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Struggling vines produce better wines

There's nothing natural about a vineyard. The grape vine's natural habitat is woodland, where it grows as a climbing plant that uses trees and bushes as a support. Left to itself, and given an adequate water supply, a vine will send out long, fast-growing shoots that search for something to climb up. The climbing shoots attach themselves firmly to the support plant by tendrils. Where a shoot touches the ground, it has the capacity to develop a new root system. And in its woodland habitat, a vine will produce relatively few small bunches of rather tart grapes that would make pretty thin, acidic wines.

In this sense, the most 'natural' vineyards are probably those that involve training the vines on pergolas, as is still the custom in Portugal's Vino Verde or Italy's Veneto regions. Here, individual vines are grown up a wooden support some distance from the ground (ladders are needed for the harvest). But the grapes that are produced by this method aren't particularly good.

More modern methods of growing grapes, in close-spaced rows on wire trellising, take advantage of the fact that making the vines struggle generally results in better quality grapes. It's a bit like people. Place someone in a near-perfect environment, giving them every comfort and all that they could ever want to satisfy their physical needs, and it could have rather disastrous consequences for their personality and physique. If you take a grapevine and make its physical requirements for water and nutrients easily accessible, then (somewhat counterintuitively) it will give you poor grapes.

Forgive my anthropomorphism, but this is because the grapevine has a choice. Given a favourable environment and it will choose to take the vegetative route: that is, it will put its energies into making leaves and shoots. Effectively, it is saying, 'This is a fine spot, I'm going to make myself at home here'. It won't be too bothered about making grapes. But make things difficult for the vine, by restricting water supply, making nutrients scarce, pruning it hard and crowding it with close neighbours, and it will take the hump. It will sense that this is not the ideal place to be a grapevine. Instead of devoting itself to growing big and sprawling, it will focus its effort on reproducing itself sexually, which for a vine means making grapes.

And this is just what the best vineyards do. Nutrients and water are necessary for growing vines, but they will be hard to find. Planted close to its neighbours, the vine will have to send roots deep into the subsoil in order to scavenge enough resources. This will have the added benefit of ensuring a steady but restricted water supply, ideal for producing high quality grapes. Another benefit of making the vine struggle a bit is that yields will be lower, and although the reasons aren't fully understood yet, lower yields generally result in better grapes.

These principles seem to be borne out by a new technique, called partial root drying (see here for an example). Normal irrigation techniques can make vines too lazy to produce top quality fruit. However, a new technique developed jointly by British scientists from Lancaster University and viticulturists from the University of Adelaide takes advantage of the fact that struggling vines concentrate more of their energies on fruit production. Partial root drying involves irrigating one side of the vine while letting the roots on the other side undergo a period of drought (lasting from 5-14 days). Then the irrigation is switched to the other side, and the cycle continues so that at any one time part of the root system is always under water stress. This causes the roots to send signals to the shoots (the plant hormone abscisic acid is involved) that tell them to concentrate on grape production, not growing leaves. As well as improving grape quality, this system also reduces the amount of water used by the plant. An added bonus is that thereís no loss in yield, a trade-off that normally has to be made with enhanced grape quality. It sounds almost too good to be true.

But, as with people, while a little stress can be beneficial, too much is bad. In hot climates, once temperatures reach a certain level the grape vine shuts down. Grape development and vegetative growth stop. So somewhat counterintuitively grapes can actually take longer to ripen in very warm areas (such as Spainís central plains) than they do in cooler climates where the vines never undergo this shutting down through heat stress. Thereís also evidence that too much environmental stress can make vines more susceptible to diseases, too. As in many walks of life, balance seems to be the key.

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