PHYSIOLOGICAL-BASIS FOR DIFFERENTIAL SENSITIVITY TO SULFENTRAZONE BY SICKLEPOD (SENNA-OBTUSIFOLIA) AND COFFEE SENNA (CASSIA-OCCIDENTALIS)

Consistent with field observations, sicklepod exhibited considerable t olerance to sulfentrazone, and coffee senna showed relatively high sen sitivity to this herbicide in greenhouse tests. Germination was not in hibited in either species at up to 12.9 mu M of the herbicide. However , the chlorophyll content of herbicide-treated coffee senna cotyledona ry leaves was greatly reduced, and seedlings died within 10 d after tr eatment, while sicklepod seedlings were not visibly affected. Shoot he ight of coffee senna was inhibited 90% by sulfentrazone at 0.5 kg ai h a(-1), while the growth of sicklepod was not affected up to 2.0 kg ai ha(-1). Root uptake of radiolabeled sulfentrazone was 74% greater in c offee senna than sicklepod, but the amount of radioactivity recovered from the shoots of both species after 12 h was not different. Eighty-t hree percent of the parent compound remained in coffee senna leaf tiss ue after 9 h root exposure to the herbicide. In contrast, sicklepod to ok up relatively less sulfentrazone through the root and metabolized s ulfentrazone in the foliage more rapidly than coffee senna, with 91.6% of the herbicide being metabolized during the first 9 h of exposure. These results suggest that the tolerance of sicklepod to sulfentrazone is primarily due to a relatively high rate of metabolism of the herbi cide compared to coffee senna.