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

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(+)/10(6) 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, 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 CO(2) 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.