Impact of water stress on plant hydraulics, gas exchange, berry composition and quality of Pinot noir wines in Switzerland

Abstract

Grapevines of the variety Vitis vinifera L. cv. Pinot Noir (clone 9-18, grafted onto 5BB) were subjected to different water regimes (irrigation treatments) over the growing season. Physiological indicators were used to monitor the plant water status (leaf and stem water potentials and relative carbon isotope composition (δ¹³C) in must sugars). Leaf gas exchange (photosynthesis A, transpiration E), leaf stomatal conductance (gs), hydraulic conductivity in petioles (K_petiole), yield components, berry composition at harvest, and the organoleptic quality of the wines were analyzed over a 7-year period, between 2009 and 2015, under relatively dry conditions in the upper Rhone river valley (canton of Wallis, Switzerland). A progressively increasing water deficit, observed throughout the season, reduced the leaf gas exchange (A and E) and gs in non-irrigated vines. The intrinsic water use efficiency (WUE_i, A/gs) increased during the growing season and was greater in water-stressed vines than in well-watered vines (irrigated vines). This increase in WUE_i was correlated with an augmenting in δ¹³C in must sugars at harvest. Drought led to decreases in K_petiole, E and sapflow in stems. A decrease in vine plant vigor was observed in vines that have been subjected to water deficits year after year. Moderate water stress during ripening favored sugar accumulation in the berries and caused a reduction of total acidic and malic acid, as well as yeast assimilable nitrogen contents (YAN) in the must. Wines produced from water-stressed vines had a deeper color and were richer in anthocyanins and phenolic compounds compared to wines from well-watered plants with no water stress. The vine water status significantly influenced the organoleptic quality of the resulting wines. Wines made from non-irrigated vines with a water deficit presented more structure and a higher quality of tannins. They were also judged to be more full-bodied and soft than those made from irrigated vines.