Nitrogen Status and Fermentation Dynamics for Merlot on Two Rootstocks

Adequate yeast assimilable nitrogen (YAN) levels in grape juice are necessary for yeast cells to complete fermentation to dryness. Nitrogen (N) uptake by grapevine roots varies seasonally; therefore, environmental conditions and cultural practices can affect grapevine N status. In addition, genetic differences between rootstock cultivars can influence root dynamics and, subsequently, N uptake, canopy biomass, and fruit composition. Two rootstock cultivars, 1103P and 101-14 Mgt, were fertilized with nitrogen during spring or fall or received no treatment. Vine biomass, leaf N concentration, fruit composition, juice amino-N levels, and fermentation kinetics were measured. The rootstock 1103 Paulsen (Vitis berlandieri x V. rupestris cv. 1103P) has a root system that tends to produce large canopies and high shoot growth. The rootstock 101-14 Millardet et de Grasset (V. riparia x V. rupestris cv. 101-14 Mgt) has a root system associated with smaller canopies and moderate shoot growth. The scion Merlot (V. vinifera L. cv. Merlot clone 1) was grafted onto the two rootstocks in an experimental block in Oakville, California. Merlot on 1103P had higher YAN levels and completed fermentation faster compared to Merlot on 101-14 Mgt. Differences in fermentation kinetics were observed within rootstock N treatments that were not explained by YAN levels, indicating that other factors related to N metabolism may play important roles in fermentation dynamics. Results indicated that Merlot grown on 1103P in the Napa Valley may require little to no N supplementation while Merlot on 101-14 Mgt may require N supplementation to avoid slow fermentations.