The roles of internal iron hydroxide precipitation, sulphide toxicity and oxidizing ability in the survival of Stratiotes aloides roots at different iron concentrations in sediment pore water.

Stratiotes aloides L. is an aquatic macrophyte that occurs in waters on reduced peaty sediments which have a relatively narrow range of free-iron content in the sediment. Comparison of different aquatic macrophytes reveals that species from reducing sediments have much lower oxidizing ability than do species from oxidizing sediments. Compared with those other species from reducing sediments, the oxidizing ability of Stratiotes aloides is very low and probably makes the species very vulnerable to sulphide toxicity and internal precipitation of iron hydroxide. Apoplastic iron contents were determined for Stratiotes roots growing in sediments with different free-iron concentrations. Roots collected from sediment with a low free-iron content appeared to have a relatively low apoplastic-iron content whereas roots from sediments with a relatively high free-iron concentration had a relatively high apoplastic-iron content. In sediments with low free-iron levels, sulphide levels are generally high. Using light microscopy, iron hydroxide precipitates were observed around the endodermis and cortical air spaces of the roots of Stratiotes aloides growing in iron-rich sediments. Internal iron oxidation outside the endodermis can prevent iron toxicity inside the stele and thus enable root apices to survive. The root hairs, however, die because of iron hydroxide precipitation at their base; the dead roots have the highest visible iron hydroxide content. Excessive internal iron hydroxide precipitation and the consequential early death of the roots probably explains the absence of Stratiotes aloides in locations with high free-iron levels in sediment pore water.