Amino Acid Composition of Grape (Vitis vinifera L.) Juice in Response to Applications of Urea to the Soil or Foliage

Abstract

Veraison-time applications of nitrogen to vineyard foliage or soil have been shown to improve grape juice yeast assimilable nitrogen concentrations and may prevent the excessive vine growth, delayed maturity, and adverse changes in fruit properties sometimes associated with high applications of N earlier in the growing season. However, the consequences of late-season foliar- and soil-applied nitrogen for grape juice yeast assimilable nitrogen (YAN) and, specifically, grape juice amino acid profiles have rarely been directly compared. Over a two year period in a drip-irrigated Merlot vineyard and Pinot gris vineyard, grape juice amino acids were measured from vines to which urea had been applied three times around veraison at a rate of 3.8 g/vine to either the foliage or the soil surface. Foliar-applied urea (applied as a 2% w/v solution) was usually more effective at boosting grape juice ammonium, and amino acid concentrations, although soil-applied urea improved some grape juice amino acids at the Pinot gris site. Changes in the amino acid profiles of grape juice, observed in response to foliar N applications but not soil N applications, may have implications for wine quality. Applications of ¹⁵N-labelled urea at the Pinot gris site demonstrated that a greater quantity of fertilizer N was incorporated into grape juice amino acids when urea was applied to the foliage than when it was applied to the soil surface. Late-season foliar applications of urea are a reliable, efficient and effective method of improving grape juice YAN. Further work is required to examine how treatment effects vary among sites, cultivars and under different management practices and to understand the implications of altered grape juice amino acid profiles for wine quality.