Variety specific thresholds for plant-based indicators of vine nitrogen status

Aim: Several plant-based indicators of vine N status are reported in the literature. Among these, yeast assimilable nitrogen in grape must (YAN) and total N concentration of petiole and leaf blades are considered to be reliable indicators and so is the chlorophyll index, measured with a device called N-tester. The N-tester index is used to measure the intensity of the green colour of the leaf blade, and therefore to estimate its chlorophyll content. The aim of this study is to measure the nitrogen content of various grapevine organs (petiole, leaf blade, grape must) and the intensity of the green colour of leaf blades, in order to establish variety specific thresholds for the interpretation of plant-based indicators of vine nitrogen status.

Methods and Results: To study the varietal effect on indicators of vine N status, the latter were measured during 4 years on 35 grapevine varieties grafted on the same rootstock and planted with replicates in an experimental vineyard in the Pessac-Léognan appellation in Bordeaux. The results of N-tester measurements carried out at mid-flowering and mid-véraison were compared with the nitrogen content of leaf blades and petioles at véraison and the concentration of yeast assimilable nitrogen (YAN) in the must at maturity. 

Conclusions: 

Strong varietal and year effects were observed for all indicators. Leaf blade nitrogen showed the lowest variability and YAN the highest. The N-Tester values recorded at mid-flowering were more consistent than those at mid-véraison.

Significance and Impact of the Study: Among the nutrients required by the vine, nitrogen is one of the most important. It is an essential factor in vegetative and reproductive development. Vine nitrogen status influences grape composition and wine quality. In addition, a low concentration of assimilable nitrogen in the must causes fermentation problems because N is one of the essential substrates for yeast growth. Vine N status depends on environmental factors (soil and climate) but can be managed through fertilisation and vineyard floor maintenance. Hence, plant-based indicators for vine nitrogen status are of utmost importance to optimize management practices for obtaining high wine quality and sustainable yields. The data generated by this experiment can help to take into account varietal specific responses to nitrogen availability when establishing thresholds for plant-based indicators of vine N-status. An example is provided for N-tester values at mid-flowering.