Research advances on potassium nutrition and berry accumulation in grapevines

Abstract: Potassium is essential for vine growth, enzyme activation, photosynthesis, osmotic regulation, yield and quality. Potassium is one of the most abundant cations in grapes, the potassium ion concentration in cytoplasm can reach 100-200 mmol·L^-1, even up to 500 mmol·L^-1 in apoplast. Potassium as a highly mobile element, can easily transfer from old tissue to a young area, and primarily concentrate in the center of the active part, such as the growing point, buds, berries and so on. Grape berries are a strong sink for K, and the potassium content has been rising in the whole growth period. Plant nutrient analysis results show that: more K is accumulated particularly during ripening. In the mature period, the potassium ion can account for about 70% of the total cation content and 66% of the total potassium content of aboveground organs. Among berry tissues, K concentration per unit fresh weight is higher in the skin than in the fleshy pericarp. The K concentration in the seeds is lower than in the skin but slightly higher than in the flesh. K concentration in the skin is 1.7-6.9 times higher than that in the pulp and 1.6-4.3 times higher than in the seeds. The concentration in the skin can reach 50% of the whole berry potassium concentration. Potassium transportation is done by ion channels. There are low affinity K+absorption systems and high affinity K+absorption systems in higher plants on the absorption of potassium, the former done in external high K+concentration and the latter in low K+concentration. In grapes, potassium ion channels belong to the Shaker gene families, and Vv KUP 1 and Vv KUP 2 are two genes encoding potassium transport carriers and are found in larger expressions before veraison and lower expressions after veraison. The ex-pression of these carriers is restricted to the skin, the tissue in the berry with the highest K concentrations.Researches on the other types of K+transport mechanisms in different berry tissues may better help in understanding the mechanism of berry K accumulation. Potassium movement occurs in both xylem and phloem. In grape berries, the xylem seems to be a minor route of K entry because xylem flow into the berry may be low due to the low transpiration rate of the berry. Potassium is the major cation in the leaf xylem sap and shoot xylem sap. While K concentration differs significantly with leaf age, leaf xylem sap flow is highest in the middle leaves and lowest in the old and young leaves. At the beginning of grape growth, potassium reflux occurs in the plant through the xylem, and remobilization of K from other organs to the berries can be reabsorbed into the phloem by apoplast. Between the veraison and mature period, the K+concentration being gradually reduced in the different organs shows that potassium is being retransported into the berries. Many factors may affect K accumulation in berries including soil, climate, variety, phytohormones and cultural practice. K deficiency may be induced in saline conditions where Na+is the predominant cation. Gravel soil is better for absorption of the potassium than sand and clay; high temperature, and shading treatment promotes the potassium accumulation in fruit; varieties differ on the potassium absorption efficiency, while higher efficiency is found in‘Cabernet Sauvignon'than in‘Cabernet Franc'. Cultivation measures affect the potassium distribution and accumulation in berries. Canopy management is a technique that results in the altered position and density of leaves, shoots, and fruit to achieve a specific desired arrangement. Reasonable leaf canopy management helps to decrease excessive potassium accumulation in the pulp, and reduces the p H of the wine. Crop load affects berry K accumulation by changing the balance of the source/sink for K. It may also have indirect effects on berry K accumulation through changes in the degree of shading in the canopy, availability of assimilates and rates of berry growth. The effects of the K addition on berry K accumulation are variable, with more K being accumulated when more fertilizers are applied. Results from other studies indicate that berry, juice, or wine K is not a positive correlation to potassic fertilizer supplement. There are data suggesting that increased irrigation tends to increase berry K accumulation. Berry and wine K concentrations were higher for irrigated treatments than for non-irrigated treatments. Irrigation enhances the dissolution of K from clay particles and its movement in the soil solution, which facilitates its supply to roots and provides higher uptake. Potassium accumulation in berries is a very complicated process. Comprehension of berry K accumulation requires an understanding of the function of K in the berries, how berry K requirements are determined, and how K is translocated into and out of the berry, while the available information is still unknown and limited. This review summarizes the potassium nutrition from the distributions, functions of potassium, potassium characteristics and uptake in grape, emphasis on the berry accumulation and relative impact factors, to better provide the basis for regulating nutritional balance in grape berries and producing scientifically.