SOLUTE ACCUMULATION BY GRAPE PERICARP CELLS .3. SUGAR CHANGES INVIVO AND THE EFFECT OF SHADING

The endogenous glucose, fructose and sucrose of the skin of the grape berry [Vitis vinifera] has been partitioned into diffusible and residual fractions during 16 day of the phase of rapid sugar accumulation following veraison. The data have been related to total soluble solids of the pericarp juice (.degree.Brix) as an index of development: strong correlations were found between the partitioned hexose fractions and .degree.Brix, but with the notable exception of non-diffusing glucose which was most variable during this period. Sucrose did not show good correlations with other attributes. A large proportion of both glucose and fructose diffused from the skin into an osmotic buffer during 30 min of efflux: at 5 .degree.Brix, 40-45% of each hexose diffused, and at 16 .degree.Brix this fraction was 70-75%. However, the degree of compartmentation of glucose consistently exceeded that of fructose by a small, constant amount for any given total level. A series of berries were shaded individually, a treatment known to lessen the rate of global sugar accumulation but, as shown here, having no qualitative effect on the relationship between glucose and fructose nor on their degrees of "compartmentation". A tentative calculation indicates a substantial concentration gradient between the diffusible and compartmented space (compartmented space having the lower concentration) both before veraison and during rapid sugar accumulation in the tissue. Accumulation seems essentially an increase in diffusible sugars, in which the primary process is probably unloading from the phloem into the apoplast rather than eventual compartmentation within the cell.