AGE-RELATED MECHANISMS OF PROPANIL TOLERANCE IN JUNGLE-RICE, ECHINOCHLOA-COLONA

Uptake and metabolism of propanil were measured in both susceptible (S ) and resistant (R) biotypes of Jungle-rice, Echinochloa colona (L.) l ink at different growth stages. Results showed that there was no signi ficant difference in uptake between S and R biotypes of E. colona at a ny given growth stage, but that uptake was significantly reduced at ol der plant growth stages in all biotypes studied. Metabolism of propani l was more rapid in R biotypes than in S biotypes at all growth stages studied. Specific and total aryl acylamidase activity, responsible fo r the first stage of propanil metabolism was higher in R biotypes than in S at all growth stages, but declined to about 50% of the maximum a t older growth stages, confirming the importance of this enzyme in con ferring resistance to this herbicide. The area of necrosis that develo ped around a single drop of propanil deposited on the adaxial leaf sur face was used to assess the degree of propanil resistance; it was foun d that resistance increased at older E. colona growth stages in contra st to the rate of propanil metabolism and amidase activity. Treatment of leaves with the amidase inhibitors, carbaryl or piperophos, simulta neously with propanil, caused a decrease in resistance at growth stage s where amidase activity was greatest. This treatment was less effecti ve at older growth stages. These results show that, in E. colona, prop anil metabolism is important for conferring resistance in younger plan ts (four-six-leaf stage). It is suggested that restricted uptake confe rs resistance in older plants.