Experimental studies on SO2 injuries in higher plants III: Inhibitory effect of suite ion on 14 CO 2 fixation

Photosynthesis decreases reversibly in plants exposed to SO₂. Photosynthesis recovers when the exposure to SO₂ is discontinued. Inactivation of a photosynthetic enzyme, ribulose-1,5-diphosphate carboxylase, by sulfonation of its SH groups was investigated as a cause of the reversible reduction of photosynthesis. The relationship between the sulfite ion concentration in the reaction mixture and ¹⁴CO₂ fixation catalized by the enzyme which was prepared from alfalfa leaves was explored by using radioactive NaHCO₃. About 50% and 85% inhibitions of ¹⁴CO₂ fixation were observed at 3 × 10^?3 M and 3 × 10^?2 M concentration of sulfite ion in the reaction mixture, respectively. The accumulation of 3 × 10^?4 M sulfite ion on the reaction site of the enzyme involved in the plants which were exposed to SO₂ could considerably reduce the CO₂ assimilation of the plant.