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  • Author or Editor: Letizia Tozzini x
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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.

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Achieving desired fruit quality at harvest in cool climate conditions is a challenge, especially for red varieties, and the typical inability of fruit to reach technological maturity is a critical contributing factor requiring examination. To probe this issue, this research investigated the impact of two levels of crop thinning and of basal leaf removal at three phenological stages in the 2011 and 2012 growing seasons in Michigan. Experiments were conducted at the Southwest Michigan Research and Extension Center (SWMREC) in Benton Harbor. Using ‘Cabernet franc’ (Vitis vinifera L.) vines, yield components (yield per vine, pruning weight, and cluster and berry weight) and basic fruit composition traits [total soluble solids (TSS), pH, titratable acidity, anthocyanins, and phenolics) were studied to investigate the effect of cluster thinning and basal leaf removal on vine performance and fruit quality at harvest. Neither of the treatments significantly impacted TSS in either of the two seasons. Cluster thinning treatment successfully altered cropload ratio, indexed as Ravaz Index (RI), independently of the time of application. Basal leaf removal increased exposed berry temperature, cluster light exposure, and subsequent anthocyanin and phenolic content of the berry in both seasons, again independent of application date, whereas cluster thinning was effective only in 2012. Crop thinning coupled with basal leaf removal resulted in an increased efficiency in anthocyanin accumulation in relation to TSS accumulation, expressed as anthocyanin:sugar, in both years. This is significant because it offers potential for vineyard management practices aiming to improve fruit quality in cool climates where the onset of anthocyanin accumulation could be reduced and decoupled from sugar accumulation.

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