The Various Mechanisms of Photosynthesis Limitation in Heat-treated Barley Seedlings of Different Ages

The effect of heat shock (40oC, 3 h) on the photosynthetic activity of 4-, 7and 11-day-old barley seedlings was studied. The rate of CO2 gas exchange in young leaves was not changed under heat shock while in 11-dayold seedlings the high temperature affected both the average quantum yield and the shape of light response curves of photosynthesis. Nevertheless, in chloroplasts of young heated leaves the rate of potassium ferricyanide recovery was reduced. The impairment of PSII activity and diminution of oxidizing ability of the Cyt b6/f complex caused the inhibition of photosynthetic electron transport. The preheating of 4-day-old leaves at 40oC led to a marked decrease of the nonphotochemical quenching of Chl fluorescence. It was related mainly to the lowering of ∆pH-dependent quenching mechanisms. Thus high temperature inhibited the photosynthetic reactions in young seedlings by the decrease of both electron transport rate and the transthylakoid proton gradient. The suppression of photosynthesis by heat shock in old leaves was caused mainly by a reduction of CO2 uptake and by a decrease of carboxylation activity. These effects led to the impairment of ATP and NADPH consumption by the Calvin cycle and an increase in the transthylakoid proton gradient. As a results, the level of plastoquinone pool reduction was also increased and electron flow was decreased. According to the data obtained, the heating of young leaves inhibited the photosynthesis by reduction of the electron transport rate while in old leaves the activity of Calvin cycle enzymes was mainly limited. The suppression of photosynthetic activity by heat shock in old leaves was more compared to the young leaves.