Cytoplasmic membrane senescence in bean cotyledons

Distinguishable patterns of cytoplasmic membrane senescence in cotyledon tissue of Phaseolus vulgaris have been elucidated by examining the behavior of four microsomal enzymes—NADH-cytochrome C reductase, NADPH-cytochrome C reductase, glucose-6-phosphatase and 5′-nucleotidase during germination. For young cotyledon tissue, specific activities for the phosphatases were similar for rough and smooth microsomal fractions, but both cytochrome C reductases were 2–3 times more concentrated in the smooth fraction. These proportionalities changed with increasing age. As senescence becomes more intense the enzyme activities change independently of one another. These changes do not appear to be influenced by the presence or absence of ribosomes on the membranes. Parallel analyses of phospholipid levels in the isolated fractions revealed that loss of microsomal enzyme activity correlates with an ultimate dismantling of the membranes into their macromolecular constituents. The data have been interpreted as indicating that functionally distinct membranes or regions of the same membrane are differentially sensitive to senescence.