alopecurus myosuroides (Huds.) seeds decreased with temperature, whereas the final proportion of germinated seeds was not significantly influenced (Colbach et al., 2002b). Minimum temperature required for seed germination is different for various weed species. Minimum temperature required for seed germination has been estimated at 0°C both for the winter annual A. myosuroides (Colbach et al., 2002a) and the summer annual P. aviculare (Batlla and Benech-Arnold, 2005). However, Masin et al. (2005) estimated the base temperature for Digitaria sanguinalis (L.), Setaria viridis (L.), P. Beauv., Setaria pumila (Poir.), Roem. & Schultes and Eleusine indica (L.), at 8.4, 6.1, 8.3, and 12.6°C, respectively. Moreover, the mean Tb recorded for summer annuals Amaranthus albus (L), Amaranthus palmeri (S. Wats.), D. sanguinalis, Echinochloa crus-galli (L.) Beauv., Portulaca oleracea (L.), and Setaria glauca (L.) was ~40% higher as compared to the corresponding value recorded for winter annuals Hirschfeldia incana (L.) and Sonchus oleraceus (L.). Optimal temperature conditions required for terminating dormancy status vary among different species. For example, Panicum miliaceum (L.) seeds lost dormancy at 8°C while P. aviculare seeds were released from dormancy at 17°C (Batlla and Benech-Arnold, 2005). The two germination response characteristics, Tb and rate, influence a species’ germination behavior in the field (Steinmaus et al., 2000). Extended models should be developed to predict the effects of environment and agricultural practices on weed germination, weed emergence, and the dynamics of weed communities in the long term. This requires estimating the baseline temperature for germination for each weed species that are dominant in a cultivated area and recording seed germination in a wide range of temperatures (Gardarin et al., 2010).

The knowledge about seed germination for the dominant weed species of a cultivated area is vital for predicting weed seedlings emergence. The possibility of predicting seedling emergence is essential for improving weed management decisions. However, weed emergence is the result of two distinct processes, i.e., germination and pre-emergence growth of shoots and roots, which react differently to environmental factors and should therefore be studied and modeled separately (Colbach et al., 2002a). In temperate regions, soil temperature is probably the most distinct and recognizable factor governing emergence (Forcella et al., 2000). Soil temperature can be used as a predictor of seedling emergence in crop growth models (Angus et al., 1981). Soil temperature can also be used for predicting weed emergence, but only if emergence can be represented by a simple continuous cumulative sigmoidal curve and the upper few centimeters of soil remain continuously moist (Forcella et al., 2000).

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Fluctuating temperatures belong to parameters that can remove the constraints for the seed germination of many weed species once the degree of dormancy is sufficiently low (Benech-Arnold et al., 2000). In particular, the extent and number of diurnal soil temperature fluctuations can be critical in lessening seed dormancy of several species. For example, alternating temperatures at 25 °C increased germination of Amaranthus retroflexus (L.), Amaranthus spinosus (L.), and Amaranthus tuberculatus (L.) from 23 to 65, 8 to 77, and 9 to 57%, respectively, as compared to non-alternating temperatures. Fluctuating temperatures from 2.4 to 15°C can terminate the dormancy situation in Chenopodium album (L.) seeds (Murdoch et al., 1989). Either four diurnal cycles of 12°C amplitude or 12 diurnal cycles of 6°C amplitude were necessary for the emergence of D. sanguinalis (King and Oliver, 1994). The number of cycles of alternating temperatures needed to end the dormancy situation has to be investigated. In Sorghum halepense (L.) Pers., a 50% increase in cycles of alternating temperatures can double the number of seeds that are released from dormancy (Benech-Arnold et al., 1990). Furthermore, if the demand for fluctuating temperatures to terminate dormancy in the seeds of this species is not satisfied, a loss of sensitivity to fluctuating temperatures occurs in a proportion of the population (Benech-Arnold et al., 1988). The variation among weed species regarding the demands for fluctuating temperatures for seed germination points out the need for further investigation regarding the effects of fluctuating temperatures in germination of noxious weed species in different regions around the world and under various soil and climatic conditions.

