METABOLISM OF RADIOLABELED GALACTOLIPIDS IN SENESCENT BARLEY LEAVES

The metabolism of galactolipids in senescent primary leaves of barley was followed upon the incorporation of [I-C-14]oleic acid during greening of etiolated shoots. In presenescent leaves the bulk of the label was present in apolar compounds of which 36% were galactolipids and 57% phospholipids. In segments induced to senesce in continuous darkness the loss of the predominant 18:3/18:3 molecular species of galactolipids was accompanied by the temporary accumulation of 18:3/18:3 phosphatidylcholine, a species which was almost absent in the presenescent leaves. A large proportion of radioactivity lost in the galactolipids was eventually released as CO2. Label was also transferred to sugars and other polar compounds. A pathway of galactolipid breakdown featuring the conversion of the diacylglycerol cores to phosphatidylcholine followed by the release of free fatty acids in glyoxisome-like microbodies and gluconeogenesis via beta-oxidation and the glyoxylic acid cycle is proposed.