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.

Elevated temperatures due to global climate change adversely affect plant growth and development, which has become a major factor restricting wheat (Triticum aestivum L.) production. Despite the introduction of dwarfing genes that have enhanced lodging resistance as well as productive potential in wheat breeding, lodging still affects wheat yields. Plant growth regulators are widely recognized as effective agents in mitigating crop lodging. Few studies have investigated the high-temperature lodging sensitivity of wheat genotypes from different breeding periods, nor have they examined how uniconazole-sucrose regulates lodging resistance under heat stress. To fill this research gap, an experiment was conducted in which two contrasting wheat genotypes from different periods, Bima 1 (BM1, ~135 cm tall, a historical genotype released in 1953, lodging-susceptible) and Shannong 28 (S28, ~75 cm tall, a modern genotype released in 2014, lodging-resistant), were exposed to high temperature stress combined with uniconazole-sucrose application. The results showed that high-temperature-induced increases in plant gravity center height, together with decreased stem diameter coefficient, stem plumpness, and lignin deposition, were the main factors responsible for the reduction in bending section factor and mechanical strength of wheat stems. These modifications are associated with reduced lodging resistance, increased susceptibility to lodging, and significant yield losses. Nevertheless, exogenous application of uniconazole-sucrose lowers plant gravity center height, enhances stem diameter coefficient, stem plumpness, and lignin content, thus alleviating lodging risk and boosting wheat yield under high temperature stress. High temperature stress was associated with downregulated relative expression levels of key genes involved in lignin metabolism and reduced activities of the corresponding key enzymes, as well as inhibited lignin biosynthesis and accumulation in stems and increased incidence of wheat lodging. Conversely, foliar spraying of uniconazole-sucrose alleviated these suppressive effects on lignin biosynthesis, thus enhancing stem mechanical strength and reducing the lodging index of wheat. Moreover, these indicators were more sensitive to heat stress or uniconazole-sucrose treatment in BM1. The two genotypes examined suggested a potential trend that S28 may exhibit reduced sensitivity to high temperature in terms of mechanical traits and lignin synthesis, which could contribute to enhanced lodging resistance under heat stress.