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

Net Cd2+ flux and Cd accumulation for four dururn 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 mu M 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 microelectrodes; Cd2+ flux to the root surface was then calculated using the MIFE (TM) 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 Cd 2, 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 approximate to G9265-AU approximate to 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 Cd 2, 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.