Lead Uptake by Roots of Four Turfgrass Species in Hydroponic Cultures

Turfgrass, which is widely grown and produces a large amount of biomass, could act as a sink for industrial pollutants in urban and suburban regions. Little research has been conducted regarding heavy metal uptake by turfgrasses. The objective of this study was to evaluate root uptake of lead (Pb) in four turfgrass species. Grasses were grown hydroponically in solutions containing from 0 to 450 mg·L Pb, at either pH 4.5 or 5.5, for 4 or 8 days. A significant quadratic relation existed between Pb accumulation in roots and solution Pb concentration within the tested range. The maximum Pb accumulation in roots of the four species was in the range of 20 mg·g dry root weight. Tall fescue (Festuca arundinacea Schreb.) and Spartina patens survived at 450 mg·L Pb solution without showing obvious damage while centipedegrass [Eremochloa ophiuroides (Munro) Hack.] and buffalograss [Buchlöe dactyloides (Nutt.) Engelm.] deteriorated or died at this concentration. This study showed that turfgrass plants can absorb heavy metals efficiently and tolerate high Pb concentration in hydroponic solutions and thus may have a potential use in environmental remediation as a biological extractor of lead. Release of pollutant metals to the biosphere has increased dramatically during the past century (Nriagu, 1979). Among the toxic metal contaminants, lead (Pb) poses a major concern because of its extensive distribution and the substantial environmental and human health problems it causes (Cunningham and Berti, 1993). Lead pollution has resulted from industrial and mining activities, as well as the use of lead-containing substances, such as paints, gasoline, explosives, and water pipes (Huang et al., 1997). Remediation of toxic metals, including Pb, and other pollutants has been an important research subject for environmental studies. Metal remediation has also become a multibillion-dollar industry in the United States, usually relying on engineering-based technologies, such as isolation and containment, and decontamination by physical, chemical, or biological treatments (Cunningham and Berti, 1993). Over the past decade, phytoremediation, defined as remediation of pollutants by using green plants, has emerged as a new strategy with great potential. Phytoremediation may involve metal absorption from the soil and transportation to the harvestable parts of the plants (phytoextraction), absorption and concentration of toxic metals in plant roots from polluted effluents (rhizofiltration), and reduction of heavy metal mobility in the soil by plants (phytostabilization) (Salt et al., 1995, 1997). Phytoremediation studies of Pb-contaminated soil have shown that some plants can accumulate Pb at high levels (Huang and Cunningham, 1996; Qureshi et al., 1985). Moreover, Pb extraction was greatly enhanced by treating the contaminated soil with synthetic chelates (Huang et al., 1997). Turfgrass is often planted as a major vegetative groundcover in many landscapes, especially in urban regions. The large acreage of turfgrass grown [e.g., 30 million acres in the United States as estimated in 1990 (Emmons, 1995)] and the high volume of biomass it produces make turfgrass potentially suitable for urban environmental remediation. Turfgrass plants may contain Pb and other heavy metals in excess of 1 mg·kg of their dry matter when grown in typical non-polluted soils (Jones et al., 1973). The heavy metal content of tall fescue (Festuca arundinacea Schreb.) grown with sewage sludge was also reported (Boswell, 1975; King, 1981). However, little research has been performed to study the tolerance of turfgrasses to high levels of heavy metals and the maximum possible accumulation in their tissues (Dushenkov et al., 1995; Li et al., 2000). The long-term objective of this research is to study the potential role that turfgrasses might play in environmental remediation of metal-contaminated soils and the mechanism behind it. As the first step towards this goal, root accumulation of Pb in four turfgrass species and its effect on plant performance were studied. Materials and Methods Plant materials and experimental conditions. Plants of centipedegrass [Eremochloa ophiuroides (Munro) Hack. ‘Common ], buffalograss [Buchlöe dactyloides (Nutt.) Engelm. ‘Common ], tall fescue (Festuca arundinacea Schreb. ‘Houndog V ) and Spartina patens (‘Common ) were grown in a loam soil at the experimental station of Nankai Univ. (Tianjin, China). Spartina patens is used in the area as a vegetative groundcover species on saline soils. Plants 15 cm in height were collected, rinsed with deionized water several times to remove attached soil, and used for the experiments. The plants were then cultivated hydroponically in Pb-containing solutions. Each hydroponic unit consisted of a glass jar (7 cm in diameter × 12 cm in height) containing 200 mL solution with varying Pb concentrations and pH values. The Pb source used for the experiments was Pb(NO3)2 (purity >99%; Tianjin Third Chemical Plant, Tianjin, China). The solutions were made with Pb-free deionized water. The pH of the solutions was adjusted with 0.1 M HNO 3 and 0.1 M NaOH to achieve a final pH of 4.5 or 5.5. To be on a comparable basis, plants with 0.5 ± 0.1 g dry root weight (as predetermined) were selected as an experimental unit for each jar. The plant roots were completely immersed in the solutions. The total volume of the solution was kept constant by adding deionized water every day to compensate for water loss through evaporation and transpiration. The plants were incubated at a temperature range of 26 to 30 °C with a 14-h day/10-h dark photoperiod and 85% relative humidity, under natural light conditions (with light intensity in the range of 500 μmol·m·s) for 4 or 8 d without aeration before sample collection for Pb analysis. Experimental design and data analysis. To study the effect of Pb concentration (0, 200, and 400 mg·L), pH value of the cultivating solution (pH 4.5 and 5.5), and duration of growth (4 and 8 d) on Pb accumulation in roots of three turfgrass species (centipedegrass, buffalograss, and tall fescue), a completely randomized design with three replications was used for Experiment A (Table 1). The lower pHs were chosen because Pb is more available to plants in soil with lower pH (Cunningham and Berti, 1993). In Experiment B, the relationship between Pb accumulation in roots and Pb concentration in the cultivating solution was investigated. Three turfgrass species (centipedegrass, buffalograss, and S. patens) were used for the experiment. The Pb concentrations tested were from 0 to 450 mg·L, in 50 mg·L increments. The Pb solution was adjusted to pH 5.0 and the HORTSCIENCE 38(4):623–626. 2003. Received for publication 8 Mar. 2002. Accepted for publication 31 Dec. 2002. The authors would like to thank Jian Wang and Nankai High School (Tianjin, China) for assistance in the experiments, and Dr. R. Cooper for critical reading and editing of the manuscript. This work was supported by a grant from Tianjin Science and Technology Council, Tianjin, China. Associate Professor; to whom reprint requests should be sent. E-mail address: [email protected] Senior Engineer. Professor. Associate Professor; to whom reprint requests should be sent. E-mail address: [email protected] 31-7234, p623-626 623 7/1/03, 3:07:33 PM HORTSCIENCE, VOL. 38(4), JULY 2003 624 MISCELLANEOUS plants were cultivated under the same conditions as in Experiment A for 8 d. ANOVA analysis and F tests were performed in Experiment A while regression analysis (Du, 1999) was carried out for Experiment B. All statistical analyses were performed using SAS statistics software (SAS Institute, 1985). Sample collection and Pb content analysis. At the end of the growth period, plants were removed from the Pb solutions for analysis and were rinsed with deionized water for 5 min. Plant roots were then sliced into small fragments and were oven-dried at 80 °C for 48 h. The dry weight of the roots of each sample was determined and the dried roots were then ashed at 550 ̊ C for 6 h. The ash of each sample was dissolved in 5 mL 1 M HNO 3 and diluted to 50 mL with deionized water in a volumetric flask. The Pb content of the samples was determined using a Hitachi 180-80 Polarized Zeeman Atomic Absorption Spectrometer (Hitachi Ltd., Tokyo) with the following parameters: wavelength 283.3 nm, lamp current 7.5 mA, slit 1.3 nm, burner standard type, burner height 15.0, fuel C2H2 2.5 L·min , oxidant air 9.4 L·min, analytical mode FAAS (CON), measurement mode ‘Integrated (5 s), equation type linear fit, background/ZAA ON. The correlation coefficient of the standard curve within the range of 0–15 mg·L Pb was 0.999 and the standard deviation of nine assays was 0.027 with a CV of 0.33%.