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 was 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 (NO3-N) levels were compared with 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 were located at the UC Kearney Agricultural Center except for Chardonnay and Cabernet Sauvignon that were on the Central Coast. A wide range of N fertilizer rates was 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. Bloom blades showed the least promise as an indicator of differences in N status in these trials. There was a tendency for the veraison and harvest samples to show greater significant differences in total-N status as compared to bloom sampling. Nitrate-N most often showed the greatest range in values from the low to the high N treatments. However, total-N showed as much statistical separation as NO3-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 NO3-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. Over half of the vine yield and fruit composition components showed 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 conducted on all of the data with goals to determine the best timing and tissue-type for N status and to develop tentative critical values for total-N and/or NO3-N for each cultivar. Four years (1996-99) of N fertilizer treatment with 3 years (1997-99) of data collection from the Chardonnay and Cabernet Sauvignon trials have shown minor or no differences. The Chardonnay vineyard was unresponsive in all measurements, indicating adequacy in N status ans/or lack of vine N uptake. The Cabernet Sauvignon vineyard showed no vine growth or fruiting response, but demonstrated significant differences in vine N status due to treatment. This included leaf petiole and blade levels of NO3-N and total N at the various vine stages.