Four Effects on Rice After Exposure to High Temperatures

Rice has different temperature requirements at each growth and development stage, and there is an optimal temperature for each. If the upper or lower critical temperature limit is exceeded, abnormal phenomena, namely physiological disorders, will appear in the growth and development of rice. Therefore, attention should be paid to the effects of high temperatures on rice.

1. Effects of high temperatures on each growth stage of rice

1. Heading and flowering stage. Generally, rice prefers a temperature of 25-30°C during the heading and flowering stage. When the temperature is below 25°C or above 30°C, large numbers of empty and shriveled grains will appear in rice. At the same time, high temperatures also damage the male reproductive organs of rice, causing variety degeneration or failure of the ovary to fertilize after flowering, forming empty and shriveled grains.

2. Grain-filling stage. The optimal temperature for the grain-filling stage of rice is 21-25°C. If the grain-filling stage of rice encounters high temperatures above 35°C, the rice grains are prone to stop developing and form half-filled grains, and the grain texture becomes loose, the white belly expands, the 1000-grain weight decreases, and the rice quality deteriorates. This is what people often call the "high-temperature forced ripening" phenomenon.

2. Effects of different paddy field locations on rice yield

Fields located on flat, open terrain with long sunshine hours, good soil fertility conditions, and a low groundwater table are most affected by high temperatures. In normal years, these fields are all high-yield fields, but due to the rare high temperatures, the advantage has turned into harm. This is related to the number of hours such paddy field soils absorb under high temperature and strong light. In contrast, mountain-valley fields and high-altitude fields are mostly half-shaded and half-sunny, which shortens the sunshine hours. In addition, the drop between field plots and the flow of irrigation water cause them to warm up slowly during the day and cool down quickly at night, increasing the diurnal temperature difference. Some fields also have a high groundwater table, and most fields are irrigated with mountain spring water, so the soil and water temperatures are relatively low, and yields have been low over the years. Under this abnormal high temperature, the low temperature rises to just the right level, thus turning harm into advantage, and the abnormal high temperature instead becomes a factor that increases the yield of such fields.

3. Effects under the same temperature conditions on different variety combinations

Among hybrid rice combinations, because the original producing areas of the male and female parents of each combination are different, their sensitivity to temperature also differs. Through hybridization, due to the action of genetic factors, these characteristics will show differences in the offspring. Even among series combinations, if one parent is the same and the other parent is different, the offspring will also differ. For late-maturing combinations with relatively stable growth periods, the chance that their heading stage and grain-filling and seed-setting stage avoid high temperatures increases, so the degree of damage from high temperatures is relatively lighter.

4. Improper cultivation management promotes the damage of abnormal high temperatures

The effects of improper cultivation management on rice are mainly shown in two aspects:

1. Improper fertilization. In all fertile paddy fields or fields with excessive nitrogen fertilizer application, especially those with late and heavy application, the seed-setting rate is relatively low, the leaves at maturity appear reddish-red, and sheath blight and rice false smut occur relatively commonly, with stalk blight occurring just as the crop matures. In contrast, fields whose leaves appear golden-yellow at maturity have a relatively higher seed-setting rate, heavier grain weight on the panicle, and only slight disease, which precisely reflects proper fertilization. There are also some poor fields where, in previous years, due to various reasons, fertilization effects were poor, fertilizer use efficiency was not high, and yield was always difficult to increase. Under such abnormal high-temperature conditions, however, some restrictive factors may be alleviated, accelerating the decomposition rate of nutrients in the paddy field and improving nutrient use efficiency, thereby achieving a rare good harvest.

2. The reason of water. This again includes both artificial and natural factors. The so-called artificial factor is that most farmers, following conventional high-yield practices, often irrigate and expose the field in the middle and late stages and manage it with moisture, but under abnormal high-temperature conditions this has done the opposite and intensified the ravages of high temperatures. Some farmers, however, have always managed extensively and allowed water to flood and flow through the fields, so they suffered less damage from high temperatures and luckily obtained a relatively good harvest. The so-called natural factor is that some low-lying areas, high groundwater tables, or cold spring irrigation have greatly helped cold-waterlogged fields and alleviated the damage of abnormal high temperatures.