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Mechanisms of terroir, part 2

Treating vines mean to keep them keen  

It is helpful to think about plants as sophisticated environmental computers. Just as we sense the world around us and then use this information to guide our actions, so do plants. It’s just that whereas we respond quickly – for example, if it is too hot in one place we move somewhere cooler – plants respond over a much longer timescale. Literally rooted to the spot, they adapt their growth form to best suit the local conditions. 

This extends to their reproductive strategies. Generally (and simplistically) speaking, if conditions are good, then plants opt for vegetative growth; if they are bad, they choose to reproduce sexually (the ‘I’m outta here’ option), which means fruit production. So viticulturalists want to treat their vines mean enough that they focus on fruit production, while giving them just enough of what they need so that they don’t suffer from water or mineral deficit, which would hamper their efforts at producing ripe fruit. Thus many viticultural interventions aim at encouraging the vine to partition nutrients to the grapes so that they ripen properly, rather than concentrating on growing more leaves and stems (vegetative growth or ‘vigour’). 

An example of this ‘environmental computing’ is seen in the growth of plant roots. Root growth is determined by interplay between the developmental program of the plant and the distribution of mineral nutrients in the soil. The roots grow to seek out the water and nutrients in the soil: to do this it appears that they sense where the various nutrients are and then preferentially send out lateral shoots into these areas. Low levels of nutrients in the upper layers of the soil results in the roots growing down to a greater depth, which is likely to improve the regularity of water supply to the vine. 

‘Vines have roots which can reach up to 3 metres in depth’, reports Davidian. ‘These deep roots can actively take up water and minerals, even though most mineral ions are more abundant at the root surface.’

A popular notion is that very old vines with deep roots express terroir better. ‘The claims often made regarding the importance of deep rooted vines are based on the assumption that the roots are then able to better exploit the underlying geology’, says Dawid Saayman. ‘In turn, this is considered by some to contribute certain minerals and thus impart a certain character to the wine. There is no scientific proof for this.’ It’s also worth mentioning here the existence of mycorrhizae. Many plant roots form an association with specific soil fungi, where the fungi hitch a ride on the roots, gaining energy from the plant, while the plant root gains an enhanced absorptive area and ability to extract mineral nutrients from the soil – this is termed a ‘mutalistic symbiosis’, because both partners benefit. 

Some people have claimed that mycorrhizae are important for terroir expression, but this is not clear from the scientific literature. Dawid Saayman points out, however, that grape vine mycorrhizae mainly assist in phosphorus uptake, an element that vines usually don’t have problems getting enough of. ‘It is highly unlikely that mycorrhizal associations are prominent enough to contribute to a terroir effect’, he concludes.

How soils have their effect  
Soils differ in their chemical and physical properties. According to Victoria Carey, a lecturer in viticulture at Stellenbosch University who specializes in terroir, the latter are more important for terroir effects. ‘The most convincing indications in the scientific literature are that the effect of soil type is through its physical properties, and more specifically, through the water supply to the grapevine,’ she suggests. 

This is a position that Richard Smart agrees with: he cites the pioneering work of French scientist Gérard Seguin, who conducted a survey of the properties of the soils in the Bordeaux region. Seguin couldn’t find any reliable link between the chemical composition of the soil and wine character or quality, and maintained that it was the drainage properties of the soil affecting the availability of water that mattered. He concluded that it is ‘impossible to establish any correlation between the quality of the wine and the soil content of any nutritive element, be it potassium, phosphorus or any other oligoelement.’ 

The verdict was that it was the physical properties of the soils, regulating the water supply to the vine, that were all important in determining wine quality. The best terroirs were the ones where the soils are free draining, with the water tables high enough to ensure a regular supply of water to the vine roots which then recedes on veraison (when the berries change colour) so that vegetative growth stops and the vine concentrates its energies on fruit ripening.

The consensus among the viticulture experts I consulted seems to be that the chemical composition of the soil – that is, nutrient availability – is only important when there is excess nitrogen, leading to excess vigour, or when there is a serious deficiency. ‘Nutrition can be instrumental to the specific growth pattern of the vine and thus can cause a specific canopy architecture and therefore ripening pattern,’ says Dawid Saayman. ‘The plant performance therefore modifies the vineyard climate creating a specific microclimate in the bunch zone, and in this way it can greatly determine the character of the wine’, he adds. ‘Overall, nutrient effects are minimal’, adds Smart.

Soil chemistry effects
But before we give up on soil chemistry as an important factor in terroir, it’s worth taking a look at recent research on the effects of mineral nutrition on plant physiology. I spoke to a number of researchers who are actively working on plant mineral nutrition, so see whether their work might shed some light on the mechanisms of terroir transduction. 

‘I wouldn't be at all surprised if soil chemistry had an effect on the expression of genes that are involved in the production of the compounds that determine flavour,’ says Professor Brian Forde of Lancaster University (UK). ‘There is certainly plenty of evidence that plants are tuned to detect and respond to soil nutrients’, he adds. ‘The balance between the nutrients (nitrogen, phosphorus, potassium, sulphur and calcium, and even the micronutrients) is likely to be important and the plant stress responses elicited by limiting amounts of one nutrient would probably be subtly different from the stress responses elicited if another nutrient is limiting.' 

Forde referred me to some publications showing that the levels of various plant metabolites were significantly altered under different nutrient regimes. At a more detailed level, it is now clear that patterns of gene expression in plants are altered by the presence and absence of various nutrients.

I spoke to Professor Malcolm Bennett and Dr Martin Broadley of Nottingham University, who recently published a paper showing the effects of phosphate deficiency on plant gene expression. Broadley feels that it won’t be too long before we have a much better idea about the influence of soils on wine flavour. ‘There is a large amount of work underway to understand the molecular biology of grapes, and scientists are identifying genes that influence wine flavour’, he explained. ‘As more grape molecular biology is known, the easier it will be to understand mechanisms of terroir on wine taste. When genes encoding for proteins that influence wine taste are identified, then the effects of different components of terroir (e.g. the availability of different minerals, soil pH, soil water content) on specific biochemical pathways can be identified and tested. This research may allow current agronomical practices to be improved to enable better-tasting grapes to be produced, or it might even allow varieties of grapes to be selected or bred more effectively.’

Concluding remarks  
Even if science leaves us with what currently looks like rather an emasculated version of terroir, I don’t think that this necessarily diminishes the importance of this cherished concept. Wine growers who use terroir as their guiding philosophical framework and focus on the importance of the soil are responsible for a disproportionately large share of the world’s most interesting wines. Perhaps Randall Grahm’s wacky rock experiments aren’t so misguided after all: although it seems clear that there is no direct link between soils and wine flavour, by framing their activities within the context of a soil-focused worldview and trying to get a bit of somewhereness and minerality into their wines, winegrowers might be vastly increasing their chances of making interesting wine. And that’s something the world needs more of.  

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