Effects of Pre-Flowering Removal of Main Leaves or Lateral Shoots

An Agroscope study shows that pre-flowering removal of the main leaves instead of the lateral shoots appears to be a viable practice with a moderate effect on both yield potential and must composition.

Leaf removal (LR) is a common practice in viticulture for limiting fungal attack and promoting grape ripening. The timing of leaf removal is critical and must be adjusted to regional climatic conditions and the winegrower’s objectives. When performed after berry set, LR does not usually affect yield. When done before flowering, it can influence grape composition and the sensory profile of the wine, particularly the aromas and polyphenols. However, pre-flowering leaf removal also reduces yield by 40 to 50% by limiting the carbon source necessary for berry set. The effects of leaf removal are thus a function of factors such as grape variety, climatic conditions, and LR timing and intensity. The choice of which leaves to remove – main leaves or lateral shoots – in the cluster area is also a key consideration for adjusting LR intensity.

A trial was conducted from 2016 to 2021 at Agroscope’s experimental vineyard in Leytron, Switzerland to highlight the physiological effects of pre-flowering leaf removal in the cluster area of the Swiss white-grape variety ‘Petite Arvine’. The experimental design followed a randomised complete four-block format with four treatments, each involving different combinations of main-leaf and/or lateral-shoot removal from the cluster area (from the base of the shoot to the sixth leaf), all applied in May at the phenological stage of ‘separated flower buds’ (Figure 1).

Effect of pre-flowering LR intensity

Pre-flowering leaf removal significantly affected vine performance, reducing berry-set rate and yield in particular. Complete removal of the lateral shoots and main leaves in the cluster area led to an average yield loss of 37% (Figure 2). By comparison, removal of 50% of the main leaves limited yield losses to 5–21%, demonstrating that moderate pre-flowering leaf removal can limit the risk of excessive yield losses. The trial also highlighted the important role played by environmental factors in yield formation.

Intensive pre-flowering leaf removal had a negligible effect on the accumulation of sugars in the grapes (23.7 ± 0.3 Brix) but increased tartaric acid concentration (Figure 3). Wine composition showed negligible differences following intensive pre-flowering LR, except for a higher concentration of polyphenols associated with the smaller size and thicker skins of the berries and their greater exposure to light. It is worth noting that removing all the leaves and lateral shoots from the cluster area tended to reduce concentrations of Cys-3MH (a thiol precursor) (-21%; p < 0.10) with respect to the control.

Comparison of main-leaf versus lateral-shoot removal

The removal of the main leaves alone resulted in a larger exposed leaf area (+15 %) compared to the removal of the lateral shoots, mainly due to the growth of the lateral shoots in the cluster area. The total photosynthetic activity of the canopy was reduced until berry set due to the higher proportion of young leaves and lateral shoots that had not yet reached maximum photosynthetic capacity. This results in a lower yield potential (-14%), mainly due to a lower number of berries per cluster (-11%). The cooler microclimate owing to the greater leaf area – resulting in less abiotic stress – probably contributed to the higher concentration of both malic acid (+0.5 g/L, +12%) and glutathione (+6 mg/L, +11%) in the musts of treatment D (removal of the main leaves) compared to those of treatment A (removal of the lateral shoots). The increase in titratable acidity (+4%), and of malic acid in particular may be desirable in the current global warming context, which strongly influences the ratio of total soluble solids (sugar) to titratable acidity. Treatment D had the lowest concentration of tartaric acid and the highest of malic acid. Removal of the main leaves alone increased the concentration of glutathione in the must with respect to the other treatments (+13%; p < 0.001). Glutathione is an asset for preserving wine aromas and colour. No significant differences were noted in terms of TSS, pH or Cys-3MH in the musts between the lateral-shoot and main-leaf elimination treatments. By contrast, yeast-assimilable nitrogen concentrations increased (+26 mg/L, +10%). The wine produced by treatment D had higher colour intensity and fewer vegetable aromas than the wine from treatment A.