Light-Stimulated Changes in the Acidity of Suspensions of Oat Protoplasts

Light induced an alkalinization and stimulated a subsequent acidification of the medium surrounding oat (Avena sativa L. cv Garry) leaf protoplasts. Blue light was less effective than would be predicted from photosynthetic action spectra. Nonetheless, 3-(3,4-dichlorophenyl)-1,1-dimethylurea prevented alkalinization and reduced acidification to the dark rate for protoplast suspensions exposed to all light regimes tested.Alkalinization increased in parallel with initial rates of O(2) evolution as the quantum flux density of white light was raised to 75 microeinsteins per square meter per second. Alkalinization was accompanied by a decrease in the CO(2) content of the medium; therefore, it was attributed to photosynthetically induced CO(2) uptake. The effect of CO(2) depletion on the acidity of the medium appeared to be mainly restricted to the first 15 minutes of exposure to light. Consequently, subsequent pH changes primarily reflected a constant net proton efflux. Acidification occurred in the dark, but rates of acidification increased in response to increased light approximately in parallel with changes in a concomitant net O(2) efflux. The results indicated that protoplasts could acidify the medium in response to nonphotosynthetic activity, but that photosynthesis mediated light stimulation of acidification.