Foliar nitrogen composition and application timing influence nitrogen uptake by, as well as partitioning within, two grapevine cultivars

Foliar spraying of fertilizers is a standard practice used in vineyards and orchards to overcome temporary mineral element deficiencies and improve fruit quality. The authors conducted a field study to compare grapevine uptake and partitioning of foliarly applied nitrogen (N). Three solutions containing equivalent total N amounts of isotopically-labeled N but with varying composition (NH4, NH4+NO3, or NO3) were applied at berry set or at veraison to leaves of mature 'Cabernet Sauvignon' and 'Chardonnay' grapevines. The shoots with the treated leaves were sampled 8 d after application, dissected into leaf, woody stem, and berry cluster components, and analyzed for total and isotopic N contents. The differential N treatments had little effect on shoot growth of either cultivar; however, N uptake from the NH4-containing treatments was higher than from the treatment containing only NO3. The leaves contained substantially greater amounts of N than did the woody stems and berry clusters at both sampling times. The interaction between cultivar and timing of application was significant. 'Chardonnay' woody stems contained higher amounts of labeled N when the N was applied at veraison than at berry set, while 'Cabernet Sauvignon' showed little difference. Leaf green color, measured using a SPAD meter, reflected relative leaf N status. 'Chardonnay' leaves had lower labeled N content and were less green than those of the 'Cabernet Sauvignon' leaves when foliar N was applied at berry set, while they contained more labeled N and were greener than 'Cabernet Sauvignon' leaves following N application at veraison.