RELATIONSHIP OF CARBOHYDRATE-METABOLISM WITH LIPID BIOSYNTHESIS IN DEVELOPING SUNFLOWER (HELIANTHUS-ANNUUS L) SEEDS

Levels of sugars, starch and the activity of a number of enzymes were studied in relation to the accumulation of oil in the developing seeds of sunflower (Helianthus annuus L.). Active accumulation of oil commenced at 10 days after flowering (DAF) and continued up to 20 DAF followed by a small increase up to maturity. The rate of oil accumulation as well as of fatty acid biosynthesis, visualized by 1-C-14-acetate and U-C-14-sucrose incorporation into total lipids, was maximal at 15 DAF. The content of soluble sugars decreased sharply while that of starch increased between 5 and 15 DAF. After 15 DAF, starch content decreased. The studies indicated that at an early stage (5 DAF) sucrose was cleaved by invertase (EC 3.2.1.26). The activity of sucrose-UDP glucosyl transferase (EC 2.4.1.13) and of alpha-amylase (EC 3.2.1.1) followed a pattern of change parallel to the accumulation of starch in the seeds. After 15 DAF, beta-amylase (EC 3.2.1.2) became active in the mobilization of starch. The activity of 6-phosphogluconate dehydrogenase (EC 1.1.1.44), NADP+-malic enzyme (EC 1.1.1.40), isocitrate dehydrogenase (EC 1.1.1.41) and succinate dehydrogenase (EC 1.3.99.1), which furnish NADPH and ATP for fatty acid biosynthesis, also showed maximal activity at 15 DAF, the phase of active lipid synthesis. It is concluded that the accumulation of starch at initial stages serves as a transient reserve material that is utilized for the biosynthesis of lipids in sunflower seeds.