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

Photosystem II (PSII) activity was examsined 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 micro mol m(-2) s(-1)) at 4 degrees C using chlorophyll a fluorescence measurements. PSII activity was inhibited in leaves of all the four plants as suggested by the decline in F(v)/F(m), 1/F(o) - 1/F(m), and F(v)/F(o) values. The changes in initial fluorescence level (F(o)), F(v)/F(m), 1/F(o) - /1/F(m), and F(v)/F(o) 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(A) 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(v)/F(o) 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.