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Block Reference: #31fad8f0-06fe-11ec-92a3-4772736b6868
Date and time: Fri, 27 Aug 2021 06:15:37 GMT
Reading assignment: Harper: Ch. 3: pp. 61-82; summary p. xiv-xv
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).
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
Block to Dormancy
Switch for Germination
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.
Vivipary: germinating while still attached to the parent plant.
Viviparous: producing offspring from within the body of the parent.
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":
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.
Solanum sarrachoides (Sendtn.) seeds germination rate was recorded over 90 % in both light and dark conditions (Zhou et al., 2005). However, seeds of several species require light for germination. Lower than 16 % germination percentages were recorded for Anchusa arvensis (L.) M. Bieb., A. Fatua, and Lamium amplexicaule (L.) in light while germination of Matricaria perforata (Mérat), Galinsoga ciliata (Raf. Blake), and Sonchus asper (L.) Hill reached the level of 99% due to exposure to light (Milberg et al., 2000). Germination of Bidens tripartita (L.), Carex flacca (Schreb.), Juncus conglomeratus (L.), and Scirpus sylvaticus (L.) was also found to be enhanced by light (Grime et al., 1981) whereas with no exposure to light, seeds of Chenopodium bonus-henricus (L.) Rchb were not able to germinate (Khan and Karssen, 1980). However, the results of (Gallagher and Cardina, 1998) indicated that light was a requirement for germination in <20% of seeds within populations of Amaranthus spp. and also that the same seeds were the most dormant out of the total amount of seeds in the soil seedbank. Light and a higher temperature of 25°C promoted faster, uniform germination of seeds of (Taraxacum officinale (L.), while in darkness, the achievement of 50% of the final germination percentage was delayed and a longer mean emergence time was needed (Letchamo and Gosselin, 1996). Other scientists noticed that germination of E. cruss-galli was increased by 65% under light conditions as compared to darkness [Boyd and Van Acker, 2004]. In contrast to the findings of the studies presented above, exposure to light has been found to have inhibiting effects on seed germination of Galium spurium (L.) (Malik and Vanden Born, 1987). In Bromus sterilis (L.) (Hilton, 1982) and some lines of A. fatua (Hou and Simpson, 1993), germination is inhibited by a single R pulse and this inhibition is abolished by a subsequent FR pulse. Therefore, light appears in some cases to function as a dormancy breaking signal when deeply buried seeds are moved to shallower soil depths, whereas in other cases exposure of seeds to light may inhibit germination in various ways depending on the species and circumstances. For the purposes of emergence modeling, the role of light requirements for seed germination needs to be investigated as they set a large and unexplored research area.
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 editor and reviewers’ affiliations are the latest provided on their Loop research profiles and may not reflect their situation at the time of review.
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.