Semester

Spring

Date of Graduation

2011

Document Type

Thesis

Degree Type

MS

College

Davis College of Agriculture, Natural Resources and Design

Department

Horticulture

Committee Chair

Jonathan Cumming.

Abstract

Soil acidity and aluminum (Al) toxicity are major factors limiting crop yield and forest productivity worldwide. Hybrid poplar (Populus spp.) was used as a model woody tree species to assess genotypic variation in Al resistance and physiological stress responses to Al. In the first study eight hybrid crosses of P. trichocarpa, P. deltoides and P. nigra were exposed to Al in solution culture. Resistance to Al varied by genotype and hybrid cross, with P. trichocarpa x P. deltoides crosses being most resistant, P. trichocarpa x P. nigra being intermediate and P. deltoides x P. nigra being most sensitive to Al. Total root Al accumulation was not a good indicator of Al resistance/sensitivity. However, the partitioning of Al into apoplastic and symplastic fractions indicated that differences in sensitivity among genotypes were associated with Al uptake into the symplasm. Aluminum treatment increased callose and pectin concentrations of root tips in all genotypes, but more prominently in Al sensitive genotypes/hybrids. In Al sensitive genotypes, higher levels of symplastic Al accumulation correlated with elevated concentrations of citrate, malate, succinate or formate in root tips, whereas organic acid accumulation was not as pronounced in Al resistant genotypes. These findings suggest that exclusion of Al from the symplast is associated with Al resistance. In the second study exudation was examined as a strategy for Al resistance in two poplar genotypes varying in Al tolerance. Increased citrate exudation in the Al tolerant genotype DTAC-7 was found to be related to increases in organically bound Al in solution as well as lower callose accumulation in root tips. A decrease in organic acid (OA) exudation over time was also observed in both genotypes. These findings suggest that, under short term Al exposure, citrate exudation is a viable mechanism to promote Al resistance in poplar genotypes, but also suggests that other strategies may exist in longer term Al exposure. Further screening of Al tolerant poplar genotypes could yield successful candidates to be utilized for sustainable reforestation/reclamation and carbon sequestration projects where soil acidity may limit tree growth.

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