| Al toxicity is a major problem in acid soils throughout the world. The exclusion of aluminum (Al) from root apices (the site of Al toxicity) by the exudation of Al-chelating compounds has been found by other researchers to be a major mechanism of Al tolerance in wheat. In chapter 2, ion chromatography was used to show that both Al-induced malate exudation and constitutive phosphate exudation occurred from the root apices of Al-tolerant Atlas 66, but not Al-sensitive Scout 66. The results presented suggest that these two processes may both contribute to Al tolerance but are not controlled by the same genetic locus. In chapter 3, three chromosome arms of Chinese Spring wheat, 4DL, 5AS and 7AS, were found to be involved in Al tolerance. Ditelosomic lines lacking one of these arms exhibited decreased Al tolerance, increased Al accumulation in root tips (as measured by ICP-MS), and decreased Al-induced malate exudation. These results suggest that the large range in rates of Al-induced malate exudation among different varieties of wheat may be due to several different genes rather than multiple alleles of a single locus.; In chapter 4 the possibility that Al-induced malate exudation might be mediated by an electrical signal was considered. Al{dollar}sp{lcub}3+{rcub}{dollar} was found to induce depolarizations of E{dollar}rmsb{lcub}m{rcub}{dollar} in Al-tolerant wheat lines but not Al-sensitive wheat lines. The depolarizations could not be attributed to the electrogenic efflux of malate{dollar}sp{lcub}2-{rcub}{dollar} across the plasma membrane. Other treatments which depolarized E{dollar}rmsb{lcub}m{rcub}{dollar} did not lead to malate exudation, suggesting that the depolarization of root cap cell membrane potentials is probably linked to, but is not sufficient to, trigger malate release.; Information regarding mechanisms of toxicity and tolerance of Al, the most common metal to which plants are exposed, may aid in understanding how plants interact with other metals and help in the optimization of phytoremediation, the use of plants to remove toxic metals from soils. In chapter 6, preliminary results demonstrating the development and use of a vibrating Cd-selective microelectrode are presented. |
