Acetolactate synthase activity and growth of rice (Oryza sativa L.) and weed species treated with the herbicide propyrisulfuron

Propyrisulfuron, a recently developed sulfonylurea herbicide, has shown high activity against broadleaf weeds, sedges, and grasses, unlike older sulfonylureas which are active only against broadleaf weeds and sedges. Greenhouse and laboratory studies were conducted to determine if the differential response of three rice cultivars (Azucena, N22 and IR64) and five weed species (Ludwigia hyssopifolia, Cyperus iria, Echinochloa colona, Echinochloa crus-galli, and Leptochloa chinensis) to propyrisulfuron is a function of its acetolactate synthase (ALS) activity. Growth and ALS activity were measured from test plants grown in sterilized soil under greenhouse conditions. Proprisulfuron inhibited in vivo ALS activity of the five weed species resulting in reduced shoot fresh weight, growth inhibition and eventual death. Ludwigia hyssopifolia and Cyperus iria were most susceptible, followed by Echinochloa colona, Echinochloa crus-galli, and Leptochloa chinensis. In weeds, inhibition of ALS activity increased progressively with time, with lowest in vivo ALS activity and greatest injury at 6 days after herbicide spraying. In contrast, minimal inhibition of in vivo ALS activity was observed in the three rice cultivars, where Azucena (japonica) showed lesser tolerance than N22 (aus) and IR64 (indica). Greater seedling injury and in vivo ALS inhibition occurred in plants treated at the two-leaf stage than in plants treated at the five-leaf stage. The I sub 50 values of in vitro ALS activity were similar in rice and weeds indicating that response to propyrisulfuron cannot be attributed to differences in ALS sensitivity at the target site. The study showed that propyrisulfuron is active not only against L. hyssopifolia and C. iria but also against E. colona, E. crus-galli and L. chinensis, with high selectivity to rice. The ability of propyrisulfuron to control grasses is a characteristic not found in older sulfonylureas. Hence, propyrisulfuron can be used as an alternative herbicide that can control grasses, broadleaf weeds and sedges in rice.