The goal of the swiss study presented by Dr Zufferey was to investigate the impact of water stress on vine hydraulics, berry composition and wine quality.
Context: how well can Pinot Noir sustain drought conditions ?
Combined effect of climatic trends associated with vineyard topography can lead to severe drought conditions in swiss vineyards in alpine valleys as reported by Zufferey and his team.
Over the past 150 years in swiss Alps, temperature has been rising while rain is decreasing. In fact vineyard data reflected that very intense drought levels reach similar levels to those reported under arid climates.
To anticipate effects of more pronounced drought expected in the future, Zufferey and his team have imposed 3 levels of water restriction to a pinot noir vineyard for 7 years.
- Treatment 1 consisted of traditional drip irrigation (9mm / week from bloom to veraison);
- Treatment 2: drought condition (no irrigation);
- Treatment 3: intense drought (no irrigation plus waterproof plastic film covering the ground).
Results: drought is no joke with Swiss Pinot Noir !
Predawn water potential: not surprisingly, season profiles over 7 years show that most negative values (- 8 bars predawn) are found where plastic is put with treatment 3 being the driest.
The technique of carbon discrimination is performed to indicate the effect of drought on vine photosynthesis performances. The more negative is the number, the easier is root water access throughout the season. As such, under treatment 1 where water is abundant, values ranged around -27. However, when drought conditions imposed a more challenging environment for Pinot Noir vines, values reach up to -22… For comparison purposes, analyzing carbon discrimination toward C13 in California, we also found similar levels within california vineyards.
Drought Consequences on Pinot noir performances:
Gaseous exchange measurements are helpful because they quantify the extent to which different drought regimes reduce stomatal conductance. As expected, higher water stress levels decrease stomatal conductance. In Switzerland, stomatal conductance values can be as low as 30 mmols.cm-2 . s-. To keep things in perspective, a typical stomatal conductance profile ranges between 120 and 400 mmoles.cm-2.s-1 over a whole season in California. As a consequence from stomatal conductance reduction, leaf photosynthesis rate decreases as well as transpiration.
Higher water stress means lower Nitrogen
A tight relationship between fruit and vine Nitrogen vs. plant water status is observed.
As treatments increased water stress, decreased values of leaf Nitrogen are observed, as well as lower nitrogen in the fruit (yeast assimilable Nitrogen dropped from 215 to 170 mg/L). This can directly affect winemaking operations.
Water stress and fruit ripening
Sugar accumulation per berry ranges from 275 to 290 mg/berry across treatments. Under non irrigated conditions (Treatment 2 and 3) moderate water stress had a positive effect on hastening sugar and color accumulation. A significant decline in malic acid per berry is observed in Treatment 3, probably because the plastic cover changed microclimate with non irrigated vines.
Water stress improves wine color and wine appreciation
Anthocyanins content in wines is significantly higher under non irrigated vines. After blind tasting, a panel of wine critic preferred wines from non irrigated vines.
Conclusion: Pinot noir wine is better under moderate drought
Drought conditions leading to a moderate water stress improve fruit and wine quality. Moderate water stress induces a decrease in shoot growth, a reduction in berry nitrogen accumulation and higher anthocyanins. However when drought is too severe, water stress and temperature stress have lead to vine embolism and yield reduction.
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