Tolerance of hairy roots of carrots to U chronic exposure in a standardized in vitro device

In soil, high variability of U bioavailability results in large range of apparent U toxic levels for plants. U toxicity on hairy roots of carrot was studied in nutrient gel with a standardized in vitro device. After exposure to 2.5 and 20 mg U L−1 for 34 days, U concentration ranged between 4 and 563 mg U kg−1 fresh weight which was in good accordance with U accumulation by roots of plant from contaminated soils. Threshold of U toxicity for root length decreased with time and a transient hormesis occurred for exposure to 2.5 and 5 mg U L−1. After 34 days and with root length as endpoint, significant toxicity appeared at a gel contamination level above 7.5 mg U L−1 corresponding to a maximum U concentration in the liquid phase of 0.8 mg L−1. The calculated EC50 for root length as a function of gel contamination was 9.4 mg U L−1. Lower threshold and EC50 were observed for biomass as endpoint (resp. 5 and 7.3 mg U L−1). The low values observed in this study could result from high sensitivity of carrot to U, high bioavailability of U in gel or absence of interferences with microorganisms. This in vitro device appeared adapted to study toxicity of U to plant roots in optimal conditions of both exposure and observations and is recommended to examine further physiological processes and the influence of microorganism interactions.