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:
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] .
Yingyin Yao , . Qixin Sun , in Plant Small RNA , 2020
11.2.1 Germination Stage
Md. Salim Azad , in Exotic Fruits , 2018
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 one of the most crucial phases in the plant growth and development, and is under the tight regulation of phy-mediated light signaling as well as two hormones, abscisic acid (ABA) and gibberellic acid (GA), that function antagonistically. Under unfavorable conditions, ABA maintains seed dormancy, while GA promotes the seed germination in response to favorable environmental conditions. Light-dependent activation of phys promotes biosynthesis of GA and represses ABA biosynthesis ( Paik and Huq, 2019 ). PIF1 and PIF8, two of the downstream signaling components of phys, are repressors of seed germination under dark conditions. PIF1 represses the seed germination either directly or indirectly through inhibition of GA signaling pathway. However, in response to light illumination, phys translocate to the nucleus and degrade PIF1 and PIF8, relieving the repression, which results in seed germination ( Oh et al., 2020; Legris et al., 2019 ). The light-induced promotion of seed germination ensures that seeds are germinated under favorable environmental conditions for increased survival.
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 ).
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.
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.
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.
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 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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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.
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.
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?
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.
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.