Asparagus mesophyll cells and the influence of CO2 availability on changes in ATP levels in response to light

Abstract

Suspensions of mechanically isolated, photosynthetically competent Asparagus sprengeri Regel mesophyll cells were stirred in 10 mL of pH 6.3 medium contained in an open glass vessel. The influence of medium aeration with normal or CO2-free air on the steady-state ATP level was determined. In illuminated cells aerated with CO2-free air, the ATP level was 1.10 nmol ATP/106 cells. However, in the presence of CO2, levels were 0.89 nmol ATP/106 cells. The transition from CO2-free to normal air resulted in ATP levels dropping 24% within 15 min. This relationship between reduced CO2 availability and increased ATP levels was also demonstrated when a change from normal to CO2-free air resulted in a 27% increase in ATP levels. When aeration with normal air ceased, the ATP level in the illuminated cell suspension rose approximately 20%. If cells were aerated with normal air, the onset of illumination resulted in a 34% decrease in the ATP level. In contrast, the onset of illumination had little influence on ATP levels in suspensions exposed to CO2-free air. With normal air, removal of illumination resulted in a 25% increase in ATP, whereas in CO2-free air, there was a 10% increase. The data demonstrate that the influence of illumination on ATP levels is determined by the availability of CO2. They are discussed with reference to conflicting reports concerning the influence of illumination on steady-state ATP levels and the likely effect of varying CO2 levels on the rate of photorespiration.