Potassium Localization in the Grape Berry Pericarp by Energy-Dispersive X-Ray Microanalysis

¹ Division of Horticultural Research, CSIRO, Private Mail Bag, Merbein, Vic. 3505, Australia.

The distribution of the major solutes, particularly potassium, in the berry pericarp of the grape variety Tarrango was investigated. The osmotic potentials () of the skin and pulp were the same, viz. -5.6 MPa. Glucose and fructose accounted for 84% of the pulp , although the contribution of these sugars to skin was significantly less, viz. 58%. The tartrate to malate ratios were 0.4 and 2.4 in the skin and pulp, respectively. The mean concentration of K in the skin was ca 300 mM which was four to five times higher than the concentration in the pulp. Although crystals of a calcium salt (tartrate?) were seen in cryo-scanning electron micrographs of hypodermal cells, crystalline K salts were never observed. The calculated contribution of skin K and complementary anion to skin was ca 20%. X-ray microprobe mapping of Tarrango hypodermal cells showed that K was not uniformly distributed across the skin. Similar observations were made for Shiraz berries. This inhomogeneity of K accumulation results in some cells with vacuolar concentrations between 0.5 and 1.0 M. It is proposed that malate is the predominant complementary anion to K in high-K hypodermal cells, and the likely salt to acid balance in the vacuole of these cells results in pH levels significantly greater than the pH level of the pulp, viz. 5 or higher. While K occurs largely as a soluble salt, the high Ca content of the skin (48 mM) probably leads to the precipitation of the greater part of the tartrate as crystalline bundles of calcium tartrate. The preferential sequestering of K in the skin is indicative of a very rudimentary homoeostatic process which assists in regulating K levels in the pulp.

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