Plant Physiol, January 2001, Vol. 125, pp. 456-463

High- and Low-Affinity Zinc Transport Systems and Their Possible Role in Zinc Efficiency in Bread Wheat¹

United States Plant, Soil, and Nutrition Laboratory, United States Department of Agriculture-Agricultural Research Service, Cornell University, Ithaca, New York 14853

There is considerable variability among wheat (Triticum aestivum L.) cultivars in their ability to grow and yield well in soils that contain very low levels of available Zn. The physiological basis for this tolerance, termed Zn efficiency, is unknown. We investigated the possible role of Zn²⁺ influx across the root cell plasma membrane in conferring Zn efficiency by measuring short-term ⁶⁵Zn²⁺ uptake in two contrasting wheat cultivars, Zn-efficient cv Dagdas and Zn-inefficient cv BDME-10. Plants were grown hydroponically under sufficient and deficient Zn levels, and uptake of ⁶⁵Zn²⁺ was measured over a wide range of Zn activities (0.1 nM-80 µM). Under low-Zn conditions, cv BDME-10 displayed more severe Zn deficiency symptoms than cv Dagdas. Uptake experiments revealed the presence of two separate Zn transport systems mediating high- and low-affinity Zn influx. The low-affinity system showed apparent K_m values similar to those previously reported for wheat (2-5 µM). Using chelate buffered solutions to quantify Zn²⁺ influx in the nanomolar activity range, we uncovered the existence of a second, high-affinity Zn transport system with apparent K_m values in the range of 0.6 to 2 nM. Because it functions in the range of the low available Zn levels found in most soils, this novel high-affinity uptake system is likely to be the predominant Zn²⁺ uptake system. Zn²⁺ uptake was similar for cv Dagdas and cv BDME-10 over both the high- and low-affinity Zn²⁺ activity ranges, indicating that root Zn²⁺ influx does not play a significant role in Zn efficiency.