Soil moisture is a key parameter affecting the seed dormancy status of many species (Benech-Arnold et al., 2000; Batlla et al., 2004). First of all, the environmental conditions existing during seed development in parent plants and seed maturation affect the relative dormancy of the seeds. Less dormant seeds of Sinapis arvensis (L.) were produced from the mother plants under water stress conditions (Wright et al., 1999) while similar results have been reported regarding either winter annual grass species Avena fatua (L.) or summer perennial S. halepense (Peters, 1982; Benech-Arnold et al., 1992). Moreover, sufficient water potential has been noticed to increase the production of dormant A. myosuroides seeds (Swain et al., 2006).

The effects of water deficits on seed germination have been encapsulated in the “hydrotime” concept. This idea was first illustrated by Gummerson (1986) and further explained by (Bradford, 1995). The model of (Bradford, 1995) accounted for dormancy loss during after-ripening through changes in the base water potential of the seeds’ environment that permits 50% germination (Ψb(50)). Christensen et al. (1996) confirmed that Ψb(50) value of the population is decreased by the change in Ψb(50) due to after-ripening. The Ψb(50) value is saved as the Ψb(50) value of the population and serves as the initial value for the next time step. The process continues until the Ψb(50) value of fully after-ripened seeds is reached. The model described is only to consider dormancy changes, not only in relation to the thermal environment, but also as a function of the soil water status. The loss of primary dormancy does not secure some species germination if moisture demands are not met. For example, adequate water conditions are demanded to promote germination of Bromus tectorum (L.) (Bauer et al., 1998). Bauer et al. (1998) assumed that the temperature-dependent after-ripening process in this winter annual occurs at soil water potentials below ~-4 MPa. Martinez–Ghersa et al. (1997) reported that increased water content promoted seed germination of A. retroflexus, C. album, and E. cruss-galli.

The seed germination response to the soil water potential of wild plants could be correlated with the soil water status in their natural habitats (Evans and Etherington, 1990). The models which aim to predict weed germination and emergence need to record seed germination in a wide range of water potentials. Seeds of various weed species require different values of water potential in order to germinate. For instance, the base water potential Ψb for A. myosuroides was estimated at −1.53 (MPa) in the study of Colbach et al. (2002b) whereas the corresponding value recorded for Ambrosia artemisiifolia (L.) was −0.8 (MPa) as observed by other scientists (Shrestha et al., 1999). The value of minimum water potential for the germination of S. viridis seeds was −0.7 (MPa) (Masin et al., 2005) whereas the corresponding value recorded for Stellaria media (L.) Villars was −1.13 (MPa) (Grundy et al., 2000). Dorsainvil et al. (2005) revealed that the base water potential for germination for Sinapis alba (L.) was at −1 (MPa). Regarding weed emergence, although seeds of many species can germinate in a wide range of water potentials, once germination has occurred the emerged seedlings are sensitive to dehydration, and irreversible cellular damage may occur (Evans and Etherington, 1991). False seedbed is a technique that aims to deplete weed seed banks by eliminating the emerged weed seedlings. Thus, it is crucial to have knowledge about water demands for germination for the dominant weed species of the agricultural area where a false seedbed is planned to be formed. If these demands are not met, then they can be secured via adequate irrigation in the meantime between seedbed preparation and crop sowing.

The Possible Effects of Light, Gaseous Environment of the Soil, Soil Nitrates Content and Soil PH on Seed Germination of Various Weed Species

The reaction of seeds to light signals is dependent on phytochromes that consist of a group of proteins acting as sensors to changes in light conditions. Cancellation of dormancy by light is mediated by the phytochromes. All phytochromes have two mutually photoconvertible forms: Pfr (considered the active form) with maximum absorption at 730 nm and Pr with maximum absorption at 660 nm. The photoconversion of phytochrome in the red light (R)-absorbing form (Pr) to the far red light (FR)-absorbing form (Pfr), has been identified as part of the germination induction mechanism in many plant species (Gallagher and Cardina, 1998). Germination can be induced by Pfr/P as low as 10 −4 and is usually saturated by

