Photosystem II inhibition by moderate light under low temperature in intact leaves of chilling-sensitive and -tolerant plants

Photosystem II (PSII) activity was examined in leaves of chilling-sensitive cucumber (Cucumis sativus L.), tomato (Lycopersicum esculentum L.), and maize (Zea mays L.), and in chilling-tolerant barley (Hordeum vulgare L.) illuminated with moderate white light (300 μmol ms) at 4°C using chlorophyll a fluorescence measurements. PSII activity was inhibited in leaves of all the four plants as suggested by the decline in F /F, 1/F-1/F, and F /F values. The changes in initial fluorescence level (F), F/F, 1/F-/1/ F, and F/F ratios indicate a stronger PSII inhibition in cucumber, maize and tomato plants. The kinetics of chlorophyll a fluorescence rise showed complex changes in the magnitudes and rise of O-J, J-I, and I-P phases caused by photoinhibition. The selective suppression of the J-I phase of fluorescence rise kinetics provides evidence for weakened electron donation from the oxidizing side, whereas the accumulation of reduced Q suggests damage to the acceptor side of PSII. These findings imply that the process of chilling-induced photoinhibition involves damage to more than one site in the PSII complexes. Furthermore, comparative analyses of the decline in F /F and photooxidation of P700 explicitly show that the extent of photoinhibitory damage to PSII and photosystem I is similar in leaves of cucumber plants grown at a low irradiance level.