The Influence of Post-veraison Foliar Potassium Applications on Table Grape Berry Quality

Sugar concentration and skin color (for pigmented table grape cultivars) are primary indicators of maturity and important for marketing purposes. Treatments that accelerate sugar and pigment development enable earlier harvest. Early harvest not only improves marketing, it reduces risks of losses from bird feeding or insect and pathogen damage because the residence time of the grapes in vineyards is reduced. In addition to accelerating maturity, increasing sugar content and improving color could also enable a larger crop to mature on the vines. Potassium is the primary soluble salt to affect xylem and phloem flows. Particularly important for sugar transport, potassium from the roots and vine are primarily transported into fruit after veraison, and it exceeds what the root system can deliver, even from well water soil with abundant potassium. Our objective was to apply a potassium spray to the berries so their content would exceed that delivered naturally to the fruit, and determine if this altered the grapes compared to those with “natural” potassium content. Previous experiments indicated that potassium applications increased soluble solids. Among the potassium sources evaluated, potassium metalosate, a common fertilizer, was effective and approved for application to grapes. In 2010, cultivars located at California State University, Fresno, were used; two blue-black cultivars, ‘Autumn Royal’ and ‘Summer Royal’; and two red cultivars, ‘Scarlet Royal’ and ‘Sweet Scarlet’. Potassium metalosate or water (control) was applied to five replicate plots of five vines each arranged in a randomized complete-block design. Two cluster-directed applications were made, each containing 1.3 g/L of potassium or water alone, beginning at veraison and three weeks later. Samples of 50 berries were collected from each plot at four weekly periods after the first application until harvest at commercial maturity. Berry firmness, size, color, soluble solids, and concentrations of fructose and glucose were measured. Potassium significantly increased soluble solids, berry firmness, and color intensity; however, it significantly decreased berry size. Soluble solids of ‘Autumn Royal’, ‘Summer Royal’, ‘Scarlet Royal’, and ‘Sweet Scarlet’ increased at harvest to 20.3%TSS, 20.0%TSS, 21.8%TSS, and 19.3%TSS, after potassium treatment, from 15.5%TSS, 18.2%TSS, 18.1%TSS, and 16.7%TSS, respectively, among the controls. It significantly increased fructose and glucose in berries, compared to controls. Fructose/glucose ratios from veraison onset to harvest time increased from about 0.65 to 0.90, independent of potassium treatment. Potassium application influence on fructose/glucose ratios varied, the only significant increase was to ‘Autumn Royal’, from 0.90 to 0.91 at harvest.