Leaf-to-fruit ratio affects the impact of foliar-applied nitrogen on N accumulation in the grape must

摘要

Aims: Agroscope investigated the impact of the leaf-to-fruit ratio on nitrogen (N) partitioning in grapevine following a foliar urea application with the aim of increasing the yeast assimilable nitrogen (YAN) concentration in the must. Methods and results: Foliar urea was applied to field-grown Vitis vinifera L. cv. Chasselas grapevines as part of a split-plot trial with two variable parameters: canopy height (90 or 150 cm) and fruit load (5 or 10 clusters per vine). Foliar application of 20 kg/ha of 15N-labelled urea (10 atom% 15N) was performed at veraison. The isotope labelling method allowed to observe foliar-N partitioning in the plant at harvest. The leaf-to-fruit ratio varied between 0.4 and 1.6 m2/kg, and strongly impacted the N partitioning in the grapevines. Total N and foliar-N partitioning was mainly affected by the variation of canopy height. The YAN concentration varied from 143 to 230 mg/L (+60 %) depending on the leaf area. An oversized canopy (+31 %DW) induced a decrease in the total N concentration of all organs (-17 %), and a decrease in YAN quantity in the must in particular (-53 %). A negative correlation between the N concentration and the carbon isotope discrimination (CID) could be pointed out in a condition of no water restriction (e.g., R2 = 0.65 in the must).Conclusion: An excessive leaf area can induce YAN deficiency in the must. Thus, a balanced leaf-to-fruit ratio – between 1 and 1.2 m2/kg – should be maintained to guarantee grape maturity, YAN accumulation in the must and N recovery in the reserve organs. Significance and impact of the study: The results of this study encourage further research to understand the role of other physiological parameters that affect N partitioning in the grapevine – YAN accumulation in the must in particular – and add new perspectives for N management practices in the vineyard. IntroductionNitrogen (N) represents 1 to 4 % of the dry weight of the vine and plays a key role in the plant development, as a component of proteins, DNA and chlorophyll. The optimization of N fertilisation practices in viticulture is required for quality and sustainability of the production while preserving the environment (Champagnol, 1984). In particular, the concentration of yeast assimilable nitrogen (YAN) in the must at harvest is a relevant parameter used to determine wine quality because both its concentration and composition affect the alcoholic fermentation and the formation of aroma compounds (Rapp and Versini, 1991; Bell and Henschke, 2005). Extreme YAN deficiency can even induce atypical ageing off-flavours in wine (Linsenmeier et al., 2007; Reynard et al., 2011). The YAN concentration is usually enhanced by the addition of diammonium phosphate to the must to improve the fermentation kinetics, but it does not appear to have any beneficial effect on wine aroma (Lorenzini and Vuichard, 2012). Indeed, the main source of aroma precursors in the must are free amino-acids (AA), which represent approximately 80 wt.% of the YAN (Ribéreau-Gayon et al., 2004). Hence, it is necessary to correct the YAN concentration, including the AA concentrations, early in the season through the foliar application of urea to grapevine canopies (Lacroux et al., 2008; Dufourcq et al., 2009; Hannam et al., 2013; Nisbet et al., 2014). However, Spring et al. (2011) observed cases of N-deficient musts produced from vigorous grapevines, despite a high level of N in the soil and an absence of water restriction. Knowing that N supply impacts the development of biomass and N allocation in the vine (Metay et al., 2014), the uptake and subsequent translocation of N are key processes in the development of good wine after the application of foliar fertiliser (Porro et al., 2010). As a consequence, the technical and physiological parameters that can improve the efficiency of foliar urea fertilisation, such as application timing (Conradie, 2005; Lasa et al., 2012), canopy height and yield (Murisier and Zufferey, 1997; Spring et al., 2011), must be optimized to increase the YAN concentration in the must at harvest. The YAN concentration in must was found to be higher when urea was applied to the vine at veraison (Verdenal et al., 2015). Though, very few studies have determined the impact of the leaf-to-fruit ratio on N partitioning with a focus on the YAN accumulation in the must (Kliewer and Ough, 1970; Schreiber et al., 2002; Peyrot des Gachons et al., 2005). The leaf-to-fruit ratio, i.e., the light-exposed leaf area per fruit quantity (m2/kg), is known as an essential parameter in grape growing: on one hand, the leaf area, as a source of nutrients, affects the leaf gas exchanges and the quantity of carbohydrates available through photosynthesis for vegetative growth and grape maturation; on the other hand, the grapes, as a sink of nutrients, affect the quantity of C and N required for their maturation (Murisier and Zufferey, 1997; Morinaga et al., 2003; Kliewer and Dokoozlian, 2005; Etchebarne et al