Because most processes related to seed germination and seedling emergence occur in the soil and within a very short time (from a few days to a few weeks, depending on the species and sowing date), understanding of what really occurs during this phase and which factors are involved is a challenging task. How can we tackle this complexity and which tools can be developed and mobilized to this objective?
Partha S. Biswas , . Jiban Krishna Biswas , in Advances in Rice Research for Abiotic Stress Tolerance , 2019
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] .
Research and innovation priorities as defined by the Ecophyto plan to address current crop protection transformation challenges in France
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Seed germination is the first phase of the growth cycle in plants ( Parihar et al., 2015 ). Salinity adversely affects seed germination, excess amount of soluble salt content into the soil reduces the water potential into the soil. As water moves from higher water potential to lower water potential, seeds are unable to take water from saline soil and causes hormonal imbalance ( Khan and Rizvi, 1994 ), reduces protein metabolism ( Dantas et al., 2007 ), nucleic acid metabolism ( Gomes-Filho et al., 2008 ) and ultimately reduces the utilization of seed reserves ( Othman et al., 2006 ). There are some evident that salinity drastically affects the seed germination in various plants like Oryza saliva ( Xu et al., 2011 ), Triticum aestivum ( Akbarimoghaddam et al., 2011 ), Zea mays ( Khodarahmpour et al., 2012 ), Brassicaspp. ( Akram and Jamil, 2007 ). Bybordi (2010) reported that with the increasing salt concentration the rate of seed germination decreases in Brassica napus ( Bybordi, 2010 ).
Seed germination is defined as the sum of events that begin with hydration of the seed and culminate in emergence of the embryonic axis (usually the radicle) from the seed coat.
The seed grows, and the radicle, or first stage of the root, emerges from the seed. Finally, the first little shoot comes out of the seed with cotyledons, the first two leaves, and photosynthesis can begin.
The process of germination is when a seed comes out of dormancy, the time during which its metabolic activity is very slow. Germination begins with imbibition, a big word for taking in water. This is the major trigger to start the period of waking up from dormancy.
Specific seed germination requirements vary depending on the plant species. But they generally include water, air, temperature, and ultimately access to light. It helps to know the specific needs for the plants you’re working on to optimize germination. Fall too far outside the requirements and you’ll either get no seeds germinating, or only a portion.
What Causes Seed Germination?
Germination is essential for what we do as gardeners. Whether starting plants from seeds or using transplants, germination has to happen for gardens to exist. But many of us take this process for granted and don’t fully understand the factors affecting germination of seeds. By learning more about the process and what seeds need, you can get better results in the garden.
As the seed takes in water, it gets bigger and produces enzymes. The enzymes are proteins that ramp up metabolic activity in the seed. They break down the endosperm, which is the seed’s store of food, to provide energy.
Understanding seed germination requirements is important for growing plants successfully from seed. Know what your seeds need before you get started so you will get a greater percentage germinating and growing into seedlings.
Dormancy is brief for some seeds—for example, those of certain short-lived annual plants. After dispersal and under appropriate environmental conditions, such as suitable temperature and access to water and oxygen, the seed germinates, and the embryo resumes growth.
Active growth in the embryo, other than swelling resulting from imbibition, usually begins with the emergence of the primary root, known as the radicle, from the seed, although in some species (e.g., the coconut) the shoot, or plumule, emerges first. Early growth is dependent mainly upon cell expansion, but within a short time cell division begins in the radicle and young shoot, and thereafter growth and further organ formation (organogenesis) are based upon the usual combination of increase in cell number and enlargement of individual cells.
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.
Until it becomes nutritionally self-supporting, the seedling depends upon reserves provided by the parent sporophyte. In angiosperms these reserves are found in the endosperm, in residual tissues of the ovule, or in the body of the embryo, usually in the cotyledons. In gymnosperms food materials are contained mainly in the female gametophyte. Since reserve materials are partly in insoluble form—as starch grains, protein granules, lipid droplets, and the like—much of the early metabolism of the seedling is concerned with mobilizing these materials and delivering, or translocating, the products to active areas. Reserves outside the embryo are digested by enzymes secreted by the embryo and, in some instances, also by special cells of the endosperm.
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.