Is Rice Resistant to High Temperatures? The Impact of High-Temperature Heat Damage and Countermeasures

High temperature damage to rice plants is closely related to the growth stage of the rice. Many studies have shown that during the flowering stage of rice, high-temperature heat damage affects pollen maturation, anther dehiscence, pollen germination on the stigma, and pollen tube elongation; the resulting failure of fertilization is the most serious harm to rice. This period is the sensitive stage of rice to high temperatures, especially when high-temperature stress occurs on the day of flowering, which easily induces spikelet sterility, causes fertilization obstacles, and seriously affects the seed-setting rate and yield. Due to intensified industrialization, increased human activity, and the influence of the greenhouse effect, the climate is gradually warming, and the frequency of high-temperature heat damage is increasing, which should draw attention in production.

1. Causes

Rice is extremely sensitive to temperature during the booting and heading stages (that is, the 10 days before and after heading). The most suitable temperature is 25°C to 30°C, and an average daily temperature above 30°C will produce adverse effects. If the booting stage encounters sustained high temperatures above 35°C, the rice floral organs develop incompletely, pollen development is poor, and vitality declines; if the heading and flowering stage encounters high temperatures above 35°C, heat damage will occur, affecting pollen dispersal and pollen tube elongation, leading to failure of fertilization and the formation of empty grains, causing a decline in the seed-setting rate, a relatively low 1000-grain weight, and even a total loss of harvest.

2. Characteristics of the impact

(1) Large affected area

In 2003, of more than 500,000 mu of rice in Lai'an County, Anhui, more than 400,000 mu were affected, with a total-loss area of more than 50,000 mu, causing direct economic losses of 100 million yuan; in 2006 alone, the area affected by high-temperature heat damage in just 4 cities, including Chuzhou, Hefei, Lu'an, and Chaohu, reached about 2 million mu, of which 500,000 mu were severely affected.

(2) Differences among varieties and combinations

Because the genetic characteristics of each rice variety determine that its temperature tolerance range is different, the degree of damage caused by high-temperature heat damage to each variety differs.

Under normal circumstances, japonica rice or parental combinations containing japonica rice genes suffer more serious high-temperature heat damage than indica rice;

hybrid rice suffers more serious high-temperature heat damage than conventional rice;

varieties that do not contain heat-tolerance genes suffer more serious damage than varieties that contain heat-tolerance genes.

3. Prevention methods

(1) Select high-temperature-tolerant varieties

Different varieties differ somewhat in the degree of damage from high-temperature heat damage. In production, farmers should first choose high-yield, high-temperature-tolerant varieties to reduce the impact of high temperatures on grain filling and seed setting. For example, high-temperature-tolerant varieties such as Hongliangyou 166 and Hongliangyou No. 5 were promoted and planted on a large scale in Anhui, Hubei, Jiangxi, and other provinces from 2003 to 2006, showing characteristics such as high-temperature tolerance, strong drought resistance, high yield, and good comprehensive traits, and were deeply welcomed by farmers.

(2) Sow and transplant at the appropriate time

Based on high-temperature conditions in previous years, sowing and transplanting should be carried out at the appropriate time so that the optimal heading and flowering period of single-season middle rice is arranged for mid-August, so as to effectively avoid the frequently occurring high-temperature summer drought weather from late July to early August.

(3) Irrigate with deep water and regulate temperature with water

Maintaining a certain water layer in the paddy field can significantly reduce the degree of yield loss under high-temperature summer drought climate conditions. Maintaining a field water layer of 5 to 10 cm during the heading and flowering stage can reduce the field microclimate temperature by 2 to 3°C and alleviate heat damage. Especially for water-deficient and drought-stricken fields, irrigation should be lifted early to increase field humidity and prevent the combined effects of drought and high-temperature heat damage. Where conditions permit, daytime irrigation and nighttime drainage or continuous-flow irrigation can be adopted.

(4) Spray fertilizer outside the roots to enhance the stress resistance of rice

Spray a 3% calcium superphosphate solution or a 0.2% potassium dihydrogen phosphate solution outside the roots, plus foliar nutrient solution fertilizer, to enhance the resistance of rice plants to high temperatures and improve the seed-setting rate and 1000-grain weight.

(5) Apply grain fertilizer as a topdressing to prevent premature senescence in the later stage

For fields with relatively mild heat damage at the booting stage, apply one supplementary grain fertilizer topdressing around the breaking stage, generally applying 3 to 5 kg of urea per mu, to restore normal grain filling and seed setting of the plants.

(6) Prevent and control pests and diseases and promote robust plant growth

Combine pest and disease prevention with heat damage prevention. The main pests and diseases to control in rice include rice planthopper, rice leaf roller, rice blast, and sheath blight. When spraying pesticide-fertilizer on the leaves, the spray concentration and spray timing must be well controlled. The solution concentration should not be too high; otherwise, excessively high concentration can easily cause damage to plant leaves and affect nutrient absorption; the best time is in the evening.

4. Remedial measures

(1) Preserve and cultivate ratoon rice. For fields that encountered high-temperature heat damage from late July to early August, with a seed-setting rate below 20% and a yield of less than 100 kg per mu, and with relatively good water sources, the stubble should be cut before August 15 to preserve and cultivate ratoon rice. When cutting the seedlings, early cutting should leave low stubble, and late cutting should leave high stubble; cutting stubble should be as early as possible. At the same time, field management of ratoon rice should be strengthened to ensure safe full heading and a relatively high yield.

(2) Switch to autumn crops. For fields with a seed-setting rate below 20% and poor water conditions, preserving ratoon rice may fail due to high-temperature summer drought. The seedlings should be cut and the field plowed, and autumn crops should be replanted in time, such as vegetables, red beans and mung beans, autumn buckwheat, fresh-eating sweet corn, and other grain and economic crops with relatively short growth periods, to compensate for the losses caused by high-temperature heat damage.