Submitted to: Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/23/2001
Publication Date: N/A
Citation: Interpretive Summary: Many soils of the North American Great Plains, where much of the U.S. wheat crop is grown, are low in available zinc (Zn). Deficient levels of Zn in crop plants have been shown to reduce yield and crop quality in the U.S. and throughout the world. While it has long been recognized that there is wide genetic variability among wheat cultivars in the capacity to grow and yield well in low zinc soils, the physiological bases for efficient use of Zn by crop plants are unknown. This study attempted to identify possible mechanisms of Zn efficiency in two wheat cultivars that differ significantly in their growth response to low soil Zn levels. The work focused on a detailed characterization and comparison of the rates of Zn uptake by roots as a potential Zn efficiency mechanism in these two cultivars. Unlike previous studies, this investigation measured Zn uptake from solutions containing Zn concentrations that are low enough to mimic those found in soil solution. The results revealed the presence of two separate Zn uptake systems that differ in their affinity for Zn: a low affinity system that had characteristics similar to those previously reported in the literature, and a high affinity system that has not been described before. This novel high affinity system functions to take up Zn at very low external Zn concentrations. However, Zn uptake was similar for the Zn efficient and Zn inefficient cultivars over both the high and low affinity concentration ranges, indicating that root Zn influx does not play a significant role in conferring Zn efficiency in wheat.
Technical Abstract: 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 have 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 Dagdas and Zn-inefficient BDME- 10. Plants were grown hydroponically under Zn sufficient and Zn deficient conditions and uptake of 65Zn2+ was measured over a wide range of Zn activities (0.1 nM - 80 uM). Under Zn deficiency stress, BDME-10 displayed more severe Zn deficiency symptoms than Dagdas. Uptake experiments revealed the presence of two separate Zn transport systems mediating high and low affinity Zn influx. The low affinity system showed Km values similar to those previously reported for wheat (2 - 5 uM). Using chelate buffered solutions to quantify unidirectional Zn2+ influx in the nanomolar activity range, we uncovered the existence of a second, high affinity Zn transport system with Km values in the range of 0.6 to 2 nM. Zn2+ uptake was similar for Zn-efficient Dagdas and Zn-inefficient BDME-10 over both the high and low affinity concentration ranges, indicating that root Zn2+ influx does not play a significant role in Zn efficiency.