TEMPERATURE-DEPENDENCE OF CHLOROPHYLL FLUORESCENCE INDUCTION AND PHOTOSYNTHESIS IN TOMATO AS AFFECTED BY TEMPERATURE AND LIGHT CONDITIONS DURING GROWTH

The temperature dependence of chlorophyll fluorescence induction and photosynthesis of tomato plants grown at different temperatures and light intensities was studied. Chlorophyll fluorescence induction and photosynthetic activity of leaf discs was determined between 0-degrees and 30-degrees-C. Two breakpoints, around 16-degrees and 24-degrees-C, were observed when the maximum of chlorophyll fluorescence induction, F(p), was plotted against temperature. DCMU was used to study the role of photosynthetic electron transport on the breakpoints of F(p). Inhibition of photosynthetic electron transport by DCMU caused the disappearance of the breakpoint at 16-degrees-C. The temperature breakpoint at 24-degrees-C remained intact. This result is used to discuss the mechanisms underlying the occurrence of the breakpoints. The low temperature breakpoint at 16-degrees-C was ascribed to electron transport limitation between photosystem I and photosystem II at temperatures below 16-degrees-C. The high temperature breakpoint was attributed to alterations in light distribution between photosystem I and photosystem II. An increased State II adaptation at temperatures above 24-degrees-C is believed to cause a decrease in F(p). Temperatures of both breakpoints were affected by growth conditions of the tomato plants. Compared with optimal growth conditions, suboptimal growth conditions caused a significant decrease in the low temperature breakpoint. Concomitantly, photosynthetic activity and leaf chlorophyll content were decreased. The possibility of using breakpoint temperatures to indicate an adaptation of the thylakoid membrane organisation and functioning to suboptimal growth conditions is discussed.