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 F_v/F_m decreased significantly under Cu stress but was not affected by Zn treatment. However, the PI_ABS 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 Q_A^- to Q_B. 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/CS_m significantly increased the ABS/RC and TR_o/RC values. When the stress intensified, the value of DI_o/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.

Potassium ions enhance photosynthetic tolerance to salt stress. We hypothesized that potassium ions, by minimizing the trans‐thylakoid proton diffusion potential difference, can alleviate over‐reduction of the photosynthetic electron transport chain and maintain the functionality of the photosynthetic apparatus. This study investigated the effects of exogenous potassium on the transcription level and activity of proteins related to the photosynthetic electron‐transport chain of tobacco seedlings under salt stress. Salt stress retarded the growth of seedlings and caused an outflow of potassium ions from the chloroplast. It also lowered qP (indicator of the oxidation state of Q_A, the primary quinone electron acceptor in Photosystem II (PSII) and Y_PSII (average photochemical yield of PSII in the light‐adapted state) while increasing Y_NO+NF (nonregulatory energy dissipation in functional and nonfunctional PSII), accompanied by decreased expression of most light‐harvesting, energy‐transduction, and electron‐transport genes. However, exogenous potassium prevented these effects due to NaCl. Interestingly, lincomycin (an inhibitor of the synthesis of chloroplast‐encoded proteins in PSII) significantly diminished the alleviation effect of exogenous potassium on salt stress. We attribute the comprehensive NaCl‐induced downregulation of transcription and photosynthetic activities to retrograde signaling induced by reactive oxygen species. There probably exist at least two types of retrograde signaling induced by reactive oxygen species, distinguished by their sensitivity to lincomycin. Exogenous potassium appears to exert its primary effect by ameliorating the trans‐thylakoid proton diffusion potential difference via a potassium channel, thereby accelerating ATP synthesis and carbon assimilation, alleviating over‐reduction of the photosynthetic electron transport chain, and maintaining the functionality of photosynthetic proteins.