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 (AN), while electron transport rate (ETR) remained unaffected. With increasing drought, g, AN, Ci and Cc further decreased, accompanied by slight decreases of ETR and of the estimated mesophyll conductance (gmes). Severe drought led to strong reductions of both g and gmes, 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 gmes.
ABSTRACT
The response of several light-energy dissipation mechanisms to water shortage was analysed in a 10-year study in field-grown, high-light-acclimated grapevines, and compared with those of greenhouse-grown, low-light-acclimated grapevines. Dissipation mechanisms, except leaf photochemistry, differ among cultivars and acclimate to the prevailing light conditions during growth. However, no additional acclimation to drought was observed. The dependence of the dissipation responses on stomatal conductance suggests that low CO2 availability in the chloroplasts during drought triggers variations in the energy dissipation pattern. In irrigated grapevines under high light, more than 50% of total absorbed energy is thermally dissipated. There is evidence that implicates the xanthophyll cycle as the main thermal dissipation processes. CO2 assimilation is the most important photochemical pathway of dissipation in irrigated plants, but is replaced by photorespiration when CO2 assimilation declines under mild drought. Under moderate to severe drought, both photosynthesis and photorespiration decline, and thermal dissipation increases to account for up to 90% of total dissipation. Involvement of other processes in light dissipation is minimal in grapevines. Even in severely-stressed leaves, the incidence of photoinhibition is very low, indicating that safe dissipation of absorbed energy is very effective in grapevines.
