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Sucrose Metabolism in the Primary Culm of Sweet Sorghum During Development¹

Sweet sorghum (Sorghum bicolor (L.) Moench] accumulates large amounts of sugar in culm parenchyma after anthesis, but the biochemical basis of this accumulation is not understood. This study was to identify the enzymes responsible for the pattern of sucrose accumulation in sweet sorghum. Sugars and four enzymes of sucrose metabolism were extracted from whole primary culms and individual internodes of primary culms of ‘Rio’ sweet sorghum at intervals during reproductive development. In whole culms, sucrose concentration increased seven-fold between the boot and mid-grain filling stages. This increase was accompanied by a decline in soluble acid invertase (pH optimum 5.0) and sucrose synthase activities. Soluble neutral invertase (pH optimum 7.0) activity was evident throughout, although its activity declined between soft dough and physiological maturity. Very low activity (<13 µmole·kg^–1 FW·s^–1) of sucrosephosphate synthase was also detected throughout development. At the flag leaf stage, the two uppermost internodes were still elongating. These internodes had the highest activities of acid and neutral invertase and sucrose synthase, and virtually no accumulation of sucrose. In succeeding growth stages, sucrose accumulated most rapidly in internodes below the top two. At anthesis, the top six internodes had similar activities, on a fresh weight basis, of all enzymes assayed. The onset of sucrose accumulation in sweet sorghum was associated with the onset of the reproductive phase of growth and the decrease in acid invertase activity, but sucrose concentration was not correlated with the activity of any enzyme. In this regard, sweet sorghum appears to be biochemically different from sugarcane (Saccharum spp.).

Key Words: Sorghum bicolor (L.) Moench • Sugarcane • Saccharum spp. • Sugar • Invertase • Sucrose-phosphate synthase • Sucrose synthase

² Plant physiologist, USDA-ARS, Plant Health and Stress Physiology, P.O. Box 267, Weslaco, TX 78596.

Received for publication January 26, 1987.