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seed germination

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.

To date, our knowledge is poor with regard to the impact of cropping practices and pedo-climatic conditions on the seed quality; do they affect the ecophysiological traits of seeds?

7.3.1.1 Seed germination

Md. Salim Azad , in Exotic Fruits , 2018

Seed germination and seedling growth are preconditions for conservation of genetic resources and sustainable uses of different products of specific species which depends on perception of genetic inconsistency, evolutionary forces, and breeding system in tree improvement ( Azad et al., 2014 ). Tamarind is commonly grown from seeds. It can also be grown from vegetative propagation (macrovegetative propagation or micropropagation). Vegetative propagation is useful for conservation of different genotypes. Germination from seed is inexpensive and very important for rural tree breeders. It can be used as root stocks to produce large number of grafted ortet. Tamarind seed germination is influenced by different presowing treatments. Different researchers noticed various responses according to the different methods used. Seed germination required 7–20 days in controlled conditions ( Azad et al., 2013 ). It can vary by seed sources, climatic requirements, and cultivars as well. On an average, it starts to germinate from 13 days of seed sowing. Sometimes it may take 30 days to complete the germination process. El-Siddig et al. (2001) recommended 45 days to allow for maximum seed germination. Azad et al. (2013) noticed 58% seed germination in the control situation, and noticed that presowing significantly enhanced seed germination. They found almost 82% seed germination in cold water treatment (immersion in cold water for 24 h at 4°C) and scarification with sand paper. El-Siddig et al. (2001) noticed acid treatment (immersion of seeds in 97% sulfuric acid for 45 min at room temperature) is an effective method for rapid and synchronous germination of tamarind.

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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.

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).

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.

Seed dormancy

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.

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 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.

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?

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.

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.

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.