Phytotoxic response of ryegrass (Lolium multiflorum L.) to extreme exposure to two anionic surfactants

Bioremediation is an effective and environment-friendly treatment used to clean up hydrocarbon-contaminated soil. However, the effectiveness of this treatment is often limited by the low bioavailability of the target contaminants. Surfactants addition thus appears as a way to increase solubility of these hydrophobic molecules and consequently improve their bioavailability. The use of biological surfactants is often favoured over synthetic ones because they are claimed to be non-toxic to the environment though few studies have addressed this issue. The present work evaluated the effects of a synthetic surfactant, sodium dodecyl sulphate (SDS) and a biosurfactant (rhamnolipids) on germination and growth of ryegrass over a wide range of concentrations, between one up to ten times their respective critical micellar concentration (CMC). Experimental results showed that SDS inhibited seed germination of Lolium multiflorum at high concentrations (10 × CMC), unlike rhamnolipids, which did not induce any toxicity symptom at germination stage. At the growth stage, high rhamnolipid concentrations induced chronic phytotoxicity by significantly reducing root length, decreasing biomass production and disrupting the enzymatic defence system. Thus, biosurfactants are less toxic than synthetic ones but their application at high doses in bioremediation treatments might still induce phytotoxicity symptoms and thus negatively affect the environment.