Variation in stomatal behaviour and gas exchange between mid-morning and mid-afternoon of north–south oriented grapevines (Vitis vinifera L. cv. Tempranillo) at different levels of soil water availability

A 3-year study was conducted to examine leaf gas exchange response of Vitis vinifera L. (cv. Tempranillo) grapevines growing in the central Iberian Peninsula as a function of soil water availability. Net CO₂ assimilation rate (A), stomatal conductance (g_s) and transpiration (E) of leaves were measured at the east and west side of vines planted in north/south orientated rows. Soil water availability was varied by three different irrigation treatments at 0.45, 0.30 and 0.15 of ETo and a fourth non-irrigated treatment. Approximately 60% of the variation in g_s over 3 years was due to changes in soil water content (θ_v); the correlation between the two was closer when examined on a season by season basis. Net CO₂ assimilation rates were significantly correlated with g_s. Stomatal conductance decreased by approximately 25–30% when measured 15:00 h (west side of vines) compared to 09:00 h (east side of vines); reductions in A were even greater than those in g_s. Leaf E increased approximately by 15–25% from morning to afternoon. The reduction in A and g_s from morning to afternoon was observed even in irrigated vines but absolute differences increased with decreasing soil water. This occurred when maximum daily g_s was less than 200 mmol m⁻² s⁻¹. These responses indicate that in hot areas training systems and row orientation, which minimize exposed leaf, area in the afternoon should be recommended.