There is evidence showing that other environmental factors, such as nitrates and gases, can also regulate seed bank dormancy (Bewley and Black, 1982; Benech-Arnold et al., 2000). For instance, germination of Sysimbrium offcinale (L.) Scop. is dependent on the simultaneous presence of light and nitrates (Hilhorst and Karssen, 1988), while in the case of Arabidopsis thaliana (L.) Heynh., nitrates modify light-induced germination to some degree (Derkx and Karssen, 1994). The seeds of summer annual species, S.officinale, showed increased sensitivity to nitrates and lost dormancy during the winter season (Hilhorst, 1990). Regarding the winter annual S. arvensis, Goudey et al. (1988) recorded maximal germination frequencies when NO 3 – content ranged from 0.3 to 4.4 nmol seed −1 for applied NO 3 – concentrations between 2.5 and 20 mol m −3 . In the same study germination was significantly lower in seeds containing more than 5 nmol NO 3 – . Although the mechanisms by which nitrates stimulate dormancy loss remain under investigation, they maybe act somewhere at the cell membrane environment (Karssen and Hilhorst, 1992). The evaluation of the effects of nitrates in regulating weed seeds’ germination and weed emergence is an area of interest for weed scientists and research needs to be carried out to get a better knowledge regarding this issue. There is also evidence that the range of pH values can promote germination of important weed species. For instance, Pierce et al. (1999) noticed that seed germination of D. sanguinalis decreased with increasing pH when soil was amended with MgCO3, whereas maximum root dry weights occurred at ranges from pH 5.3–5.8. A pH range of 5–10 did not influence seed germination of E. indica (Chauhan and Johnson, 2008). Cyperus esculentus (L.) germination rate at pH 3 was 14% as compared to 47% at pH 7, while germination of Sida spinosa (L.) was highest at pH 9 (Singh and Singh, 2009). In the experiment by (Lu et al., 2006) Eupatorium adenophorum (Spreng.) germinated in a narrow range of pH (5–7) whereas other researchers recorded a 19–36% germination rate for Conyza canadensis (L.) Cronquist. over a pH range from 4 to 10 (Nandula et al., 2006). As a consequence, another area available for research is the role of soil pH on seed germination and weed emergence especially in fields where false seedbed technique has been planned to be applied.

Oxygen and carbon dioxide are two of the most major biologically active gases in soil. Oxygen concentration in soil air does not usually fall below the limit of 19% (Benech-Arnold et al., 2000). During storage of seeds in soil, oxygen can have both detrimental and beneficial effects on the dormancy status of weed seeds. Results of an early study carried out by Symons et al. (1986) revealed that introduction to the cycle of secondary dormancy in the seeds of A. fatua was attributed to hypoxia. Hypoxic conditions did also cause a decrease in the germination capacity and rate of Datura stramonium (L.) (Benvenuti and Macchia, 1995). Moreover, B. tripartita seeds showed increased germination rates under 5 and 10% oxygen concentration as compared to the germination rate recorded under 21% oxygen concentration (Benvenuti and Macchia, 1997). Germination of E. crus-galli was increased with oxygen concentrations in the range among 2.5 and 5% and declined when the oxygen concentration level was above 5% citepbib20. However, low oxygen concentration or the inability to remove anaerobic fermentation products from the gaseous environment directly surrounding the seed may inhibit seed germination. The results of Corbineau and Côme (1988) indicated that low oxygen concentrations, or even hypoxia, can terminate dormancy situation in the seeds of Oldenlandia corymbosa (L.). The results of Experiment 1 carried out by Boyd and Van Acker (2004) revealed that oxygen concentration of 21% highest led to 31, 29, and 61% increased germination of Elymus repens (L.) Gould. as compared to oxygen concentrations of 5, 10, and 2.5%. In the same experiment, the greatest germination rate for Thlaspi arvense (L.) was also recorded with 21% oxygen concentration.

