Yeast assimilable nitrogen (YAN) in the grape must is a key variable for wine quality as a source of aroma precursors. In a situation of YAN deficiency, a foliar urea application upon the vine at veraison enhances YAN concentration and facilitates must fermentation. In 2013, Agroscope investigated the impact of leaf-fruit ratio on the nitrogen (N) assimilation and partitioning in grapevine Vitis vinifera cv. Chasselas following foliar-urea application with the aim of improving its efficiency on the YAN concentration. Two factors, canopy height with two levels (90 and 140 cm), and crop load with two levels (5 and 10 clusters per vine), were combined in a split plot trial (5 vines per treatment). All treatments received 20 kg/ha of 15N-labelled foliar urea (10 atom% 15N) at veraison. An extra 5-vine control treatment (150-cm canopy and 5 bunches per vine) received no foliar urea. As a result, the leaffruit ratio had a strong impact on the grape maturity at harvest and on the labelled-N partitioning after urea application. The YAN varied from 143 ± 17 mg/L when the leaf-fruit ratio was 1.6 m2/kg (light exposed leaf area / fruit quantity), up to 230 ± 25 mg/L when the leaf-fruit ratio was 0.4 m2/kg. The grapes were the strongest sink of all the vine organs, with more than 20 % of their total organic N originating from the urea treatment. Whereas a too small leaf-fruit ratio affected the grape maturity and the accumulation of labelled N in the reserve organs, a large canopy induced a diminution of the total N concentration (% dry weight) in all organs comparable to a “dilution” in the plant. Thus a balanced leaf-fruit ratio – between 1 and 1.5 m2/kg – should be maintained in order to guarantee the grape maturity, the accumulation of YAN in the must and the storage of N in the reserve organs. This study fosters further research at the isotopic molecular level to unravel other mechanisms controlling the source-sink relationship and the specific N partitioning between grapevine organs.
