Water use efficiency and carbon uptake of international cultivars under water stress

Abstract

Intrinsic water use efficiency (WUE_i) plays a key role in studying the physiological adaptation of grapevines to different environments, so it must be thoroughly assessed when selecting the most suitable cultivars for each region in the context of climate change. WUE_i is defined as the ratio of net assimilation (A_n) to stomatal conductance (g_s). This implies that different cultivars may have similar WUE_i despite having very different A_n and g_s values. While WUE_i and g_s are often jointly evaluated, WUE_i and A_n are rarely considered together. Thus, this study examines WUE_i and A_n in six international grapevine cultivars under gradual water stress. For two consecutive summers (2021 and 2022), weekly measurements of midday stem water potential (Ψ_st) and gas exchange (A_n and g_s) were taken in Chardonnay, Garnacha Tinta, Merlot, Riesling, Syrah, and Tempranillo vines grown in an experimental vineyard in Tomelloso, La Mancha, Spain, under gradual water stress conditions. While no significant differences were generally found across cultivars in terms of A_n and g_s, differences in their WUE_i were observed. Garnacha Tinta and Syrah stood out by showing the highest and lowest WUE_i, respectively, at 118.50 and 88.67 µmol CO₂ mol⁻¹ H₂O. These results are consistent with their traditional categorization as iso- and anisohydric cultivars, respectively. Additionally, the relationship between WUE_i and Ψ_stwas analyzed. Most cases fitted very significantly to a linear model, according to which WUE_i increased as water stress increased. However, this increase did not occur in the same way for all cultivars. Under non-stress conditions (Ψ_st close to 0 MPa), Merlot and Riesling exhibited a high WUE_i (between 56.38 and 56.99 µmol CO₂ mol⁻¹ H₂O). As stress increased, however, their WUE_iincreased at a slower rate –i.e., with a lower slope– than that of other cultivars, such as Garnacha Tinta (slope ≈ –80 µmol CO₂ mol⁻¹ H₂O MPa^–1), which exhibited a much lower WUE_i under non-limiting conditions (23.85 µmol CO₂ mol⁻¹ H₂O). These results were used to establish a classification based on each cultivar’s suitability according to its WUE_i in relation to the water stress threshold. Under low water stress conditions, Merlot and Riesling stood out for their high WUE_i. However, under more intense stress conditions, Garnacha Tinta stood out as the most efficient cultivar. Finally, when considering the relationship between A_n and Ψ_st, the outstanding cultivars were different, as expected. Overall, Merlot, Syrah, and Tempranillo were the best cultivars for carbon uptake.