Primary productivity and physiological responses of Vitis vinifera L. cvs. under Free Air Carbon dioxide Enrichment (FACE)

Abstract Two Vitis vinifera L. cultivars, Riesling and Cabernet Sauvignon, were grown in the Geisenheim VineyardFACE (Free Air Carbon dioxide Enrichment) system under rain-fed conditions to investigate the effects of elevated CO 2 on the productivity of grapevines for three consecutive years (2014–2016) following planting in 2012. The FACE system consisted of six 12 m diameter rings, with three at ambient CO 2 (aCO 2 , 400 ppm), and three rings at elevated CO 2 (eCO 2 , + 20% of aCO 2 level). Vegetative growth, single leaf gas exchange and yield parameters were monitored for the three growing seasons. Vegetative growth parameters responded differently to CO 2 treatments depending on biomass components. Trunk cross section area, as an indicator of perennial growth, showed a significant increase for Riesling under eCO 2 but not for Cabernet Sauvignon. Fresh biomass as lateral leaf area and fresh weight of summer pruning were stimulated by elevated CO 2 for both cultivars. Leaf gas exchange measurements for both cultivars showed a significant increase in net assimilation rate and an improved intrinsic water use efficiency for all three years under eCO 2 conditions. However, contrary to expected stomatal behaviour of grapevines, transpiration rate and stomatal conductance were higher under elevated CO 2 for Riesling and Cabernet Sauvignon in all three seasons. Higher values of pre-dawn leaf water potential recorded under eCO 2 point towards an interaction with soil water availability and root system development. Elevated CO 2 resulted in higher yield in terms of higher bunch weight, but did not affect average number of bunches per vine or sugar content of must at harvest date. Accordingly, bunch architecture was altered under elevated CO 2 levels. The increase in primary productivity of grapevines under eCO 2 indicates yield gains that can be expected under even modest near-future CO 2 scenarios. However, higher water use, particularly if maintained as grapevines transition to maturity, may have critical implications for the future adaptation of non-irrigated viticulture to increasing temperature and periods of rainfall deficit.