2. The Causes of Cold Injury and Late Lodging in Spring Wheat and Their Relationship with Variety
2.1 Winter Wheat Freezing Injury
2.1.1 Genetic Factors
Cold resistance in wheat varieties is a complex trait influenced by both genetic and environmental factors. From a genetic perspective, it is a quantitative trait controlled by multiple genes, often showing additive effects. The interaction between genotype and environment plays a significant role. Generally, winter wheat varieties are more cold-resistant than spring types. This trait has a moderate heritability, ranging from 30.7% to 81.4%. Hybrid offspring may exhibit super-parental segregation, where F1 generations show intermediate or parent-like cold resistance, while F2 generations display continuous variation, sometimes exceeding the parent's resistance level. Due to the high heritability in early generations, varieties with strong cold resistance may lose this trait after several planting cycles.
2.1.2 Spike Differentiation Process and Weather Conditions
The process of young spike differentiation directly affects a plant’s ability to withstand cold. As growth cones elongate and enter the young panicle stage, cold tolerance begins to decrease, though not drastically. However, during the dihedral phase, cold tolerance drops significantly. Frost damage becomes severe when young spikes reach 5–10 mm in length. In 2007, an unusually warm spring led to rapid temperature increases. Some wheat fields experienced premature growth due to early sowing and high planting density, causing the spike differentiation process to advance abnormally. This reduced the plants’ ability to "resist cold," resulting in severe frost damage on March 6 and April 3, which affected leaves, young ears, and floral parts.
2.1.3 Cultivation Practices
Excessive growth before and during winter, as well as in early spring, can lead to poor cold resistance. High temperatures before winter and fast warming in early spring caused some fields to be sown too early (e.g., September 23) with excessive seeding rates (150–225 kg/ha), leading to overly vigorous growth and early spike development. This weakened the plants’ cold resistance. On March 6 and April 3, 2007, severe frosts damaged semi-winter wheat that had been over-fertilized, sown too early, or planted at too high a density. In contrast, properly managed fields with suitable sowing dates and reasonable inputs remained largely unaffected. Therefore, cold injury is not solely determined by variety but also by cultivation practices and weather conditions. Proper sowing time and balanced fertilization can significantly reduce frost damage. Overly aggressive sowing or excessive use of water and fertilizer can weaken cold resistance, making it difficult to assess variety quality based on such practices.
2.2 Late Wheat Lodging
2.2.1 Fertility Tolerance
Lodging resistance is a key characteristic of high-fertilizer wheat varieties. It is closely related to the plant's response to nitrogen fertilizers. Nitrate reductase activity, a critical enzyme in nitrogen metabolism, is inversely correlated with lodging resistance. Higher nitrate reductase activity leads to weaker stem strength and increased lodging risk. Excessive nitrogen application enhances this activity, reducing the plant’s ability to resist lodging.
2.2.2 Stem Traits
Stem characteristics, including height, thickness, internode length, and vascular bundle structure, play a major role in lodging resistance. Stronger stems with thicker walls and shorter internodes are more resistant to lodging. Additionally, the balance between stem and ear development influences overall stability. Varieties with well-distributed photosynthetic products between stems and ears tend to have better resistance.
2.2.3 Cultivation Conditions and Weather Factors
Cultivation practices, such as sowing date, density, and fertilizer application, significantly affect lodging resistance. In 2006–2007, many wheat fields in Jining City suffered from lodging due to excessive growth caused by early sowing and high fertilization. This resulted in weak stems, long internodes, and poor wind resistance. Heavy winds and rain in May 20–22 caused widespread lodging. While variety traits influence lodging resistance, improper management and adverse weather conditions can severely reduce it. Therefore, large-scale lodging is often due to cultivation errors rather than variety limitations alone.
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