An Investigation into the Role of Photosynthesis in Regulating ATP Levels and Rates of H+ Efflux in Isolated Meosphyll Cells

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Abstract
Aerated and stirred 10-ml suspensions of mechanically isolated Asparagus sprengeri Regel mesophyll cells were used for simultaneous measurements of net H+ efflux and steady-state ATP levels. Initial rates of medium acidification indicated values for H+ efflux in the light and dark of 0.66 and 0.77 nanomoles H+/106 cells per minute, respectively. When the medium pH was maintained at 6.5, with a pH-stat apparatus, rates of H+ efflux remained constant. Darkness or DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea] however, stimulated H+ efflux by 100% or more. Darkness increased ATP levels by 33% and a switch from dark to light reduced ATP levels by 31%. In the absence of aeration, illumination prevented the accumulation of respiratory CO2 and the buffering capacity of the medium was about 50% less than that found in the nonilluminated nonaerated medium. As a result, rates of pH decline were similar even though the dark rate of H+ efflux was approximately 50% greater. Proposals that photosynthesis stimulates H+ efflux are based on changes in the rate of pH decline. The present data indicate that photosynthesis inhibits H+ efflux and that changes in rates of pH decline should not be equated with changes in the rate of H+ efflux.