During the period of 2003 to 2007 my laboratory examined the concept of nitrogen use efficiency for Merlot on two wine grape rootstocks, Vitis berlandieri x V. rupestris cv 1103P and V. riparia x V. rupestris cv 101-14 Mgt. Both are extremely important rootstocks of relatively wide distribution in the North Coast and other grape growing regions of California. There are numerous ways one might define nitrogen use efficiency (NUE). The classical definition in physiology generally concerns the instantaneous rate of photosynthetic carbon dioxide assimilation per unit of leaf nitrogen. We defined NUE in more agronomic terms concerning the quantity of nitrogen (N) absorbed and its influence on total above ground productivity and yield, plus N translocated into the fruit, and therefore must, and its influence on fermentation dynamics and wine quality. Further, the quality of must N was evaluated in terms of its amino acid composition and yeast assimilable nitrogen content (YAN). Two N fertilization regimes were sustained that generally consisted of application of 17 or 35 kg N per hectare in either spring or fall following harvest, or no nitrogen applied. These treatments are important since storage in trunks and roots, atmospheric N deposition and soil mineralization pools of N could supply sufficient N for sustainable viticulture in this region, but information is lacking concerning longterm N fertility experiments.
Overall, our results indicated that the two rootstocks differed vastly in their ability to acquire N and translocate it to fruit and must. Rootstock 1103P generally had more than two times the concentration of amino acids and thus two times YAN content in must as 101-14 Mgt. This held true for every vintage where amino acids and YAN were evaluated, and was independent of N fertilization rate or whether the N fertilizer was applied in Spring or Fall. Changes in AA contents generally resulted in more substantial alterations to the glutamine family of amino acids (glutamate, glutamine, arginine, proline) and this has provided background for a major research initiative and collaboration being led by Dr. Douglas Adams with respect to its influence on phenolics metabolism. Fermentation dynamics were much faster for fruit harvested from 1103P in comparison to 101-14 Mgt. The combined effect was that N had a major influence on wine quality irrespective of irrigation. Our results ultimately indicate that a re-evaluation of the UC critical values concept of vine N nutrition in favor of an approach based on the nutrient budget concept (Bowen 1990).