Comparative assessment of heavy metal accumulation and bio-indication in coastal dune halophytes

This study aimed at assessing heavy metals (Fe, Mn, Zn, Cu, Cr and Pb) in four perennial halophytes (viz. Heliotropium bacciferum, Halopyrum mucronatum, Ipomoea pes-caprae and Salsola imbricata) growing at two sites on the Karachi coast. Site - II, closer to the Industrial area had higher bioavailability as well as translocation factor (TF) for most of the heavy metals and Na⁺ where soil sediments had lower pH (approximately 7.5), higher salinity (EC) and organic matter (OM). Site - I which was far from Industrial area had comparatively higher bio-concentration factor (BCF) and lower TF for metal ions and soil pH of 8.1-9. Metal accumulation in plants was both site and species specific. Extractable concentration of shoot Pb in all tested halophytes was above normal of the threshold values (i.e., >0.3 mg kg^-1) while Mn (<50 mg kg^-1) and Cu (<40 mg kg^-1) were within permissible limits. Salsola imbricata had highest Na⁺ at both sites (site - I = 73; site - II = 98 mg kg^-1) with and 10 mg kg^-1 extractable shoot Pb at site - I. Ipomea pes-caprae also accumulated shoot Pb higher than normal (site - I = 3.3; site - II = 0.8 mg kg^-1) with lowest Na⁺ content. Heliotropium bacciferum had higher extractable Pb (site - I = 10.5; II = 2.75) with >20 mg kg^-1 Na⁺ in shoot while maintaining > 1 TF for Pb, Cu, Mn and Zn at site - I and all tested metals at site - II. Halopyrum mucronatum had highest shoot Fe (644 mg kg^-1), Zn (63 mg kg^-1) and Cr (9.2 mg kg^-1) at site - II and above threshold values of Pb at both sites (site - I = 8.2; site - II = 2.5 mg kg^-1) which makes this species an ideal bio-indicator candidate while other species could be potentially used for Pb phytoremediation.