Abstract

The effect of diffusional and photochemical limitations to photosynthesis was assessed in field-grown water-stressed grapevines (Vitis vinifera L.) by combined measurements of gas exchange and chlorophyll fluorescence. Drought was slowly induced, and the progressive decline of photosynthesis was examined in different grapevine cultivars along a continuous gradient of maximum mid-morning values of stomatal conductance (g), which were used as an integrative indicator of the water-stress conditions endured by the leaves.

Initial decreases of g were accompanied by decreases of substomatal CO₂ concentration (Ci), the estimated chloroplastic CO₂ concentration (Cc) and net photosynthesis (A_N), while electron transport rate (ETR) remained unaffected. With increasing drought, g, A_N, Ci and Cc further decreased, accompanied by slight decreases of ETR and of the estimated mesophyll conductance (g_mes). Severe drought led to strong reductions of both g and g_mes, as well as of ETR. The apparent carboxylation efficiency and the compensation point for CO₂ remained unchanged under severe drought when analysed on a Cc, rather than a Ci, basis, suggesting that previously reported metabolic impairment was probably due to decreased g_mes.