Abstract
The direction of photosynthate movement among plant organs is determined by "sink strength", a model involving the importation of sucrose (or other sugars) by hydrolysis or sequestration. In most plants, sink strength is determined by the activities of two sucrose-cleaving enzymes: sucrose synthase(SS) and invertase(INV). In addition, activities of sucrose phosphate synthase(SPS), sucrose phosphate phosphatase( SPP), and tonoplast-bound ATPase could affect sink strength. Increased activities of membrane-bound sucrose transporters or decreased vacuolar pH could also enhance accumulation of soluble solids. In fruits of many commercial crops, increases in soluble solids have been recorded during drought conditions. This research investigates the determinants of citrus fruit sink strength through drought stress. Potted Hamlin orange trees were grown under watered or drought-stressed conditions and fruit harvested and analyzed for Brix and acids. Stressed fruits had higher acid content and soluble solids, and lower pH than controls. The following components of fruit sink strength were measured: SS, INV, SPS, SPP, ATPase, PPase. In addition, isolated and purified membranes from fruit were tested for the presence of a sucrose symport at the plasmalemma and an antiport at the tonoplast. Increased sink strength appeared to be the result of SS, since SS activity was higher in drought-stressed versus well-watered fruit. Activities of other enzymes and transporters were not significantly different between control and treated fruit. We concluded that SS is the predominant factor controlling Brix levels in citrus fruit, although the altered pH could have contributed to sink strength by enhancing acid hydrolysis.
