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

Abstract

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.