The Latest Growth Characteristics of Rice and Its Environmental Requirements
In rice, the period from seed germination to the production of new rice seeds is usually regarded as one growth cycle of rice, namely the growth period. The growth period can be divided into the seedling stage, the greening stage, the tillering stage, the long panicle stage (panicle differentiation stage), and the grain-filling stage. Generally, the seedling stage is completed in the seedbed; after transplanting, the period during which the seedlings recover and survive is called the greening stage. After greening, tillering begins (some have already begun tillering in the seedbed), and then panicle differentiation (stem elongation) begins. Before young panicle differentiation, the stage dominated by root, stem, and leaf growth is the vegetative growth period; from panicle differentiation to maturity, the stage 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 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 of 40°C-42°C. However, during seedling raising, it must not fall below 5°C, or 0°C; at low temperatures, seed rot, bud rot, and seedling rot will occur. First, the seedbed should be selected as a field plot that faces the sun, is sheltered from the wind, and has irrigation and drainage. If low temperatures occur, measures such as covering with plastic film can also be added to reduce or avoid rot. The temperature for emergence and seedling growth is 2°C higher than that 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 During the Greening Stage and Tillering Stage
The greening stage is the buffer stage after transplanting, from the seedbed to survival in the main field, lasting about 4 days. Shallow water is required, because if the water is too deep, it submerges the growing point (young leaf), ventilation is poor, and seedling rot may also occur, or survival may be slow. After greening, the vegetative growth period centered on tillering begins, with roots and leaves growing.
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 affect the total number of effective panicles from tillering; therefore, 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 tiller number. Under natural light, tillering begins 3 days after greening; if there is only 50% natural light, tillering begins only 13 days after greening; 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 only a saturated state in the paddy field is required, or shallow water is most favorable for tillering. Under high temperature conditions (26°C-36°C), when soil water-holding capacity is 80%, the most tillers are produced. If irrigated deeply, when the water layer exceeds the field by 8 cm, light at the tillering nodes becomes weak, oxygen is insufficient, and when the temperature is also low, tillering is inhibited. However, if the field is excessively dry, when 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 faster and greater tillering and 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 especially needed. A topdressing combination of nitrogen, phosphorus, and potassium is most beneficial.
Requirements of the Stem Elongation and Booting Stage on External Conditions
The stem elongation 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 largest, and the leaf area of the rice plant reaches its maximum. At the same time, the rice panicle begins to differentiate. Stem elongation and booting is the key period that determines the number of grains per panicle, the period that consolidates the number of effective panicles per mu, and the period that determines grain weight. The main factors lie in the influence of external conditions.
1. Nutrient requirements. During young panicle differentiation, the root mass of water roots increases continuously, and the last 3 leaves grow one after another. 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 primordia. This fertilization can also be called “flower-promoting fertilizer”; 10-20 days before heading, fertilizer can be sprayed once, namely during the period of male and female flower 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 a day temperature of 35°C and a night temperature of 25°C 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 can 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 experiments, shading with two layers of gauze during the young panicle differentiation stage (transmittance about 1/6-1/8 of natural light) resulted in 30% more glume flower degeneration than the control. If there are many rainy days, little sunshine, excessive canopy closure, and poor ventilation. Therefore, in Guizhou there is relatively little sunshine time, so a reasonable rice density should be ensured to promote a certain amount of photosynthesis, increase organic matter, and allow the growth of rice with large panicles and 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, field water-holding capacity must be maintained above 90%; if water is lacking, it will affect glume flower development. However, excessive water and flooding will also cause adverse effects; if completely submerged, it will also die.
Environmental Requirements During the Heading and Grain-Filling Stage
During the rice heading and grain-filling stage, vegetative growth basically stops. This period is mainly dominated by reproductive growth, and all tasks are focused on the key period of maintaining grain number and weight. Management should mainly ensure that rice does not senesce early, does not remain green too long, and does not lodge.
1. Heading and flowering.
(1) Rice heading. After young panicle differentiation in rice, 1-2 days later the rice panicle is extracted from the leaf sheath of the flag leaf. When 50% has been extracted, it is the heading stage; when 80% has been extracted, it is the full heading stage. During heading, due to low temperature or insufficient fertilizer and water, the rice panicle often cannot be fully extracted. In production, this phenomenon is called “enclosed panicle” or “neck-enclosed panicle”. The enclosed part causes blighted grains or empty husks. For example, when the temperature during heading of hybrid rice is below 20°C, it will cause 100% occurrence of “enclosed panicles” and no harvest.
