RESUMO
The role of organic acids in aluminum (Al) tolerance has been the object of intensive research. In the present work, we evaluated the roles of organic acid exudation and concentrations at the root tip on Al tolerance of soybean. Exposing soybean seedlings to Al3+ activities up to 4.7 &mgr;M in solution led to different degrees of restriction of primary root elongation. Al tolerance among genotypes was associated with citrate accumulation and excretion into the external media. Citrate and malate efflux increased in all genotypes during the first 6 h of Al exposure, but only citrate efflux in Al-tolerant genotypes was sustained for an extended period. Tolerance to Al was correlated with the concentration of citrate in root tips of 8 genotypes with a range of Al sensitivities (r2=0.75). The fluorescent stain lumogallion indicated that more Al accumulated in root tips of the Al-sensitive genotype Young than the Al-tolerant genotype PI 416937, suggesting that the sustained release of citrate from roots of the tolerant genotype was involved in Al exclusion. The initial stimulation of citrate and malate excretion and accumulation in the tip of all genotypes suggested the involvement of additional tolerance mechanisms. The experiments included an examination of Al effects on lateral root elongation. Extension of lateral roots was more sensitive to Al than that of tap roots, and lateral root tips accumulated more Al and had lower levels of citrate.