Mycorrhizae confer aluminum resistance to tulip-poplar seedlings

Aluminum (Al) toxicity may limit the growth and nutrient acquisition of sensitive tree species in regions receiving acidic deposition. Symbioses between tree roots and mycorrhizal fungi may offset the negative impacts of Al in the root zone. Liriodendron tulipifera L. (tulip-poplar) is an important tree species in the Appalachian Mountains of the southeastern United States and may be at risk from the high levels of acidic deposition in that area. Mycorrhizal and non-mycorrhizal tulip-poplar seedlings were exposed to Al levels of 0, 50, 100, and 200 µM in sand culture for 6 weeks. Mycorrhizal plants accumulated two to seven times the shoot and root biomass of nonmycorrhizal plants and demonstrated no decreases in biomass with Al exposure. Nonmycorrhizal plants exhibited significant reductions in biomass at and above 100 µM Al. Aluminum toxicity in non-mycorrhizal plants appears to be the result of the disruption of P translocation to leaves and Ca, Mg, P, Cu, and Zn uptake in roots. Mycorrhizal plants accumulated 2 and 1.5 times the concentration of Al in shoots and roots, respectively, indicating that Al resistance was not associated with the exclusion of Al from the plant. Patterns of labile Al in solution, nutrients, and Al accumulation in tissues suggest that arbuscular mycorrhizal fungal ecotypes may alter the form or compartmentation of Al within the rhizosphere and plant, thus protecting seedlings from the effects of exposure to Al in the soil solution.