Foliar Spraying of Glycine Betaine Lowers Photosynthesis Inhibition of Malus hupehensis Leaves under Drought and Heat Stress

Apple is often affected by environmental stresses, particularly the drought and heat stress in the field in northern China. Rootstocks play a major role in tree resistance, and Malus hupehensis Rehd. var. pinyiensis Jiang (Pingyi Tiancha, PYTC) is a widely used rootstock for apple cultivars in China. In recent years, exogenous glycine betaine (GB) has been used to increase crop stress resistance. In this study, the impact of drought and heats stress applied alone and in combination on photosynthesis of PYTC seedlings was evaluated under GB (10 mM) application to find out the possible physiological and biochemical mechanisms of GB in improving the resistance of PYTC seedlings to these stresses. Potted seedlings (sand culture - water culture) were exposed to conditions of drought (30.0% PEG-6000(w/v), 25 degrees C, 3 days), heat stress (42 degrees C, 3 h) and their combination. Results showed that under combined drought and heat stresses, the photosynthesis inhibition degree of the leaves of PYTC was significantly higher than that under drought or heat stress alone. The increased inhibition of photosynthesis by the combined stresses was not simply the additive stress effect of separate heat- and drought treatments; different responses in plant physiology to drought and heat stresses were also found. Heat stress decreased the chlorophyll contents, net photosynthetic rate, apparent quantum efficiency and calboxylation efficiency of leaves more than drought stress. Drought stress decreased the transpiration rate, stomatal conductance and intercellular CO2 concentration more than heat stress. In conclusion, the foliar-applied GB improved the photosynthesis of leaves of PYTC not only under single drought or heat stress but also under their combination. The improvement of water status and antioxidant enzyme activities were the possible mechanisms related to the promotion photosynthesis under stress conditions by GB application. (C) 2020 Friends Science Publishers