METABOLISM OF POLAR LIPIDS DURING SENESCENCE OF WATERCRESS LEAVES

The metabolism of polar lipids in senescent leaves of watercress (Rori ppa nasturtium-aquaticum) was monitored, following the incorporation o f [1-C-14]acetate prior to senescence induction. Analysis of changes i n contents and radioactivities of the various lipid constituents was p erformed during senescence, either in permanent darkness or under a ph otoperiod of 14 h. During senescence under these conditions, the leave s 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), phosphatidylg lycerol (PG), and phosphatidyl-inositol (PI). The decrease in GL conte nt (MGDG and DGDG) was more pronounced than that of PL. The first PL t o decrease during dark senescence was PC, followed by PE and PG. where as PI level did not change significantly. Thus, the changes in total P L and in their main constituents, PC and PE, during dark senescence re sulted from both enhanced degradation and reduced synthesis rate. On t he other hand, the increase in PI level and the constant PG levels obs erved during senescence under photoperiodic conditions, resulted from reduced degradation and continuation of biosynthesis. Hence, the senes cence-retarding effect of light was exhibited through the retardation of Chl loss and the catabolic processes of all polar lipids, as well a s through enhancement of biosynthesis of PG and PI. It seems therefore , that the senescence-related changes in the various lipid constituent s, which occurred both in darkness and in the light, resulted from the different proportions of anabolic and catabolic reactions in the turn over of different lipids.