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98 WINES&VINES November 2016 GRAPEGROWING PRACTICAL WINERY & VINEYARD For each variety grafted on Ramsey at the Loxton site, the mean percentage of total berry K content in skin, pulp and seed was 41.5%, 53% and 6%, respectively. 20 On a concentration basis, because skins are only a small percentage of total berry weight, K concentrations can be much higher in berry skins than in pulp. For example, in fruit from Padthaway, South Australia, and Merbein, Victoria, skin K con- centrations were 4.0 to 4.5 times higher than in the pulp for Chardonnay and 4.0 to 5.5 times higher than in pulp for Shiraz. 6 Potassium concentration in fermenting musts In a study involving four varieties during fermentation on skins, there was a 50% to 120% increase in must K concentration during the first two or three days of fermentation, after which concentrations plateaued over the next 10 days. During juice fermentation (no skin contact), the increase over the same period was smaller (0% to 38%), followed by a plateau or decline. 20 This suggests that sig- nificant K is released from skins into the must during early stages of fermentation. The subsequent plateau in must K concentrations may be explained by pressing off the skins after three days. The R.R. Walker et al. study 20 also showed that K concentration in ferments was higher from Ramsey-grafted vines than from own- rooted vines, most likely reflecting higher K concentrations in the pulp and skin of berries from Ramsey-grafted vines than from own- rooted vines, as demonstrated in the study by H. Gong et al. 6 Juice K/wine K relationship in red and white wine Using data obtained from the study of Char- donnay and Shiraz grown on own roots and grafted to eight different rootstocks at four different sites in Australia, and where wine was made from grapes harvested at the various sites, it was observed that K concentrations in wines (measured on average 10 months after completion of vinification) showed a decrease of more than 50% for white wines and around 20% for red wines relative to concentrations in grape juice. This agrees with observations by T.C. Somers, 18 although recently for Shiraz, we observed a wide range, from increases of around 20% to decreases of around 40% in K concentrations between grape juice and wine. The main influencing factors are likely to involve formation and precipitation of potas- sium bitartrate during vinification, 3,4 extraction of K from skins and time on skins during the making of red wines, 20 and possible adsorption of K on pomace. 7 Summary Potassium is essential for vegetative growth and berry development. There is an interac- tion between rootstock, scion, management and environment in determining tissue con- centrations. Grape juice pH is positively cor- related with grape juice K, and high grape juice pH requires more pH adjustment in winemaking with higher cost to bring to re- quired levels. Potassium accumulates in berry pulp and skins, and leakage of potassium from skins to must during fermentation on skins contributes to higher wine potassium and pH. Higher pH correlates with higher color hue, which is associated with less bright wines. Canopy management to reduce excessive shade—and monitoring of soil and plant potas- sium to ensure optimum but not excessive supply—is essential for sustaining plant health and production. Rootstocks selected for potas- sium uptake at the lower end of the adequate range may assist to achieve low grape and wine potassium concentrations. Rob Walker, chief research scientist at CSIRO Agricul- ture, Waite Campus in Urrbrae, South Australia, spe- cializes in plant response to abiotic stress, plant salt tolerance and salt exclusion, grapevine potassium partitioning and function. Peter Clingeleffer, chief re- search scientist at CSIRO Agriculture, specializes in grapevine scion and rootstock breeding, grapevine management and the interaction between genotype, environment and management. This text was edited from the original publication in the March 2016 issue of Australian & New Zealand Grape- grower & Winemaker, with permission of Winetitles. The authors acknowledge support from CSIRO and the Grape and Wine Research and Development Corp. (now Wine Australia). Grape juice pH is positively correlated with grape juice K, and high grape juice pH requires more pH adjustment in winemaking with higher cost to bring to required levels. References for this article are available online at winesandvines.com. SUBSCRIBE ONLINE & SAVE! winesandvines.com/subscribe Print + Digital Magazine 12 monthly issues a year for $28 winesandvines.com/subscribe WINESANDVINES.COM OCTOBER 2016 Packaging Design Winners Reorganizing a Barrel Cellar Under-Trellis Cover Crop IN THE BLACK Dark Designs Evoke Elegance