Inhibition of Photosynthesis by Bentazon in Intact Plants and Isolated Cells in Relation to the pH

Bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4 (3H)-one 2,2-dioxide] causes an increase in the chlorophyll fluorescence in the leaves of mustard plants [Smapis alba (L.) ] as a result of its in­hibitory effect on the photosynthetic electron transport. The rate of this fluorescence induction depends on the pH of the suspension medium. As the concentration of hydrogen ions diminishes, the maximum fluorescence that indicates a total inhibition of photosynthesis is reached at a later time. In the case of single cells isolated from the leaves of soybean plants [Glycine max (L.) Mer “Harosoy”], bentazon inhibits photosynthetic CO1, assimilation. The amount of inhibition is depen­dent on the pH of the suspension medium. The uptake of CO2 is inhibited more at pH 6 than at pH 7 or 8. Bentazon is more readily absorbed by cells at pH 6 than at pH 7 or 8. The inhibition of CO2 assimilation is proportional to the amount of bentazon taken up.