How sunlight shapes wine, from leaf to glass

We talk about “sun-kissed” wines as if more sun always meant better wine. It doesn't. The sun does four separate jobs in a vineyard, and the most useful thing a wine lover can learn is that sunlight and heat are not the same.

The short answer: light on the leaves makes the sugar that becomes alcohol. Light on the grapes deepens colour and reduces green, bell-pepper flavours. Light on the buds helps set next year's crop. But too much heat on the grapes burns them and can even reduce colour. Growers manage the canopy to get the light without the burn.

1. Sunlight is the vine's fuel

Leaves use sunlight, water and carbon dioxide to make sugar: photosynthesis. That sugar travels to the grapes, and later yeast turns it into alcohol. So a sunny, healthy canopy means riper grapes and, usually, wines with more alcohol and body. More on this in why grapevine leaves matter.

2. Light on the grapes: colour and aroma

Two field studies show what light on the bunches does:

• Colour: in Merlot grown in Washington State's Yakima Valley, sunlight raised the grapes' anthocyanins (the red and purple pigments) whatever the temperature, and sun-exposed bunches had almost ten times the flavonols of shaded ones (Spayd and colleagues, 2002).
• Green flavours: in a Cornell University study of Cabernet Franc, exposing the bunches cut the build-up of the bell-pepper compound IBMP (a methoxypyrazine) by 21–44% before véraison. After véraison, exposure no longer helped (Ryona and colleagues, 2008). Timing matters: shade early means green flavours later.

That is why a cool, shaded Cabernet Franc or Sauvignon Blanc can taste of green pepper and cut grass, while the same grape from a sunny, open canopy tastes riper.

3. Sun is not heat

In the same Merlot study, grapes in direct sun were up to 13°C hotter than the air around them. When researchers cooled sun-exposed bunches, colour went up further; heat worked against it. Bunches facing east, which get the gentler morning sun, had more colour than those facing the hot afternoon sun in the west.

• Sunburn: too much exposure in a heatwave scorches the berries, which shrivel and taste cooked or bitter.
• Acidity: warmth burns off malic acid as grapes ripen. Hot-climate grapes often need acid added; cool-climate ones keep their snap.
• What growers do: in warm regions they often open up the canopy on the morning side and keep leaves as shade on the afternoon side.

4. Next year's crop

The buds that will grow next spring form during this summer. Sunlight and warmth reaching those buds help decide how many flower clusters they will hold, which growers call bud fruitfulness. A dense, shaded canopy this year can mean fewer bunches next year.

How growers manage the sun

• Choosing the slope: in cool places, vineyards face the sun. The best slopes of the Mosel face south; Burgundy's Côte d'Or faces east to south-east.
• Row direction: it decides which side of the vine gets morning or afternoon sun.
• Canopy management: removing leaves around the bunches, positioning shoots, trimming. Done early, it reduces green flavours; done carelessly in a heatwave, it causes sunburn.
• Altitude: high vineyards, as in Mendoza or Salta in Argentina, get intense light but cool nights, keeping acidity fresh.

What you taste in the glass

• Cool, cloudier places: higher acidity, lighter body, sometimes green or herbal notes. Loire Cabernet Franc, Sancerre.
• Sunny, warm places: riper, darker fruit, more alcohol, softer acidity. Barossa Shiraz, Napa Cabernet Sauvignon.
• The magic in between: plenty of light, cool nights, careful canopy work. That is what many great vineyards share.

Next time a wine tastes bright and fresh, or warm and generous, think of the light and the heat it grew in. That's a story worth telling at the table. See also a vine's year, stage by stage.

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