Effects of High Temperature and Water Deficit on Photosystem II in Hordeum vulgare Leaves of Various Ages

Structural and functional characteristics of photosystem II (PSII) were examined in leaves of 4-, 7-, and 11-day-old barley seedlings exposed to high temperature (40°C, 3 h) and water deficit (replacement of nutrient medium with 3% PEG 6000 solution, 45 h). In young seedlings, the effective quantum yield of PSII photochemical reactions decreased upon heat treatment but did not change after dehydration. Both stressful factors diminished the Q_B-reducing capacity of PSII in 4- and 7-day-old plants. This was caused by the increase in the reduction level of plastoquinone and by the impairment of the Q_B-binding site of the D1 protein. The increase in the content of plastoquinol after the heat shock was due to the impaired oxidizing capacity of cytochrome f (Cyt f). The dehydration did not alter the content of functionally active Cyt f but elevated the microviscosity of the lipid bilayer in thylakoid membranes, which presumably impeded the lateral diffusion of plastoquinones and reduced their reoxidation rate. The heating and dehydration of old leaves reduced the amount of -type PSII reaction centers, thereby inhibiting the linear electron transport. It is concluded that PSII responses to heat treatment and water deficit are subject to variations depending on leaf age. Measurements of the redox potentials for plastoquinones, Cyt f, and cytochrome b₅₅₉ upon senescence, hyperthermia, and water deficit allowed us to propose that heat and water stresses activate cyclic electron transport around PSII.