Effects of cadmium-zinc interactions on hydroponically grown bean (Phaseolus vulgaris L)

Bean (Phaseolus vulgaris L. cv. Morgan) plants were grown 14 days in nutrient solution at two concentrations of cadmium (2 and 5 mu M) and two of zinc (10 and 25 mu M) singly and in factorial combination. The decrease of growth was more pronounced in shoots for Cd treatments and in roots for Zn. The combination of Cd and Zn resulted in synergistic and additive, but not protective, effects on the growth. Plant concentrations of Mn, Cu and Mg were slightly involved in the interaction, while Fe, Ca and K were unaltered with all treatments. However, some combinations of Cd + Zn increased the contents of Cd and Zn, respectively, in shoots and in roots. This was confirmed by the time course of metal accumulations in short-term and no-mineral competition experiment. Fourteen-day-old seedlings growing in full nutrient solution were transferred to distilled water supplemented with 10 mu M Cd, 50 mu M Zn and 10 mu M Cd + 50 mu M Zn for 24 h. When the two metals were simultaneously applied: the roots retained less cadmium and the Cd percent in shoots increased. Inversely, zinc was higher in the roots to the detriment of its allocation to shoots. These results, together, suggest that synergism and additivity of Cd-Zn interactions might be, essentially, related to an inadequate compartmentation of cadmium and zinc burdens. The metal levels increased in the sensitive sites, where they would be relatively 'excluded' in the case of individual stress regime. (C) 1997 Elsevier Science Ireland Ltd.