Effect of potassium and nitrogen supply on the distribution of minerals and organic acids and the composition of grape juice of Sultana vines

Fruiting Sultana vines grown under glasshouse conditions were supplied with 2 K+ levels (0.05 or 0.15 g K+/week) and fertilised with either NH4+ [as (NH4)2SO4] or NO3- [as Ca(NO3)2] as a nitrogen source. Enhanced K+ supply (0.15 g/week) increased K+ concentration in laminae by 1896, in petioles by 59% and in stems by 23%, and reduced Mg2+ of laminae and petioles by 18 and 24% respectively, while Ca2+ was not effected. K+ supply had no effect on tartrate or malate concentration in laminae, stem or roots. Use of NO3- instead of NH4+ as a nitrogen source led to 220% higher citrate and 168% higher malate concentrations in laminae. In roots NO3- supply caused 61% higher citrate, 41% higher tartrate and 26% higher malate concentrations. Higher K+ fertilisation (0.15 g/week) had no effect on fruit yield or sugar content, but increased grape juice pH from 3.95 to 4.08, malate concentration from 0.94 to 1.20 g/ L and K+ concentration from 46.8 to 56.8 mmol/L, thereby causing lower grape juice quality.