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