Ecophysiological responses of water hyacinth exposed to Cr3+ and Cr6+

Due to its wide industrial use, chromium (Cr) is considered a serious environmental pollutant of aquatic bodies. In order to investigate the ecophysiological responses of water hyacinth [Eichhornia crassipes (Mart.) Solms] to Cr treatment, plants were exposed to 1 and 10 mM Cr2O3 (Cr3+) and K2Cr2O7 (Cr6+) concentrations for two or 4 days in a hydroponic system. Plants exposed to the higher concentration of Cr6+ for 4 days did not survive, whereas a 2 days treatment with 1 mM Cr3+ apparently stimulated growth. Analysis of Cr uptake indicated that most of the Cr accumulated in the roots, but some was also translocated and accumulated in the leaves. However, in plants exposed to Cr6+ (1 mM), a higher translocation of Cr from roots to shoots was observed. It is possible that the conversion from Cr6+ to Cr3+, which immobilizes Cr in roots, was not total due to the presence of Cr6+, causing deleterious effects on gas exchange, chlorophyll a fluorescence and photosynthetic pigment contents. Chlorophyll a was more sensitive to Cr than chlorophyll b. Cr3+ was shown to be less toxic than Cr6+ and, in some cases even increased photosynthesis and chlorophyll content. This result indicated that the Fv/F0 ratio was more effective than the Fv/Fm ratio in monitoring the development of stress by Cr6+. There was a linear relationship between qP and Fv/Fm. No statistical differences were observed in NPQ and chlorophyll a/b ratio, but there was a tendency to decrease these values with Cr exposure. This suggests that there were alterations in thylakoid stacking, which might explain the data obtained for gas exchanges and other chlorophyll a fluorescence parameters.