Increased Heat Sensitivity of the Photosynthetic Apparatus in Triazine-Resistant Biotypes from Different Plant Species

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Abstract
Leaf fragments from five species of triazine-resistant and -susceptible plants were exposed for brief periods to temperatures above 38°C. In resistant leaves, a greater increase in temperature-induced fluorescence emission was observed than in susceptible leaves, indicative of greater heat sensitivity of the PS II electron transfer linked to triazine resistance. The kinetics of fluorescence induction under low or strong light excitation revealed two distinct effects of heat exposure; A DCMU-type inhibition which was reversed upon cooling and a decrease in the variable fluorescence, due mainly to an increase in the constant fluorescence (O level) in resistant leaves, which was only partially reversible. A greater deterioration of electron transfer activity in chloroplasts isolated from resistant plants also was detected, under exposures to a high pH, temperature of 25–35°C and to ferricyanide. This indicates that there is greater instability of the oxygen evolving system in triazine-resistant plants.