Effect of Pb toxicity on leaf growth, physiology and ultrastructure in the two ecotypes of Elsholtzia argyi.

Hydroponics experiments were conducted to study the effects of Pb on mined ecotype (ME) and non-mined ecotype (NME) of Elsholtzia argyi from Pb/Cu mining and the non-contaminated agricultural areas, respectively. The results showed that at 200 microM Pb treatment, although concentrations of Pb in leaves and stem of the ME were 2.6 and 4.5 times higher than those of the NME, these plants exhibited higher tolerance to excessive levels of Pb in the growth medium. In both the ecotypes, Pb caused inhibition of leaf growth and photosynthesis, and induced the membrane damage which was more obvious in the NME. Pb treatment decreased the activities of superoxide dismutase (SOD) and guaiacol peroxidase (G-POD) while activity of catalase (CAT) and levels of total soluble proteins (TSP), ascorbic acid (AsA) and glutathione (GSH) were significantly induced after Pb exposure, however, increase was sharp in the ME plants. Leaf ultrastructural analysis of the spongy mesophyll cells revealed that the excessive Pb concentrations caused adverse effects on chloroplast ultrastructure of both ecotypes whereby internal damage was more severe in NME. The higher tolerance to Pb displayed by ME is mainly attributed to maintenance of its leaf growth and physiology, induction of GSH and integrity of cell organelles especially chloroplast ultrastructure.