Rice Growth Characteristics and Requirements for Environmental Conditions
In rice, the period from seed germination to the production of new rice seeds is usually called one growth cycle, that is, the growth period. The growth period can be divided into the seedling stage, the regreening stage, the tillering stage, the panicle development stage (panicle differentiation stage), and the grain-filling stage. Generally, the seedling stage is completed in the seedbed. The period after transplanting during which the seedlings recover and survive is called the regreening stage. After regreening, tillering begins (some have already begun tillering in the seedbed), and panicle differentiation begins (jointing). Before young panicle differentiation, the period dominated by root, stem, and leaf growth is the vegetative growth period, and from panicle differentiation to maturity, the period dominated by panicle, flower, and grain growth is the reproductive growth period.
Seedling Stage
The seedling stage is the stage from seed germination to the three-leaf stage, and is generally further divided into the seed germination stage and the seedling growth stage. For seed soaking and germination, the required temperature for germination is 12°C for indica rice and 10°C for japonica rice, with an optimum temperature of 30°C—32°C and a maximum temperature reaching 40°C—42°C, but during seedling raising it must not fall below 5°C, or 0°C; at low temperatures, seed rot, bud rot, and seedling rot occur. First, the seedbed should be selected as a field plot facing the sun, sheltered from the wind, with irrigation and drainage, etc. If low temperatures occur, measures such as covering with plastic film can also be added to avoid rot or reduce it. The temperature for emergence and seedling growth is 2°C higher than for germination, that is, 14°C for indica rice and 12°C for japonica rice; above 16°C, both indica and japonica rice can emerge smoothly.
Environmental Requirements for the Regreening Stage and Tillering Stage
The regreening stage is the buffering stage after transplanting, from the seedbed to survival in the main field, lasting about 4 days. It requires shallow water, because if the water is too deep and submerges the growth point (heart leaf), aeration is poor, which can also cause seedling rot, or slow survival. After regreening, the vegetative growth period follows, centered on tillering and growing roots and leaves.
1. Temperature requirements. The highest optimum temperature for rice tillering is 30~32°C, and the optimum water temperature is 32°C~34°C. The maximum air temperature is 38°C~40°C, the maximum water temperature is 40°C~42°C, the minimum air temperature is 15°C~16°C, and the minimum water temperature is 16°C~17°C. When the water temperature is below 22°C, tillering becomes relatively slow. Low temperatures delay tillering and also affect the total number of effective panicles from tillering, so transplanting is required to begin when the temperature is above 15°C.
2. Light requirements. During the tillering stage, sufficient sunlight is needed to increase the photosynthetic intensity of leaves, produce organic matter, and promote an increase in the number of tillers. Under natural light, tillering begins 3 days after regreening; if there is only 50% natural light, tillering begins only 13 days after regreening; if there is only 5% natural light, not only will no tillers be produced, but even the seedlings will die.
3. Water requirements. The tillering stage is the period most sensitive to water, but it only requires the paddy field to be in a saturated state, or shallow water is most favorable for tillering. Under high-temperature conditions (26°C~36°C), when soil water-holding capacity is 80%, the greatest number of tillers will be produced. If irrigated with deep water, when the water layer exceeds the field by 8 cm, the light at the tillering nodes is weak, oxygen is insufficient, and when the temperature is also low, tillering is inhibited. However, if the field is too dry and the water-holding capacity is below 70%, tillering will also stop.
4. Nutrient requirements. During tillering, more nutrients are needed, and there are also more effective tillers. More nutrients can promote tillering and fast, abundant growth. If nutrients are insufficient, tillering will also be reduced or stop. Among the required nutrients, nitrogen, phosphorus, and potassium are the main ones, with nitrogen fertilizer being the most needed. It is best to apply nitrogen, phosphorus, and potassium together as topdressing for the most benefit.
Environmental Requirements During the Jointing and Booting Stage
The jointing and booting stage is a period when vegetative growth and reproductive growth advance simultaneously. During this period, rice growth and development increase rapidly, the root mass is at its largest, and the leaf area of the rice plant reaches its maximum. At the same time, the rice panicle begins to differentiate. Jointing and booting is the key period determining the number of grains per panicle, the period for consolidating the effective panicle number per mu, and the period determining grain weight, with the main factors lying in the influence of external conditions.
1. Nutrient requirements. During the process of young panicle differentiation, the root mass of water roots continuously increases, and the last 3 leaves grow successively. This is a period when both vegetative growth and reproductive growth require nutrients. If nutrients are lacking during this period, it will have an adverse effect on young panicle differentiation. Therefore, in production, intertillage and topdressing should be carried out, about 30~40 days before heading, to promote glume flower differentiation and increase the number of secondary branch stems. This fertilization can also be called "flower-promoting fertilizer"; about 10~20 days before heading, fertilizer can be sprayed once, that is, during the period of pistil and stamen core formation and the meiosis period of pollen mother cells, when fertilizer is most needed to prevent glume flower abortion and ensure more grains. This topdressing is "flower-preserving fertilizer."
2. Temperature requirements. The optimum temperature for young panicle differentiation is 26°C~30°C, and 35°C during the day and 25°C at night are more favorable for forming large panicles. The external temperature for young panicle differentiation is 15°C~18°C, but the most sensitive period is the meiosis period. During the meiosis period, both high-temperature and low-temperature damage will cause massive abortion and sterility of glume flowers.
3. Light requirements. Light intensity is closely related to young panicle differentiation, and strong light is favorable for young panicle differentiation. According to experimental evidence, during the young panicle differentiation period, shading with two layers of gauze (transmittance about 1/6~1/8 of natural light) resulted in 30% more glume flower degeneration than the control. If there are many cloudy and rainy days, little sunshine, excessive canopy closure, and poor aeration. Therefore, Guizhou has relatively less sunshine time, so a reasonable rice density should be ensured to promote a certain amount of photosynthesis and increase organic matter, allowing the growth of large panicles and rice grains with many grains.
4. Water requirements. From young panicle differentiation to heading is the period when rice needs the most water in its entire life, especially during the meiosis period of pollen mother cells, when it is most sensitive to water. Therefore, during this period, the field water-holding capacity must be maintained above 90%; if water is lacking, it will affect glume flower development. However, too much water and flooding will also cause adverse effects, such as death if completely submerged.