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

Abstract

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 UL-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 UL-1. After 34 days and with root length as endpoint, significant toxicity appeared at a gel contamination level above 7.5 mg UL-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 UL-1. Lower threshold and EC50 were observed for biomass as endpoint (resp. 5 and 7.3 mg UL-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. (C) 2008 Elsevier B.V. All rights reserved.