EFFECT OF CADMIUM ON NITROGEN-FIXING PEA-PLANTS GROWN IN PERLITE AND VERMICULITE

The effect of cadmium (Cd) on nitrogen (N2) fixing pea plants was studied. Perlite and vermiculite were compared as substrates in a hydroponic system where the plants were grown. In both experiments, there was no significant effect on nitrogenase activity when referred to fresh weight of nodules, but there was a reduction in the nodulation ratio (number of nodules developed per plant), even at low concentrations of available Cd. In plants grown in vermiculite, the nitrogenase activity expressed in terms of activity per nodule increased as Cd application increased; while in plants cultivated in perlite, that activity decreased when the applied Cd increased at the highest levels of Cd in the nutrient solution. Vermiculite had a significant interaction with the Cd in the nutrient solution, and therefore, we assume there was less Cd available to the plants than when perlite was used as the substrate. The loss of N2 fixation by the presence of Cd in plants grown in perlite has been related to a change in the distribution of manganese (Mn) in the plant tissues. There was a reduction of Mn content both in the root and nodules. The reduction was greater in the nodules, while the concentration of Mn increased in the shoot.