Soil acidity is a worldwide problem for agriculture and forestry. Under acidic conditions, soil aluminum (Al) solubility increases. This increase in soil Al is a major factor in limiting crop yield and forest productivity. Al inhibits root growth in plants and disrupts nutrient and water uptake. Crop plants, forest trees and genotypes within species have shown large variation in Al resistance. Many studies have investigated physiological mechanisms of Al resistance in crop species; however detailed physiological mechanisms of forest tree Al resistance are less numerous. Hybrid poplar (Populus spp.) is a good model system for forest trees due to their ease of propagation, rapid growth and sequenced genome. The objective of the current study was to investigate physiological differences in Al resistance among eight hybrid poplar (Populus spp.) genotypes. Eight hybrid poplar genotypes consisting of three different hybrid crosses were exposed to five Al concentrations 0, 50,100,200 and 500 µM for thirty days. Resistance to Al was quantified by growth and callose production. Physiological Al resistance mechanisms such as root cell wall pectin and tissue organic acid accumulation were quantified. Finally total tissue Al and cellular fractionation of Al were measured.

Results/Conclusions

Within all hybrid crosses used in this study, resistant and sensitive genotypes could be identified bases on Al-induced changes in growth. At 500µM Al sensitive genotypes exhibited up to 97% and 98% reductions in shoot and root biomass respectively, resistant genotypes exhibited 49% and 60% reductions in shoot and root biomass respectively. Al tolerant genotypes exhibited higher apoplastic fractionation of Al while sensitive genotypes accumulated higher symplastic Al levels in their root tips. Callose accumulation in root tips also reflected resistance of genotypes, with tolerant genotypes accumulating less callose in response to Al than sensitive genotypes. Pectin showed a similar trend that was more distinct at higher Al treatments. Al sensitive genotypes accumulated significantly higher amounts of citrate and formate in root tips. This study illustrates that Al susceptibility in hybrid poplar can be quantified from physiological markers such as callose production. Concurrently Al resistance mechanisms such as organic acid production are correlated to cellular fractionation of Al driving resistance. These Al tolerant genotypes can be utilized for more sustainable reforestation and carbon sequestration techniques in acidic soil environments. Furthermore the physiological responses of these trees to Al lay the groundwork for further elucidation of precise mechanisms of forest tree Al resistance.