The Uptake of Surcrose and its Conversion to Starch in Detached Ears of Wheat
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Detached ears of wheat were cultured on solutions of 14C-sucrose and the distribution of carbon-14 in the ear was followed. Within 8 h radioactive sucrose was found in all the tissues of the cultured ear, and con siderable amounts of carbon-14 had accumulated in other ethanol-soluble compounds. Carbon 14 accumulated rapidly in the starch deposited in the endosperm, but little was found in the starch of the pericarp, or in other material insoluble in ethanol in the vegetative organs. During 48 h the specific radioactivity of the sucrose in the endosperm increased in a hyper bolic pattern and was equal to that of the starch produced. Carbon-14 in the glucose and fructose accumulated more slowly and in a linear fashion. Experiments with sucrose containing carbon-14 in both moieties equally, or in the fructosyl moiety exclusively, confirmed that both moieties are converted into starch and at about the same rate. As sucrose in endosperm provided with asymmetric sucrose retained a considerable degree of asymmetry, it seems as though inversion is not a necessary step in the transport of sucrose into the grain. In ears provided with 14C-sucrose at 30 mg ml-1 the rate of accumulation of 14C-sucrose in the culm, rachis, and floral organs was about 0-6 times the value at 50 mg ml-1. However, in the sucrose of the endosperm, and in the starch deposited there, the rates of accumulation of carbon-14 from both levels were identical. This finding supports the concept that the transport of sucrose is limited during the final stages of its passage into the grain.