Effects of Plant Acclimation on Electron Transport in Chloroplast Membranes of Cucumis sativus and Cucumis melo

In this work, we have studied photosynthetic electron transport in chloroplasts of two “contrasting” species of Cucumis genus, the shade-tolerant species Cucumis sativus (cucumber) and the light-loving species Cucumis melo (melon). Plants were acclimated to moderate (50–125 μmole photons m⁻² s⁻¹) or high light (850–1000 μmole photons m⁻² s⁻¹). Parameters of a fast induction of chlorophyll a fluorescence, emitted from photosystem 2 (PS2), were determined using a conventional OJIP test. For monitoring the turnover of photosystem 1 (PS1) reaction centers \({\text{{Р}}}_{{700}}^{ + }\), we used electron paramagnetic resonance. The shade-tolerant (C. sativus) and light-loving (C. melo) species, acclimation to high or low light irradiation, revealed substantial difference in their response to variations of light intensity. Photosynthetic activity of shade-tolerant species C. sativus revealed higher sensitivity to light intensity during acclimation as compared to C. melo. In the course of the long-term acclimation (more than 2 months) of С. sativum to high light (≥ 500 μmole photons m⁻² m⁻¹), a photochemical activity of PS2 decreased. This was not the case, however, for leaves of C. melo. In С. sativus leaves, a decrease in photochemical activity of PS2 caused by acclimation to high light was reversible, demonstrating the recovery after the attenuation of irradiation intensity. Plants of both species acclimated to high and low light also revealed significant differences in the two-phase kinetics of \({\text{{Р}}}_{{700}}^{ + }\) redox transients. In the leaves of plants acclimated to strong light, we observed a lag-phase in the kinetics of \({\text{{Р}}}_{{700}}^{ + }\) photooxidation that could be attributed to cyclic electron transport (CET) around PS1. The ratio of the signals induced by white light and far-red light (707 nm) was higher in plants acclimated to strong light. This effect can be explained by the enhancement of CET and optimization of the energy balance at excess of light, protecting plants from oxidative stress. The data obtained are discussed in the context of the problem of photosynthesis optimization upon fluctuations of light intensity.