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

The processes of electron transport in the leaves of two species of plants of the genus Cucumis, the shade-tolerant species Cucumis sativus (cucumber) and the light-loving species Cucumis melo (melon), grown under moderate (50–125 µmol photons m^–2 s^–1) or strong illumination (850–1000 µmol photons m^–2 s^–1) were studied. The parameters of fast and slow induction of chlorophyll a fluorescence were used as indicators characterizing the activity of photosystem 2 (PS2). The functioning of photosystem 1 (PS1) was monitored by changes in the electron paramagnetic resonance signal from the oxidized reaction centers of PS1, \({\text{P}}_{{700}}^{ + }\). A significant difference was revealed in the dynamics of changes in photosynthetic parameters of shade-tolerant (C. sativus) and light-loving (C. melo) species during their acclimation to moderate or high light intensity. In the shade-tolerant species C. sativus, photosynthetic indicators characterizing the activity of PS2 showed a noticeable sensitivity to increased illumination compared to the light-loving species C. melo, indicating a weakening of the activity of PS2 with an increase in light intensity during acclimation of plants. During a long-term (more than 1–2 months) acclimation of C. sativus to high intensity light (≥500 µmol photons m^–2 s^–1), their PS2 lost photochemical activity, which, however, was not observed in the leaves of C. melo. The weakening of the activity of PS2 in the leaves of C. sativus was reversible, that is, after returning to light of moderate intensity, the activity of PS2 was restored to the level characteristic of the leaves of C. melo. In the leaves of plants of both species, differences in the kinetics of photoinduced redox transformations of the reaction centers of PS1 were manifested, depending on the acclimation conditions. In plants acclimated to strong light, there was a clearly pronounced delayed phase of signal from \({\text{P}}_{{700}}^{ + }\) growth, which presumably could be caused by cyclic electron transport (CET) around PS1. The ratio of the amplitudes of EPR signals from \({\text{P}}_{{700}}^{ + }\) under the white and far-red light (707 nm) became higher in plants grown under strong light. This might be due to an increase in CET, which helped to optimize the energy balance and reduce light stress when there was an excess of illumination. The results obtained are discussed in the context of the task of optimizing photosynthetic processes during plant acclimation.