Developing a rapid method for determining grapevine nitrogen status

Optimisation of nitrogen (N) supply and other nutrients in vineyards is critical to quality and quantity of fruit and wine production. However, there are major challenges to balance N availability for canopy growth in spring, reproductive development, fruit maturity rates and optimum grape composition at harvest. Therefore, frequent monitoring of nutrient status is essential to better predict and target inputs and consequentially maximise profitability. Rapid and convenient assessment of grapevine N status in the field can help identify N requirements during the season. A replicated experiment was undertaken in a glasshouse using two-year-old 'Shiraz' vines from dormancy to veraison that were pruned back to four two-bud spurs. The vines were previously water stressed, re-potted in washed river sand and then placed into a cool room at approximately 5-6°C for 14 days. From the end of dormancy, different nutrient solutions were applied to create a range of leaf nutrient levels for N, supplying approximately 100, 50 and 0% of N required for the bud burst to veraison period. Spectrometer measurements (reflectance) and leaf samplings were conducted at three times (at flowering, four weeks before and after) and three locations along the shoots (opposite the basal bunch, and two and four nodes above the basal bunch). The leaves were washed, separated into blades and petioles, ground and analysed for N concentration. The reflectance, particularly around 550 nm, showed considerable difference between N treatments, and these were more pronounced on the older leaves and later in the season. Leaf and petiole N concentrations were closely related, allowing the calibration of the indirect measurements with a spectrometer to establish petiole standards for flowering. Better understanding of changes in leaf N concentrations during vine development will allow for more accurate predictions of N requirements throughout the growing season.