Photosystem II Function Response to Drought Stress in Leaves of Two Alfalfa (Medicago sativa) Varieties

摘要

The two main alfalfa varieties cultivated in northern China, Medicago sativa CV. Zhaodong and Medicago sativa CV. WL353HQ, were used to study the photosynthetic responses to drought stress. Changes in the content of chlorophyll and water in leaves during drought stress were studied using potted plants with natural drought treatment, and the effects of drought stress on the photosystem II(PSII) functions of leaves were also studied using a rapid chlorophyll fluorescence kinetics technique. Under drought stress, the chlorophyll and water content of WL353HQ leaves, although reduced, were significantly greater than those of Zhaodong leaves. On day 4 of drought stress, with the relative water content of soil at 56.3%, the maximum photochemical efficiency (Fv/Fm) of PSII and the photosynthetic performance index (PIABS) based on light energy absorption of the leaves started to decrease, and PIABS was more sensitive than Fv/Fm. The standardized OJIP curves of the two types of leaves showed that under drought stress, the values of the relative variable fluorescence of point J at 2 ms(VJ), were significantly increased. The relative variable fluorescence of point K at 0.3 ms(VK), and point I at 30 ms(VI), incurred only small changes, which indicated that the main reason for the drought-induced decrease of PSII photochemical activity was related to the obstruction of electron transfer from QA to QB on the acceptor side of PSII. After 10 days of drought treatment, with a relative soil water content of 29.9%, the relative variable fluorescence of point L at 0.15 ms, VL, showed no significant changes throughout the drought process, and there was no significant difference between the two varieties, which indicated that there was no dissociation of thylakoid membranes in the leaves. However, the light energy absorption per unit reaction center (ABS/RC) of the two leaf types were significantly increased, thus indicating that the number of reaction centers in the leaves were greatly reduced. Drought stress also incurred a decrease in the proportion of absorbed light energy for electron transfer, while the proportion of energy dissipation was increased in both leaf types, which was an important reason for the decrease of photochemical activity in the leaves. The degree of PSII function in Zhaodong alfalfa leaves under drought stress was significantly less inhibited than those of WL353HQ, therefore, Zhaodong alfalfa should be considered an important alfalfa variety in arid regions of China.