Canopy Management: How Leaves and Light Shape Grape Polyphenols

What the canopy is and what it controls

A vine's canopy is the whole of its shoots and leaves. It intercepts the sunlight that drives photosynthesis, and it also sets the microclimate of the fruit zone: how much light reaches the clusters, how warm they become and how quickly they dry after rain or dew.

Growers change that microclimate with a set of tools: shoot thinning, shoot positioning, trimming, leaf removal and dividing the canopy. This article follows the light from the leaf wall to the berry, through shade, skin flavonols, the green-pepper compound IBMP and the risk of sunburn.

Dense canopies and the shade inside them

When shoots are crowded, the outer leaves intercept most of the light and the inside of the canopy stays in deep shade. Fruit from such shaded canopies tends to have less sugar, anthocyanin and phenolics, and more potassium and malic acid with a higher pH. Dense canopies also stay humid for longer, which favours fungal disease.

Canopy work aims to keep the leaf wall thin. Shoot thinning removes surplus shoots, shoot positioning spreads the remaining ones into a thin wall, trimming cuts shoots that outgrow the wall, and leaf removal opens the fruit zone.

Sunlight, UV-B and skin flavonols

Sunlight on the skin changes the berry's phenolic make-up, but not every compound responds in the same way. Ultraviolet B is the part of sunlight between 280 and 315 nm. Flavonols such as quercetin glycosides respond to it most clearly: in a field study of Merlot, Spayd and colleagues (2002) found higher flavonol levels in sun-exposed clusters, and screening out ultraviolet lowered them.

Anthocyanins, the red pigments, behaved differently. In the same study, the clusters exposed on the side of the row that takes the afternoon sun ran hottest, and the authors attributed their lower anthocyanin levels mainly to temperature rather than to light. More generally, flavonol synthesis depends strongly on light, while high temperature can reduce anthocyanin accumulation, so the benefit of exposure depends on how hot the fruit becomes.

IBMP: why the timing of exposure matters

3-Isobutyl-2-methoxypyrazine (IBMP) is the methoxypyrazine behind green-pepper and herbaceous aromas in grapes such as Cabernet Franc, Cabernet Sauvignon and Sauvignon Blanc. It builds up in the berries before véraison and declines afterwards.

Ryona and colleagues (2008) followed IBMP in exposed and shaded Cabernet Franc clusters. Exposed clusters accumulated 21–44% less IBMP before véraison, but light did not speed up its decline after véraison. Light exposure, in other words, changed how much IBMP accumulated, not how fast it was broken down.

The practical conclusion is that exposure has to come early, between fruit set and véraison, if the aim is lower IBMP. Leaves removed after véraison add light and heat to the fruit without hastening the loss of IBMP. Sunlight does not simply burn off pyrazines late in ripening.

Heat and sunburn in warm climates

In hot, sunny regions, full exposure can do harm. Berries in direct sun can become considerably warmer than the surrounding air, and heat combined with strong radiation causes browning and, in severe cases, death of skin tissue (Gambetta and colleagues, 2021). Hot berries lose anthocyanins even when they receive plenty of light (Spayd and colleagues, 2002).

Growers therefore match leaf removal to the sun's path. On rows that run north–south, the west side of the canopy takes the afternoon sun, when the air is hottest, so in warm sites leaves are often kept on that side and removed mainly on the side that receives the morning sun. Shade cloth over the fruit zone and particle films of kaolin clay sprayed onto the fruit to reflect radiation are further options, and fruit exposed early in the season copes better with sun than fruit exposed suddenly in mid-summer.

Training systems: VSP and divided canopies

The trellis sets the starting shape of the canopy. Vertical shoot positioning (VSP) trains the shoots upward between foliage wires into a narrow, vertical wall, with the fruit zone along the bottom. On fertile sites with vigorous vines the wall thickens, and the fruit zone falls into shade.

For vigorous vines, dividing the canopy is an option. Systems such as the Geneva Double Curtain and the lyre split the leaf wall into two thinner curtains, so that a larger share of the leaves and fruit is exposed to light.

What reaches the glass

Because each mechanism acts on a different compound, canopy work leaves several traces in a wine. Fruit with less IBMP gives less green-pepper character in varieties prone to it. Exposed skins carry more flavonols, and fruit that stayed cool enough keeps more of its anthocyanins, whereas sunburnt berries bring browned, damaged skins to the fermenter. Fruit from shaded canopies tends to arrive with a higher pH, more malic acid and less colour.

None of these is a single setting to maximise. The right canopy depends on the climate, the vigour of the site and the variety: more light in cool, cloudy places, protection from afternoon heat in hot ones, and early timing wherever IBMP is the concern.

Practice question 1: shade, IBMP and timing

Question
A Cabernet Franc vineyard has a dense, shaded fruit zone, and its wines show strong green-pepper aromas. Explain what IBMP is, how cluster shading affects it, and when leaf removal should be done to lower it.

Model answer
IBMP (3-isobutyl-2-methoxypyrazine) is the methoxypyrazine responsible for green-pepper aromas. It accumulates in the berries before véraison and declines after it. Ryona and colleagues (2008) found that exposed Cabernet Franc clusters accumulated 21–44% less IBMP before véraison than shaded clusters, while light did not speed up the decline after véraison. Leaf removal in the fruit zone should therefore be done between fruit set and véraison. Removing leaves after véraison will not lower IBMP and may expose the fruit to heat. Opening the canopy also lets the fruit zone dry faster, which reduces the risk of fungal disease.

Mark scheme (self-check, 10 points)
- Identifies IBMP as the source of green-pepper aroma and describes accumulation before véraison and decline after it (3 points)
- Explains, with evidence, that exposure reduces accumulation but does not speed up degradation after véraison (4 points)
- Recommends early leaf removal and explains why late removal does not help (3 points)

Practice question 2: leaf removal in a hot climate

Question
A grower in a hot, sunny region plans to strip all the leaves around the clusters on both sides of north–south rows. Explain the risks and propose a better approach.

Model answer
Full exposure in a hot climate can overheat the fruit. Berries in direct sun can be much warmer than the air, and heat combined with strong radiation causes sunburn browning or, in severe cases, necrosis of the skin (Gambetta and colleagues, 2021). High berry temperature also lowers anthocyanins even when light is plentiful (Spayd and colleagues, 2002), so stripping the fruit zone may cost colour rather than add it. A better approach is to remove leaves mainly on the side that receives the morning sun and keep them on the west side, which takes the afternoon sun; to do any leaf removal early, since fruit exposed early copes better with sun than fruit exposed suddenly later; and, where needed, to use shade cloth or a kaolin particle film. Early leaf removal on the morning side still lets light reach the clusters before véraison, which also limits IBMP.

Mark scheme (self-check, 10 points)
- Explains sunburn and the loss of anthocyanins under heat (4 points)
- Proposes leaf removal on the morning side, done early (3 points)
- Names at least one further measure, such as shade cloth or kaolin (3 points)