The effects of heavy metal on the tolerance and root anatomy of Typha domingensis.

MUFARREGE, M. M.; HADAD H. R.; DI LUCA, G. A.; MAINE, M. A.

Barcelona

Congreso; 3rd Wetland Pollutant Dynamics and Control, WETPOL; 2009

A free water surface wetland was built to treat wastewater containing Cr, Ni, Zn, and nutrients from a tool factory in Santo Tomé, Argentina. Typha domingensis Pers. showed a high tolerance to the conditions of the effluent, reaching 70% of cover in the wetland after 5 years of operation (Maine et al., 2009). The aim of this study was to investigate the variations in root anatomy, chlorophyll concentrations and plant height of T. domingensis exposed to high concentrations of heavy metals (Cr, Ni, Zn). This is key knowledge not only to understand the macrophyte behaviour but also to optimize effluent depuration using constructed wetlands.   METHODS A greenhouse experiment was carried out. Plants were collected from a natural environment, placed with sediment in plastic containers and acclimatized in the greenhouse. At the beginning of the experiment, the plants were pruned and solutions of heavy metals were added to obtain treatments with the following water concentrations, arranged in triplicate: 1) Zn(II): 80 mg l-1; 2) Cr(III): 60 mg l-1 3) Ni(II): 40 mg l-1 and  4) control: without metal additions. These concentrations were selected according to the effects of metals on chlorophyll concentrations in Typha latifolia L. (Manios et al., 2003). Plant height was measured daily during the 30 days of experiment. The concentrations of metals in water, roots and leaves, surface and rhizospheric sediment, and chlorophyll a concentration were measured at the beginning and end of the experiment. The cross-sectional areas (CSA) of the whole root, stele, and metaxylem vessels were measured using a micrometric ocular. In addition, the number of metaxylem vessels per section was recorded. One-factor ANOVA was performed to determine if there were significant differences for plant height and chlorophyll concentrations. Duncan’s test was used to differentiate means. Kruskal-Wallis analysis was applied to check the differences in the root anatomic parameters. When statistically significant differences were found, Wilcoxon’s test was applied. In all comparisons a level of p<0.05 was used. RESULTS and discussion Metal removal from water at the end of the experiment was 98.5, 99.5 and 98.5% for Zn, Cr and Ni, respectively. The concentrations of Cr in surface and rhizospheric sediment were significantly higher than those of the other metals. In Zn and Ni treatments, the metal concentration in the surface sediment was significantly higher than that in the rhizospheric sediment.   The concentration of Ni in roots was the highest and the Cr root concentration was the lowest. Leaves showed significantly lower metal concentrations than roots, indicating a scarce translocation from roots (Fig. 1), in agreement with Cardwell et al. (2002). Plant height increased in all treatments during the experiment, but the control presented the highest values from day 10 to the end of the experiment (Fig. 2). The lowest chlorophyll concentration was observed in the treatment of Ni (Fig. 3). Plants exposed to Cr exhibited the highest concentration of this pigment, probably because that metal was retained mainly by the sediment and not by the plant tissues. In the case of Zn, chlorophyll was not affected, indicating a lower toxicity. Manios et al. (2003) suggested an increase in chlorophyll a hydrolysis due to the accumulation of Ni, Zn, Cd, Cu and Pb in T. latifolia. The root CSA in the Ni treatments was significantly lower than those of Zn and Cr, but statistically significant differences were not observed among the metal treatments and control. There were no statistically significant differences in the stele CSA among Zn, Ni and control, being the Cr treatment the one showing the lowest value. The lowest number of vessels was observed in the Zn treatment and no statistically significant differences were observed among Cr, Ni and control. Metaxylematic vessel CSA in the Zn and Cr treatments were significantly higher than in the control (Fig. 4). Probably, a longer exposure time is required to generate noticeable root anatomy changes.   CONCLUSIONS Internal signs of cell death and plant senescence were not recorded, but plant height was affected in all treatments, and chlorophyll concentration was affected only by Ni. Changes in root anatomy did not present a clearly defined pattern. However, the number of vessels in the Zn treatment was reduced. We may conclude that T. domingensis shows tolerance to extreme toxic conditions which makes it suitable to be used in constructed wetlands for the treatment of industrial effluents.