Temperature and pH effects on plant uptake of benzotriazoles by sunflowers in hydroponic culture.

This article describes a systematic approach to understanding the effect of environmental variables on plant uptake (phyto-uptake) of organic contaminants. Uptake (and possibly phytotransformation) of xenobiotics is a complex process that may differ from nutrient uptake. A specific group of xenobiotics (benzotriazoles) were studied using sunflowers grown hydroponically with changes of environmental conditions including solution volume, temperature, pH, and mixing. The response of plants to these stimuli was evaluated and compared using physiological changes (biomass production and water uptake) and estimated uptake rates (influx into plants), which define the uptake characteristics for the xenobiotic. Stirring of the hydroponic solution had a significant impact on plant growth and water uptake. Plants were healthier, probably because of a combination of factors such as improved aeration and increase in temperature. Uptake and possibly phytotransformation of benzotriazoles was increased accordingly. Experiments at different temperatures allowed us to estimate an activation energy for the reaction leading to triazole disappearance from the solution. The estimated activation energy was 43 kJ/mol, which indicates that the uptake process is kinetically limited. Culturing plants in triazole-amended hydroponic solutions at different pH values did not strongly affect the biomass production, water uptake, and benzotriazole uptake characteristics. The sunflowers showed an unexpected capacity to buffer the solution pH.

Phytotransformation of benzotriazoles.

Plant roots interact with organic pollutants and some of these contaminants can be phytotransformed. Root uptake of 1-H-benzotriazole and its derivatives, tolyltriazole, 5-methyl benzotriazole, and 1-hydroxy benzotriazole was studied. At levels below the toxic threshold of about 100 mg/L, triazoles appear to be incorporated into plant tissue. Their concentration in the aqueous phase of the culture decreases with time and they cannot be extracted from the plant material using methanol. Hydroponic studies with sunflowers (Helianthus annuus) were used to investigate the behavior of the solution concentration versus time and to determine kinetic parameters for plant uptake of triazoles. Plants actively take up the triazoles at a rate greater than predicted by transpiration stream-concentration factor and plant-water uptake. Analyses of the data for phytotransformation rate versus concentration were performed to establish the kinetic model for the removal process. Except for 1-hydroxy-benzotriazole, triazole disappearance in plant systems followed the Michaelis-Menten kinetic model (commonly found for enzyme-catalyzed reactions) better than a first order rate model. However, the fit for the first order model was improved when normalizing to the plant fresh weight, which was assumed to be an approximate measure of the changing root surface area. Experiments with other plant species are in progress.