Changes of root morphology and Pb uptake by two species of Elsholtzia under Pb toxicity

Elsholtzia argyi and Elsholtzia splendens, which are Chinese endemic Pb/Zn mined and Cu mined ecotype respectively, were investigated in the aspect of their response to Pb toxicity in the presence or absence of EDTA addition. After 8 d's Pb treatment, root length, root surface area and root volume of E. splendens decreased much more than those of E. argyi, and reduced considerably with increase of Pb, while no marked change was noted for root average diameter. Compared to E. argyi, length of root with diameter (D)<0.2 mm was significantly reduced for E. splendens as Pb increasing. Root with cross-sectional area of D<0.1 mm for E. splendens was at Pb> or =10 mg/L, while for E. argyi, it was at Pb> or =25 mg/L. DW of E. splendens decreased much more than that of E. argyi with increase of Pb. E. argyi exhibited much more tolerance to Pb toxicity than E. splendens. Treatment with 100 mg/L Pb plus 50 mmol/L EDTA significantly decreased the length and surface area of D< or =0.2 mm root, increased the length and surface area of 0.2< or =D< or =0.8 mm root for the case of E. argyi, while for E. splendens, length and surface area of D<0.6 mm root reduced, as compared to 100 mg/L Pb treatment, alone. At 100 mg/L Pb, shoot Pb accumulation in E. splendens and E. argyi were 27.9 and 89.0 microg/plant DW respectively, and much more Pb was uptaken by the root and translocated to the stem of E. argyi as compared to E. splendens. Treatment of the plant with 100 mg/L Pb plus 50 mmol/L EDTA increase leaf Pb accumulation from 16.8 to 84.9 g/plant for E.splendens and from 18.8 to 52.5 g/plant for E. argyi, while both root and stem Pb pronouncedly reduced for both Elsholtzia species. The increased translocation of Pb to the leaf of E. splendens than that of E. argyi at the treatment of 100 mg/L Pb plus 50 mmol/L EDTA should be further investigated.