Effect of drought and rewatering on the photosynthetic characteristics in Morus alba leaves

In order to detect the effects of drought and rewatering on the photosynthetic capacity and adaptive mechanism in mulberry leaves, a mulberry variety named ‘Qiuyu’ was employed as the experimental material, and the characteristics of photosynthesis, and fluorescence were investigated using pot tests with a water control (CK). The results showed that the water contents in the potted mulberry plant leaves were significantly lower than that in the CK; leaf wilting, curling and drooping, and the leaf base angle was increased when the soil water content was reduced to 26.7%, and the net photosynthetic rate, stomatal conductance, and transpiration rate were close to zero, while the values of stomatal limitation and intercellular CO2 concentration were increased. In addition, the excess light energy, ratio of dependent thylakoid membrane on both sides of the proton gradient, proportion of xanthophyll cycle, fluorescence quantum yield, heat dissipation quantum yield, and the original fluorescence were increased. Additionally, the maximum fluorescence, proportion of absorbed light energy in to the photochemical reaction yield, and the electron transport rate were decreased. After rewatering, the indicators of leaf growth, main parameters of photosynthesis and fluorescence in the mulberry leaves recovered quickly and returned to close to those in the CK. Our results indicated that the decrease in the net photosynthetic rate under drought stress might be caused by the stomatal and non-stomatal limitations. The PSII response center was partly inactivated, the excitation energy in mulberry seeding leaves was reduced; these responses mainly depended upon increasing ratios of the xanthophyll cycle and the quantum yields of fluorescence and heat dissipation. After rewatering, the photosynthetic apparatus in the mulberry leaves had a perfect mechanism for adapting to drought stress and repairing after injury, which could help mulberry to recover quickly in a shorter time.