The germination of any plant seed needs water, air and suitable temperature. However, the seeds of different plants have different requirements for these three conditions during germination. Compared with the dry weight of seeds, the water requirements of some cultivated plants are: rice 40%, wheat 45%, pea 107%, soybean 1 10%. Various cultivated plants have different requirements for sowing temperature: sorghum, corn, soybean, millet and so on. When the ground temperature of the sowing layer is stable at 65438 02℃, the seeds can be sown. When rice, cotton and other seeds germinate, it is required that the environmental temperature is high and the ground temperature is stable at 12 ~ 15℃ before sowing the layer. The seeds of various cultivated plants have different requirements for air when they germinate. Soybean and cotton need a lot of oxygen when they germinate, so the soil should be loose when sowing. Rice seeds need less oxygen when they germinate, and they can germinate even if they are submerged in water. Mature seeds begin to germinate under suitable environmental conditions. After a series of growth processes, the radicle of the seed first breaks through the seed coat and grows down to form the main root. At the same time, the cells of hypocotyl also grow and elongate correspondingly, pushing the embryo or embryo together with cotyledons out of the dough, and the embryo sticks out of the soil to form stems and leaves. Cotyledons such as cotton and rape extend out of the soil with the germ, turn green after unfolding and carry out photosynthesis. After the young leaves of the embryo open to perform photosynthesis, the cotyledons will wither and fall off. At this time, a seedling that can live independently will all grow up, which is a seedling. Ecological conditions for seed germination (1) Water content 1, minimum water requirement for seed germination The minimum water requirement for seed germination refers to the percentage of the minimum water content during seed germination to the original weight of seeds (it can also be expressed by water content). The water requirement of seed germination is closely related to chemical composition. Starch seeds and oily seeds need less water, for example, the minimum water requirement for rice seeds germination is 26%, while the water requirement for protein seeds is higher, for example, the minimum water requirement for soybean seeds germination is 107%. 2. Factors affecting seed water absorption The rate and quantity of seed water absorption are mainly affected by seed chemical composition, seed coat permeability, external water conditions and temperature. Generally, seeds germinate in liquid water, and seeds planted in soil may absorb soil moisture, with a diameter of about 1cm. When the water pressure and osmotic pressure of the soil around the seed increase, the water absorption rate of the seed will be low. At a certain stage of water absorption, temperature will obviously affect the water absorption of seeds. Generally, when the ambient temperature rises 10℃, the water absorption will increase by 50 ~ 80%. (2) Temperature Seed germination requires a certain temperature, and the requirements of various plant seeds for germination temperature can be expressed by the lowest, optimum and highest temperature. The lowest and highest temperatures refer to the lowest and highest temperature limits at which at least 50% of seeds can germinate normally, and the optimum temperature refers to the temperature at which seeds can germinate quickly and reach the highest percentage. Most crops can germinate well in the range of 15 ~ 30℃, but the specific requirements of different crops are different. The germination temperatures of warm crops or summer crops are 6 ~ 12℃, 30 ~ 35℃ and 40℃ respectively, and the germination temperatures of cold-tolerant crops or winter crops are 0 ~ 4℃, 20 ~ 25℃ and 40℃ respectively. (3) Oxygen Oxygen is an indispensable condition for seed germination, and most seeds need sufficient oxygen for germination. When seeds germinate, aerobic respiration is particularly strong, which requires sufficient oxygen supply, and some enzyme activities also need oxygen. The oxygen supply of seed embryo during germination is affected by the external oxygen concentration, the solubility of oxygen in water, the permeability of seed coat to oxygen and the affinity of enzyme to oxygen in seed. According to the research, the change of oxygen demand during seed germination is similar to that in water absorption stage. When seeds absorb water, their water demand increases rapidly with the increase of water absorption. When the seed is in stagnant water, its oxygen demand is also more, but when the seed radicle breaks through the seed coat, its oxygen demand increases sharply. If there is insufficient oxygen supply and high temperature during this period, the seeds will fall into anoxic breathing and produce alcohol to kill the seeds. In recent years, this situation often occurs in the process of accelerating rice germination, so we should pay special attention to it. (4) Other factors 1, light Most seeds are insensitive to light when they germinate, and they can germinate normally under light and dark conditions. However, a few plant seeds are sensitive to light when they germinate, and they need light or dark conditions to germinate. 