Aluminum tolerance in nodulated N₂-fixing legumes species native to two contrasting savanna sites

Abstract

Eight native legume species were analyzed for their tolerance to high soil aluminium (Al) levels in terms of the Al, iron (Fe) and calcium (Ca) contents in different plant organs and spatial distribution in two contrasting savanna sites in Venezuela. The less fertile soils in site 1 (S-1) are characterized by a pH of 4 and contents of Al 8.4, Fe 0.9 and Ca 0.2 cmol kg⁻¹. The relatively more fertile soils in site 2 (S-2) have a pH of 5.4, with Al 1.3, Fe 0.4 and Ca 2.2 cmol kg⁻¹. Examples of nodulated Chamaecrista flexiuosa, Clitoria guianensis, Galactia jussieuana, Stylosanthes elegans and Zornia curvata were harvested at S-1. Nodulated examples of the same species plus examples of Centrosema pubescens, Chamaecrista tetraphilla and Phaseolus gracillis were harvested at S-2. All S-1 species were categorized as Al-tolerant. In turn, C. pubescens, C. tetraphilla and P. gracillis were classified as non-tolerant to soil Al contents above 1.3 cmol kg⁻¹. With the exception of S. elegans, all S-1 species accumulate Al in underground organs, mainly in secondary roots, and were considered root-Al accumulators. The accumulation of Al in aboveground organs of S. elegans allowed the classification of this species as leaf-Al accumulator. Over all the species, the Al contents in underground and aboveground organs were 11 and 8-fold higher in S-1 than in S-2 species, respectively. However, the accumulation of Al in roots of S-1 species did not inhibit their symbiotic performance as indicated by the absence of significant differences in the relative abundance of ureides between S-1 and S-2 species. This situation could be partially explained by the exclusion of Al from nodule tissues. In S-1 and S2 species, Al contents in nodules were below detectable values. Significant differences in Ca content were also encountered between S-1 and S-2 species, with the highest values in S-2 species. Except for S. elegans, there was a significant positive correlation between Al and Fe contents in the different organs of S-1 and S-2 species, as well as a significant negative correlation between Al and Ca contents in S-1 species.