| Aluminum (Al)-resistance mechanisms in loblolly pine (Pinus taeda) and slash pine (Pinus elliottii) seedlings were studied in solution and soil cultures. To probe variation in Al-sensitivity, seedlings of six full-sib families of each species were exposed to 4.4 mM or 0.01 mM AlCl{dollar}sb3{dollar} in a solution culture. Shoot growth of slash pine was more sensitive to high concentrations of Al than shoot growth of loblolly pine. Loblolly pine exhibited greater intra-specific variation in Al-sensitivity than slash pine. To test responses of both pine species to a severe Al or acidity stress, similar to one resulting from solid rocket motor testing, five of the same full-sib families of each species were grown in a nursery bed soil. After 20 weeks of soil treatments, survival in AlCl{dollar}sb3{dollar} (pH 2.5) and HCl (pH 0.6) treatments was 50% for loblolly pine and 70% for slash pine seedlings. Families Al-resistant in solution culture maintained higher growth (but not survival) in soil culture. Inoculation with the ectomycorrhizal fungus Pisolithus tinctorius improved survival of loblolly pine in AlCl{dollar}sb3{dollar} and HCl treatments, and growth of Al-sensitive families of both species in AlCl{dollar}sb3{dollar} treatments. Inoculation lowered soil exchangeable Al, sustained higher soil pH{dollar}sb{lcub}rm H2O{rcub},{dollar} and limited downward movement of soil exchangeable Ca and Mg in AlCl{dollar}sb3{dollar} treatments. Root-based physiological Al-resistance mechanisms were investigated by growing seedlings in solutions with 100 {dollar}mu{dollar}M AlCl{dollar}sb3{dollar} and analyzing Al distribution among several cellular fractions in 5 mm long root tips. Total Al was lower in loblolly pine than slash pine root tips. On average, 13% of Al absorbed by root tips was in the symplasmic fraction, 30% in the cell-wall-bound fraction and 57% in the cell-wall-labile fraction. For the Al-resistant slash pine family, 57% of Al was in the cell-wall-bound fraction compared to only 18% for the Al-sensitive family. Immobilization of Al in cell walls of slash pine roots may partially explain Al-resistance in the families studied. The basis for differential Al-resistance in loblolly pine was uncertain. |
