Chlorophyll-. a fluorescence is an effective tool for measuring photosynthesis. Photosynthesis is particularly sensitive to the presence of zinc or other metals. Plant age and developmental stage strongly influence the effects of metals on the photosynthetic apparatus. However, there is little information on how this may affect toxicological assessment in plants. In this study, three . Lemnaceae species were exposed to a range of zinc concentration in static, batch exposures for seven days, as detailed for . Lemnaceae toxicity testing in OECD guidelines. Effects of zinc on photosynthetic performance in the three species were measured over the seven days. The maximum quantum efficiency of photosystem II, . Fv/Fm, the effective quantum efficiency, . Y(II), and photochemical quenching, . qP, were measured in mature and young fronds, as well as along a developmental gradient within fronds. . Fv/Fm and . Y(II) in young, emerging . Landoltia punctata fronds were more severely impacted by zinc than in developed, mature fronds. Furthermore, younger proximal sections of . L. punctata fronds were more impacted than older distal frond sections. Overall, . Fv/Fm and . Y(II) also tended to be more affected by zinc in young, compared to mature, . Lemna gibba and . Lemna minor fronds. Single colony, time-point or leaf-zone analyses may not, therefore, show the full biological picture of the impact of a toxicant, while species-specific differences need also to be considered. Therefore, selection of . Lemnaceae species and developmental stages of fronds should be given careful consideration when using chlorophyll-. a fluorometry for assessing chemical toxicity in . Lemnaceae. © 2012 Elsevier B.V.