Different Physiological and Photosynthetic Responses of Soybean and Mungbean at the Initial Flowering Stage under Low-Temperature Stress to Uniconazole S3307

Low-temperature stress is a destructive abiotic stress that limits the production and quality of tropical and subtropical crops. In this study, we investigated the physiological responses of the soybean ( Glycine max (L.) Merr. variety ‘Kenfeng 16’ and the mungbean ( Vigna radiata (L.) R.Wilczek) variety ‘Lvfeng 2’ to exogenous uniconazole S3307 and low-temperature stress at the initial flowering (R1) stage. Uniconazole S3307 increased the photosynthetic parameters [net photosynthetic rate ( P n ), transpiration rate ( T r ), stomatal conductance ( G s ), and intercellular CO 2 concentration ( C i )], sucrose phosphate synthetase (SPS) activity, and the contents of sucrose (SUC) and starch (STA) in ‘Kenfeng 16’ and ‘Lvfeng 2’ under low-temperature stress; but it decreased the activities of acid invertase (AI), neutral invertase (NI), total invertase (IN) as well as the contents of glucose and fructose. Additionally, sucrose synthase (SS) activity was increased in ‘Kenfeng 16’ but decreased in ‘Lvfeng 2’ with uniconazole S3307 treatment. The observed changes of P n , SUC, and STA in ‘Kenfeng 16’; the AI and IN activities in ‘Lvfeng 2’; and the glucose and fructose contents in both varieties were quickly recovered after they were subjected to the 4-day recovery period. Compared with the controls, the yield and yield components of ‘Lvfeng 2’ were significantly ( P ≤ 0.05) higher on day 1 (D1), D2, and D3 of the cold-stress conditions, respectively. The yield per plant was positively correlated with the number of grains per plant, number of pods per plant, G s, SUC, and STA in ‘Lvfeng 2’ and ‘Kenfeng 16’. S3307 enhanced the G s and the accumulation of SUC and STA, but it decreased the NI activity, glucose content, and fructose content, thereby alleviating cold stress to increase the yield.