The uptake and accumulation of platinum metals by the water hyacinth ( Eichhornia crassipes)

Abstract

The biological effects of the platinum group metals on plants have been studied by treating plants with chlorocomplexes of each metal at a range of concentrations. The vascular aquatic plant Eichhornia crassipes (water hyacinth) was selected for detailed study because of its remarkable ability to assimilate high levels of transition elements from solution. Water hyacinths are capable of recovering platinum group metals even from dilute solution though to varying degrees depending on the metal: ▪ When this is compared to the relative order of toxicity at the 10 ppm level for each metal, some similarities emerge: ▪ The relationship between phytotoxicity and position in the periodic table is tenuous but appears to be linked with the oxidation state and hence electron configuration of the metal ion. The two least toxic ions, Ir 3+ and Rh 3+, have the electron configuration ( d 6); it is significant too that Pt 4+( d 6) is far less toxic than Pt 2+( d 8). A similar relationship has been found for the phytotoxicity of 1st row elements: ▪ Included in the soft acid classification is Pt 2+ and Pd 2+ whilst Rh 3+ and Ir 3+ are considered borderline between hard and soft, along with Fe 2+, Co 2+, Cu 2+ and Zn 2+. This approach goes some way to explaining the relative toxicity of the platinum metals. The anomaly is Pt 4+, which is classified as soft, but which is relatively non-toxic; however, some softness is lost when Pt 2+ is oxidized to Pt 4+. The most prominent toxic symptom at low levels was the appearance of reddish-brown streaks in the leaves of Eichhornia crassipes. Such phytotoxic symptoms have been observed in beans ( Phaseolus vulgaris) and soybeans ( Glycine max) treated with high quantities of zinc. Cd 2+, Co 2+ and Ni 2+ are reported to cause similar symptoms. In contrast to the toxic effects of Pt 2+, Rh 3+ appears to exhibit a tonic effect. When treated with 10 ppm Rh 3+ applied as Na 3[RhCl 6], water hyacinth increased its biomass some 6.7% more than control plants, grown under the same conditions. When the South African grass Setaria verticillata was treated with 0.5 ppm Pt 2+ (as K 2[PtCl 4]), vascular discolouration was absent and the roots were growth stimulated some 65% more than controls. Thus the phytotoxic symptoms of platinum vary according to which species is treated, though with water hyacinth, some stimulation of vegetative reproduction was apparent with platinum complexes, at low levels. When applied as the antitumor complex cis[Pt(NH 3) 2Cl 2] at low levels, some 47.9% of the platinum found in the leaves of water hyacinth was associated with α-cellulose and lignin; 16.1% was removed by the proteolytic enzyme pronase and 20.8% found with water soluble pectates. A similar distribution of platinum was found in the floats of water hyacinth. In the roots of treated plants, the values were 35%, 9.5% and 14.2% respectively; in addition to this, a further 23.1% was removed with low molecular weight alcohol soluble materials and 12.0% with polar water soluble materials. Thus in water hyacinth, the cell wall acts as a ion exchange column trapping most of the platinum, though some is found bound to water soluble pectates. Together, this accounts for 49.2% of the platinum found in the roots and this figure rises to 68.7% in the leaves. The platinum released by pronase may represent that which is bound to protein from a number of sources including organelle protein, membrane protein and cell wall glycoprotein.