Abstract

We addressed the question of whether results gathered from single- species laboratory bioassays can be extrapolated to field populations by evaluating the response of 14 geographically isolated clones of Lemna gibba L. to the toxic compound simazine. The plants were exposed to a herbicide stress and several effects were studied in laboratory bioassays lasting 14 days. The ranges of the EC50 values registered among the clones were 0.31- 0.62, 0.26-0.55, 0.29-0.53 and 0.33-0.53 mg a.i.l-1 for the frond number, dry weight, total chlorophyll and surface area, respectively. The orders of the sensitivity and dissimilarity relationships between the clones (as well as the ecotoxicological parameters) were exclusive of each growth response. However, the groups of more sensitive clones and more resistant clones were the same for each of the effects analysed. Within the effects studied the highest variation between the clones was observed at concentrations of >0.1 mg a.i.l-1. The results show that the extrapolation considered is influenced by two main factors: the herbicide concentration and its temporal variation in the aquatic medium. The broad phenotypic plasticity probably reduced the variability of the ecotoxicological parameters between clones considering the important genotypic differences of L. gibba clones previously studied. In order to extrapolate the results from laboratory studies to natural populations of L. gibba it seems more important to consider the variation in environmental conditions than the genetic diversity of natural populations.