KINETIC-PARAMETERS OF ZN UPTAKE BY WHEAT ARE AFFECTED BY THE HERBICIDE CHLORSULFURON

Kinetic parameters of Zn uptake were determined for wheat plants (Trit icum aestivum cvs Excalibur and Gatcher, and Triticum turgidum conv. d urum cv. Durati) pre-grown at deficient or sufficient Zn supply and wi th 0 or 4 mg m(-3) sulphonylurea herbicide chlorsulfuron triazin-2-yl) amino)carbonyl)-benzenesulphonamide]. Net Zn uptake generally showed a saturable response to increasing solution Zn concentrations; this res ponse fitted a modified Michaelis-Menten equation incorporating the C- min term (solution concentration when net uptake is greater than or eq ual to 0). Zinc deficiency caused an increase in I-max (maximum net up take rate) in the Zn-efficient genotype Excalibur (grows better than Z n-inefficient genotypes in environments with low Zn availability), but not in Zn-inefficient Durati. Zinc deficiency over a longer period (2 4 d) increased I-max and K-m in Excalibur plants more than over a shor ter period (18 d). Exposure to chlorsulfuron only during the 6 h uptak e period had no measurable effect on net Zn uptake. In Zn-deficient pl ants, 4 d chlorsulfuron exposure decreased Imax and increased C-min. A deleterious chlorsulfuron effect on I was more obvious in Zn-deficien t plants after longer (10 d) than shorter (4 d) exposure to chlorsulfu ron. The conclusion is that a greater capacity of Zn-deficient in comp arison to Zn-sufficient Excalibur plants to take up Zn may be one of t he mechanisms of Zn efficiency of Excalibur because Zn-inefficient Dur ati does not show an increase in I-max following a period of Zn defici ency. Prolonged exposure to chlorsulfuron (4 d) eliminates the capacit y of Zn-deficient plants to increase I-max and net Zn uptake.