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

TLDR: It is hypothesized that both stomatal and mesophyll conductance are involved in the adaptation of the  CO 2 supply to the CO 2 demand at the site of carboxylation in chloroplasts.

Abstract: From simultaneous determination of net CO 2 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 CO 2 concentration in intercellular spaces (Ci) and in chloroplasts (Cc) were estimated. CO 2 assimilation was saturated at about Ci = 340 ppm. At increasing ambient CO 2 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 CO 2 assimilation rates (R 2 = 0.999). When Ca was raised, the CO 2 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 O 2 and 400 ppm CO 2 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 CO 2 supply to the CO 2 demand at the site of carboxylation in chloroplasts.