Factors Affecting Cadmium Uptake and Distribution in Barley

Abstract

Crop plants often accumulate heavy metals to concentrations that exceed recommended food standard guidelines. Cadmium (Cd) is particularly problematic because of its high toxicity and its ability to be accumulated and retained in kidney and bones for decades. In most cases, reduction in the concentration of toxic metals in soil is impractical, so plant-based strategies aimed at restricting Cd accumulation in edible plant parts need to be considered. What follows is a study of the factors that affect the uptake and distribution of Cd within a representative cereal, barley (Hordeum vulgare cv. Schooner). Since Cd is a non-essential metal in plants, the transport characteristics were compared with those of an essential micronutrient, Ni in a series of supplementary experiments. The effects of nutrient status and cation competition on Cd uptake were investigated to analyse the main route for Cd entry into the plants. Cd uptake into whole plants (hydroponically grown) was measured by radiotracer studies and elemental analysis. Fe and Zn were found to have large effect on the uptake of Cd both via deficiencies and by the competition for uptake. This strongly suggests that the main route for Cd uptake is via Fe and Zn transporters. The inhibition of Cd influx only by FeII (but not by FeIII) suggests that Cd uptake into the root occurs through divalent cation transporters. At the same external concentration more than twice as much Cd was absorbed as Ni. Ni translocation to the shoot was also much lower than for Cd. The transport studies on protoplasts showed that transporters in the shoot respond to plant nutrient status but differently to that of the root. Comparison of the concentrations of 109Cd in whole protoplasts and vacuoles isolated from shoots demonstrated that the majority of the cellular Cd was located in the vacuole. In vacuolar transport assays, the addition of ATP alone significantly increased Cd uptake into the vacuole, but more so when both ATP and GSH were supplied together. This suggests that Cd may be partly sequestered as Cd2+ ions via divalent cation transporters, but predominantly as Cd-GSH complexes, most likely via ABC-type transporters. Cd distribution within the plant was investigated following 109Cd supplied through the roots or applied to individual leaf surfaces. The preferential movement of Cd to the tip of the youngest leaf was noticed in both cases. Following foliar application, a significant amount of Cd was rapidly distributed to roots instead of shoots. Over 48 h, 12% of this root Cd was effluxed into the external solution. This active excretion may be a detoxification strategy, in addition to sequestration into vacuoles. The bidirectional movement of Cd within the plant indicates that Cd is highly mobile in both xylem and phloem. This study has provided a detailed picture of the movement of Cd in barley at various levels. Prevention of Fe and Zn deficiencies and higher Fe/Zn nutrition levels should reduce Cd accumulation.