Tree Physiology Advance Access originally published online on January 13, 2009
Tree Physiology 2009 29(3):423-436; doi:10.1093/treephys/tpn035
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Rhizosphere carbon deposition, oxidative stress and nutritional changes in two poplar species exposed to aluminum
1 Department of Biology, West Virginia University, Morgantown, WV 26506, USA
2 Corresponding author (Dhiraj.Naik{at}mail.wvu.edu)
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Abstract |
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Species and hybrids in the genus Populus have become the focus of investigation for use in biofuels production and their capacity to sequester carbon (C) in the environment. The identification of species resistant to marginal edaphic sites may be important in both of these endeavors. Plant growth, total dissolved organic carbon (TOC) and low molecular weight organic acid (OA) production, antioxidative enzyme activities and mineral content were assessed in Populus tremuloides L. and Populus trichocarpa Torr. & Gray seedlings under exposure to aluminum (Al). Both species were sensitive to Al, with significant reductions in shoot and root biomass at and above 50 µM Al. Exposure to Al induced 40-fold increases in TOC deposition in P. tremuloides and 100-fold increases in P. trichocarpa. In P. tremuloides, Al treatment induced root exudation of malic and citric acids, while Al increased exudation of citrate and oxalate in P. trichocarpa. Organic acids accounted for 20–64% of total C released upon Al exposure, with the proportion of OAs increasing in P. tremuloides and decreasing in P. trichocarpa. Dose-dependent responses of catalase and ascorbate peroxidase were observed in both root and leaf tissues, indicating that Al exposure induced oxidative stress in poplar. Treatment at and above 100 µM Al reduced the concentrations of calcium (Ca) and magnesium (Mg) in roots and leaves, whereas Al at or above 50 µM reduced root and leaf phosphorous (P) concentrations. The majority of Al taken up was retained in the root system. Even with the induction of OA exudation and accumulation, P. tremuloides and P. trichocarpa remained sensitive to Al, as evidenced by elevated antioxidative enzyme activities, which may reflect inhibition of Ca or P uptake and destabilization of cell homeostasis in these poplar species. Although plants exhibited reductions in growth and evidence of oxidative and nutritional stress, total C rhizodeposition rates for both species increased with increasing Al exposure concentration. Estimated C deposition rates of 16 mg C plant–1 day–1 were four-times larger than previously reported values for forest tree species, indicating that edaphic stress plays an important role in C flux to the rhizosphere.
Keywords: exudates, mineral nutrition, organic acids, Populus tremuloides, Populus trichocarpa, reactive oxygen species, total organic carbon
Received September 22, 2008; Accepted November 2, 2008