Improved Tissue Analysis Methods for Nitrogen Assessment of Wine Grape

The limitations of petiole nitrate-N as a criteria for vine N status are widely recognized. The purpose of this study is to search for improved N tissue sampling and analytical methods which can be used for many wine cultivars under different growing conditions. Total-N and nitrate-N levels are being compared in leaf petiole and blade samples taken at bloom, veraison and harvest in 7 cultivars–French Colombard, Chenin blanc, Ruby Cabernet, Barbera, Grenache, Chardonnay, and Cabernet Sauvignon. All of the trial blocks are located at the UC Kearney Agricultural Center except for Chardonnay and Cabernet Sauvignon which are on the Central Coast. A wide range of N fertilizer treatments are being imposed in order to establish large differences in vine N status and potential plant response. Fertilizer treatment was initiated one year ahead of the beginning of data collection to provide carry-over N in the vines. Significant differences in N determinations for each tissue and sampling stage from N fertilizer treatment were found in 5 cultivars at Kearney, with the exception of bloom blade total-N. This tissue and stage was not significantly different for total-N in 4 cultivars?Barbera, Grenache, French Colombard and Chenin blanc. Thus far, bloom blades have shown the least promise as an indicator of differences in N status. There was a tendency for the veraison and harvest samples to show greater significant differences in N status as compared to bloom sampling. As expected, nitrate-N has most often shown the greatest range in values from the low to the high N treatments. However, total-N showed as much statistical separation as nitrate-N by the Duncan’s Multiple Range Test. This indicates the possibility of developing useful critical values for total-N, as well as the traditional nitrate-N. Also, petioles tend to show as good, if not better, statistical separation for total-N as compared with blades. This is encouraging, as it would be very useful to be able to use petioles rather than blades because of value of petiole samples for other nutrient determinations. Some vine yield and fruit composition components have shown significant differences due to fertilizer treatment. This provides the opportunity to correlate vine response with leaf tissue N values. Correlation and regression analyses are being performed after 2 full years of vine and laboratory data. The goal is to develop some tentative critical values for total-N and/or nitrate-N for important wine cultivars. The two years of data from the Chardonnay and Cabernet Sauvignon trials show minor or no differences. Thus, they are too preliminary for any conclusions at this time. One more year of data collection is planned for possible N treatment differences. The Barbera, Grenache, French Colombard, Chenin blanc, and Ruby Cabernet trials are now complete. We are presently conducting correlation and regression analyses on the data to compare plant tissue, time of sampling and type of N analysis for the most effective diagnostic method.