Our previous work demonstrated that exogenous gibberellins (GAs) applications during rapid fruit growth significantly increases sink demand and results in a larger fruit in Japanese pear. In an attempt to unravel the mechanism of increased sink demand by applied GAs, the histology, cell wall components of the flesh, and carbon accumulation in the fruit were assessed for Japanese pear (Pyrus pyrifolia, cultivar 'Kousui'), as were the activities of sucrose- and sorbitol-cleaving enzymes. Our results show that most vascular tissues occurred in core tissue with very little vascular tissue in the flesh. Application of a mixture of GA(3) + GA(4) in lanolin paste significantly increased the amount of ethanol-insoluble solids, e.g., total pectins, hemicellulose, and cellulose in the cell walls. There was a significantly increased sink demand (assessed by C-13 accumulation in the fruit) by the applied GAs, and this increased sink strength was closely related to increased activities of cell wall-bound invertase in the core, neutral invertase and NAD-dependent sorbitol dehydrogenase in the flesh during rapid fruit growth. As well, concentrations of sorbitol and sucrose in the flesh were decreased by GA application, while glucose concentration increased. Most importantly, the fact that sink activity can be increased by GA application implies that endogenous GAs are likely to be important modulators for sugar metabolism. Hence, selecting for genotypes with elevated GA production in the growing fruit and increased activities of key enzymes for sugar metabolism could result in increased fruit size.
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