2. Carbon dioxide usually only contains 0.03% of CO 2 in the atmosphere, which has no effect on germination. Only when the CO 2 in the germination environment increases to a fairly high concentration will the germination be seriously inhibited. The inhibition of CO 2 on germination is related to temperature and oxygen concentration, especially when the ambient temperature is not suitable or the oxygen content is low. Second, the process of seed germination seed germination involves a series of physiological, biochemical and morphological changes, and is affected by the surrounding environmental conditions. According to the general law, the process of seed germination can be divided into four stages. (1) imbibition stage imbibition is the initial stage of seed germination. Generally, in the storage stage of mature seeds, the water content is in the range of 8% ~ 14%, and all parts of the tissue are relatively solid and dense, and the substances contained in the cells are in a dry gel state. Seeds directly contact with water or in high humidity air, quickly absorb water and expand (except for a few seeds) until the water inside the cell reaches a certain saturation level, the cell wall is in a state of tension, and the seeds are protected outside. When seeds imbibe, the seed coat will expand greatly due to the increase of the volume of all cells, which may lead to seed coat rupture. When the seed absorbs a certain amount of water, the seed imbibition stage ends, and the ratio of imbibition volume to air-dried volume is called imbibition rate. Generally, the imbibition rate of starch seeds is 130% ~ 140%, while that of bean seeds is about 200%. (2) Germination stage Germination is the second stage of seed germination. On the basis of initial imbibition, the water absorption of seeds usually stops for several hours or days. After a period of time, although the water absorption stopped, the metabolism inside the seeds began to strengthen and turned into a new physiological state. During this period, on the basis of the activation and repair of biological macromolecules and organelles, embryonic cells resumed growth. When the volume of embryonic cells expands and stretches to a certain extent, the tip of radicle breaks through the seed coat, which is called seed germination. Seed germination is commonly known as "exposure" in agriculture, which means that embryonic tissue appears from the breakthrough of seed coat. In seed physiology, the arrival of the morphological change stage of seed germination is often regarded as the completion of seed germination. When seeds germinate, the growth of embryos varies with water supply: when water is low, radicle comes out first; When there is more water, the germ comes out first. This is because germ is more sensitive to hypoxia than radicle. In a few cases, after some inanimate seeds are fully imbibed, the radicle will also extend out of the seed coat due to volume expansion. This phenomenon is called false germination or false germination. (3) After the seeds germinate in the germination stage, the embryos start or accelerate the division and differentiation, and the growth rate is obviously accelerated. When radicle and embryo extend out of seed coat and develop to a certain extent, it is called germination. The definition of germination in China and international seed inspection regulations is that seeds are called germination when they develop into seedlings with normal main structure. Seeds are in this period, the metabolism of embryos is extremely vigorous, the respiratory intensity reaches the highest limit, and a lot of energy and metabolites are produced. If the oxygen supply is insufficient, it will easily lead to anoxic breathing and release harmful substances such as ethanol, which will suffocate and paralyze the embryo and lead to poisoning and death. This often happens if crop seeds germinate improperly after sowing or are affected by harsh conditions. (4) According to the emergence of cotyledons after germination in seedling stage, seedlings can be divided into two types. 1, when the seeds of cotyledon exotic plants with dicotyledons germinate, their hypocotyls are significantly elongated and arched at the initial stage. After coming out of the ground, the distribution of auxin changes correspondingly under the induction of light, which makes the hypocotyl gradually straighten, the growing embryo leaves the seed coat, the cotyledons expand rapidly, turn green after seeing the light and start photosynthesis. Later, true leaves and main stems grow from the embryo between two cotyledons. Only a few monocotyledonous plants, such as onion and garlic, belong to this type, and 90% of cotyledon seedlings belong to this type. Common cotton, soybean, rape, etc. 2. Cotyledon-soil-conserving cotyledon-soil-conserving dicotyledonous plants germinate, the epicotyl is elongated and unearthed, and then the true leaves grow into seedlings. Cotyledons remain in the soil until internal storage and nutrient depletion. Most monocotyledonous seeds, such as cereals, and a few dicotyledonous seeds, such as broad beans, peas and tea, belong to this type. The unearthed part of cereal seed seedlings is actually a bullet-shaped coleoptile. After the coleoptile is unearthed, it splits under the light, and the true leaves inside gradually protrude for photosynthesis. Without the protection of coleoptile, the seedlings will be blocked when they are unearthed. In addition, because the vegetative storage tissues and some lateral buds of the seedlings left in the soil are still left in the soil, once the seedlings above the soil are damaged by pests and low temperature, it is still possible to re-grow seedlings from the soil.