Cu and Zn Stress affect the photosynthetic and antioxidative systems of alfalfa (Medicago sativa)

Cu and Zn are common and potentially harmful heavy metals to plants, animals and humans. Herein, we investigated the effects of Cu and Zn stress on the photosynthesis and tolerance mechanism of alfalfa plants to ROS using fluorescence and biochemical methods. The results showed that Cu stress significantly reduced the chlorophyll content of the leaves, while Zn stress only reduced the Chl a content. The Fv/Fm decreased significantly under Cu stress but was not affected by Zn treatment. However, the PIABS of the leaves were sensitive to Cu and Zn stress. Both Cu and Zn stress resulted in the weakening of the ability of PQ library to accept electrons, the damage of OEC and the inhibition of the electron transfer from QA- to QB. Moreover, Cu stress also dissociated the thylakoids of leaves, but Zn stress did not significantly damage it. In Cu and Zn stressed leaves, the reduction of RC/CSm significantly increased the ABS/RC and TRo/RC values. When the stress intensified, the value of DIo/RC increased indicated a plant self-protection mechanism that eliminates excess energy in the PSII reaction center and increases the energy for heat dissipation per unit reaction center. Cu stress significantly increased the O2- production rate, H2O2 content, and MDA accumulation in the leaves. However, Zn stress exhibited a minimal effect on the ROS production and oxidative damage in the alfalfa leaves but increased the O2- production rate at the concentration of 800 μmol·L−1. Cu stress increased the activities of SOD, POD, CAT, APX, and GPX in the leaves; however, leaves adapts to Zn stress by enhancing the activities of SOD and GPX. Thus under Cu stress, the degree of photoinhibition and oxidative damage in alfalfa leaves were significantly higher than under Zn stress.