MODE OF ACTION OF CHLORSULFURON IN A SENSITIVE WHEAT (TRITICUM-AESTIVUM) CULTIVAR - PRIMARY AND SECONDARY EFFECTS ON NITROGEN ASSIMILATION

Chlorsulfuron (15 g a.i. ha(-1)) inhibited growth of wheat (Triticum a estivum L. cv. Rongotea) especially on high nitrate (NO3-) supply. Dec reased growth at high NO3- was associated with higher concentrations o f reduced nitrogen (N) and NO3- in the shoots. Seven days after sprayi ng (DAS), shoot dry weight (dry wt) of sprayed plants was similar with NO3- or branched chain amino acids as main N supply but 28 DAS, shoot dry wt was greater with the amino acid treatment. One DAS, chlorsulfu ron caused substantial decreases in extension of the youngest leaf and acetolactate synthase activity and valine content of shoots of plants supplied with NO3- or branched chain amino acids. Total amino acid co ntent of shoots was greater in sprayed plants than in unsprayed plants 1 DAS. Acetolactate synthase activity of sprayed plants supplied low NO3- returned to normal 14-21 DAS. For sprayed plants transferred from low to high NO3- supply 7, 14 or 21 DAS, shoot dry wt 50 DAS increase d with increased time of transfer to high NO3- while shoot NO3- conten t decreased. Shoot NO3- content of sprayed plants transferred to high NO3- supply 7 or 14 DAS was similar to that in unsprayed plants at app lied NO3- concentrations which inhibited growth. It is concluded that inhibition of acetolactate synthase is likely to be the primary mode o f action of chlorsulfuron in this wheat cultivar; data are consistent with the proposal that subsequent NO3- accumulation can also inhibit g rowth.