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

Dependence upon Photosynthesis ¹

Department of Botany, University of Massachusetts, Amherst, Massachusetts 01003

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 O₂ 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 CO₂ content of the medium; therefore, it was attributed to photosynthetically induced CO₂ uptake. The effect of CO₂ 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 O₂ efflux. The results indicated that protoplasts could acidify the medium in response to nonphotosynthetic activity, but that photosynthesis mediated light stimulation of acidification.