Summary Researchers are beginning to understand how some plant genotypes can maintain reasonable growth and yields under low soil Zn, a trait termed zinc efficiency (ZE). Several studies have shown no correlation between ZE and root Zn uptake, Zn translocation to shoot, or shoot Zn accumulation. Furthermore, it has not been possible to conclusively link differences in leaf subcellular Zn compartmentation with ZE. However, biochemical Zn utilization, including the ability to maintain the activity of Zn requiring enzymes in response to Zn deficiency may be a key component of ZE. The next logical step in investigations of this trait is research on the genetic and molecular basis for ZE, in order to better understand the relationship between Zn utilization and ZE, and to identify the gene(s) controlling ZE. Progress in this research area could provide the knowledge to facilitate the engineering of Zn‐efficient plant varieties, which could help both crop production on marginal soils as well as possibly improve the micronutrient density of food crops to help address significant human nutrition problems related to micronutrient deficiency.

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 Zn 2ϩ influx across the root cell plasma membrane in conferring Zn efficiency by measuring short-term 65 Zn 2ϩ 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 65 Zn 2ϩ was measured over a wide range of Zn activities (0.1 nm-80 m). 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 m). Using chelate buffered solutions to quantify Zn 2ϩ 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 Zn 2ϩ uptake system. Zn 2ϩ uptake was similar for cv Dagdas and cv BDME-10 over both the high-and low-affinity Zn 2ϩ activity ranges, indicating that root Zn 2ϩ influx does not play a significant role in Zn efficiency.