Increasing nitrogen availability at veraison through foliar applications: Implications for leaf assimilation and fruit ripening under source limitation in 'Chardonnay' (Vitis vinifera L.) grapevines

Viticulture in Michigan is often limited by cool and humid climate conditions that impact vine growth and the achievement of adequate fruit quality at harvest. Sugars, pH, acids, and yeast available nitrogen (YAN) are indices of quality and, as such, of suitability for wine production. The aim of this study was to evaluate the efficacy of foliar nitrogen (N) fertilization applied as a 1% w/v urea solution at veraison as a method to increase canopy N availability during the fruit ripening stage. To test the effect on different source sink conditions, we imposed three levels of defoliation (0%, 33%, and 66% of leaves removed per vine) and measured net photosynthetic rate (Pn), leaf efficiency parameters, yield components, and fruit quality parameters. Apical leaf Pn was increased by the 33% defoliation (+12% from the undefoliated control) and by the urea application (+6%) 2 weeks after veraison. In basal leaves we observed a reduction in chlorophyll content (SPAD) and maximum photochemical efficiency of PSII (Fv/Fm) as a result of the defoliation treatment and secondarily by the N application, which resulted in a reduction in Pn. Therefore, mean shoot Pn was unaffected by the treatments. Although neither main nor lateral shoot growth was increased by any defoliation treatment, both percent soluble solids (%SS) and berry weight were significantly reduced by the 66% defoliation treatment. Application of urea increased yeast available amino acids by 20% but did not impact %SS or other chemical parameters indicating a different accumulation pathway for sugars and amino acids in the berry.