Accumulation and Transformation of Sugars in Sugar Cane Stalks

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Abstract
Comparison of the rates of accumulation of C14-labelled sugars into sugar cane internodal tissue showed decline with increasing maturity. Most of the label appearing in inner space sugars was in sucrose. The glucose and fructose moieties of sucrose accumulated by young internodal tissue immersed in glucose-C14 were equally labelled. Synthesis of sucrose from glucose occurred at a locus not in rapid equilibrium with the free fructose pool. Studies of efflux rates indicated that immature tissue was 8 to 10 times more permeable to reducing sugars than to sucrose. The rate of efflux of sugars was not closely correlated with sugar content of the tissue. The rate first decreased then increased with increased tissue maturity. The presence of sucrose in the external medium did not affect efflux rates. Defoliation experiments on stalks previously labelled by supplying C14O2 showed that stored sugar was rapidly mobilized, 75% being removed from a young internode 48 hours after defoliation. The rate of loss was of the same order as for tissue discs in water. Possible mechanisms of sugar accumulation and transformation are discussed. The results indicate a carrier mediating accumulation is located in a permeability barrier which separates the outer and inner space. The barrier is logically identifiable with the tonoplast membrane.