Enhanced susceptibility of photosynthesis to high leaf temperature in triazine-resistant Solanum nigrum L. Evidence for photosystem II D₁ protein site of action

We have used nuclearly isogenic lines of Solanum nigrum, that differ genetically in the triazine-resistance trait, to investigate the mechanistic basis for the greater susceptibility of photosynthesis in resistant biotypes to elevated leaf temperature. Based on measurement of CO₂-dependent oxygen evolution by leaf discs, of electron transfer partial reactions in isolated thylakoid membranes, and of chlorophyll fluorescence induction parameters of both leaves and thylakoid membranes, we conclude that: (1) The single base substitution in the chloroplast psb A gene that is responsible for triazine resistance is almost certainly also responsible for the decreased tolerance to high temperature; (2) The high temperature-induced inhibition of photosynthesis in the resistant biotype is caused by an inhibition of electron transfer between the primary and secondary quinone acceptors of photosystem II; (3) The enhanced temperature susceptibility is likely to be a significant contributing factor in the competitive disadvantage of triazine-resistant biotypes.