ABSTRACT
Populus trichocarpa Torr. and Grey x P. balsamifera L. TT32 lines were regenerated from calli that had been maintained under differing in vitro conditions for sixteen months. In the final months, calli were maintained with one of six concentrations of 2,4-dichlorophenoxyacetic acid (2,4-D, 0.1, 0.2, 0.3, 0.4, 0.5 or 0.6 mg l(-1))and regenerated with 0.25, 0.50 or 1.0 mg l(-1) benzylaminopurine (BA). Regenerant lines were obtained from 15 of these 18 treatments. The spectrum of variation in several morphological, physiological and leaf gas exchange traits was evaluated in the primary regenerants in 1986, and in their secondary vegetative propagules in the two subsequent years, in relation to differences in the original culture conditions. The results indicate that somoclonal variation was induced largely as a result of prolonged culture in the presence of 2,4-D, but that the terminal maintenance and regeneration phases also induced changes in the regenerants. Qualitative differences among the regenerant lines were detected by the end of 1986. For most traits, these differences were statistically confirmed within the 3-year period. The treatment lines ultimately diverged sufficiently to produce lines showing general performance that was either above or below that of the original TT32 clone. An early visible indicator of this divergence was variation in leaf shape (leaf length/width ratio), which could be related to 2,4-D-BA interactions in the final stages of culture. Graphic illustration of the independent effects of either 2,4-D or BA on stem height and gas exchange parameters suggested an inverse relationship with BA concentration and a complex interaction with 2,4-D. Significant correlations were detected between gas exchange parameters and morphological characteristics representing leaf form and stem development. Overall, the results indicate the presence of somaclonal lines that offer potential for the selective improvement of growth using morphological and gas exchange parameters as screening tools.
ABSTRACT
Populus hybrid TT32 lines produced from 15 treatment tissue culture regimes exhibited somaclonal variation in morphological and gas exchange parameters. Within four years of regeneration, discrete lines showing statistically validated superior, or inferior, growth performance relative to the parental reference clone were identified. Significant differences in the ratio of leaf length/width between treatment lines provided the earliest reliable indicator of the divergence in overall growth performance. Despite discernible variation in leaf phenotype among primary regenerants and secondary propagules within individual lines, the leaf length/width ratio was identified as a potential parameter for predicting growth performance. Its subsequent use led to the recognition of four distinct leaf morphotypes; cordate, ovate, reniform and oval. Two or more of these morphotypes were distributed within each of the original 15 treatment lines. Regrouping the data on the basis of leaf morphotype resulted in a clear segregation of the morphological traits, and revealed differences that were not readily apparent by statistical analysis based on treatment groups. The demonstration of similar relative performances by individual morphotypes with respect to a range of growth and gas exchange parameters confirmed that variation in leaf morphology was indicative of differential photosynthetic performance. Somaclonal variants with a leaf morphology was indicative of differential photosynthetic performance. Somaclonal variants with a leaf morphotype closest to that of the parental line showed the highest overall potential for selection, suggesting that the greatest benefits accrue from a minimal disturbance of the parental leaf phenotype.
