Chemical changes in selected wetland plant species with increasing Fe supply, with specific reference to root precipitates and Fe tolerance.

A range of wetland plant species (44) was screened in solution culture experiments for sensitivity to large concentrations of supplied Fe. Two distinct root precipitates were observed (see 'Appendix' for a tabulated summary of symptoms). Energy dispersive spectroscopy showed that an ochreous precipitate was probably a hydrated ferric oxide whilst a pale yellow or yellow-grey one was an Fe-P solid, possibly a ferric phosphate. Ochre precipitates were found almost exclusively cm the roots of Fe-tolerant species and were particularly intense on the most tolerant ones, Monocotyledonous species (generally more Fe-tolerant than dicotyledonous species and with higher root porosities), tended to produce ochre most freely. The Fe-P precipitate was frequently found on roots of the less tolerant species and was particularly abundant on the most sensitive ones. Blackening of roots (with flaccidity I, another common response of roots of Fe-sensitive plants exposed to large Fe concentrations, was not a result of any obvious external precipitate. There was no evidence that differential pH changes induced by roots helped to determine either Fe tolerance or the nature of the root precipitate observed. Roots of all species tested caused the pH of an agar medium to rise. Elevated Ft- supply resulted in greater external acid phosphatase activity in roots of two test species. The increase corresponded with the species' mean relative growth rate (RGR) (and presumed P requirement), and with me fertility of sites in which it grows. Chemical analyses of shoots and roots of four species of differing Fe tolerance showed that, with increasing Fe supply, mean shoot Fe concentrations increased significantly whereas for most other elements (except Na and Zn) a significant reduction in concentrations in the shoot was observed. Mean root concentrations of most elements also fell significantly with increasing Fe supply, though for three of the four test species there was a significant increase in root P concentrations. It is clear that only some wetland plant species form ochreous root precipitates when grown in Fe-rich conditions, and have an effective Fe-exclusion mechanism. Growth of those species that lack this capacity is affected adversely by high Fe concentrations. This might be owing both to direct effects of Fe uptake and to indirect effects, particularly of Fe upon P metabolism. No evidence was found to suggest chat the Fe-tolerant species examined could accommodate higher tissue Fe concentrations than the Fe-sensitive species.