Kinetics of phyto-accumulation of hexavalent and trivalent chromium in rice seedlings

Phyto-accumulation kinetics of chromium (Cr) was investigated using hydroponic experiments with excised roots or intact rice seedlings exposed to potassium chromate Cr(VI) or chromium nitrate Cr(III). Results showed that the accumulation kinetics was all described by Michaelis-Menten function, whereas very distinct half-saturation constant (KM) and maximal accumulation capacity (vmax) was obtained for the respective Cr species. The comparison of the results from intact rice seedlings to detached roots indicated that transpiration has a stronger influence on Cr(III) transport by rice seedlings. Rice seedlings exposed to Cr(III) showed significantly higher potential for Cr accumulation in plant tissues than these exposed to Cr(VI). Although roots were the major site for Cr accumulation in both Cr treatments, restriction of root-to-shoot Cr translocation was more evident in Cr(III) treatments than Cr(VI) treatments. A remarkable difference in subcellular distribution of Cr in plant materials was observed between the two Cr treatments. Cell walls of rice seedlings exposed to Cr(III) accumulated more Cr in both roots and shoots than organelles and cytosol, whereas cytosol fraction was the largest in roots from the Cr(VI) treatments. Our results suggest that significant differences in uptake, accumulation and translocation pathways for Cr(VI) and Cr(III) exist in rice seedlings.