In the process of seed germination, water is absorbed by the embryo, which results in the rehydration and expansion of the cells. Shortly after the beginning of water uptake, or imbibition, the rate of respiration increases, and various metabolic processes, suspended or much reduced during dormancy, resume. These events are associated with structural changes in the organelles (membranous bodies concerned with metabolism), in the cells of the embryo.
Environmental factors play an important part not only in determining the orientation of the seedling during its establishment as a rooted plant but also in controlling some aspects of its development. The response of the seedling to gravity is important. The radicle, which normally grows downward into the soil, is said to be positively geotropic. The young shoot, or plumule, is said to be negatively geotropic because it moves away from the soil; it rises by the extension of either the hypocotyl, the region between the radicle and the cotyledons, or the epicotyl, the segment above the level of the cotyledons. If the hypocotyl is extended, the cotyledons are carried out of the soil. If the epicotyl elongates, the cotyledons remain in the soil.
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.
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.
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.
N. Priyanka , . Perumal Venkatachalam , in Advances in Phytonanotechnology , 2019
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 a parameter of the prime significance, and fundamental to total biomass and yield production and consists of a complex phenomenon of many physiological and biochemical changes leading to the activation of embryo ( Parihar et al., 2014 ). A significant negative correlation generally exists between the seed germination percentage, time for seed germination and level of salinity ( Kaveh et al., 2011 ). During seed germination, salinity results in many disorders and metabolic changes such as solute leakage, K + efflux and α-amylase activity ( Shereen et al., 2011 ). Firstly, salinity reduces moisture availability by inducing osmotic stress and, secondly, creates nutrient imbalance and ionic toxicity ( Munns and Tester, 2008; Rajendran et al., 2009 ). Cell membranes are the hotspots for controlling active and passive transfer of solutes, and regulating plant nutrient uptake ( Munns and Tester, 2008 ). An imbalance of mineral nutrients under salinity stress generally alters the structural and chemical composition of the lipid bilayer membrane, and, hence, controls the ability of the membrane for selective transport of solutes and ions inwards and, the membrane could become leaky to the solutes they contain ( Cushman, 2001; Lodhi et al., 2009 ).
Role of Engineered Zinc and Copper Oxide Nanoparticles in Promoting Plant Growth and Yield: Present Status and Future Prospects
Seed germination , which determines when the plant enters natural or agricultural ecosystems, is a crucial process in the seed plant life cycle and the basis for crop production. The germination of freshly produced seeds is inhibited by primary dormancy, which helps the seeds equip for environments with unfavorable conditions [1–3] . The seeds will enter a germinating state from the dormant state at an appropriate time when the dormancy is lost through moist chilling (stratification) or after-ripening  . Therefore, seed germination is a accurately timed checkpoint to avoid unsuitable weather and unfavorable environments during plant establishment and reproductive growth  . Finally, seed germination in crops will affect seedling survival rates and vegetative growth, which are accordingly associated with ultimate yield and quality. Considering agronomic production, crop cultivars must be prepared for rapid and uniform germination at sowing, which will improve the crop yield and quality; however, this selection during crop breeding usually results in weak dormancy, which is one of the factors leading to PHS in the rainy season, which tends to overlap with the harvest season [6, 7] . Hence, to improve crop agronomic performance, the crop cultivars during breeding must be prepared for uniform and rapid germination at sowing while preventing PHS [7a] .
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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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.
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.
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.
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.
What Causes Seed Germination?
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.
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.