The levels of carbon dioxide in soil air ranges between 0.5 and 1% (Karssen, 1980a,b). When soils are flooded, the ratio of carbon dioxide to oxygen typically increases and can have detrimental effects on seed germination and seedling emergence. In very early studies, concentrations of carbon dioxide in the range of 0.5 and 1% have been reported to have a dormancy breaking effect in seeds of Trifolium subterraneum (L.) and Trigonella ornithopoides (L.) Lam. & DC. (Ballard, 1958, 1967). Elevated carbon dioxide concentrations combined with low oxygen concentrations may further strengthen the signal to germinate and promote germination below the surface during periods of high soil moisture content (Yoshioka et al., 1998), and this hypothesis was supported by the results of (Boyd and Van Acker, 2004). Ethylene, a gas with a well-known role as a growth regulator, is also present in the soil environment, with its usual value of the pressure ranging between 0.05 and 1.2 MPa (Corbineau and Côme, 1995). At these concentrations, it has break-dormancy effects on seeds of T. subterraneum (Esashi and Leopold, 1969), P. oleracea, C. album, and A. retroflexus (Taylorson, 1979). According to Katoh and Esashi (1975), at low concentrations in the soil ethylene promotes germination in Xanthium pennsylvanicum (L.) and similar observations have been made regarding A.retroflexus (Schönbeck and Egley, 1981a,b). However, these are results of old studies and it should be noted that a newer study stated that the role of ethylene in governing seed germination and seedling emergence cannot be clearly explained (Baskin and Baskin, 1998). The findings of another study where strains of a bacterium were evaluated as stimulators of emergence for parasite weeds belonging to Striga spp. were interesting. The bacterium Pseudomonas syringae (Van Hall) pathovar glycinea synthesizes relatively large amounts of ethylene. In the study of Berner et al. (1999) strains of P. syringae pv. glycinea had a stimulatory effect on the germination of seeds of the parasite weeds Striga aspera (Willd.) Benth. and Striga gesnerioides (Willd.) Vatke. Consequently, whether oxygen, carbon dioxide, and ethylene influences weed seeds’ germination and seedlings emergence is not yet clarified since variation has been reported among gases’ concentrations and various weed species. Thus, the role of the gaseous environment of the soil in seed germination and weed emergence needs to be further explained.

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The Importance of Tillage as Stimulator of Weed Emergence and as Weed Control Method in False Seedbed Technique

The movement of the weed seeds within the soil profiles as a consequence of tillage creates variations in the dormancy of seeds (Ghersa et al., 1992). There is evidence that weed species’ timing and duration of emergence varies (Stoller and Wax, 1973; Egley and Williams, 1991), suggesting that timing of tillage interferes with the timing of species germination and acts as an assembly filter of weed communities (Smith, 2006). The results of Crawley (2004) revealed that the frequency of Papaver dubium (L.), A. thaliana, and Viola arvensis (Murray) was increased by 62.5, 66.5, and 72%, respectively, due to fall cultivation. In the same study, spring cultivation increased the frequency of C. album, Bromus hordeaceus (L.), and Galinsoga parviflora (Cav.) by 48, 88, and 92.5 %. Spring tillage acts as a filter on initial community assembly by hindering the establishment of later-emerging forbs, winter annuals, C3 grasses, and species with biennial and perennial life cycles, whereas fall tillage prevents the establishment of early-emerging spring annual forbs and C4 grasses. Species adapted to emerge earlier are therefore able to exploit the high availability of soil resources and be more competitive as compared to species that usually emerge later in the growing season when soil resource availability is restricted at a significant point (Davis et al., 2000).

Tillage events confined to the top 10 cm can provoke greater weed emergence than the corresponding events usually observed in untilled soil (Egley, 1989). Although no direct evidence exists of the effect of tillage on dormancy through modification of temperature fluctuations or nitrate concentration, it is well-known that tillage exposes seeds to a light flash before reburial, allows for 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 (Mohler, 1993). Tillage promotes seed germination, and this is a fundamental principle in which innovative management practices such as stale seedbed techniques that target the weed seed bank are based (Riemens et al., 2007). Weed emergence is an inevitable result of shallow soil disturbances in crop production, as it is indicated by Longchamps et al. (2012). Disturbances as small as wheel tracking can enhance seedling emergence. Results from past studies point out that promotion of seedling emergence is more dependent on the density of a given recruitment cohort rather than flush frequency (Myers et al., 2005; Schutte et al., 2013), and that the stimulatory effect of a particular shallow soil disturbance event dissipates over time and flushes occurring afterward feature seedling densities are similar to flushes recorded in untilled soils (Mulugeta and Stoltenberg, 1997; Chauhan et al., 2006). Plants react to the low fidelity between germination cues and recruitment potential and have become able to produce seed populations with different germination demands not only in qualitative but also in quantitative points to secure the longevity of the population. Thus, only a fraction of a population can germinate after performing shallow tillage operations (Childs et al., 2010). Soil type can also affect seedbank dynamics as it was shown by the results of a study conducted in Ohio. When the soil was sampled at 15 cm depth, the concentration of seeds was reduced with depth but the effect of tillage on seed depth was not the same for all three soil types that received the same tillage operation (Cardina et al., 1991).

