Accumulation and distribution of potassium and its association with water balance in the skin of Cardinal table grapes during storage

• Low temperature storage causes bound water loss in the skin tissues of berries.
• Changes in water status are linked to soluble K+ accumulation in the grape skin.
• A non-uniform K+ accumulation in skin cells is evident by SEM–EDX analysis.
• Outer skin cell layers are compressed in grapes stored in air at low temperature.
• Air with high CO2 levels mitigated the cellular water stress and K+ accumulation.

Although potassium participates in distinct mechanisms that influence grape growth and development, including osmoregulation, little is known about the association between water and potassium in grape during storage at low temperature. We analyzed the relationship between potassium and the bound water fraction in the skin of early-harvested Cardinal table grapes (Vitis vinifera L.) from two different harvest years, both of which were stored at 0 °C for 3 days in air (20% O2 + 0.03% CO2) or in air + CO2 (20% O2 + 20% CO2). The relative K+ content and distribution in the skin cells was determined by energy dispersive X-ray microanalysis, revealing a non-uniform accumulation of K+ in grape skin cells. Storage at 0 °C in air causes a significant decrease in bound water levels and greater soluble-water K+ accumulation, irrespective of the harvest year. Furthermore, low temperature-scanning electron microscopy images revealed that the epidermal and the first hypodermal layers of the cells were compressed in the skin of fruit stored in air. However, when exposed to air plus 20% CO2, there was no decrease in the bound water content or in the associated K+ accumulation, nor were the outer skin cells compressed.