What are the causes of wheat seedling death during overwintering

In recent years, winter wheat seedling death during overwintering has occurred almost every year. Because wheat populations have strong self-regulating ability, a death rate below 10% has little impact, but a rate above 20% will cause yield reduction. The causes are analyzed as follows:

1. Low-temperature freeze injury. Low temperature is the main cause of wheat seedling death during overwintering. Weather that alternates between cold and warm, with large, rapid and frequent temperature changes, causes wheat seedlings to freeze too quickly and thaw too rapidly. In this alternating freeze-thaw process, the seedlings cannot adapt in time, their physiological functions decline, and the seedlings die. Winter wheat that has undergone cold-hardening before overwintering can generally tolerate a low temperature of -23°C; at lower temperatures, seedling death becomes severe. Winter wheat that has not undergone cold-hardening has noticeably weaker low-temperature tolerance. When the winter climate is cold, the frozen soil layer is very thick, and soil moisture freezes, the roots cannot absorb water from the soil, or their water-absorbing capacity is very weak, while the stems and leaves of the seedlings still transpire and lose water, causing the wheat to gradually dehydrate and die.

2. Soil drought. Annual total precipitation, summer precipitation and winter precipitation all have obvious effects on seedling death. When soil moisture is insufficient, the plants cannot get an adequate water supply during the overwintering period; especially in years with changeable weather, frequent winds and many cold waves, the seedlings die from drought and lack of water. In wheat fields where straw is returned to the field, the soil is loose, cold air can enter easily, and wheat freeze injury is even more likely to occur.

3. Weak seedlings overwintering. Weak seedlings are caused by late sowing, poor fertility, excessive density, or grazing. The individual seedlings grow thin and weak, have little growth before winter, and accumulate little organic nutrients, which cannot meet the needs of wheat overwintering. When winter cold occurs, this can cause seedling death during overwintering, and it is even more serious in weak winter varieties with poor cold resistance.

4. Winter irrigation too late. Irrigating overwintering water too late has three effects: first, the water content in the wheat plant is high, and when a cold wave hits, it easily causes ice to form inside the plant and kills the seedlings; second, the surface of the frozen soil cracks, the roots are torn apart, and cold air enters the lower soil layer and kills the seedlings; third, the lower soil layer freezes, so after watering the water cannot infiltrate downward, the surface freezes, and after a long time the soil lacks oxygen and the wheat suffocates to death. 5. Improper selection of varieties or too shallow land preparation and sowing, etc.