False seedbed technique is based on the principle of using soil disturbance to provoke weed emergence and use shallow tillage instead of herbicide as a weed control method before crop establishment. False seedbed by inter cultivation decreased weed density and dry weight in finger millet (Patil et al., 2013). It is well-established that 5 cm is the maximum depth of emergence for most cropping weeds. If tillage overpasses this boundary, non-dormant seeds from deeper soil profiles are placed in germinable superficial soil positions. Re-tillage must be as shallow as 2 cm. Spring tine can be used in false seedbeds and multiple passes are suggested for more efficient weed control in cereal crops, while milling bed formers are more suited to vegetable crops (Merfield, 2013). Johnson and Mullinix (1995) found that shallow tillage was efficient against weeds like C. esculentus, Desmodium tortuosum (L.), and Panicum texanum (L.) in peanuts in a false seedbed. Similar results have also been observed in soybeans (Jain and Tiwari, 1995). An issue remaining under investigation is if the timing of weed elimination can affect the efficacy of such techniques. The results of Sindhu et al. (2010) were not clear regarding which treatment was superior among the stale seedbed prepared for seven days and the one prepared for 14 days before controlling weeds with tillage operations.

Conclusions

Important parameters that influence weed seeds’ germination and seedlings’ emergence can affect the efficacy of false seedbed as weed management practice. These parameters consist of environmental factors such as soil temperature, soil water potential, exposure to light, fluctuating temperatures, nitrates concentration, soil pH, and the gaseous environment of the soil. Soil temperature and soil water potential can exert a great influence on weed diversity of a cultivated area. Estimating minimum soil temperatures and values of water potential for germination for the dominant weed species of a cultivated area can give researchers the ability to predict weed infestation in a field and also the timing of weed emergence. Predicting weed emergence can answer the question of how much time weed control and crop sowing should be delayed in a specific agricultural area where false seedbed technique is about to be applied. As a result, if it was possible in the future to use environmental factors to make such predictions, this could maximize the efficacy of false seedbed technique as weed management practice. Timing, depth, and type of tillage are important factors affecting weed emergence and, subsequently, the efficacy of false seedbed. The importance of shallow tillage as a weed control method in the false seedbed technique has also been highlighted. In general, estimating the effects of environmental factors and tillage operations on weed emergence can lead to the development of successful weed management practices. Further research is needed to understand the parameters that influence weed emergence in order to optimize eco-friendly management practices such as false seedbeds in different soil and climatic conditions.

Author Contributions

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Weed Seed Dormancy

In this unit we will learn about seed dormancy as seen by many respected seed biologists, the way most scientists in the seed discipline view this complex area of plant biology. The concepts and organization of this unit follow this traditional way of looking at seed dormancy. It is essential that you understand these concepts, and see them from this perspective, if you are to have an understanding of weed seed biology and the scientific literature on seed dormancy.

Having said that, it is important that you realize that I am not in complete agreement with this established model of seed dormancy. Will will evaluate these different views in our classroom discussions. For starters, I view the term dormancy as the “biology of what isn’t“:

Dormancy: a state in which viable seeds, spores or buds fail to germinate under conditions favorable for germination and vegetative growth.

What this definition of dormancy obscures is what important phenomena are hidden from our view, it tells us nothing about what is happening in that seed, or its potential to germinate, its a “black box” definition. Many different seed phenomena, potentially caused by a multitude of different mechanisms, all fall under this vague term What we call dormant is a range of germinability states, from those right on the edge of germination and those profoundly dormant..

Germinability: the capacity of an seed, bud or spore to germinate under some set of conditions.

The dormant seed requires after-ripening for it to become capable of germination. After-ripening of weed seed usually occurs in the soil from the time it is shed in the growing season until it germinates, often over one or more winters (in the north temperate regions like Iowa). In the soil, physiological, chemical and physical changes occur and after-ripening proceeds.

After-ripening: period after dispersal when the seed, spores and bud cannot germinate, even under favorable conditions, and during which changes occur allowing it to germinate.

