DEGRADATION OF PROTEINS FROM THYLAKOID MEMBRANES IN SENESCING WHEAT LEAVES AT HIGH-TEMPERATURE

This investigation determined whether thylakoid proteins would be degr aded more rapidly or not in senescing wheat (Triticum aestivum L. em. Thell.) leaves concurrently exposed to high temperatures. Excised leav es were pulse-labelled with [S-35]-methionine for a 12 h period, and t hen incubated at 22, 32 or 42-degrees-C for 0, 1, 2, or 3 d, before ex tracting a thylakoid enriched membrane sample. After electrophoretic s eparation, two prominent [S-35]-labelled protein bands were chosen for further analyses. Band A contained the D-1 thylakoid protein and band B contained thylakoid proteins of the light harvesting complex (LHCII ) associated with photosystem II (PSII). Total protein, [S-35]-labelle d protein, band A protein, and band B protein within the thylakoid enr iched membrane samples were measured. Unlabelled thylakoid enriched me mbrane samples, extracted from leaves given similar treatments, were u sed to measure uncoupled whole-chain and photosystem II (PSII) electro n transport and chlorophyll fluorescence. Accentuated decline in whole -chain and PSII electron transport, increasing F(o) values, and decrea sing F(max) values were a result of high temperature injury in leaves treated at 42-degrees-C. None of the thylakoid enriched membrane prote in fractions were degraded more rapidly in high-temperature treated le aves. Degradation of the total [S-35]-labelled membrane proteins and b and B was inhibited by the 42-degrees-C treatment. The results indicat e that high temperature stress may disrupt some aspects of normal sene scence.