Frond-level analyses reveal functional heterogeneity within heavy metal-treated duckweed colonies

Duckweeds (Lemnaceae) rapidly produce clonal populations that make them ideal models in plant physiological and ecotoxicological research. Yet, despite their genetic homogeneity, duckweed colonies are clusters of successively produced fronds with different ontogenetic states, and this heterogeneity has rarely been studied. We analyzed frond-level photosynthetic responses of three duckweed species (Spirodela polyrhiza, Landoltia punctata and Lemna minor) to hexavalent chromium and nickel by means of chlorophyll fluorescence imaging. Our aim was to test whether fronds, or frond zones in their different developmental stages respond differently to heavy metal stress. Different response patterns were found for the three tested species. Young fronds of S. polyrhiza were more sensitive to chromate but less affected by nickel compared to mature ones, while in La. punctata the opposite was found. In Le. minor cultures, young fronds were more sensitive to both heavy metals compared to mature ones. Within-frond patterns indicated that chromate inhibited the marginal frond regions strongest. In La. punctata and Le. minor, nickel was most detrimental in the developing frond regions. In contrast, in S. polyrhiza the middle zone of both young and mature fronds was affected strongest by nickel. The observed patterns suggest that internal redistribution of toxicants plays a key role in shaping duckweed responses.Results highlight that ontogenetically different parts of duckweed plants respond differently to environmental stimuli. Since duckweed-based impact studies and economical applications rely strongly on the interactions between the duckweed colony and its environment, frond-, and within-frond level analysis of duckweed responses comprise a unique tool to reveal uptake, re-distribution and mode of action of environmentally relevant substances

Species- and Metal-Specific Responses of the Ionome of Three Duckweed Species under Chromate and Nickel Treatments

In this study, growth and ionomic responses of three duckweed species were analyzed, namely Lemna minor, Landoltia punctata, and Spirodela polyrhiza, were exposed for short-term periods to hexavalent chromium or nickel under laboratory conditions. It was found that different duckweed species had distinct ionomic patterns that can change considerably due to metal treatments. The results also show that, because of the stress-induced increase in leaf mass-to-area ratio, the studied species showed different order of metal uptake efficiency if plant area was used as unit of reference instead of the traditional dry weight-based approach. Furthermore, this study revealed that μXRF is applicable in mapping elemental distributions in duckweed fronds. By using this method, we found that within-frond and within-colony compartmentation of metallic ions were strongly metal- and in part species-specific. Analysis of duckweed ionomics is a valuable approach in exploring factors that affect bioaccumulation of trace pollutants by these plants. Apart from remediating industrial effluents, this aspect will gain relevance in food and feed safety when duckweed biomass is produced for nutritional purposes