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

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Abstract
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 Cmin term (solution concentration when net uptake is ≥0). Zinc deficiency caused an increase in Imax (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 Imax and Km 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 Cmin. A deleterious chlorsulfuron effect on Imax 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 Imax following a period of Zn deficiency. Prolonged exposure to chlorsulfuron (4 d) eliminates the capacity of Zn-deficient plants to increase Imax and net Zn uptake.

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