A Detailed Explanation of the Effects of Low-Temperature Cold Damage on Rice and Defensive Measures

Rice cold damage refers to the disaster in which rice is affected by low temperatures below the minimum critical temperature for growth, resulting in the inability of rice to grow and develop normally and thus causing reduced yields. Depending on the period of damage, it can be divided into delayed-type cold damage, sterility-type cold damage, and mixed-type cold damage.

I. Types and Characteristics of Rice Cold Damage

1. Delayed-type cold damage: During the vegetative growth period of rice, and sometimes also during the reproductive growth period, the plant is harmed by relatively low temperatures over a longer period, which weakens the physiological activity of the rice plant, delays its development, postpones heading and flowering, and prevents full grain filling and maturity, resulting in significantly reduced yields. There are also cases where temperatures are normal in the early stage and heading is not delayed, but abnormal low temperatures in the later stage lead to delayed flowering, pollination, and grain filling and maturity. When rice suffers delayed-type cold damage, the number of blighted grains increases, the 1000-grain weight decreases, grain quality is poor, and yield reduction is severe.

2. Sterility-type cold damage: During the reproductive growth period of rice, the plant suffers short-term abnormally relatively strong low temperatures, which damage the physiological mechanisms of the floral organs, causing spikelet sterility, producing large numbers of empty husks, and resulting in severe yield reduction. Depending on the period when low-temperature damage occurs, it is further divided into cold damage at the booting stage and cold damage at the heading and flowering stage. If the temperature at the booting stage is below 17°C for 5 to 6 days, it will cause large-scale spikelet degeneration or failure to form normal pollen grains and egg cells, resulting in large numbers of empty husks; if the temperature at the heading and flowering stage is below 20°C, large numbers of glumes will fail to open, anthers will not dehisce, pollen will not be shed or the pollen germination rate will drop sharply, thus causing sterility and yield reduction.

3. Mixed-type cold damage: In the same year, delayed-type cold damage and sterility-type cold damage occur simultaneously. Low temperatures in the early growth stage delay the growth and development of roots, stems, leaves, and tillers, delay rice panicle differentiation, and postpone heading, affecting yield. Low temperatures again at the booting, heading, and flowering stages cause spikelet sterility or partial sterility, delay maturity, and produce large numbers of empty and blighted grains.

II. Effects of Low-Temperature Cold Damage on the Nutritional Physiology of Rice

1. Low temperatures weaken photosynthesis, mainly because low temperatures cause protein denaturation in chloroplasts and reduce enzyme activity, thereby lowering the photosynthetic intensity of the plant.

2. Low temperatures reduce respiration intensity. Respiration is an indispensable condition for maintaining plant growth and development. During the growth process of rice, for every 10°C decrease in temperature from the optimum, its respiration intensity decreases by 1.6 to 2.0 times.

3. Effects of low temperatures on the absorption of mineral nutrients: The energy for roots to absorb mineral elements comes from respiration. Low temperatures weaken root respiration, reducing the energy supplied for the plant to absorb mineral nutrients, which leads to a weakened absorption rate of nutrients by the plant and disrupts the nutrient balance of the plant.

4. Effects of low temperatures on nutrient transport: Low temperatures not only reduce the photosynthetic intensity of rice plants and affect the absorption of mineral nutrients by the root system, but also hinder the transport of photosynthetic products and nutrient elements to growing organs, reducing the transport speed.

III. Measures to Defend Against Low-Temperature Cold Damage

1. Establish and improve the improved-seed breeding system, purify and rejuvenate existing varieties, maintain and enhance the varietal characteristics and resistance of the original varieties, and introduce and breed early-maturing, high-quality varieties with strong cold resistance.

2. Autumn land preparation, autumn soil preparation, and autumn seedbed preparation. After the beginning of spring, make the most of accumulated temperature, raise seedlings early, and in early spring clear snow and cover with film to promote the rise of ground temperature. When the temperature steadily passes 5 to 6°C, begin raising seedlings and sowing; when the temperature steadily passes 13°C, begin transplanting.

3. Fertilization techniques for defending against cold damage. (1) Control the application of nitrogen fertilizer. In a cold-damage year, the total amount of nitrogen fertilizer should usually be reduced by 20 to 30%, with 70 to 80% of the remaining nitrogen fertilizer used as base fertilizer and tillering fertilizer, and 20 to 30% applied as panicle fertilizer 10 to 20 days before heading; (2) Increase the application of phosphorus fertilizer. Phosphorus can increase the content of soluble sugars in the rice plant, thereby improving the cold resistance of rice, and can also promote early maturity. Apply the entire amount as base fertilizer at one time into the soil layer where the roots are dense, which can defend against low-temperature cold damage; (3) Combine with potassium fertilizer and trace elements. Potassium can promote the metabolic synthesis of carbohydrates and improve crop stress resistance. Apply 60 to 70% of the total amount as base fertilizer throughout the entire layer; apply the remaining 30 to 40% as topdressing later. At the same time, appropriately combine with some trace elements to improve the quality of rice.

4. Increase water temperature and ground temperature. Setting up water-warming ponds, extending the water route, covering channels with film, widening and raising the water inlet, and running water into the field to increase temperature, as well as adopting return-flow irrigation, can all raise field water temperature and ground temperature during the day. During the sensitive period of sterility-type cold damage, carry out deep-water irrigation with a water depth of 17 to 20 cm.