Low solar radiation, caused by climate change or dense planting patterns, now limits wheat production. Although wheat breeding has increased lodging resistance and yield potential through the introduction of dwarfing genes, it still reduces wheat yields. Few studies have been conducted to clarify the lodging sensitivity to shading of different-era wheat cultivars in China’s Huang-Huai-Hai region, as well as the characteristics of lodging resistance as affected by paclobutrazol under shading stress. To address this gap, the experiment included two wheat cultivars released in different decades, grown under shade and treated with or without paclobutrazol. The results showed that reductions in filling degree and lignin content, together with increases in length of the basal internode and gravity center height, markedly reduced the section modulus and breaking strength of shaded wheat culms. These changes impaired lodging resistance and raised lodging risk. However, paclobutrazol application effectively reduced lodging incidence and increased wheat yield under shading stress. Furthermore, these responses were more pronounced in the old cultivar (YZM) than in the modern cultivar (S28). This indicates that the culm mechanical parameters of the old cultivar were more shade-sensitive than those of the modern cultivar. Moreover, shading downregulated the relative expression levels of key genes associated with lignin biosynthesis to decrease the activities of key enzymes, thereby inhibiting the biosynthesis and deposition of lignin in culms to increase the risk of wheat lodging. Paclobutrazol application alleviated the inhibitory effects of shading on lignin biosynthesis, thereby strengthening culms and enhancing lodging resistance. These findings may provide a basis for exploring cultivation regulation methods to enhance wheat lodging resistance under overcast and low-sunshine conditions, and to offer guidance for the breeding of wheat cultivars with lodging resistance and shade tolerance.

Lodging is a major constraint limiting wheat (Triticum aestivum L.) yield and quality globally. Despite dwarfing genes reducing plant height and mitigating lodging risk and losses, lodging still severely limits wheat yield. Few studies have examined how wheat breeding has altered the sensitivity of yield to lodging, especially its penalties in grain number and grain weight at specific spike positions. Two separate experiments were conducted in the Huang-Huai-Hai region of China: a lodging-period experiment (two genotypes, five periods) during the 2011–2013 growing seasons, and a lodging-angle experiment (three genotypes, five angles) during the 2019–2021 growing seasons. The results showed that grain number per m² (GNO), average grain weight (AGW), and grain yield (GY) all increased linearly with genotype release year. Lodging significantly reduced GNO and AGW, and consequently GY in all genotypes, but these losses declined linearly with the year of genotype release, indicating that modern genotypes suffer less yield penalty under lodging. Furthermore, lodging at any stage reduced the weight of both superior and inferior grains within the spike, whereas only pre-anthesis lodging decreased grain number per spike. Inferior grains, owing to their higher environmental sensitivity, showed larger reductions in both grain number and grain weight per spike than superior grains. Across all genotypes, lodging decreased grain number and grain weight per spike in the order apical > basal > central, and the relative losses declined linearly with year of genotype release. Moreover, lodging-induced losses in ¹³C assimilation followed the order old > intermediate > modern across genotype eras; the reduction in ¹³C assimilation within the spike ranked apical > basal > central, and that in superior grains exhibited the same pattern. Concurrently, the allocation of ¹³C assimilates to inferior grains was markedly inhibited across all spike positions, with the reduction magnitude significantly exceeding that in superior grains. Lodging-induced differential changes in ¹³C assimilation among grain positions within each spike layer are associated with the uneven reductions in grain number and average grain weight across the spike. These findings may provide a basis for lodging-resistant wheat production and cultivar breeding. Artificial lodging induction can serve as a reliable strategy to efficiently evaluate yield stability and lodging tolerance. However, its applicability to other wheat regions needs further verification via multi-location and multi-cultivar trials.