[3] The principal mode of propanil's herbicidal action against weeds is inhibition of their photosynthesis and CO2 fixation.

Propanil inhibits the electron transport chain reaction and its conversion of CO2 to carbohydrate precursors.

The reason for the selectivity is that rice contains a high level of the enzyme aryl acylamidase (AAA), which rapidly metabolizes propanil to relatively nontoxic 3,4-dichloroaniline.

Susceptible weeds lack the gene(s) coding for the AAA enzyme and thus succumb to propanil.

Monsanto had conducted a number of tests and submitted test data to the Patent Office that indicated that propanil was superior to other similar chemicals, including one chemical that differed from propanil only in eliminating the CH2 group to the left of the CH3 group at the far right of the diagram shown at above right.