Aluminum rhizotoxicity in soybean: A physiological approach to cellular localization, tolerance mechanisms and alleviation by basic cations (Cation amelioration)

Aluminum toxicity is a serious constraint to crop productivity in extensive areas of acid soils worldwide. The mechanisms of Al toxicity and tolerance in plants are poorly understood. The purpose of the present research was to evaluate the physiological basis of Al toxicity and potential tolerance mechanisms in several soybean (Glycine max L. Merril) genotypes. In situ localization experiments using fluorescence confocal laser scanning microscopy revealed that substantial amounts of Al accumulated in the cell wall and in the interior of meristematic cell at the root tip. Co-localization of Al and DNA fluorescence signals indicated that, while most symplastic Al accumulated in the nuclear material, it accumulated to a lesser extent in the Al-tolerant than in the Al-sensitive genotype. Superior tolerance to Al rhizotoxicity by soybean genotypes was found to be closely related to an increased concentration of citrate in the root tip as well as root citrate efflux. Citrate exudation was not observed in control plants and was specific for Al since the trivalent toxic La failed to trigger significant citric acid release by roots. Lateral roots were more sensitive to Al injury, accumulated more Al and had a lower root tip citrate concentrations than tap roots. Aluminum rhizotoxicity was greatly reduced in a complete nutrient solution in comparison to a simple Ca solution, and this protective effect was mostly due to the presence of low levels of Mg in the complete solution. The positive effect of Mg for soybean was species-dependent because no significant improvement of root elongation was observed for wheat grown under similar conditions. When present at micromolar concentrations Mg was more effective than Ca in alleviating Al inhibition of soybean root elongation, and its protective action could not be accounted for by changes in root electrical potential and Al3+ activity at the surface of root cells plasma membrane. Improvements in root elongation with millimolar concentrations of Ca suggests a protective action against Al mechanistically distinct from Mg, possibly through an electrostatic effect. Manganese failed to improve growth of Al-intoxicated roots. Soybean genotypes that largely differed in Al tolerance presented only a small differential root growth response to La, and inhibition of root elongation by La could not be alleviated by the presence of Mg. Magnesium protection against Al damage to roots was associated with an increase in citrate production and exudation by roots and a concomitant decrease in Al accumulation in root tips. A decrease in Mg, but not Ca concentration, in root tips of Al intoxicated soybean plants suggests that Al rhizotoxicity may be associated to the blockage of Mg transport from mature root tissue to the fast growing root tip.