Abstract
Effects of macronutrient cations -K, Ca and Mg- on uptake of Cd in whole rice plants were examined to compare them with the antagonistic effect found in the rice radicles previously. The plants were grown in culture solution with Cd and the cations of various concentrations. From early tillering stage to flowering stage, the growth rates of Cd-treated plants were as high as those of untreated plants, and concurrently with the growth, increase of the uptake of Cd and the cations was high. During this period, the effect of the cations on uptake of Cd was found to be the reverse of that in the radicles, and the cations in media finally showed synergistic effect on Cd uptake in the whole plants. Increase of Cd uptake was observed in accordance with increased uptake of the cations which was induced by increase of concentration of the cations in media. High positive correlations were shown between content of Cd and those of divalent macronutrient cations in the aerial parts. Therefore, there exists a parallelism between translocation of Cd and those of the cations. Cumulative amounts of transpiration in this period were not parallel to those of Cd. From these results, we judged that Cd was translocated by exchange adsorption with macronutrient cations on inner surface of xylem vessels. Thus, we assume that the synergistic effect found in this work should be brought about through the exchange adsorption in xylem vessels which developed concomitantly with plant growth, because in the exchange adsorption the translocation of Cd must necessarily be accelerated with increased uptake of the cations by higher cation treatments.
