The Relative Contribution of Nitrogen Originating From Two Seasonal Supplies to the Total Nitrogen Pool Present in the Bleeding Sap and in WholeVitis viniferacv. Pinot noir Grapevines at Bloom Time

Abstract

The temporal mobilization of nitrogen reserves and their relative contribution to whole plant growth, have been studied in grapevines (<i>Vitis vinifera</i> L. cv Pinot noir). Four-year-old grapevines received in addition to nutrient medium the isotopic tracer ¹⁵N (as ¹⁵N-NO₃K) as a ten fold increase of the natural abundance, for a three week period. Two periods of ¹⁵N application were made either three weeks before leaf fall (in autumn) or three weeks preceding the beginning of shoot growth (at early spring). After collection of the spring xylem sap, at pruning-time, the same plants were sampled at bloom in order to determine both the amount and the redistribution of the isotopic tracer in the different parts of the plant. During the bleeding period, the autumnal N reserves represented 4% of the total pool of nitrogen present in the xylem sap, while 40% of the same pool originated from the spring supply. At flowering time, high amounts of "autumn" nitrogen remained stored in the perennial parts (30% of the total "autumn" storage), whereas the figure was only 17% in the case of spring ¹⁵N feeding. Thus, a major part (83%) of the N assimilated at spring was used to ensure the new shoot growth, and almost 60% was accumulated in the subapical part of these shoots. The relative specific allocation (RSA) values for N compounds originating from autumn or spring assimilation periods were calculated in the different parts of the grapevine at flowering time. The highest values of RSA observed in the base and the clusters indicated that they were fed preferentially at their earlier stages of growth by a highly labeled xylem sap. The lower values of RSA evidenced in the subapical part of the shoot, which developed later, could be explained by an important dilution of spring or autumn labeled nitrogen by unlabeled N compounds recently synthetized in the course of shoot elongation. This experiment underlined the importance of the stored nitrogen, particularly "spring" nitrogen, to ensure the first steps of new shoot growth until the leaves of the shoot base have been fully expanded.