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Foliar urea applications can increase berry yeast-assimilable N in wine grapes (Vitis vinifera L.)

Neilsen, G.H., Neilsen, D., Hannam, K., Millard, P., Midwood, A.
Acta horticulturae 2013 no.984 pp. 427-433
Vitis vinifera, amino acids, diammonium phosphate, fermentation, foliar spraying, fruit yield, grape juice, soil, soil treatment, urea, urea nitrogen, vineyards, wine grapes, British Columbia
Wine grapes (V. vinifera L.) in western North America commonly have deficient yeast-assimilable N concentrations (YANCs), necessitating the addition of diammonium phosphate to the must prior to fermentation. To examine strategies for alleviating this condition, N treatments were applied three times, centered around veraison, in replicated trials on seven commercial vineyards in the Okanagan Valley of British Columbia, Canada. In 2010, treatments included: 1% or 2% (w/v) foliar urea sprays; urea-N applied to the soil at the same rate as the 2% foliar spray; and a combination treatment in which half the N was applied as a foliar spray and half was applied to the soil. In 2011, an efficiency-enhanced soil N product was also applied. Treatments had few effects on fruit yield or berry quality but grape juice YANCs were significantly increased by the 2% foliar spray at six of the seven sites in both years. After 2% foliar spray application in 2010, grape juice YANCs approached but did not exceed the 140 mg N/L threshold generally considered sufficient to avoid stuck fermentation. After 2% foliar spray application in 2011, however, grape juice YANCs exceeded this threshold at four sites. The 2% foliar spray also reduced the ratio of proline:arginine (an indicator of N quality) in grape juice at most sites. Soil applications of N were infrequently effective at improving grape juice YANC at the rates applied. Ongoing work is using 15N-labelled urea to determine which grape juice amino acids are enhanced by N application treatments.