Effects of high temperature on dry matter accumulation and sugar metabolism in different soybean varieties

High temperature is one of the major environmental factors limiting soybean growth and development. Investigating the effects of high temperature at different growth stages on dry matter accumulation and sugar metabolism can help elucidate the physiological mechanisms underlying heat-induced yield loss, thereby providing a theoretical foundation for breeding heat- tolerant soybean varieties. In this study, two soybean varieties—Liaodou 24 (heat-insensitive) and SN22-15 (heat-sensitive)—were grown under pot conditions and subjected to high-temperature treatments at different developmental stages. Dry matter accumulation, photosynthetic parameters, and sugar content were measured. Under high-temperature conditions, Liaodou 24 exhibited significantly higher leaf color index, net photosynthetic rate, stomatal conductance, intercellular CO2 concentration, transpiration rate, and dry matter weight of stems, leaves, petioles, and pods compared to SN22-15. High-temperature stress during the R1 and R3 stages, Liaodou 24 also showed significantly higher levels of soluble sugar content, sucrose content, and starch content in various plant parts than SN22-15. High-temperature stress during the V4, R1, and R3 stages, Liaodou 24 produced more pods per plant, more grains per plant, and greater seed weight per plant than SN22-15. Notably, high-temperature stress during the R3 stage caused a marked yield reduction in SN22-15. Excessive heat impaired photosynthesis in soybean leaves, inhibited the synthesis of photosynthetic products, reduced dry matter and sugar accumulation, and ultimately led to yield decline. Liaodou 24, being relatively heat-tolerant, showed moderate reductions in dry matter accumulation, sugar metabolism, and yield under high temperatures, only during the R3 stage. In contrast, SN22-15, as a heat-sensitive variety, experienced significant reductions in these traits across all growth stages, with the most severe impact occurring during the R3 stage. These findings suggested that in soybean breeding programs, developing varieties with appropriate maturity periods could help avoid yield loss due to high temperatures during the early podding stage.