The influence of the combined effects of acute gamma-radiation, sodium bromate and sodium nitrate on lettuce (Lactuca sativa) seedling root growth

Among special industrial reservoirs used for the storage of liquid radioactive waste of Mayak PA, Russia, one of the most radioactively contaminated is the R-17 reservoir, so-called 'Staroye Boloto' (the total β-activity of water ranged in the observation period from 0.4 MBq/l to 4.5 MBq/l, the total a-activity ranged from 43 to 420 Bq/l). Also this reservoir is characterized by high level of chemical contamination, in particular, the concentration of nitrates in water is 2.5-4,4 g/l, sodium bromate - up to 35 mg/l. One of the interesting questions is interaction of radiation and chemical contamination in their effect on living organisms in this reservoir. In laboratory experiments seeds of Lactuca sativa were used; the effect of the studied factor on the length of the sprout's root was estimated. To assess the effect of chemical toxicants the solutions of each salt in 7 different concentrations were used, distilled water was used as a control. For evaluation of acute effects of external gamma irradiation the seeds after exposure for 24 hours in distilled water, were irradiated at 7 different doses using gamma-unit on the basis of Cs-137 with the dose rate of 0.62 Gy/min. To assess the combined effects of acute external gamma irradiation, of nitrates and bromates, seeds after 24 hour exposure at each test concentration of the salts solutions were irradiated using gamma-unit. To calculate the effective concentrations or doses was used drc package for R software. To calculate the dose rate to aquatic organisms in the R-17 was used ERICA Assessment Tool 2012. It was found out that the EC50 of sodium nitrate for lettuce was 2.69 g/l, which is comparable to the concentration of nitrates in the 'Staroye Boloto'. This indicates that nitrate can have significant toxic effect on aquatic higher plants of the reservoir. The EC50 of sodium bromate was 14.6 mg/l. This is less than the maximum concentration of the substance in the R-17, which suggests that sodium bromate has a toxic effect on macrophytes. The ED50 of gamma irradiation was 10.5 Gy. According to our estimates, the dose of ionizing radiation to the vascular plants of R-17 in average is about 0.03 Gy/hr. The accumulated dose for 5 days (the period corresponding to the duration of laboratory experiments) is 3.6 Gy, which does not exclude significant radiation exposure for macrophytes. In case of combined effect of three factors (nitrates, bromates and gamma-irradiation), the effect is generally located in an antagonistic interaction area, although these changes were not significantly different from the effect of additive interaction. For the reservoir R-17 it may indicate that the effects to aquatic organisms may correspond to the sum of effects of three main polluting factors - nitrates, bromate and radioactive contamination - or be slightly less. Document available in abstract form only. (authors)