Impact of reduced post-veraison leaf area on the relationship between petiole and fruit composition

Petiole nutrient concentrations are widely used to assess the nutritional status of grapevines. Nitrogen (N) allocation to ripening berries contributes to the juice yeast assimilable N (YAN) concentration. In addition to supplying the ripening berries with assimilates, leaves can contribute as N source towards berry N accumulation during maturation. The aim was to evaluate the metabolite composition of petioles during berry ripening; to gain an enhanced understanding of the translocation of leaf sourced nitrogenous compounds and soluble sugars towards the berries. Potted grapevine leaf area was adjusted to 25 leaves per vine shortly after veraison, and compared to a full leaf area control (100 primary leaves). Petioles and berries were sampled every 9-10 days between veraison and harvest, and the leaf area and berry fresh weight per vine, and juice YAN and total soluble solid (TSS) concentrations determined. Total N concentration was assessed in the petioles, while evaluation of sugars, sugar alcohols and amino acids were conducted through GC/MS. Although not affecting the berry fresh weight, defoliation increased juice YAN concentrations, and YAN was positively related to petiole N concentrations. Several amino acids, predominantly GABA, accumulated in remaining petioles after defoliation. Berry TSS accumulation continued after defoliation, albeit at a reduced rate. Sucrose and sugar alcohols (myo-inositol, mannitol and dulcitol) depleted rapidly in remaining petioles following defoliation, while hexoses accumulated in petioles under full leaf area. This study suggests that limiting the post-veraison leaf area may be beneficial towards juice YAN accumulation. Furthermore, evaluating the petiole N concentration during berry ripening could be used to estimate juice YAN development. Limiting the post-veraison canopy leaf area additionally enhances assimilate distribution towards berries, as sucrose and sugar alcohols, from the remaining leaves to contribute to berry sugar accumulation.