Abstract: 【Objective】 The response of photosynthesis of apple plants to drought stress which simulated drought stress conditions for roots under localized irrigation and alternate partial rootzone drying. Moreover, stomatal and non-stomatal limitations in photosynthesis under water stress were investigated. 【Method】Water stress was simulated using PEG6000. Compared with good water condition, micropropagated ‘Royal Gala’ apple plants were subjected to half root stress (HRS, to simulate localized irrigation), alternate half root stress (AHRS, to simulate alternate partial rootzone drying), whole root stress (WRS). Their net photosynthesis rate (Pn) and the related photosynthetic parameters, photosynthetic pigment, chlorophyll fluorescence, SOD activity, and malondiadehyde (MDA) content were studied. 【Result and conclusion】Pn, stomatal conductance, substomatal CO2 decreased significantly and leaf temperature increased significantly for all plants under water stress, when compared with CK plants. There were no significant differences of photosynthetic pigment and MDA content, SOD activity and chlorophyll fluorescence response between CK and HRS, indicating that decreased Pn of HRS plants was due to mainly stomatal limitation. Similar foliar photosynthetic pigment and MDA content, SOD activity were found between AHRS and CK while AHRS resulted in higher foliar MDA content and lower Fv/F0 and Fv/Fm than CK from 2nd day after the plants were under water stress. It is suggested that the decreased Pn of AHRS plant should be due both stomatal and non-stomatal limitations. Significantly lower foliar photosynthetic pigment content, SOD activity, Fv/F0 and Fv/Fm, but higher foliar MDA content, were observed on WRS 4 or 5 day after the plants were under drought stress than CK, indicating that the decreased Pn in the first few days after water stress was applied should be due to mainly stomatal limitation, and non stomatal limitation should play an important role thereafter in regulating Pn for WRS plants.

Key words: Water stress, Photosynthesis, Stomatal limitation, Non-stomatal limitation