Response of leaf acetolactate synthase from different leaf positions and seedling ages to sulfonylurea herbicide

Responses of acetolactate synthase (ALS) from grass and broadleaf weed to sulfonylurea (SU) herbicide were compared in relation to the leaf position in a seedling and seedling age. The responses of Echinochloa crus-galli (L.) P. Beauv. and Eclipta prostrata L., dominant grass and broadleaf weed in paddy fields in Korea, respectively, to azimsulfuron were examined. In this study, in vivo ALS assay was used to verify the responsibility of selected weed species at different leaf stages to SU-herbicides. The data from in vivo ALS assay could be used for discriminating the degree of tolerance between weeds showed different susceptibility. In E. crus-galli and E. prostrata there was no apparent relationship between the chlorophyll concentrations and herbicide concentrations treated on leaves. Both in E. crus-galli and E. prostrata, the free amino acid concentrations, however, were increased as herbicide concentration increased in the younger leaves. The free amino acid concentrations were generally higher in older leaves than young leaves and were significantly increased concomitantly with increasing herbicide concentration. The ALS activity was decreased rapidly with higher azimsulfuron rates in old but not senescent leaves compared to juvenile leaves. Generally, ALS activity was less sensitive at the early leaf stage than late leaf stage. The activity of ALS in E. prostrata was highly responsive to application time and more susceptible to the herbicide as compared to E. crus-galli. The highest levels of acetoin were observed in the uppermost and youngest leaf in all species tested.