Effects of Chromium (VI) and Humic Substances on Selected Physiological Responses of Azolla caroliniana

Abstract

The independent and interactive effects of Cr(VI) and humic substances were inves- tigated on several physiological and biochemical responses of Azolla caroliniana. Cr(VI) concen- trations of 1 and 2 mg 1-~ were found to cause a reduction in growth and chlorophyll a and b concentrations of Azolla with and without the coexistence of humic substances. The Cr(VI) tox- icity on these parameters was less pronounced in the presence of humic substances. Carotenoid concentrations were not significantly affected by Cr(VI) treatments while humic substances, in the absence of Cr(VI), influenced an increase in carotenoid concentration. Anthocyanins were in- creased in treatments receiving Cr(VI) and humic substances, both individually and together. In- creases in anthocyanins were less pronounced in treatments receiving humic substances with Cr(VI) in comparison to treatments receiving Cr(VI) in the absence of humic substances. Cr(VI) and humic substances influenced an increase in sucrose, starch, and total nonstructural carbo- hydrate (TNC) accumulation. In the coexistence of Cr(VI) and humic substances, accumulation of carbohydrates was reduced in comparison to those receiving Cr(VI) in the absence of humic sub- stances. Humic substances in the absence of Cr(VI) influenced a significant decrease in the activ- ities of sucrose phosphate synthase (SPS) and invertase and a significant increase in the activity of sucrose synthase (SS). Cr(VI)was insignificantly influenced SPS activity at 1 mg 1-1 and at 2 mg 1-'. Invertase activity was significantly increased at both Cr(VI) treatments. Humic substances and Cr(VI) interactively resulted in increased SS activity insignificantly above those treatments re- ceiving Cr(VI) in the absence of humic substances. Phosphate synthase activity was reduced at both Cr(VI) treatments when humic substances were coexistent in comparison to treatments re- ceiving Cr(VI) in the absence of humic substances. Invertase activities were reduced in treatments receiving Cr(VI) and humic substances together in comparison to those receiving Cr(VI) in the absence of humic substances.