Phytotoxicity on Rice Plant of Herbicide Propanil in Combination with Carbamate Insecticides

Rice plant (Oryza sativa L. var. Kimmaze) shows leaf burn when propanil (3, 4-dichloropropionanilide) is applied after carbamate insecticides. The phytotoxicity can be explained by an inhibitory action of the carbamate insecticides residue on propanil hydrolyzing enzymes. Total and internal residues of carbamate insecticides in rice plants were extracted with methylene chloride, cleaned up through Florisil columns, and analyzed with a colourimetry method. The internal residual amount of MTMC (m-tolyl-N-methylcarbamate) and PHC (2-isopropoxyphenyl-N-methylcarbamate) were greatest immediately after the spray, decreased in a day, and not detected after 5 to 8 days, while that of carbaryl (1-naphthyl-N-methylcarbamate) was greatest in the next day of the spray to decrease to 0.5ppm after 20 days. Inhibitory activity of various carbamate insecticides on propanil hydrolyzing enzymes in vitro was positively correlated to the herbicidal activity of propanil when propanil was applied to the rice plant in combination with each carbamate, except BPMC (o-tert-butylphenyl-N-methylcarbamate) which caused slight leaf burn despite of relatively strong inhibitory activity to the enzyme. During 5 days after the spray, activity of the propanil hydrolyzing enzymes in the rice plant sprayed with carbaryl (in vivo) was in accord with the activity in the enzyme preparations from non-sprayed samples treated in vitro with carbaryl of corresponding residual amounts. However, the sprayed samples showed a higher activity than the non-sprayed sample after 8 days of the spray.