Effects of lead and chelators on growth, photosynthetic activity and Pb uptake in Sesbania drummondii grown in soil

Abstract

Effects of lead (Pb) and chelators, such as EDTA, HEDTA, DTPA, NTA and citric acid, were studied to evaluate the growth potential of Sesbania drummondii in soils contaminated with high concentrations of Pb. S. drummondii seedlings were grown in soil containing 7.5 g Pb(NO₃)₂ and 0–10 mmol chelators/kg soil for a period of 2 and 4 weeks and assessed for growth profile (length of root and shoot), chlorophyll a fluorescence kinetics (F_v/F_m and F_v/F_o) and Pb accumulations in root and shoot. Growth of plants in the presence of Pb + chelators was significantly higher (P < 0.05) than the controls grown in the presence of Pb alone. F_v/F_m and F_v/F_o values of treated seedlings remained unaffected, indicating normal photosynthetic efficiency and strength of plants in the presence of chelators. On application of chelators, while root uptake of Pb increased four-five folds, shoot accumulations increased up to 40-folds as compared to controls (Pb only) depending on the type of chelator used. Shoot accumulations of Pb varied from 0.1 to 0.42% (dry weight) depending on the concentration of chelators used.

Introduction

Lead (Pb) contamination in soil is a widespread phenomenon and originates from automobiles, metal smelting plants, mines, lead-contaminated sewage sludge, industrial wastes, etc. (Zakrzewski, 1991). Pb exposure to plants causes effects such as the disturbance in mitosis (Liu et al., 1994, Wierzbicka, 1994), induction of leaf chlorosis (Johnson and Proctor, 1977), depression of photosynthetic rate, (Bazzaz et al., 1974), inhibition in root and shoot growth (Fargasova, 1994, Liu et al., 1994), and inhibition and activation of enzymatic activities (Van Assche and Cliisters, 1990). Severe Pb contamination in soils may lead to a variety of environmental problems – loss of vegetation, ground water contamination, and ultimately Pb toxicity to animals and humans (Body et al., 1991). Thus, there is an urgent need for remediation of contaminated sites using an effective and environment-friendly technology such as phytoremediation.

In recent years, phytoremediation has emerged as a viable biotechnology to decontaminate the heavily polluted sites (Blaylock et al., 1997, Huang et al., 1997, Kirkham, 2000, Sharma et al., 2004). This strategy makes use of hyperaccumulator plants, which have the inherent potential to survive and accumulate excessive amounts of metal ions in their biomass without incurring damage to basic metabolic functions (Cunningham et al., 1997). With successive cropping and harvesting of accumulator crops, the levels of contaminants can be reduced substantially. For a plant species to be efficient in lead phytoextraction it should accumulate metal concentration >0.1% of shoot dry weight, besides having high biomass productivity (Kirkham, 2000). A balance between metal accumulation and plant biomass productivity is critical for a plant species to be used in Pb phytoextraction (Huang and Cunningham, 1996). From this standpoint, plant species such as Indian mustard, pea, and corn were focused recently for Pb phytoremediation research. These species accumulate high amounts of lead, and produce satisfactory biomass (Huang et al., 1997, Blaylock et al., 1997, Epstein et al., 1999). Another interesting Pb accumulator is Sesbania drummondii, a perennial large bushy plant with greater biomass productivity than the above plant species (Ruley, 2004). S. drummondii grows naturally in seasonally wet places of the southern coastal plains of the United States and tolerates high concentrations of soil Pb. It demonstrated a unique potential of Pb accumulation in aerial parts from an aqueous solution (Sahi et al., 2002).

To compensate for the relatively low metal accumulation capacities of Indian mustard, corn, pea and other potential plant species, chelates such as ethylenedinitrilotetraacetic acid (EDTA), N-(2-hydroxyethyl)ethylenediaminetriacetic acid (HEDTA), diethylene trinitrilopentaacetic acid (DTPA), trans-1,2-cyclohexylenedinitrilotetraacetic acid (CDTA) and ethylenebis [oxyethylenetrinitrilo] tetraacetic acid (EGTA) were supplemented to the Pb-contaminated soils (Blaylock et al., 1997, Huang et al., 1997, Epstein et al., 1999, Kirkham, 2000, Sarret et al., 2001). Application of chelators induces metal desorption from minerals and boosts translocation of Pb from root to shoot. A chelate-assisted increase of 100 to 200 folds in shoot Pb accumulation was noticed in Indian mustard (Blaylock et al., 1997, Epstein et al., 1999) while several-fold increases were observed in pea and corn (Huang et al., 1997). Kirkham (2000) reported a significant increase in shoot Pb when sunflower plants were grown in soils contaminated with sewage sludge. Chelators not only facilitate Pb uptake and translocation, but also protect plants from oxidative stress that is produced as a result of heavy metal (Pb) exposure, as reported in Sesbania seedlings grown in vitro (Ruley et al., 2004). Studies show how exposure of Pb or other heavy metals affect growth and photosynthetic activities in plants (Xiong, 1997, KrishnaRaj et al., 2000). Chlorophyll a fluorescence, a non-destructive marker of the photosynthetic apparatus, has been used extensively in screening for abiotic stresses, such as heat, chilling, drought, salinity and heavy metal stresses (Becerril et al., 1988, Krause, 1991, KrishnaRaj et al., 2000, MacFarlane, 2003). In the present investigation, we have utilized chlorophyll a fluorescence parameters as a quantitative marker to assess and compare the tolerance of Sesbania sp. when exposed to Pb and different chelators.

Therefore, in order to understand the effects of high concentrations of Pb and chelators, this study was focused to determine 1) growth profile, 2) chlorophyll a fluorescence kinetics [F_v/F_m and F_v/F_o], and 3) Pb accumulation in Sesbania drummondii seedlings grown in soils contaminated with a high concentration of Pb in the presence or absence of synthetic chelators, such as EDTA, DTPA, HEDTA, NTA and citric acid. Comparing the efficacy of different chelators on Pb accumulation by Sesbania was also aimed in this study.