Heat-induced modifications of photosynthetic electron flows in Hordeum vulgare leaves of different age

Abstract

Temperature is one of the major environmental constraints that limits photosynthetic activity, affecting plant growth and productivity. One of the unresolved issues in studies of thermal adaptation of plants is the question of the difference in adaptive strategies to heat of the photosynthetic apparatus at different stages of development. The aim of this study was to investigate how photosynthetic electron flows respond to hyperthermia (40 °C, 3 h) at different stages of leaf development (four-, seven- and eleven-day-old primary leaves of Hordeum vulgare L.; young, mature and aging, respectively). Chlorophyll fluorescence parameters (including imaging), characteristics of the redox state of P700 and plastoquinones as well as transcription of ndhA and ndhF genes were examined. It was shown that heat inhibited the ferredoxin‐plastoquinone reductase (FQR)-dependent cyclic electron flow (CEF) in all tested variants. In eleven-day-old leaves, an inhibition of the linear electron flow (LEF) was also detected. Heat-induced decrease of CEF and LEF in seven- and eleven-day-old plants (but not in four-day-old seedlings) was compensated by the activation of ‘NADH dehydrogenase-like complex’ (NDH)-dependent electron flow as well as increase of ndhA and ndhF transcription (genes encoding NDH). A hyperthermia decreased the level of plastoquinone pool reduction in all tested leaves. In seven- and eleven-day-old leaves, exposure to heat decreased the photoactive and increased the non-photoactive plastoquinone pools, respectively. Infiltration of leaves by 2,6-dichlorophenolindophenol (DPIP) prevented the decrease of the plastoquinone reduction. It also inhibited the FQR-dependent CEF and qE changes in all age groups. At the same time, the DPIP treatment did not affect the heat-induced suppression of LEF and the redistribution of plastoquinones from the photoactive to the non-photoactive pool in the eleven-day-old leaves. Based on the data obtained, it can be concluded that a decrease in the efficiency of electron donation from plastocyanin to PSI and electron acceptance by ferredoxin could be the reason of heat-induced suppression of FQR-dependent CEF, while a decrease in the size of photoactive plastoquinone pool is potentially a reason of heat-induced LEF inhibition.