Imagine a seed at the end of this after-ripening period and process, it is still dormant but it is almost ready to germinate. If we were able to place this single seed in many slightly different but “favorable” germination conditions (parallel universes?), it might germinate in some and not others. This dormancy term just doesn’t tell us much. The ability to germinate is often a “window” of sensitivity to environmental conditions. As germination conditions in the soil become more favorable, the “window” widens and more weed seed germinate.

Weed seed, and their germination requirements, are very diverse. They range from the deeply and profoundly dormant (imagine a velvetleaf seed buried 18 inches in some cold Minnesota soil, its hard coat protecting it, low oxygen is preventing oxidative stress and germination: happy as a clam and able to last a hundred years) to viviparous (imagine a foxtail seed germinating right on the panicle in a warm, moist, foggy field along the edge of the Chesapeake Bay in Maryland).

Vivipary: germinating while still attached to the parent plant.
Viviparous: producing offspring from within the body of the parent.

In this unit we will learn the established model of seed germination and dormancy. We will also discuss how a better model might be developed. Warning signs that you are headed in the wrong direction in your thinking can be signaled by the following terms, often used. BEWARE!:

Breaking Dormancy
Block to Dormancy
Dormancy Trigger
Biochemical Trigger
Dormancy Switch
Switch for Germination

Finally, it is important to realize there is very good reason for this confusion, why seed biologists like nothing better to do than invent new terms: dormancy is an extremely complicated area of biology. No one has figured out the mechanism of embryo dormancy. Most of the literature doesn’t differentiate between dormancy imposed by the embryo and surrounding (often inhibiting) envelopes when they discuss experimental seed germination results. And then there is the problem of figuring out how these seed behave in the soil: agricultural soil seed bank dynamics.

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The Traditional Seed Dormancy Model

Reading assignment: Harper: Ch. 3: pp. 61-82; summary p. xiv-xv

  • Introductory concepts
  • Innate dormancy
  • Enforced dormancy
  • Induced dormancy

Introductory Concepts

  • Dormancy: 1. A state in which viable seeds (or buds; spores) fail to germinate under conditions of moisture, temperature and oxygen favorable for vegetative growth (Amen, 1968); 2. A state of relative metabolic quienscence
  • Seasonal dormancy: in an environment in which favorable growth conditions are seasonal, dormancy is usually clocked by solar rhythm; consequences to the population: predictive
  • Opportunistic dormancy: 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; consequences to the population: responsive
  • Innate dormancy: the condition of seeds as they leave the parent plant, and is a viable state but prevented from germinating when exposed to warm, moist aerated conditions by some property of the embryo or the associated endosperm or maternal structures
  • Induced dormancy: is an acquired condition of inability to germinate caused by some experience after ripening; in the opportunistic dormancy category
  • Enforced dormancy: is an inability to germinate due to an environmental restraint: shortage of water, low temperature, poor aeration, etc.; in the opportunistic dormancy category
  1. Dormancy occurs during periods of unfavorable conditions; is more resistant to environmental hazards
  2. Dormancy can be seen as “dispersal in time”
  3. Dispersal phase usually dormant; dehydrated seeds weight less (esp. wind dispersal) and are metabolically slower
  4. Dormancy:
    a. rhythmic adaptation of weeds to the temporal rhythms in the environment
    b. strategic alternative to dispersal
  5. Seed-Environment interactions: Seed dormancy a product of the interaction of the seed (embryo, envelopes like a seed coat, seed food reserves like the endosperm or cotyledons) and the environment (temperature, gases, water, light, soil, temperature)

Evolutionary and environmental context of weed seed dormancy as an adaptive strategy in the struggle for existence

1. Dormancy weak solution to problem of adaptation in changing environment: time lost in capturing resources, in struggle for existence with neighbors

