In nature, a plant organism is exposed to many stress factors that negatively affect the photosynthetic apparatus and especially PSII, which is most vulnerable to stress. Determination of the site of toxic action of stress factors in the electron transport chain (ETC) of PSII was the aim of this work. The functional state of the of PSII ETC in the leaves of wheat seedlings subjected to the complex toxic effect of Co2+ and photoinhibition was determined based on various characteristics of delayed chlorophyll a fluorescence (ms DF Chl a). The effect of Co2+ was expressed in a sharp decrease in the characteristic value of ms DF Chl a of the PSII reaction center (RC) and in a weaker blocking of the donor side characterizing the state of the Mn4O5Ca cluster and Yz. Reactive oxygen species (ROS) generated in the process of photoinhibition also blocked to a greater extent the acceptor side of the PSII ETC. With an increase in the adaptation time, a significant decrease in activity on the donor side of the ETC of PSII was observed. The combined effect of both factors had little effect on the change in fluorescent characteristics, which remained almost at the level of Co2+ action. It was shown that the adaptive capabilities of photochemical reactions occurring in the PSII ETC under combined stress are stimulated by the low-molecular antioxidant Na-ascorbate. Restoration by Na-ascorbate of the processes suppressed by the simultaneous action of photoinhibition and Co2+ occurs during the induction period of the ms DF Chl a, and is apparently expressed as a result of effective neutralization of the formed ROS. This indicates that the mechanism leading to a change in the character of the induction pattern of the ms DF Chl a as a result of the action of both factors has a single nature. The stress resistance of the photosynthetic apparatus increases due to an increase in the activity of antioxidant enzymes or the effectiveness of low-molecular antioxidants. As a result, the photosynthetic apparatus switches to the adaptive program, which ensures an increase in its stress resistance. It is assumed that Na-ascorbate plays a decisive role in protecting chloroplasts from oxidative stress by quenching O2– and *OH1.
Keywords: Triticum aestivum L., PSII, ETC, Со2+, photoinhibition, ROS, Na-ascorbate
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