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SUMMARY1 he iron plaque on roots of Aster tripolium L. growing in waterlogged salt marsh soil adsorbed appreciable amounts of Zn and Cu,with maximum Zn/Fe and Cu/Fe ratios of 0-1. When concentrations of Zn or Cu adsorbed 'n the iron plaque are expressed as mg metal kg"' FeOOH (assuming that iron plaque consists mainly of I'eOOH), the Zn and Cu concentrations ot the iron plaque was up to 680 and up to 2900 times higher than in the surrounding sediment, respectively. The Zn concentration in red roots (with iron plaque) was higher than in white roots (without iron plaque), Zn concentrations in field sampled roots were correlated with the amount of Zn on "le roots and the Zn concentration in the soil, whereas Cu concentrations in the roots were only significantly correlated to the Cu concentration in the soil. In vitro experiments showed that red roots take up more Zn than •white roots. Measurement of Zn uptake by excised roots showed that the uptake of Zn into the xylem fluid was significantly higher in roots with 500-2000 nmol Fe cm"'' on the root surface compared to roots with less than 500 Or more than 2000 nmol Fe cm"'^ on the root surface. The results indicate that iron plaque enhances uptake of Zn ny the roots but may act as a barrier when large amounts of Fe are deposited on the root surface. The role of the "•on plaque on roots of salt marsh plants growing in soil contaminated with heavy metals is discussed

Dimethylsulphoniopropionate (DMSP) is produced in high concentrations in many marine algae, but in higher plants only in a few salt marsh grasses of the genus Spartina, in sugar canes (Saccharum spp.), and in the Pacific strand plant Wollastonia biflora (L.) DC. The high concentrations found in higher plants (up to 250 micromol g(-1) dry weight) suggest an important role, but though many functions have been suggested (including methylating agent, detoxification of excess sulphur, salt tolerance, and herbivore deterrent), its actual functions remain unclear. The fact that the ability to produce DMSP in high concentrations is found in species that have no taxonomic or ecological relationship suggests that the compound evolved independently and serves different functions in different plants. This is supported by observations that DMSP in W. biflora behaves differently from that in Spartina species. While DMSP concentrations in W. biflora have been found to increase with increasing salinity, suggesting a role in osmotic control, such a relationship has not been found for DMSP in Spartina species. Recent observations on tissue culture showed that, while undifferentiated tissue of W. biflora produced DMSP, such material of Spartina alterniflora Loisel. did not. Ongoing studies with tissue culture of both species have opened up new avenues of research on DMSP in higher plants, ultimately to elucidate the functions of this enigmatic compound.

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