Table 221

LC-XANES Fittings of Sesbania drummondii Exposed to 500 mg/L Pb(NO3)2 Using Lead (II) and (IV) Model Compounds

Samples % PbNO3 % PbSO4 % Pbmetal % PbS % Pb acetate

Source: Sharma, N.C. et al., Environ. Toxicol. Chem., 23, 134, 2004. With permission.

22.3.2 Characteristics of EXAFS in Sesbania Samples

EXAFS features (Figure 22.2A and B) and FEFF fittings (Table 22.2 and Table 22.3) shown by Sharma et al. [18] indicate that lead in the root and leaf samples is bound into a chemical moiety with interatomic distances similar to lead (II) acetate. Though the root sample of Sesbania displays spectra comparable to the lead (II) acetate type of structure, the presence of a secondary complex complicates the interpretation. The FEFF fittings of root sample, however, indicate a ten-coordinate system that has a mixture of different compounds including lead particles. At the same time, the leaf sample shows an octahedral coordination, making EXAFS interpretation easy. XANES fittings of leaf sample are simple in composition, showing absence of lead particles. Though both samples are complex, they contain compounds of similar structure and composition, as indicated by XANES structure.

Presence of lead acetate in large proportions (>50%) in both the samples (Table 22.1) suggests transport of lead via Pb-organic acid complex. Metal-organic acid complexation has been reported as a phase of metal transport in a variety of plants [17,40,41]. In Phaseolus vulgaris, formation of various lead complexes including Pb-salicylate was reported when plants were grown in medium containing Pb and EDTA. Cerussite (lead carbonate) was found to be the predominant Pb species in leaves when Phaseolus plants were grown in solution containing Pb(NO3)2 alone.

The evidence for sulfur ligands is a potentially unique aspect of lead speciation in Sesbania species. Though sulfur is present in both the samples, the proportion in leaf is double the root content. The nature of sulfur complex also varies. Leaves contain PbSO4 and PbS, but roots have only PbS. These ligand properties are indicative of glutathione (y-Glu-Cys-Gly) and phytochelatins, small metal-binding polypeptides with the amino acids sequence (y-Glu-Cys)n- Gly, where n = 2 to 14. Phytochelatins (PCs) play an important role in heavy metal homeostasis and detoxification [42] and their induction has been reported as an effective strategy against Cd toxicity [12,43]. A recent study suggested that PCs might also be involved in arsenic detoxification [44]. Reports of PC induction in response to Pb toxicity are not common in higher plants [16]. However, Pb-induced PC synthesis was confirmed in a common microalga, Stichococcus bacillaris [45].

22.4 ANTIOXIDATIVE DEFENSE IN A LEAD ACCUMULATOR 22.4.1 Catalase Activity in Sesbania Seedlings

Catalase activities of Sesbania drummondii seedlings grown in the absence or presence of 500 mg/L Pb(NO3)2 were determined by Ruley [10]. The enzyme activities in Pb-treated seedlings increased dramatically, >300% higher than the respective values of controls at week 4 (Figure 22.3). In Thlaspi caerulescens, a constitutive Cd hyperaccumulator, an elevated catalase induction in response to Cd stress has been demonstrated [46]. An accumulator of Pb, Pisum sativum [7], also demonstrated an elevated activity of CAT in response to 104 or 207 mg/L Pb [47].

FIGURE 22.2 (A) Extended X-ray absorption fine structure of lead model compounds: lead (II) nitrate, lead (II) sulfide, lead (II) sulfate, and lead (IV) oxide. (B) Extended X-ray absorption fine structure of lead-treated Sesbania drummondii samples and lead (II) acetate. (From Sharma, N.C. et al., Environ. Toxicol. Chem., 23, 134, 2004. With permission.)

FIGURE 22.2 (A) Extended X-ray absorption fine structure of lead model compounds: lead (II) nitrate, lead (II) sulfide, lead (II) sulfate, and lead (IV) oxide. (B) Extended X-ray absorption fine structure of lead-treated Sesbania drummondii samples and lead (II) acetate. (From Sharma, N.C. et al., Environ. Toxicol. Chem., 23, 134, 2004. With permission.)

22.4.2 SOD Activity in Sesbania Seedlings

Superoxide dismutase (SOD) activities in the absence and presence of 500 mg/L Pb(NO3)2 were also reported by Ruley [10]. SOD activities in Pb-treated seedlings gradually increased and were recorded to be >200% higher than the respective values of controls at week 4 (Figure 22.4). Similar

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