Effects of exogenous 6-BA on photosynthesis, physiological characteristics of alfalfa leaves at flowering stage and rate of podding

The role of exogenous 6-benzylaminopurine (6-BA) in the regulation of photosynthesis and the antioxidant system in alfalfa leaves was evaluated to provide a theoretical and research basis the application of this cytokinin in the growth and development of alfalfa. Xinmu No. 4 alfalfa (Medicago sativa ‘Xinmu No. 4’) plants were examined in this study. 6-BA solutions at concentrations of 10, 20, and 30 mg·L^–1 were sprayed onto the leaves of alfalfa during the squaring period; the control plants were sprayed with water. The effects of exogenous 6-BA on photosynthetic parameters, fluorescence parameters, antioxidant enzyme activities, osmotic adjustment substances, and rate of podding were examined during the flowering stage of alfalfa. The results showed that after treatment with 20 mg·L^–1 6-BA, the photosynthetic rate, transpiration rate, and maximum photochemical efficiency of the alfalfa leaves were significantly increased and the activities of superoxide dismutase, peroxidase, and catalase in the leaves of alfalfa were enhanced. The increase accumulation of proline and soluble sugars and the reduced accumulation of hydrogen peroxide in the leaves significantly improved the fruiting rate and seed yield per plant. In the 30 mg·L^–1 treatment, although the antioxidative enzyme activity was increased and the main fluorescence parameters were higher than those of the control, the rate of photosynthesis was decreased compared to the control. The decrease in the photosynthetic rate under this treatment was dominated by non-stomatal limiting factors. In conclusion, 20 mg·L^–1 6-BA can increase the photosynthetic rate and antioxidant enzyme activity as well as reduce damage to the photosynthetic mechanism of alfalfa leaves at the flowering stage. Moreover, increasing photosynthesis in the alfalfa leaves can increase the fruiting rate and seed yield.