Net Cd2+ flux at the root surface of durum wheat (Triticum turgidum L. var. durum) cultivars in relation to cultivar differences in Cd accumulation

Abstract

Net Cd2+ flux and Cd accumulation for four durum wheat cultivars were measured to determine whether the net Cd2+ flux to the roots plays a role in cultivar differences in Cd accumulation. Plants were germinated for 4 d in the dark and then grown hydroponically for 2 d before being placed in a Petri dish with their roots immersed in a 1.0 µM Cd solution. Net Cd2+ flux to the root was measured at ten positions located 0.5 mm to 25 mm from the root tip. Cadmium concentrations were measured using Cd2+-selective microel ectrodes; Cd2+ flux to the root surface was then calculated using the MIFE™ system. The flux was greatest nearest the root tip (within 1.5 mm of the tip) and there were no significant differences in net Cd2+ flux among the various cultivars — likely because of the absence of a Casparian strip in the root tip cells. Net Cd2+ flux decreased with increasing distance from the root tip, with significant differences among cultivars. The mean Cd2+ flux to the roots in the 2–25 mm region increased in the following order: Arcola ≈ G9265-AU ≈ Kyle < G9265-BF, with the low grain Cd accumulators (Arcola and G9265-AU) generally exhibiting lower fluxes than the high grain Cd accumulators (Kyle and G9265-BF). However, net Cd2+ flux did not correspond to total plant Cd content, which increased in the order: G9265-AU < G9265-BF < Kyle < Arcola. We conclude that while there are durum wheat cultivar differences in the net Cd2+ flux at the root surface, their contribution to cultivar differences in total plant Cd accumulation is generally overshadowed by one or more other Cd accumulation factors. Key words: Cadmium, microelectrodes, plant cadmium content, cadmium flux, durum wheat, cultivar