Impact of High-Temperature Heat Damage on Rice Production in Fanchang County and Defensive and Remedial Measures

In the process of rice production, the climate factors with the greatest impact and the hardest to control are temperature and water. Reasonable irrigation can supplement and regulate water, but temperature is difficult to control, and it exerts a major influence on the entire life of rice.

Rice is the main crop in grain production in Fanchang County, ranking first among grain crops in both planting area and total output. At present, the rice planting area in Fanchang County is about 16,360 hectares (including the multiple cropping index), of which about 13,191 hectares are middle-season indica rice (mainly hybrid rice), about 1,514 hectares are early rice, and about 1,655 hectares are double-cropping late rice. Therefore, growing rice well and achieving high yields plays a very important role in developing the county's rural economy, "two-high and one-optimal" agriculture, increasing farmers' income, improving agricultural efficiency, and stabilizing market supply.

1 Relationship between temperature and rice production

1.1 Effect of temperature on vegetative growth of rice Although rice originated in the tropics, through continuous cultivation and improvement of varieties, rice cultivation areas have now spread across tropical and temperate regions, and rice can even be grown in some cold high-altitude marginal areas of northern China. This shows that rice has strong adaptability to different climatic zones. However, the production and development of rice has its most suitable temperature range, and temperatures that are too high or too low are both unfavorable to its growth.

1.1.1 Germination temperature of rice seeds. Generally, the lower limit of germination temperature for rice seeds is 8°C (cold-tolerant varieties can also germinate at around 0°C); below this, germination is slow and the seeds may rot. The upper limit is 43°C. Therefore, the temperature range for rice seed germination is 0-43°C, with the most suitable temperature at about 18-33°C, but germination capacity decreases at 15-30°C. Therefore, in a sense, 15°C and 33°C are not within the physiologically suitable temperature range.

1.1.2 Growth temperature during the seedling stage. The most suitable temperature for seedlings in wet seedbed raising is 25-30°C. In the early seedling stage, when the temperature reaches 20°C, growth accelerates as the temperature rises, and at 30°C the seedlings emerge from the water layer fastest. The optimum temperature for radicle elongation and rooting in the soil is 25-30°C. When it reaches 35°C, roots mainly grow in the water layer, which easily causes seedlings to float up and root unstably; therefore, the roots at the seedling stage are more sensitive to high temperatures.

In Fanchang County, the seedlings of single-cropping and double-cropping late rice are basically in relatively suitable water temperatures during growth, but for early rice seedbeds sown early, the seedlings are weak in resistance to the external environment during the seedling stage and grow slowly, and are especially difficult to root. If they encounter severe weather, such as spring cold waves and low temperatures (below 12°C), coupled with improper management, seedling rot is likely to occur, so some protective measures should be taken.

1.1.3 Temperature of seedlings after transplanting. Under normal circumstances, the critical temperature required for rice transplanting is 15°C, and the most suitable temperature for root development is 25-28°C. After transplanting, the growth of the aboveground part of rice rises linearly within the average temperature range of 18-33°C, and growth decreases significantly above or below this range. The temperature during the tillering stage of rice is 15-33°C, and outside this range tillering is unfavorable, but the suitable temperature is 25°C during the day and 20°C at night.

1.2 Effect of temperature on reproductive growth and maturity stage of rice The reproductive period of rice is relatively long, lasting about 60 d from the beginning of panicle differentiation to grain maturity. The formation of effective panicles before heading, and the grain filling and fullness of filling after heading, are the 3 key processes determining rice yield, so special attention should be paid to climatic conditions during the reproductive growth period.

1.2.1 Temperature for panicle number formation. Without considering factors such as water, temperature, sunshine, and soil, the suitable temperature for spikelet formation in rice is 20-23°C. Studies have shown that 15-20 d before heading, within a temperature of 25-30°C, the number of rice tillers that die is the greatest. 20°C is most suitable for panicle formation, and 35°C is most suitable for tiller formation.

1.2.2 Temperature during heading and maturity. The suitable average temperature from heading to maturity is about 23°C, and damage to maturity in the low-temperature range is more serious than in the short-term high-temperature range. The double-cropping late rice in Fanchang County is sometimes attacked by cold waves at the late heading stage, which greatly reduces the seed-setting rate, so double-cropping late rice must reach full heading before September 23. Under the condition of a water temperature of 25°C during the day and 20°C at night, the seed-setting rate of rice is relatively high. When the water temperature rises to 35°C, it has a certain effect on the seed-setting rate of rice; when the water temperature rises to 40°C, the seed-setting rate is about 50% lower than under normal temperature; when the daytime temperature is 35°C and the nighttime temperature is 30°C, it has a serious effect on the seed-setting rate of rice, because high temperatures day and night cause a low amount of pollen and failure of pollen dehiscence, increasing the probability of sterility.

2 Occurrence of high-temperature heat damage and its harm to rice growth

For several consecutive years, during the growth period of single-season hybrid rice in Fanchang County, rare high-temperature weather with little rain occurred, especially the continuous high temperature from mid-to-late July to early August, which caused serious disaster to middle-season indica rice that was heading and flowering. In normal years, the average seed-setting rate of rice is above 80%, but in 2003 it was generally only 50%-60%, with moderate cases at 25%-35%, and severe cases below 10%, basically resulting in total crop failure and causing a sharp reduction in grain output.

2.1 Continuous high temperature causing heat damage makes rice flower but not set grain Rice is most sensitive to temperature during the 10 d before and after the booting to heading and flowering stage, and the most suitable temperature is 25-30°C. When the daytime temperature is higher than 30°C, there is obvious damage to flowering and seed setting. In production, the heat damage index for rice at the heading and flowering stage is usually a daily average temperature and daily maximum temperature higher than 35°C. If the booting stage encounters continuous high temperature above 35°C, the floral organs of rice develop incompletely, pollen is poor, and vitality declines. If the heading and flowering stage encounters continuous high temperature, heat damage occurs, water is rapidly lost, affecting flowering, pollen dispersal, and pollen tube elongation, resulting in failure of normal pollination, or the pollen dying before it opens, forming empty grains, that is, "flowering but not setting." Rice that heads at a constant temperature of 38°C all fails to set grain, and high temperature can also directly kill pollen.

2.2 Harm and loss degree of heat damage to middle-season indica rice in Fanchang County Continuous high-temperature heat damage for several years has led to a decline in the seed-setting rate of middle-season indica rice and reduced yields in Fanchang County, with varieties sown too early and early-maturing varieties suffering the most obvious damage. According to survey statistics, calculated at an average yield of 8,250 kg/hectare, affected farmers suffered a per capita income reduction of 900-1,200 yuan/hectare.