Jay Ram Lamichhane , . Pierre Ricci , in Advances in Agronomy , 2019
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Seed germination is a transit process when an active plant with photosynthesis grows from a quiescent embryo, generated in the fertilized ovule. The process of seed germination includes the following five changes or steps: imbibition, respiration, effect of light on seed germination, mobilization of reserves during seed germination, and role of growth regulators and development of the embryo axis into a seedling. All five of these stages result from a interplay of several metabolic and cellular events, coordinated by a complex regulatory network that includes seed dormancy, an intrinsic ability to temporarily block radicle elongation to optimize the timing of germination. The primary plant hormones including abscisic acid (ABA) and gibberellin (GA) antagonistically regulate seed dormancy and germination [8–10] . ABA is synthesized during seed maturation and decreased before the onset of germination; it plays key roles in inhibiting germination and establishing and maintaining seed dormancy  . In contrast to ABA, GA significantly increases to promote germination by causing the secretion of hydrolytic enzymes that weaken the structure of the seed testa [12, 13] .
11.2.1 Germination Stage
Shereen et al. (2011) conducted experiments to study the effects of salinity on seed germination of six rice varieties differing in salt tolerance by treating them with 0, 50, 75, 100, 200 mM NaCl solutions. The results revealed that salinity caused a delay in germination of rice seeds with 3–6 days of delay in treatments containing 100 and 200 mM NaCl respectively, advocating a strong negative relationship between salinity and seed germination. The rice cultivators exhibiting minimal leakage of solutes showed relatively higher germination under high salinity stress of 100 and 200 mM NaCl compared to the cultivars exhibited higher solute leakage. Similarly, Jamil et al. (2012) investigated the effects of salinity on seed germination of three different rice genotypes and found that the rice cultivars differed in their germination response to salt stress. Increase in salinity from 0 to 150 mM adversely affected the seed germination percentage and significantly delayed seed germination.
Seed germination is the most important stage in a plants life cycle. Water, air, temperature and light are all essential for the seed germination process starting from imbibition, activation and succeeding manifestation. Rice seed germination is affected greatly by temperature. Temperatures colder than the favorable range (18–33°C) retards the germination process. Cold temperatures slow down the diffusion process which causes disrupted imbibition and escape of solutes from the seeds. The effect of cold stress is more pronounced at the imbibing phase which is regarded as the most sensitive phase. The exposure of rice seeds to cold stress during this phase causes an increased escape of solutes from the seeds. The standard temperature for rice seed germination is considered to be 30°C. The minimum critical temperature of rice germination is considered as 10°C ( Yoshida, 1981 ). Soil temperatures below 10°C can result in complete failure of germination ( Yoshida, 1981 ). Temperatures below 20°C decrease both the speed and percentage of seed germination ( Yoshida, 1981 ), lower crop stands, and consequently reduce grain yield ( da Cruz and Milach, 2004 ; Cruz et al., 2006 ; Sharifi, 2010 ). Germination speed is related to seedling vigor and it could be a significant determinant of good field performance ( da Cruz and Milach, 2004 ).
Seed germination and seedling emergence are the most important and vulnerable phases of a crop cycle. A poor quality of seed and sowing conditions have both direct (e.g., the lack of seed germination translates either into the need to re-sowing with further costs or into a reduced plant density thus a reduced yield) and indirect (e.g., lower competitiveness of crops toward weeds and more favorable conditions for the development of diseases) impacts on crop health as it affects seed germination and seedling emergence. Consequently, reducing the exposure of young radicle and seedlings to biotic (soil-borne pests) and abiotic (drought, heat and mechanical) stresses at such a vulnerable stage is of paramount importance via any form of seed treatments or cropping practices. In this regard, the following issues should be taken into account:
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In many seeds the embryo cannot germinate even under suitable conditions until a certain period of time has lapsed. The time may be required for continued embryonic development in the seed or for some necessary finishing process—known as afterripening—the nature of which remains obscure.
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In some seeds (e.g., castor beans) absorption of nutrients from reserves is through the cotyledons, which later expand in the light to become the first organs active in photosynthesis. When the reserves are stored in the cotyledons themselves, these organs may shrink after germination and die or develop chlorophyll and become photosynthetic.
Germination sometimes occurs early in the development process; the mangrove (Rhizophora) embryo develops within the ovule, pushing out a swollen rudimentary root through the still-attached flower. In peas and corn (maize) the cotyledons (seed leaves) remain underground (e.g., hypogeal germination), while in other species (beans, sunflowers, etc.) the hypocotyl (embryonic stem) grows several inches above the ground, carrying the cotyledons into the light, in which they become green and often leaflike (e.g., epigeal germination).
The seeds of many species do not germinate immediately after exposure to conditions generally favourable for plant growth but require a “breaking” of dormancy, which may be associated with change in the seed coats or with the state of the embryo itself. Commonly, the embryo has no innate dormancy and will develop after the seed coat is removed or sufficiently damaged to allow water to enter. Germination in such cases depends upon rotting or abrasion of the seed coat in the gut of an animal or in the soil. Inhibitors of germination must be either leached away by water or the tissues containing them destroyed before germination can occur. Mechanical restriction of the growth of the embryo is common only in species that have thick, tough seed coats. Germination then depends upon weakening of the coat by abrasion or decomposition.
The seeds of many plants that endure cold winters will not germinate unless they experience a period of low temperature, usually somewhat above freezing. Otherwise, germination fails or is much delayed, with the early growth of the seedling often abnormal. (This response of seeds to chilling has a parallel in the temperature control of dormancy in buds.) In some species, germination is promoted by exposure to light of appropriate wavelengths. In others, light inhibits germination. For the seeds of certain plants, germination is promoted by red light and inhibited by light of longer wavelength, in the “far red” range of the spectrum. The precise significance of this response is as yet unknown, but it may be a means of adjusting germination time to the season of the year or of detecting the depth of the seed in the soil. Light sensitivity and temperature requirements often interact, the light requirement being entirely lost at certain temperatures.
Germination, the sprouting of a seed, spore, or other reproductive body, usually after a period of dormancy. The absorption of water, the passage of time, chilling, warming, oxygen availability, and light exposure may all operate in initiating the process.
Figure 1. Monthly maximum and minimum air and soil temperature data in the Australian Alps (mean ± STDEV). Air temperatures are averages from the BOM database, soil temperatures were measured with iButtons (see methods). Bars indicate experimental incubator temperatures (mean ± STDEV). The equivalent time of year (season), incubator temperature regime (°C, 12/12 h, light/dark) and duration of treatment (weeks) as indicated in the top panel.
Steadman, K. J., Eaton, D. M., Plummer, J. A., Ferris, D. G., and Powles, S. B. (2006). Late-season non-selective herbicide application reduces Lolium rigidum seed numbers, seed viability and seedling fitness. Aust. J. Agric. Res. 57, 133–141. doi: 10.1071/AR05122
Kullman, L. (2004). The changing face of the alpine world. Glob. Change Newsl. 57, 12–14.
To date, there has been little published evidence of intra-specific variability in dormancy mechanisms within alpine seed collections such as would explain the staggered germination strategies observed here. Germination staggered over time may also be explained by varying levels of seed maturity among individuals in the population at the time of collection, and/or could reflect a dimorphic strategy within a single plant associated with position on the plant or timing of development.
Hoyle, G. L., Steadman, K. J., Daws, M. I., and Adkins, S. W. (2008b). Pre-and post-harvest influences on seed dormancy status of an Australian Goodeniaceae species, Goodenia fascicularis. Ann. Bot. 102, 93–101. doi: 10.1093/aob/mcn062