Potassium application to table grape clusters after veraison increases soluble solids by enhancing berry water loss

Abstract Application of aqueous potassium solutions to grapevine ( Vitis vinifera L.) canopies and clusters was reported to increase soluble solids concentration (SSC) in grape berries, although detailed study of the effect was lacking. The objective of this study was to evaluate its repeatability across genotype and year, further optimize its means of application, and provide information on its likely mode of action. Grape clusters were treated in the field with glycine-complexed potassium or water at veraison and then 3 weeks later in 2 separate years to ‘Autumn Royal’, ‘Scarlet Royal’, and ‘Sweet Scarlet’ or in a single year to ‘Summer Royal’. At harvest the mean SSC of the untreated berries was 17.3%, while those treated with potassium was 19.8%. Dates of commercial maturity were advanced in these varieties from 4 to 11 d by potassium treatment. Berry weight tended to be slightly reduced by potassium treatment and the grapes often became darker in color, while the effect on firmness was inconsistent. Field treatment of ‘Flame Seedless’ grape clusters with potassium sorbate at veraison or both at veraison and prior to harvest indicated that a dual potassium application enhanced soluble solids more than a single application. Dipping ‘Flame Seedless’ grape clusters in potassium sorbate while attached to the vine increased the subsequent concentration of SSC to the same degree as spraying both clusters and the leaf canopy, while application to the canopy alone had no significant effect. Analysis of the elemental composition of the grape juice indicated that potassium was significantly enhanced in concentration by cluster application of potassium as was phosphorus, magnesium, boron and copper. Potassium sorbate, potassium bicarbonate and glycine-complexed potassium were effective at increasing SSC, while potassium phosphate and potassium phosphite had no significant effect. The increase in SSC among detached berries treated with potassium salt solutions was positively correlated with the rate of weight loss. Those salts that caused the most rapid loss in berry weight were those that caused the largest increase in SSC. Those that did not increase weight loss did not increase SSC. Potassium salts with basic pH were most effective in inducing weight loss. Potassium application consistently enhanced soluble solids concentration in grapes and most likely acts by increasing water loss from the grape berries.