INHIBITION OF THE DEGRADATION OF CHLOROPLAST MEMBRANES DURING SENESCENCE IN NUCLEAR STAY GREEN MUTANTS OF SOYBEAN

Near-isogenic lines of soybean (Glycine max [L.] Merr.) cv. Clark carr ying nuclear 'stay green' genes were examined to determine the effects of these genes on the breakdown of thylakoid membranes during senesce nce. In order to accelerate their senescence, mature leaves were excis ed and incubated in darkness for 7 days. The homozygous combination of the recessive alleles d1 and d2 (at two different nuclear loci), with or without G (a dominant allele in another locus that causes green se ed coat) inhibited the loss of chlorophyll and thylakoid proteins duri ng senescence. Electron micrographs of leaves of cv. Clark during the yellowing process showed chloroplasts in various stages of disintegrat ion; their thylakoid network was disrupted and abundant osmiophilic gl obuli formed. These senescent leaves also showed evident signs of dete rioration of the plasma membrane, including discontinuities, invaginat ions and membrane 'whorls'. In contrast, leaves carrying d1d1d2d2 and GGd1d1d2d2 did not show signs of plasma membrane degradation, and thei r chloroplasts appeared intact, with a continuous, unbroken thylakoid network and tightly stacked grana. Exogenous applications of abscisic acid (1 and 10 mu M), methyl jasmonate (10 mu M) or ethylene (1 and 10 mu l l(-1)) accelerated chlorophyll degradation in cv. Clark, but had no appreciable effect in d1d1d2d2 and GGd1d1d2d2, which indicates tha t their phenotypes are not due to a deficiency in any of these hormone s. The nuclear 'stay green' genotypes d1d1d2d2 and GGd1d1d2d2 exhibit a general incompetence for the degradation of chloroplast membranes an d, thus, they may constitute useful tools in the study of the biochemi stry and regulation of leaf senescence.