METABOLISM OF POLAR LIPIDS DURING SENESCENCE OF WATERCRESS LEAVES

The metabolism of polar lipids in senescent leaves of watercress (Rorippa nasturtium-aquaticum) was monitored, following the incorporation of [1-C-14]acetate prior to senescence induction. Analysis of changes in contents and radioactivities of the various lipid constituents was performed during senescence, either in permanent darkness or under a photoperiod of 14 h. During senescence under these conditions, the leaves showed concomitant changes in the contents of chlorophyll (Chl), and of phospholipids (PL) and galactolipids (GL) and their constituents [digalactosyldiglyceride (DGDG) and monogalactosyldiglyceride (MGDG)], phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylglycerol (PG), and phosphatidyl-inositol (PI). The decrease in GL content (MGDG and DGDG) was more pronounced than that of PL. The first PL to decrease during dark senescence was PC, followed by PE and PG. whereas PI level did not change significantly. Thus, the changes in total PL and in their main constituents, PC and PE, during dark senescence resulted from both enhanced degradation and reduced synthesis rate. On the other hand, the increase in PI level and the constant PG levels observed during senescence under photoperiodic conditions, resulted from reduced degradation and continuation of biosynthesis. Hence, the senescence-retarding effect of light was exhibited through the retardation of Chl loss and the catabolic processes of all polar lipids, as well as through enhancement of biosynthesis of PG and PI. It seems therefore, that the senescence-related changes in the various lipid constituents, which occurred both in darkness and in the light, resulted from the different proportions of anabolic and catabolic reactions in the turnover of different lipids.