  • Probability of suffering greater hardship by continuing growth; evolutionary solution: annual habit, dormancy
  • The effort or cost of a seasonally dimorphic phenotype; evolutionary solution: e.g. desert shrubs with somatic polymorphism, large leaves in wet season, small leaves and scales in dry season
  • The cost of producing a homeostatic growth form that is tolerant of the whole range of environmental conditions: cost of wide tolerance is great, obtained at cost of reduced efficiency at optimum conditions; evolutionary solution: e.g. evergreen trees
  • a. Grain: the way in which an individual plant experiences the heterogeneity of the environment
  • Fine-grained environment: individual exposed to environmental factors in small doses, short-termed flucuations; each individual in each year experiences the same range of environments in the same frequencies and there is no uncertainty; example: long-lived perennial tree experiences entire range of environmental factors in yearly changes
  • Coarse-grained environment: each individual spends its whole life, or a long period at least, in a single environmental alternative; example: annual weed gets a dry year, or a wet year
  • Optimal adaptive strategies in a heterogeneous environment:
    -if environments experienced are not too different, the optimal adaptive strategy is a single type of best suited to some intermediate environment
    -if the environments experienced are very different, and acts in a fine-grained way, the optimal strategy is a single type of dormancy adapted to the more common environment
    -if the environments experienced are very different, and act in a coarse-grained way, the optimal strategy is polymorphism; seed dormancy in weeds in temperate climates good example here

Innate Dormancy

  • Innate dormancy conferred by the fact that the process of growth of an embryo to a stage fit for germination has not been completed while the the embryo was still borne on the parent plant, it is shed morphologically incomplete
  • Example: Heracleum sphondylium: development of embryo continues at the expense of extra-embryonic food reserves for several months after seed is shed
  • This process imposes a necessary time lag between dispersal and germination

Control by a biochemical trigger
[Beware!]

1. A biochemical process may need to be stimulated before the germination process can begin

2. Often this trigger is a seasonally related stimulus which can switch on germination at an adaptively appropriate time of year

  • Cues and triggers involved in breaking innate dormancy do not produce a clear “all or nothing” effect: only a portion of the seed germinate at one time; a spectrum of requirements by seeds in a single sample which may reflect:
    -different genotypes
    -different maternal influences
    -different ages and ripening conditions (influence of different environmental conditions at different times during reproductive phase, in same plant: Cavers)
  • Light and phytochrome
    1. Example Betula pubescens (UK)
    a. require light and long days for germination
    b. length of dark period critical: germination declines with increasing dark period length
    c. temperature dependence complicated light dependence
    -at 20C light dependence lost
    -with chilling treatment light dependence lost
    2. Several other species follow variations on this same theme: e.g. many dicot weeds (Isikawa, 1954)
  • Temperature: chilling or flucuating temperature-
    example: Papaver spp.: diurnal flucuation between 10 and 30C breaks dormancy; occurs in upper layers of UK soils in April and May, fixes time of germination
  • Nitrate ion: NO3-
    -nitrate concentration of the soil solution often rises quite sharply as the soil temperature increases in the spring (Russell, 1962)
    -stimulation of Chenopodium album seed germination in the field, and several other species, stimulated by nitrate
  • Germination stimulants
    -e.g. ethanol, anesthetics, etc.
    -[ecological, agricultural relevance?]
  • Triggering of biochemical process which destroys a germination inhibitor: breakdown process of inhibitor which occurs within the tissues of the seed
  • Physical leaching, or removal of the source of, and inhibitor: leaching or destruction of inhibitor by an external agent

Physical restriction of gas exchange and growth

  • Impermeable (or relatively impermeable) seed or fruit coat may prevent water or gas uptake by seed and prevent germination until physical damage occurs to this barrier
    -example: Avena fatua (wild oat) seed dormancy broken easily by pricking pericarp
  • Common innate dormancy in species inhabiting sand dunes; abrasion by sand
  • Scarification: seeds that require abrasion tend to break dormancy at different times rather than in a sudden flush
    -example: Avena fatua (wild oat) seed dormancy broken easily by pricking pericarp
    -example: Abutilon theophrasti hard seed coat: germination broken readily with treatments cracking hard seed coat (50C for 15 minutes; 10-15 minutes in sulfuric acid); hard seed coat confers very long dormancy and viability in soil seed bank

2. Dormancy caused by mechanical restriction of growth by embryo coverings (pericarp, testa, perisperm, endosperm)
-example: cocklebur: upper seed (of two in capsule) radicle is restricted, insufficient thrust to rupture testa and germinate

