Root length of aquatic plant, Lemna minor L., as an optimal toxicity endpoint for biomonitoring of mining effluents

Abstract

Lemna minor, a free-floating macrophyte, is used for biomonitoring of mine effluent quality under the Metal Mining Effluent Regulations (MMER) of the Environmental Effects Monitoring (EEM) program in Canada and is known to be sensitive to trace metals commonly discharged in mine effluents such as Ni. Environment Canada's standard toxicity testing protocol recommends frond count (FC) and dry weight (DW) as the 2 required toxicity endpoints-this is similar to other major protocols such as those by the US Environmental Protection Agency (USEPA) and the Organisation for Economic Co-operation and Development (OECD)-that both require frond growth or biomass endpoints. However, we suggest that similar to terrestrial plants, average root length (RL) of aquatic plants will be an optimal and relevant endpoint. As expected, results demonstrate that RL is the ideal endpoint based on the 3 criteria: accuracy (i.e., toxicological sensitivity to contaminant), precision (i.e., lowest variance), and ecological relevance (metal mining effluents). Roots are known to play a major role in nutrient uptake in conditions of low nutrient conditions-thus having ecological relevance to freshwater from mining regions. Root length was the most sensitive and precise endpoint in this study where water chemistry varied greatly (pH and varying concentrations of Ca, Mg, Na, K, dissolved organic carbon, and an anthropogenic organic contaminant, sodium isopropyl xanthates) to match mining effluent ranges. Although frond count was a close second, dry weight proved to be an unreliable endpoint. We conclude that toxicity testing for the floating macrophyte should require average RL measurement as a primary endpoint.

Comparing the acute sensitivity of growth and photosynthetic endpoints in three Lemna species exposed to four herbicides

“…In the past, little attention has been paid to the roots in Lemna since it was generally considered that root fragility made their handling for measurements difficult and that it was impractical to obtain sufficient numbers of individual plants with identical root lengths to initiate tests. However, more recently the ecotoxicological significance of the root endpoint has been re-evaluated and root length was shown to be a sensitive, precise and ecologically significant endpoint in comparison with more traditional frond growth and biomass endpoints (Park et al, 2013;Gopalapillai et al, 2014). Compared with results obtained from measurements of frond area, root re-growth showed paraquat to be the most toxic of the tested herbicides to all three Lemna species, with mean EC 50 values of 7.1, 7.9 and 10.6 mg L À1 for L. gibba, L. paucicostata and L. minor, respectively.…”

“…(see reviews by Wang, 1990). Recently, Gopalapillai et al (2014) reported root length of Lemna minor as to be the optimal endpoint for biomonitoring of mining effluents. The authors considered average root length (RL) the ideal endpoint for three reasons: accuracy (i.e., toxicological sensitivity to the contaminant), precision (i.e., lowest variance), and ecological relevance (metal mining effluents) (Gopalapillai et al, 2014).…”

“…Recently, Gopalapillai et al (2014) reported root length of Lemna minor as to be the optimal endpoint for biomonitoring of mining effluents. The authors considered average root length (RL) the ideal endpoint for three reasons: accuracy (i.e., toxicological sensitivity to the contaminant), precision (i.e., lowest variance), and ecological relevance (metal mining effluents) (Gopalapillai et al, 2014). A well-defined toxicant concentrationdependent inhibition of root re-growth has also recently been shown with the root re-growth test using three Lemna species (Park et al, 2013).…”

“…Specifically, data obtained from root re-growth and Chl a fluorescence measurements are compared with those based on a traditional endpoint of frond area. The accuracy and precisions (sensu Gopalapillai et al, 2014) of the three endpoints are evaluated.…”

β-Radiation Stress Responses on Growth and Antioxidative Defense System in Plants: A Study with Strontium-90 in Lemna minor

“…Highly sensitive stress responses on L. minor root growth were also observed in metal and pesticide toxicity. Following this, L. minor root growth/length has been suggested as an alternative bioassay for toxicity testing [ 42 , 43 , 44 ]. All endpoints, including frond biomass, frond number and frond area, showed a clear decrease at the highest tested dose rate (97 ± 8 mGy·h −1 ).…”