Effects of exogenous ascorbic acid on photosynthesis and xanthophyll cycle in alfalfa (Medicago sativa L.) under drought and heat stress

Abstract

Alfalfa is an important leguminous plant, yield and quality depend on the growing environment, while effects of drought and heat stress on alfalfa leaves are unknown. This study was conducted to evaluate the effect of exogenous ascorbic acid (AsA) on photosynthesis, chlorophyll fluorescence, and xanthophyll cycle in alfalfa leaves subject to under drought and heat stress. The results suggest that drought and heat stress caused decreases in the net photosynthetic rate (Pn) in alfalfa leaves, but stomatal conductance (gs), transpiration rate (Tr), and intercellular CO2 concentration (ci) were increased. The application of AsA could alleviate these changes to some extent. Besides, the decreased photosystem II (PSII) maximum photochemical efficiency (Fv/Fm) and violaxanthin (V) contents and significantly increased non-photochemical quenching (NPQ) levels. The increased NPQ corresponds to the de-epoxidation state (DPS) of xanthophyll pigments. In the AsA-pretreated alfalfa plants, the Fv/Fm and the NPQ were elevated, indicating that AsA could alleviate the adverse effects on photosynthesis induced by this stress. The violaxanthin de-epoxidase (VDE) enzyme activity was inhibited by drought and heat stress, and AsA significantly increased VDE enzymatic activity on the 2nd and 8th days. In summary, photoinhibition of PSII occurred in alfalfa leaves under drought and heat stress, resulting in decreased photosynthetic activity. Exogenous AsA can enhance the photosynthetic capacity of the plant, and enhance the drought and heat resistance of alfalfa.

Keywords

• exogenous substances
• abiotic stress
• leaf gas exchange
• climate change