  • Innate dormancy appears to be under strict and simple genetic control; often modified by maternal effects (i.e. endosperm effects from mother; maternal origin of ovary)
  • Commercial crop seed have lost dormancy present in wild relatives in process of domestication; some dormancy left as protection from precocious germination of crop seed while still in inflorescence (?) in wet weather near harvest time
  • Genetically controlled polymorphism: distinctly different dormancy genotypes
    -example: Spergula arvensis: 3 different seed coats, each control different levels of seed dormancy
    -example: Nicandra physaloides: presence or absence of isochromosomes determines whether the seed is non- dormant (2n = 20) or dormant (2n = 19)

Somatic polymorphism and innate dormancy

Somatic polymorphism: Production of seeds of different morphologies or behavior (phenotypes) on different parts of the same plant; not a genetic segregation but a somatic differentiation

  • Adaptive advantage to producing seed on one plant with different qualities
  • Common adaptation limited to weedy species usually
  • Seed dormancy somatic polymorphism is common in weedy species of Gramineae, Compositae, Chenopodiaceae and Cruciferae families
  • A quality lacking in genetic polymorphisms: continuum of responses, not just a few genetic alternatives
  • Proportion of morphs can be subtly and directly altered by environmental conditions experienced by the parent plant
  • Water stress in mature leaves plus short days may induce abscissic acid production
  • ABA may have an effect on developing seeds as they differentiate histologically in developing seed
  • Differences in dormancy in seed may be a function of water stress at time of seed development
  • Germinability of seeds as a function of maternal environment (Gutterman, Y. 1978. Acta Horticulurae 83:41-55)
  • 1. example: Rumex crispus (curled dock)
    -progeny of individual plants vary enormously in ability to germinate in darkness or at common temperature
    -variation is greater between plants than between habitats, no one germination response
  • Example: Xanthium spp. (Cocklebur);
    -seed borne in pairs in capsule: large and small seed dispersed together
    -upper seed in capsule usually dormant, lower germinates first due to differences in testa permeability to water entry, leaching of endogenous germination inhibitors
    -dormancy breaking requirements different for 2: 12 month difference insurance second will become established if first year unfavorable
  • Example: Avena fatua (wild oat), and Avena ludoviciana
    -grains borne on different parts of the spikelet have different germination requirements
    -first grain of spikelet lacks dormancy, remainder have deep dormancy
  • Example: Compositae germination behavior differentiated by seed size, seed formed in ray versus disc flowers
  • Example: Chenopodium album (common lambsquarters) may produce 4 different kinds of seed on same plant
    -two color categories: brown and black; two seed coat categories: reticulate and smooth
    -brown: thin-walled, larger, germinate quicker than black, even at low temperatures; killed by winter, but if they survive have the capacity to produce very large plants with high reproductive output; only 3% of seed on a plant; among the first to be produced by a plant
    -black: require cold treatment, nitrate to break dormancy
    -ratio of brown:black govered by environmental conditions
  • Example: common purslane seed varied from nondormant to dormant on same plant (Egley, 1974)

Enforced Dormancy

  • Imposed dormancy: state of seed dormancy maintained by the absence of necessary conditions for germination
    -E.g. shortage of water, temperature, unfavorable soil atmosphere, etc.
    -E.g. seed buried deeply in soil by tillage, etc., commonly has enforced dormancy
  • Carbon dioxide narcosis in soils common factor in enforced dormancy; e.g. high respiration sites in soil elevate CO2 (seed respiration, soil microorganisms)
  • Lowered oxygen tension in the soil also important here; e.g. severe oxygen starvation in waterlogged or compacted soils
  • Temperate agricultural regions: low temperature enforces dormancy

Induced Dormancy

A seed has acquired dormancy which is not innate and which does not require continued enforcement

CO2 narcosis: example: Brassica alba dormancy induced by high CO2 treatment

  • ilum acts as a hygoscopically activated valve
  • when air is dry the hilum valve opens and allows water loss from seed
  • in wet air it closes,
  • embryo progressively dries to a value equal to that of the driest environment it experienced
  • hard seed dormancy only broken by seed coat scarification
  • white and red clover seed
  • collected seed from soil in dark after burial treatment
  • later seed of many species (buckhorn plantain, corn spurry, field poppy) tested had light requirement for germination which was not needed when freshly harvested

Cold treatment induced light requirement for germination of Stellaria media (Chickweed); one way autumn shed seed acquire light requirement by spring

High temperature exposure of imbibed seeds coupled with restriction of oxygen availability induced dormancy (Villiers, 1972).

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