Photoinhibition of photosynthesis was studied in young and mature detached sun needles of cypress under high irradiance (HI) of about 1 900 μmol m-2 s-1. The degree of photoinhibition was determined by means of the ratio of variable to maximum chlorophyll (Chl) fluorescence (Fv/Fm) and electron transport measurements. Compared with the mature needles, the young needles, containing about half the amount of Chl a+b per unit area, exhibited a higher proportion of total carotenoids (Car) as xanthophyll cycle pigments and had an increased ratio of Car/Chl a+b. The potential efficiency of photosystem (PS) 2, Fv/Fm, markedly declined in HI-treated young needles without significant increase of F0 level. In contrast, the Fv/Fm ratio declined with significant increase of F0 level in mature needles. In isolated thylakoids, the rate of whole chain and PS2 activity markedly decreased in young HI-needles in comparison with mature needles. A smaller inhibition of PS1 activity was observed in both needles. In the subsequent dark incubation, fast recovery was found in both needle Types that reached maximum PS2 efficiencies similar to those observed in non-photoinhibited needles. The artificial exogenous electron donors DPC, NH2OH, and Mn2+ failed to restore the HI-induced loss of PS2 activity in mature needles, while DPC and NH2OH significantly restored it in young needles. Hence, HI-inactivation was on the donor side of PS2 in young needles and on the acceptor side of PS2 in mature needles. Quantification of the PS2 reaction centre proteins D1 and 33 kDa protein of water splitting complex following HI-exposure of needles showed pronounced differences between young and mature needles. The large loss of PS2 activity in HI-needles was due to the marked loss of D1 protein of the PS2 reaction centre in mature needles and of the 33 kDa protein in young needles. and N. La Porta ... [et al.]
Photoinhibition of photosynthesis was investigated in Vitis berlandieri and Vitis rupestris leaves under field conditions at different sampling time in a day. The degree of photoinhibition was determined by means of the ratio of variable to maximum chlorophyll fluorescence (Fv/Fm) and photosynthetic electron transport measurements. When the photochemical efficiency of PS2, Fv/Fm, markedly declined, F0 increased significantly in leaves of V. berlandieri, while F0 did not increase in V. rupestris leaves. Isolated thylakoids of leaves of V. berlandieri showed significant inhibition of whole chain and PS2 activities at midday. A smaller inhibition was observed for V. rupestris. Later, the leaves reached maximum PS2 efficiencies similar to those observed early in the morning during sampling at evening. The artificial exogenous electron donor Mn2+ failed to restore PS2 activity in both species, while DPC and NH2OH significantly restored PS2 activity in V. rupestris midday leaf samples. Quantification of the PS2 reaction centre protein D1 and 33 kDa protein of water splitting complex following midday exposure of leaves showed pronounced differences between V. berlandieri and V. rupestris leaves. The marked loss of PS2 activity noticed in midday samples was mainly due to the marked loss of D1 protein in V. berlandieri while in V. rupestris it was the 33 kDa protein. and M. Bertamini, N. Nedunchezhian.
Photoinhibition under irradiance of 2 000 µmol m-2 s-1 (HI) was studied in detached control (C) and water deficit (WD) leaves of grapevine (Vitis vinifera L.) plants. The degree of photoinhibition was determined by means of the ratio of variable to maximum chlorophyll (Chl) fluorescence (Fv/Fm) and electron transport measurements. The potential efficiency of photosystem (PS) 2, Fv/Fm, marginally declined under HI in WD-leaves without significant increase of F0. In contrast, Fv/Fm ratio declined markedly with significant increase of F0 in C-leaves. In isolated thylakoids, the rate of whole chain and PS2 activity under HI were more decreased in C-than WD-leaves. The artificial exogenous electron donors diphenyl carbazide, NH2OH, and Mn2+ failed to restore the HI-induced loss of PS2 activity in both C-and WD-leaves. Thus HI operates at the acceptor side of PS2 in both leaf types. Quantification of the PS2 reaction centre protein D1 following HI exposure of leaves showed pronounced differences between C-and WD-leaves. The marked loss of PS2 activity under HI of C-leaves was due to the marked loss of D1 protein of the PS2 reaction centre. and M. Bertamini ... [et al.].