Kinetic parameters of Zn uptake by wheat are affected by the herbicide chlorsulfuron

Kinetic parameters of Zn uptake were determined for wheat plants (Triticum aestivum cvs Excalibur and Gatcher, and Triticum turgidum conv. durum cv. Durati) pre-grown at deficient or sufficient Zn supply and with 0 or 4 mg m(-3) sulphonylurea herbicide chlorsulfuron [2-chloro-N-(((4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino)carbonyl)-benzenesulphonamide]. Net Zn uptake generally showed a saturable response to increasing solution Zn concentrations; this response fitted a modified Michaelis-Menten equation incorporating the C-min term (solution concentration when net uptake is greater than or equal to 0). Zinc deficiency caused an increase in I-max (maximum net uptake rate) in the Zn-efficient genotype Excalibur (grows better than Zn-inefficient genotypes in environments with low Zn availability), but not in Zn-inefficient Durati. Zinc deficiency over a longer period (24 d) increased I-max and K-m in Excalibur plants more than over a shorter period (18 d). Exposure to chlorsulfuron only during the 6 h uptake period had no measurable effect on net Zn uptake. In Zn-deficient plants, 4 d chlorsulfuron exposure decreased Imax and increased C-min. A deleterious chlorsulfuron effect on I was more obvious in Zn-deficient plants after longer (10 d) than shorter (4 d) exposure to chlorsulfuron. The conclusion is that a greater capacity of Zn-deficient in comparison to Zn-sufficient Excalibur plants to take up Zn may be one of the mechanisms of Zn efficiency of Excalibur because Zn-inefficient Durati does not show an increase in I-max following a period of Zn deficiency. Prolonged exposure to chlorsulfuron (4 d) eliminates the capacity of Zn-deficient plants to increase I-max and net Zn uptake.