Physiological and molecular mechanisms of adzuki bean (Vigna angularis) responding to low light and screening of shade-adaptable varieties

Global dimming-induced reduction in surface solar radiation threatens crop productivity, particularly for light-sensitive leguminous crops such as adzuki bean (Vigna angularis). This study aimed to characterize the responses of adzuki bean to low-light stress and identify associated varietal differences. Six adzuki bean varieties were subjected to three shading gradients (0%, 25%, and 50%) throughout the entire growth period. Parameters related to light signal perception, photosynthetic carbon metabolism, antioxidant defense, photosynthetic traits, yield components, and grain quality were systematically determined. Partial least squares path modeling (PLS-PM) and principal component analysis (PCA) were used to explore regulatory pathways and index correlations, respectively. Results showed that shading significantly suppressed the expression of genes related to light signaling, photosynthetic electron transport, and carbon metabolism. This was accompanied by reduced activities of key photosynthetic enzymes (38.1%–52.3% under 50% shading) and net photosynthetic rate, as well as a negative relationship between antioxidant protection and photosynthetic carbon fixation, ultimately leading to a gradient decrease in yield. Distinct varietal differences were observed: V4 (Tangshan Red Adzuki Bean) exhibited relatively strong low-light tolerance, while V6 (Long Bean 9) was highly sensitive. Additionally, shading altered grain quality indices in a pattern consistent with changes in photosynthetic carbon metabolism. Conclusions This study characterized the physiological and molecular responses of adzuki bean to low-light stress and delineated the specific varietal differences. These findings provide a theoretical basis for evaluating low-light adaptability, screening shade-tolerant germplasm, and guiding adzuki bean cultivation under low-light conditions.