Response of wheat cultivars to chlorsulfuron and the effect of nitrogen availability

Sensitivity of wheat cultivars to the herbicide chlorsulfuron was tested and mechanisms to explain their differential response determined. In addition, a sensitive wheat cultivar, Rongotea, was selected and the effect of different levels of nitrogen on its response to chlorsulfuron was evaluated, and the mode of action of chlorsulfuron in this sensitive wheat cultivar was investigated. Thirteen wheat cultivars showed differential sensitivity to post-emergence applications of chlorsulfuron at rates from 0 to 90 g a.i. ha⁻¹. Five cultivars were chosen and tested in further experiments under glasshouse and field conditions. Cultivar 'Kotare' was found to be tolerant of chlorsulfuron and showed no injury symptoms at application rates of 15 or 60 g a.i. ha⁻¹. Cultivars 'Lancer' and 'Rongotea' showed early damage in pot and field experiments at both rates of the herbicide, and there were yield reductions in pot experiments. Cultivars' Abele' and 'Jasper' were intermediate in their response to chlorsulfuron. Retention, uptake, translocation and distribution pattern of chlorsulfuron was similar between the five cultivars studied. Metabolism of chlorsulfuron was found to be more rapid in 'Kotare' than 'Lancer' or 'Rongotea'. Within 48 h of application, Kotare metabolised 92.2% of 14C- chlorsulfuron, while Lancer and Rongotea metabolised only 43.5% and 63% of the herbicide, respectively. The concentration of chlorsulfuron in young tissues of Kotare, Lancer and Rongotea at this time was calculated as 1.2, 31.9 and 15.6 ng g⁻¹ dry weight, respectively. It was concluded that differential rates of metabolism were the main basis for differences in sensitivity to chlorsulfuron between the wheat cultivars tested. Rongotea wheat showed greater reduction in dry weight following chlorsulfuron application when grown at high (5 mol m⁻³) than low (1 mol m⁻³) nitrate. No differences were found in retention, uptake, translocation or metabolism of the herbicide between plants grown at either nitrate level. Moreover, plants grown at low nitrate and transferred to high nitrate at spraying showed similar growth reductions to plants grown at high nitrate throughout. It was concluded that low nitrate prior to spraying does not result in increased tolerance to chlorsulfuron and that another mechanism must be involved. Chlorsulfuron limited the capacity of wheat to respond to additional nitrate. This was not through an interference with uptake or assimilation of nitrogen as these processes were not affected by chlorsulfuron. Sprayed plants accumulated reduced N indicating an inability to utilize assimilated nitrogen for growth. Supplying branched chain amino acids (BCAA), which are associated with the mode of action of chlorsulfuron, partially overcame the restriction on growth imposed by chlorsulfuron. This was not a specific BCAA effect as supplying glutamine plus glutamate gave a similar response. Leaf extension, acetol act ate synthase (ALS) activity, valine content and its proportion of the total free amino acid pool was decreased rapidly following chlorsulfuron application. This is strong evidence that the initial effect of chlorsulfuron in restricting growth of Rongotea wheat was through inhibiting the activity of ALS. Moreover, plants kept on high nitrate following spraying accumulated nitrate to toxic levels. This was shown to be the main reason for decreased growth in the long term. Plants kept on low nitrate and transferred to high nitrate three weeks after spraying could respond to nitrate. In addition, it was found that ALS activity returned to normal at this time. It is likely therefore, that accumulation of nitrate in high-nitrate-fed plants was related to the effect of chlorsulfuron on the target enzyme. However, other possibilities are not discounted.