Disinfection by-products and their biological influence on radicle development, biomass accumulation, nutrient concentration, oxidative response and lipid composition of two tomato (Solanum lycopersicum) cultivars

Abstract

Trihalomethanes are disinfection byproducts of chlorinated waters, and there is a growing interest to understand plant responses to organohalogens. This study investigates the effects of increasing trihalomethane dose on the physiology of tomato (Solanum lycopersicum) and determines whether the extent of physiological impacts of trihalomethane exposure on seedling radicle length, biomass accumulation, concentration levels of 12 key nutrients, oxidative stress, fatty acids and α-tocopherol content in membrane lipids of tomato correlated with either the number of bromine or chlorine atoms in the trihalomethane molecules. The 2 x 4 x 5 factorial experiment was laid out in CRD with four replications. Two cultivars of tomato were exposed to 4 levels of trihalomethanes (bromodichloromethane, bromoform, chloroform and dibromochloromethane) and 5 levels of concentration (0.0, 2.5, 5.0, 7.5, and 10.0 mg.L-1) in a green house. The decrease in seedling biomass and the inhibition of radicle growth increased with increasing trihalomethane concentrations in a dose dependent manner. Also, both these parameters decreased in response to an increase in the number of bromine atoms in the trihalomethane molecule. However, in growing plants the decrease in concentration levels of seven essential nutrients namely nitrogen (N), phosphorus (P), potassium (K), sulphur (S), copper (Cu), zinc (Zn) & boron (B) correlated to an increase in the number of chlorine atoms. Increase in trihalomethane dose also induced a decrease in all the above mentioned nutrients with the addition of manganese (Mn), although the decrease in P and S were not significant at P ≤ 0.05. The increase in trihalomethane dose induced an increase in oxidative stress parameters such as the total phenolic content, ferric reducing antioxidant power (FRAP), oxygen radical absorbance capacity (ORAC), ascorbate peroxidase (APX), guaiacol peroxidase (GPX) and lipid peroxidation. The increase in the above parameters correlated to an increase in the number of chlorine atoms, however, no such correlations were observed in superoxide dismutase (SOD) activity, general lipid peroxidation, α-tocopherol content and totalsoluble proteins. In plant membrane lipids, increase in the saturated fat hexadecanoic acid was observed in both tomato cultivars that correlated to the degree of chlorination in the trihalomethane molecule. The increase in α-linolenic acid stress signaling correlated with an increase in the degree of chlorination in only one tomato cultivar suggesting variable tolerance between cultivars to chemical action. Membrane lipids adjustments in tomato plants exposed to increasing trihalomethane dose were based on two factors; first the adjustments of membrane fluidity with the increase in plant sterols and fatty acids content and secondly, the increase in lipophyllic antioxidants such as phenols, quinones and α-tocopherol content. The phenolic lipophyllic antioxidant was tentatively identified to be 2,2’-methylenebis [6-(1,1-dimethylethyl)-4-methyl] phenol. In conclusion, the magnitude of plant responses to trihalomethanes is more dependent on the halogenation number of the molecule and less on its concentration.