[The control of sugar accumulation within strawberry aggregate fruit by invertase and hexokinase]

The glucose, fructose and sucrose contents and the activities of invertase, sucrose synthase (SS), sucrose phosphate synthase (SPS), hexokinase (HXK) and fructokinase (FRK) of 'Tochiotome' strawberry aggregate fruit were analyzed at various stages of fruit development. The sugar accumulation exhibited the difference between various parts of the fruit. The sugar content is the highest in the top part of ripe 'Tochiotome' strawberry aggregate fruit, followed by the central part, and the basal part was the lowest. Changes in activity of invertase corresponded to the changes of sugar concentration gradient within various parts of the ripe strawberry aggregate fruit. The SS and SPS activities within various parts of aggregate fruit did not showed obvious change that corresponded to the sugar content. At the stage of fruit maturation, the HXK and FRK activities within various parts of strawberry aggregate fruit, however, exhibited an opposite gradient to sugar content. The basal part of aggregate fruit had the highest activities of HXK and FRK, while the top part had the lowest activities. These results suggested that the higher activity of invertase in the top part of aggregate fruit may play a role for rapid cleavage of sucrose and help to form a sucrose gradient in apoplastic space from the basal part to the top part of strawberry aggregate fruit, thus enhance photosynthate translocation from the basal part to the top part of aggregate fruit, which led to higher sugar accumulation in the top part of fruit. In the basal part of aggregate fruit, the higher activities of hexose-metabolizing enzymes may promote the consumption of hexose, and thus resulted in the lowering of sugar content in the basal part than in the top part of strawberry aggregate fruit.