Impact of heterophylly on the sensitivity of Myriophyllum aquaticum biotests

The heterophylly of M. aquaticum affects significantly the sensitivity of aquatic macrophyte biotests and should be taken into account in the development of a standardized test design. The 2,4-D results show the importance of an additional macrophyte biotest to the Lemna test, where the no observed effect concentration (NOEC) value is more than 30 times lower (7 μg/L) than the NOEC value of Lemna in the literature (270 μg/L). Furthermore, the growth rate endpoint in macrophyte biotests should not be misinterpreted. Rapid shoot elongation of amphibious macrophytes, which become submerged, is mainly caused by ethylene-triggered endogenous processes that are not connected to photosynthetic carbon assimilation and appear to be part of a stress reaction to avoid adverse environmental conditions.