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November 2016 WINES&VINES 95 PRACTICAL WINERY & VINEYARD GRAPEGROWING ies to compare with these greenhouse studies on K translocation efficiency, since destructive harvest and analysis of whole vines from the field is required. Accumulated K in grapevines can be re- translocated to other plant parts. This was demonstrated by S. Kodur et al. in studies using rubidium (Rb) as an analogue of K that involved an initial loading of Rb into leaves. 11 For example, concentrations of Rb in leaves decreased significantly during a 48-hour pe- riod after loading and increased significantly in lateral shoots, stem and roots (there were no clusters on these vines). W.J. Conradie provided evidence that leaves, shoots and roots could each potentially contribute to ac- cumulated K in bunches. 2 Potassium accumulation is also higher with higher soil K supply. For example, in an experiment involving fruiting Sultana grape- vines in a greenhouse receiving either low K supply (0.05 g of K per week as KH 2 PO 4 ) or high K supply (0.15 g of K per week as 0.05 g of KH 2 PO 4 and 0.10 g of K 2 SO 4 ), the high K supply vines accumulated significantly higher concentrations of K (56.8 mmol/L) and malate (1.20 g/L) and had higher pH (4.08) in grape juice relative to low K supply vines (46.8 mmol/L K, 0.94 g/L malate and 3.95 pH). Tartaric acid concentrations in grape juice were similar between high and low K supply vines. 16 Critical role of potassium in plant stress response Today there is an emerging appreciation of K's role in plant resistance to biotic and abi- otic stresses. Balanced fertilization and effi- cient K usage in combination with other nutrients not only contribute to a crop's growth, yield and quality, they also influence plant health and reduce the impacts from environmental stresses. 24 Canopy effects on K accumulation Fruit from vines subjected to different shad- ing treatments were shown to have the high- est K concentrations at harvest in the most heavily shaded treatment and lowest in the totally exposed treatment. 14 Results sug- gested that the effect on fruit K concentra- tion was driven more by leaf shading than cluster shading. Further, high-must K has been shown to be associated with a shaded microclimate. 17 It is well known that the degree of shading in the canopy increases as vine vigor increases. 13 In an attempt to compare K uptake and growth characteristics between a relatively low-vigor rootstock (110 Richter) and a rela- tively high-vigor rootstock (Rupestris St. George), B.A. Swanton and W.M. Kliewer used a flowing nutrient solution system to make the comparison. 19 Total root surface area and leaf blade K (percent weight basis) were higher in Rupestris St. George than in 110 Richter. They used total quantity of K taken up divided by the total transpiration volume, termed xylem K approximation, as a non- destructive measure of xylem sap K. They found that this was significantly higher in Rupestris St. George than in 110 Richter and a better indicator of the foliage K concentra- tion than K influx (K uptake per cm 2 root surface area per hour). Fruit was shown to have the highest K concentrations at harvest in the most heavily shaded treatment and lowest in the totally exposed treatment. S U P P O R T R E S E A R C H & W I N E I N D U S T R Y N E E D S T H R O U G H T H E AMERICAN VINEYARD FOUNDATION AMERICAN VINEYARD FOUNDATION Finding Solutions Through Research P.O. Box 5779, Napa, CA., 94581 • T: (707) 252-6911 • Visit our web site at www.avf.org for information on funding and current research projects For a wealth of useful viticulture and enology research and information, visit AVF.org, ngwi.org, ngr.ucdavis.org, asev.org, or iv.ucdavis.edu For a wealth of useful viticulture and enology research and information, visit AVF.org, ngwi.org, ngr.ucdavis.org, asev.org, or iv.ucdavis.edu