Water status response of Vitis vinifera L. cv Cabernet Sauvignon during the first years within the long-term VineyardFACE (Free Air Carbon dioxide Enrichment) study 

Abstract

Understanding grapevine responses to increasing atmospheric CO₂ (aCO₂) concentrations is crucial for assessing the impact of climate change on viticulture. Previously, at the VineyardFACE (Free Air Carbon dioxide Enrichment) experiment in Geisenheim, leaf gas exchange measurements were made as Vitis vinifera cv. Cabernet Sauvignon established from planting (2014 to 2016) under aCO₂ or elevated CO₂ (eCO₂, aCO₂ + 20%) concentrations. Contrary to many preceding observations with grapevines and other perennial plant species the young vines showed an increased intrinsic water use efficiency (WUE_i) that was mainly associated with an increase in net assimilation (A) rather than a decrease in stomatal conductance (g_s) under eCO₂.
To determine the impact of this stomatal conduction response to eCO₂ on whole vine water use, sap-flow gauges were installed for subsequent seasons (2016/2017), with complementary measurements of leaf gas exchange, pre-dawn leaf water potential (Ψ_pd), soil water content and recording of weather data. Furthermore, a vineyard water balance model was used to test the implications of the eCO₂ response, and possible explanations. Net assimilation and intrinsic water use efficiency values were higher for Cabernet Sauvignon under eCO₂ conditions, likewise transpiration rates (E) and stomatal conductance. Results were supported through whole vine transpiration measurements, pointing to a higher water use of young vines under eCO₂. The difference in daily vine water use between both CO₂ treatments was higher on days of high evaporative demand. Further, pre-dawn leaf water potentials were slightly lower under aCO₂, indicating a potentially lower risk for drought stress for young vines under eCO₂.