(2) Flowering; under normal conditions, the rice panicle can flower once it is extracted. Before flowering, first the two lodicules inside the glume absorb water and swell 3-5 times in volume. After sufficient absorption, they open the lemma, and the angle between the lemma and palea is about 25°C-30°C. The opening process takes about 10-20 minutes. After fully opening, the angle of 25°C-30°C can be maintained for 30 minutes, so the entire flowering process takes about 1-2 hours. Temperature has a relatively large effect on flowering time; flowering time is short at high temperature and long at low temperature, sometimes reaching more than 2 hours. When the lemma and palea open, the filaments extend, the anthers dehisce, and pollen is shed onto the stigma for pollination. About 10-15 minutes after pollination, the anthers gradually wither, and at the same time the lodicule water also shrinks in volume due to evaporation, and the lemma and palea close again.
2. Factors affecting grain filling and grain setting.
(1) Light. Light intensity and light duration affect the photosynthesis of rice leaves and the transport of carbohydrates to grains. For high-yield rice, more than 90% of the substances filling the grains after heading are carbohydrates produced by leaf photosynthesis. Therefore, photosynthesis during the grain-filling period will directly affect rice yield.
(2) Temperature. Temperature is closely related to grain filling and grain setting. Generally, the most suitable air temperature for grain filling is 20°C-22°C. After the first 15 days of grain filling, a day temperature of 29°C, a night temperature of 19°C, and a daily average temperature of 24°C are suitable. For the latter 15 days, a day temperature of 20°C, a night temperature of 16°C, and a daily average temperature of 18°C are good. A suitable grain-filling temperature helps prolong the time for accumulating nutrients, slows cell aging, reduces respiration and consumption, and gives good rice quality. Both high temperature and low temperature are unfavorable for normal grain filling of rice grains, and they also affect rice quality.
(3) Water. Water during the grain-filling period is second only to water during the stem elongation, long panicle, and tillering stages. If water is insufficient, it will affect the assimilation capacity of leaves and the transport of grain-filling substances, resulting in insufficient grain filling and reduced yield. Insufficient water during the grain-filling period will affect photosynthesis, reduce the efficiency of material transport, shorten the time, and deteriorate the physical properties of rice.
(4) Mineral nutrients. During the grain-filling period, there is a close relationship between leaf nitrogen content and photosynthetic capacity. Appropriate nitrogen application can increase leaf area photosynthesis, maintain maximum leaf area, prevent early senescence, and improve root activity, which has a great impact on increasing rice yield. Therefore, foliar topdressing is often used in production. At the full heading stage, fertilizer can be supplemented according to seedling conditions, or phosphorus and potassium fertilizers can be supplemented, to ensure that the grain-filling process proceeds normally.
Factors Affecting Rice Root Growth
1. Temperature
The optimum soil temperature for rice root growth is 30°C-32°C. Above 35°C, growth is hindered, aging is accelerated, and absorption capacity declines; above 37°C, there is significant decline; below 15°C, both growth and absorption capacity are greatly weakened; below 10°C, growth stops.
2. Light
Light plays an important indirect role in root development and absorption capacity. Because sufficient light strengthens photosynthesis and transpiration, increases the nutrients supplied to roots, thereby promoting root development, improving root activity, and increasing root absorption of inorganic nutrients. When light is insufficient, it not only affects root development, but also significantly reduces root absorption of various inorganic nutrients.
3. Soil nutrients
Among the three major fertilizer elements, nitrogen has the greatest effect on root growth and absorption capacity. An appropriate amount of nitrogen fertilizer can effectively increase the differentiation of root primordia and the rooting ability of rice plants, increasing the number of roots per plant and shortening root length. With nitrogen deficiency or excessive nitrogen supply, root mass is small in both cases. Combined application of phosphorus and potassium fertilizers has a greater promoting effect on root growth; root number, root mass, root length, and so on all increase, and the distribution of the root system also deepens.
4. Soil aeration
The occurrence, elongation, nutrient absorption, transformation, and other physiological activities of the rice root system all require sufficient oxygen. If oxygen is lacking, root growth and physiological functions will be affected, and root activity will also be lost.