Nitrogen Assimilation and Distribution in Grapevine as a Function of Crop Load

A study on Chasselas grapevines shows that yield regulation by cluster thinning after foliar fertilisation does not increase grape nitrogen concentration but does encourage N accumulation in the perennial parts of the vine.

In viticulture, nitrogen excess and deficiency can both impair grape yield and quality. Nitrogen affects plant vigour, plant susceptibility to disease and fruit ripening. It also plays a key role in wine fermentation and aroma due to its impact on the yeast-assimilable compounds. Nevertheless, our understanding of the relationship between plant nitrogen status and fruit nitrogen composition remains incomplete. Plant nitrogen assimilation depends on several factors: environmental conditions, plant material and cultivation practices. Fruit-load regulation is a practice that aims to limit fungal diseases and encourage grape ripening; however, its effect on the amount and composition of nitrogen in the berries and in the plant remains unknown.  

In 2017-2018 a study was conducted on the Chasselas grape variety using a large fruit-load gradient obtained by grape removal at the ‘cluster closure’ stage (0.5-5.0 kg per plant). The study analysed the effect of crop load and N fertilisation on vegetative growth, fruit composition and the distribution of N in the vine over a two-year period. For this, 20 kg/ha of nitrogen in the form of urea was applied to the canopy in four passes around veraison. The isotopic labelling of nitrogen (10% ¹⁵N) allowed us to quantify the nitrogen assimilated by the vine and to follow its distribution among the leaves, fruits, trunk and roots as a function of yield.

Crop load influenced nitrogen distribution

Plant crop load strongly influenced the leaf-to-fruit ratio, which ranged from 0.4 to 3.0 m²/kg (Table 1). Crop-load regulation improved grape ripening by encouraging sugar accumulation (+1.3 Brix) and reducing titratable acidity (‒0.8 g/L) compared to the high-yield condition but had no impact on the concentration of assimilable nitrogen in the must at harvest (Table 1). Conversely, although fertilisation did not affect ripening it increased the concentration of assimilable nitrogen in the must (+34 mg N/L) compared to the non-fertilised control. Foliar fertilisation at the veraison stage did not have a significant effect on either vine growth or yield (Table 1).

Fertilisation encouraged nitrogen accumulation in the grapes (+24 mg N/L) without modulating plant vigour or total biomass. However, foliar N assimilation varied as a function of crop load (34% of the applied N on average in the high-yield conditions versus just 25% in the low-yield conditions (Figure 1). In other words, limiting yield reduced fertilisation efficiency. N uptake was higher for the fruits than for the canopy and perennial parts (roots and trunk).

Crop load influenced nitrogen distribution between the fruits and the perennial parts of the vine (Figure 2): 40% of the nitrogen fertiliser was found in the fruits at harvest in high-yield conditions, as against just 24% in low-yield conditions. Nitrogen distribution remained constant in the canopy. The amount of nitrogen exported from the vineyard was linked to the amount of grapes harvested. Consequently, the nitrogen reserves in the perennial parts at the time of harvest were lower in high-yield conditions (17% of total assimilated N) than in low-yield conditions (31%).