Stomatal and mesophyll conductances control CO2 transfer to chloroplasts in leaves of grapevine (Vitis vinifera L.)

From simultaneous determination of net CO2 assimilation and transpiration at the abaxial side and of the photosynthetic electron transport rate at the adaxial side of fieldgrown, light-saturated leaves of grapevine (cv. Riesling) photorespiration, stomatal conductance for CO2, mesophyll conductance and the CO2 concentration in intercellular spaces (Ci) and in chloroplasts (Cc) were estimated. CO2 assimilation was saturated at about Ci = 340 ppm. At increasing ambient CO2 concentration (Ca) photorespiration decreased (less negative values); stomatal conductance decreased significantly (- 45 %) limiting CO2 uptake into intercellular spaces. Rates of total photosynthetic electron transport were constant between Ci = 340 and 800 ppm and decreased by 34 % at low Ci. Electron flow to carboxylation was closely correlated to CO2 assimilation rates (R2 = 0.999). When Ca was raised, the CO2 concentration in chloroplasts (Cc) increased but at smaller rates than Ci. Presumably due to the distinct decline of the mesophyll conductance Cc remained constant at Ci >340 ppm. At Ca = 400 ppm the Cc/Ca ratio was 0.46 - 0.48, corroborating data reported for other species (CORNIC and FRESNEAU 2002). At 2 % ambient O2 and 400 ppm CO2 decreased rates of photorespiration (- 69 %) were associated with a decline of total photosynthetic electron flow (- 6 %); higher stomatal and mesophyll conductances, however, led to increases of Cc and CO2 assimilation rates (+ 49 %). It is hypothesized that both stomatal and mesophyll conductance are involved in the adaptation of the CO2 supply to the CO2 demand at the site of carboxylation in chloroplasts.