Plant nitrogen assimilation and partitioning as a function of crop load

Aims: The optimization of nitrogen use efficiency (NUE, i.e. uptake, assimilation and partitioning) is a solution towards the sustainable production of premium wines, while reducing fertilization and environmental impact. The influence of crop load on the accumulation of N compounds in fruits is still poorly understood. The present study assesses the impacts of bunch thinning on NUE and the consequences on the free amino N (FAN) profile in fruits.

Methods and Results: A large crop load gradient was imposed by bunch thinning (0.5 to 2.5 kg m^–2) in a homogeneous plot of 225 vines. Isotope-labelled foliar urea (10 atom % ¹⁵N) was applied on the canopy of the fertilized treatment at veraison. The plants were excavated at four phenological stages over the two seasons (bud burst, flowering, veraison and harvest) and were individually split into five plant parts (roots, trunk, canopy, pomace and must). Total nitrogen and its stable isotope composition were determined in each part, with the aim of monitoring NUE as a function of crop load and fertilization.

The N concentration in fruits either at veraison or at harvest was not related to crop load variation. N concentration was maintained in the must to the detriment of N content in the roots. The root dry weight was 15 % lower and the root N quantity 27 % lower under high yielding conditions (HYC, compared to low yielding conditions LYC). The fertilizer N uptake was 41 % higher under HYC than under LYC. Consequently, urea supply had a positive impact on the yeast assimilable N concentration in the must (+55 mg L^-1) only under HYC. However, the must FAN profile was significantly affected by the crop load, suggesting a possible modification of the aroma potential, independently from fertilization and grape maturation.

Conclusion: 

Using a ¹⁵N-labeling method, we demonstrate that grapevine has a strong ability to regulate nitrogen uptake and reserve mobilization to maintain a constant fruit N concentration despite changes in crop load. Foliar-urea fertilization at veraison was more efficient under HYC and helped to fulfill grape N demand, while limiting the mobilization of N reserves. However, the crop load affected the must FAN profile, inducing a possible modification of the fruit aroma. 

Significance and Impact of the Study: These findings highlight the great capacity of plants to adapt their N metabolism to constraints, e.g. bunch thinning in this case. These results are important to improve perennial fruit crop production through higher fertilization efficiency and lower environmental impact. Without fertilization, plant nutrition can be enhanced through the optimization of agricultural practices. The root activity appears to be key for understanding the mechanisms that balance N nutrition in plants