Poster P3: Toxicity and Detoxification of Cadmium in the Aquaticmacrophyte Ceratophyllum Demersum; session "Metal homeostasis and detoxification"

The heavy metal cadmium (Cd) is an important pollutant and poisonousto many organisms. We studied the effects of Cd on C.demersum under environmentally relevant conditions. High, moderateand low concentrations of Cd had different effects. Lethally toxicconcentrations (100–200 nM) led to growth stop and the plants’ability to perform photosynthesis (measured as Fv/Fm) decreased morethan twofold, consistent with decreased pigment content. Moderatelytoxic concentrations (10–50 nM) led to reduced growth, slightlyreduced pigment content, but hardly affected photosynthesis (measuredas O2 exchange and as Fv/Fm). Lower concentrations(0.2–5 nM) even had beneficial effects, like enhanced growth rate.When applied in low concentrations, Cd was homogeneously distributedin the whole cross-section of the leaves like a nutrient.Moderate and high Cd concentrations led to sequestration of Cd in thevascular bundle and the epidermis cells, where Cd does not affectphotosynthetic molecules. At toxic Cd concentrations, Zn was redistributedand mainly found in the vein along with Cd, indicatinginhibition of Zn transporters. Consistently, by metalloproteomics(HPLC-ICP-MS) we found that during Cd toxicity concentrations of Cdincreased in fractions of the major photosynthetic complexes while Mgdecreased, suggesting the replacement of Mg by Cd in chlorophylls.Furthermore, the induction of phytochelatins was not proportionalto metal concentration, but had distinct thresholds, specific for each PCspecies. PC3 especially was switch-like induced already at 20 nM Cd,which was previously regarded as non-toxic to most plants. Phytochelatinlevels at the lowest Cd concentrations were not detectable orbelow 0.1 % of the level at sublethally toxic concentrations, suggestingthat they do not have another function than metal detoxification