Aquatic macrophytes have been used to monitor waters polluted by heavy metals. The phytoremediation capacity of Alternanthera philoxeroides (Mart.) Griseb. (Amaranthaceae) and Polygonum ferrugineum Wedd. (Polygonaceae) was therefore evaluated. These two species were submitted to treatments of 0, 25 and 50 mg L-1 Cd in nutritive solution. Seven days after treatment application, leaf gas exchange and chlorophyll fluorescence were measured in mature and fully expanded leaves. Fragments of roots, shoots and leaves were fixed in FAA 70, embedded in historesin and cut with a rotating microtome, and the sections stained with 1% toluidine blue. The Cd content in plant biomass was determined using ICP-MS. In P. ferrugineum exposed to 25 and 50 mg L-1Cd, an increase in the central leaf-vein- and root-aerenchyma lacunae was observed. Furthermore, in plants exposed to 50 mg L-1Cd, an increase in leaf glandular trichomes and mesophyll compacting was observed. In the stem of A. philoxeroides exposed to 50 mg L-1Cd, the nuclei of the cortex cells became elliptic, and at 25 and 50 mg L-1Cd, the number of druses in the cells of the central leaf vein was reduced. Cd accumulation was higher in the roots, followed by the stem and leaves in A. philoxeroides. The net photosynthesis rate of P. ferrugineum was more affected by Cd than that of A. philoxeroides. There was no variation in photosystem II (Fv/Fm) activity between the two species and the Cd treatments. A. philoxeroides was more cadmium-tolerant than P. ferrugineum, which indicates its capacity for use in phytoremediation programs.
Alternanthera philoxeroides; chlorophyll fluorescence; heavy metal; leaf gas exchange; phytoremediation; Polygonum ferrugineum
