Light-induced alkalization of the chloroplast stroma in vivo as estimated from the CO 2 capacity of intact sunflower leaves

Abstract

Rapid CO 2 gas exchange by Helianthus leaves was analysed kinetically using a computer model which distinguished different components of the gas exchange by different time constants. A rapid phase of CO 2 uptake was ascribed to the solubilization of CO 2 in all leaf compartments and to the conversion of the dissolved CO 2 to HCO 3 − in the chloroplast stroma which contains carbonic anhydrase. From stromal HCO 3 − CO 2 ratios the stroma pH of darkened leaves was estimated to be close to 7.5. Occasionally, values as high as 8 or as low as 7 were also obtained. If fast HCO 3 − formation also occurs in the cytosol, pH values may be lower by about 0.3 pH units than those calculated under the assumption that carbonic anhydrase is localized in chloroplasts only. Illumination with a light intensity close to saturation of photosynthesis caused an increase in CO 2 solubilization which indicated the alkalization of the chloroplast stroma by about 0.6 pH units. This is an underestimation, if the pH of cytosol decreases in the light liberating CO 2 by the action of carbon anhydrase. An alkalization of the stroma by 0.6 pH units indicates the export of about 450 nmol H +/mg chlorophyll from the stroma. This forms the basis of a large transthylakoid pH gradient which drives light-dependent ATP synthesis. A pH gradient between stroma and cytosol is capable of supporting secondary gradients between these compartments in the light, such as a gradient in the ATP ADP ratio. On darkening, the stroma alkalization was reversed. The rate of stroma acidification was much higher in the presence of CO 2 than in its absence.