PHOTOSYNTHETIC DECLINE FROM HIGH-TEMPERATURE STRESS DURING MATURATION OF WHEAT .1. INTERACTION WITH SENESCENCE PROCESSES

Photosynthetic capacity decreases rapidly when temperate species are exposed to heat stress during reproductive development. We investigated whether injury in wheat (Triticum aestivum L.) resulted from general acceleration of senescence processes or specific heat-induced lesions. In situ photosynthetic capacity of leaf discs and thylakoid reactions were measured using flag leaf tissue from two cultivars maintained at 20 and 35°C during maturation. Photosynthetic rates of leaf discs decreased faster at 35 than at 20°C and were more photolabile in cv Len than in cv Waverly at high temperature. Patterns of thylakoid breakdown also differed in the two wheat genotypes at 20°C: intersystem electron transport and photosystem II activity decreased linearly during postanthesis development in Len wheat, whereas coupling of photophosphorylation to electron transport declined late during senescence in Waverly wheat. Heat stress induced early loss of intersystem electron transport followed sequentially by decreased silicomolybdic acid, + 3-(3,4-dichlorophenyl)-1-dimethylurea-mediated photosystem II activity and 2,5-dichloro-p-benzoquinone-mediated photosystem II activity in Len. Stress accelerated the uncoupling process, but loss of intersystem electron transport and photosystem II activities was slower in Waverly than in Len. We conclude that high temperature initially accelerated thylakoid component breakdown, an effect similar to normal senescence patterns. Thylakoid breakdown may induce a destabilizing imbalance between component reaction rates; an imbalance between photosystem II and cytochrome f/ b6-mediated activities would be particularly damaging during heat stress.