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weed seed dormancy and germination

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Date and time: Thu, 24 Jun 2021 16:34:35 GMT

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

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).

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 <0.03 Pfr/Pr (Benech-Arnold et al., 2000). The quality of light received by seeds may be more important than the quantity. There is evidence that Far-red light (FR, about 735 nm) can inhibit germination (Ballaré et al., 1992). Regarding the way weed emergence is influenced by light, given that FR or the ratio of FR to red light (R, about 645 nm) increases as plant canopies develop and solar elevation decreases with time after the summer period, emergence of sensitive species should be inhibited during the summer period. However, the practical significance of FR exposure for emergence in field settings is not well-known (Forcella et al., 2000).

Author Contributions

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 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.

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.

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).