Membrane Lipids in Senescing Green Tissues2

At a late stage in sensescence cucumber cotuledons lose fresh weight rapidly; at the same time there is an increase in apparent free space and large quantities of electrolyte leak out when discs of cotyledon tissue are floated on water. It is concluded that tonoplast and plasma membrane become leaky at this time. Phosphatidyl choline, the major phospholipid present, begins to disappear once the cotyledons reach maximum fresh weight; by the time rapid water loss starts, 56 per cent has gone, and phosphatidyl ethanolamine and inositol start to disappear. Only at maturity is there enough of these phospholipids to furnish more than two complete membranes around each cell; it is suggested that the decline in phospholipid level at senescence destroys membrane integrity and allows leakage. The glyohpids begin to disappear at the same time as chlorophyll, 2 weeks before weight loss starts. The minor lipid phosphatidyl glycerol is the first to disappear from the cotyledons.