Effect of cadmium on iron uptake in cucumber roots: A Mössbauer-spectroscopic study

The uptake and accumulation of iron in cucumber roots exposed to cadmium were investigated with Fe sufficient and deficient cucumber plants using Mossbauer spectroscopy, Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and ferric chelate reductase activity measurements. Both Fe sufficient and Fe deficient plants were applied. In the case of Fe sufficient cucumber roots grown in nutrient solution with 10 μM Cd no changes were found in the occurrence of Fe species (mostly hydrous ferric oxides and ferric-carboxylate complexes) compared to the control where no Cd was added. In the Fe deficient roots pretreated with 0, 0.1, 1, 10 and 100 μM Cd for 3 h then supplied also with 0.5 mM 57Fe-citrate for 30 min, FeII was identified in a hexaaqua complex form. The relative amount of FeII was decreasing simultaneously with increasing Cd concentration, while the relative occurrence of FeIII species and total Fe concentration were increasing. The results support the inhibitory effect of Cd on Fe-chelate reduction. Although the reductase activity at 10 and 100 μM Cd treatment was lower than in the iron sufficient control plants, FeII could be identified by Mossbauer spectroscopy whereas in the Fe sufficient control, this form was below detection limit. These data demonstrate that the influx and the reoxidation of FeII was decreased by Cd, consequently, they refer to the competition of Cd2+ and Fe2+ during the membrane transport and the inhibition of the reoxidation process.