Effect of Nitrogen Foliar Application On Translocation of 15 N and 13 C in Seyval Blanc Grape

Nitrogen availability is critical to support canopy development in the early phenological phases of grapevines. It is also necessary to maintain photosynthetic efficiency and productivity of the canopy later in the season. At veraison, when cluster sink activity is elevated, it is fundamental to ensure high leaf source productivity to sustain clusters during rapid growth. Foliar applications can increase leaf nitrogen content, stimulate vegetative growth and modify the vine balance. A field experiment was carried out at the Horticulture Research Center of Michigan State University (East Lansing, MI) to evaluate the effect of a foliar spray of nitrogen on the carbon and nitrogen partitioning at veraison. Single cordons of Seyval blanc vines were thinned after fruit set in two separated group of 4 shoots on each cordon, and cropload was manipulated to high and low (2 or 6 cluster per 4 shoots, respectively). At veraison low and high cropload shoots were treated with a low rate nitrate solution (0.5% KNO₃, 10% ¹⁵N), and with labeled ¹³CO₂. Methods described by Kappes and Flore (1989) were used to pulse ¹³CO₂. Shoot growth rate, final shoot length, leaf area and yield per shoot were not affected by the level of cropload imposed. Nitrogen application did not increase shoot growth or photosynthetic rates post-veraison. Basic fruit chemistry parameters (brix%, pH, TA) were not significantly affected by cropload levels, or nitrogen application. Mature leaves, apical leaves and berries were sampled at harvest (30 days after veraison) for a ¹³C and ¹⁵N enrichment analysis. All tissues sampled in labeled shoots had increased percentages of enrichment for both ¹⁵N and ¹³C when compared to control shoots (non labeled). Within labeled shoots, translocation of ¹⁵N and ¹³C from mature leaves to berries and to apical leaves was not affected by the cropload. Non labeled shoots with high and low cropload did not show a significant difference in enrichment of both ¹⁵N and ¹³C. We suggest that vegetative sinks and fruit quality parameters could be affected by a redistribution of photosynthetates within the vine that compensated according to the sink demand of each single shoot. However, the pattern of distribution of stable isotopes does not support this hypothesis, rather suggesting that translocation of both carbohydrates and nitrogen is limited to the single shoot.