DIFFERENTIAL TOXICITY OF HEAVY-METALS TO GROWTH AND NITRATE REDUCTASE-ACTIVITY OF SESAMUM-INDICUM SEEDLINGS

The response observed for biomass accumulation in Sesamum indicum L cv PB-1 seedlings during early growth phase induced by 1.0 mM Pb2+ suppl y in our earlier studies was not persistent when 1.0 mM CU2+ and Cd2were supplied either alone or in the various combinations of Pb2+, Cu2 + and Cd2+. Root and leaf fresh weights of the seedlings were hardly a ffected due to supply of 1.0 mM Pb2+, whereas Cu2+ and Cd2+ caused a d rastic decrease in the tissue fresh weight which was more pronounced i n the roots. The dry weight of the roots showed a similar effect, howe ver, leaf dry weight was increased with the metal supply. In vivo nitr ate reductase activity (NRA, EC 1.6.6.1) was significantly inhibited i n roots in the order Pb2+ > Cu2+ > Cd2+. Leaf NRA was inhibited with C u2+ supply and with the combinations of the metals. The in vitro enzym e activity, on the other hand, was increased in most of the cases due to the metal supply. Further, total organic nitrogen of the young root s and in several cases in the leaves also increased with the metal sup ply, possibly because of increased translocation of nitrogen from seed to growing seedlings. The results show that growth and nitrate reduct ion of this cultivar of sesame respond differentially to Pb2+, CU2+ an d Cd2+, and the effect was specific to the tissue examined.