GROWTH AND NITRATE REDUCTION BY SESAMUM-INDICUM CV PB-1 RESPOND DIFFERENTIALLY TO LEAD

Lead (0.01-2 mM) supplied to five-day-old Sesamum indicum cv PB-1 seedlings differentially affected growth and nitrate assimilation. In root, stem and leaf, accumulation of Pb2+ increased with increasing concentration of the metal in the nutrient media. In vivo nitrate reductase (NRA, EC 1.6.6.1) activity was greatly inhibited by Pb2+ supply in roots, whereas this was less affected in the leaves, and the effect correlated with the accumulation of Pb2+ in the respective organs. In vitro NRA, however, was not inhibited by the metal. Total organic nitrogen of the young roots, on the other hand, increased with increasing Pb2+ supply, possibly because of increased translocation of N from seed to the growing roots during Pb2+ enrichment. Seeds of this cultivar were very tolerant to this metal regarding germination, and treated seedlings accumulated more dry mass during the early growth phase. Fresh weight of the growing roots and leaves, however, was slightly inhibited at higher levels, i.e. 2 mM of Pb2+ supply. The tolerance shown by this variety of sesame to Pb2+ differ from the response of another previously studied Pb2+ - sensitive cultivar of sesame, i.e. HT-I.