Physical And Biochemical Assessment Of Mexican Palo Verde Response To Chromium Toxicity

In the United States, about 1/3 of the 1400 superfund sites have been reported to be chromium contaminated, making chromium the second most common metal found in superfund chemical contamination sites. Chromium removal from soil has commonly been carried out via chemical, physical, and thermal processes. However, these methods are expensive and/or invasive to local ecosystems. In contrast to these techniques, phytoremediation (the use of plants to remove toxic substances from areas containing contamination at low or medium levels) appears to be a cost-effective and environmentally friendly process for chromium removal. However, most reports on chromium phytoremediation suggest that this element is stored in roots and very few amounts are translocated to the frond. Parkinsonia aculeata (Leguminosae, Fabaceae), a shrub/tree native to the southwestern United States and northern Mexico, where it is commonly known as the Mexican Palo Verde (MPV), was selected as a model plant to investigate its potential for chromium phytoremediation in desert areas. In these studies, plants were exposed to chromium(III) and chromium(VI) ions under different experimental conditions, including the use of phytohormones in hydroponics and soil. The objectives of this research were to understand the Cr uptake, distribution, and accumulation in MPV, as well as the effect of phytohormones in response to Cr stress on MPV. This project was performed in four phases. In phase one, MPV plants were treated with different concentrations of Cr(III) and Cr(VI) in hydroponics to assess its tolerance to Cr stress, as well as to determine the main signs of Cr toxicity. The results from phase one were used in phase two, where plants were sown in soil with a range of Cr(III) and Cr(VI) concentrations to determine the real potential of MPV for Cr absorption. In phase three, MPV plants were grown both in soil containing tolerable concentrations of Cr(III) and Cr(VI), as well as different concentrations of phytohormones so as to determine their influence on MPV\u27s response to Cr stress. In phase four, a 6-month long experiment was carried out to evaluate the long term effect of Cr stress in MPV plants and the Cr distribution pattern in plant organs. A series of inductively coupled plasma-optical emission spectrometry and other spectroscopic techniques as well as biochemical assays were used in this research. Results demonstrated that Cr(VI) treatment produced shorter and weaker plants compared to Cr(III) treatment. In both cases the translocation of Cr into the stems increased with time. Results have also shown that the uptake of nutrient elements varied with time and Cr species. For instance, in Cr(III)-treated plants, Fe accumulation in stems decreased with time but no changes were observed in leaves, although Cr(VI)-treated plants displayed similar Fe accumulation in all tissues throughout the 6-month growth period. The IR spectra of tissues revealed that Cr altered plant lignin in the cortex and xylem, and the degree of lignification and protein content increased under Cr stress. The study also showed that the phytohormone kinetin (KN) at 250 µM reduced catalase (CAT) but increased ascorbate peroxidase (APOX) activity in the roots of Cr(VI)-treated plants. Both CAT and APOX were reduced in the leaves of plants treated with Cr(III) and KN. However, under Cr(VI) stress neither CAT nor APOX were reduced by the presence of KN. The electron probe microanalyzer proved to be a feasible and practical tool for metal analysis in plant tissues. X-ray fluorescence images showed that for both of the Cr(III) and Cr(VI) treatments, Cr was mainly located at the cortex section in the root, and Cr distribution was essentially homogenous in stems. However, Cr(VI)-treated roots and stems had more Cr accumulation than the corresponding part in Cr(III) treatments. X-ray absorption spectroscopy data showed that Cr(VI) was reduced to Cr(III) in/on plant roots and transported as Cr(III) to the stems and leaves. Overall, this research demonstrated not only that MPV can uptake and store Cr inside plant tissues, but also, that kinetin is capable of increasing the translocation of Cr from roots to stems in plants treated with both Cr ions, improving the MPV\u27s ability to phytoremediate Cr in moderately impacted soils