INDUCTION AND STABILITY OF SOMACLONAL VARIATION IN GROWTH, LEAF PHENOTYPE AND GAS-EXCHANGE CHARACTERISTICS OF POPLAR REGENERATED FROM CALLUS-CULTURE

Populus trichocarpa Torr. and Grey x P. balsamifera L. TT32 lines were regenerated from calli that had been maintained under differing in vi tro conditions for sixteen months. In the final months, calli were mai ntained with one of six concentrations of 2,4-dichlorophenoxyacetic ac id (2,4-D, 0. 1, 0.2, 0.3. 0.4, 0.5 or 0.6 mg l-1) and regenerated wit h 0.25, 0.50 or 1.0 mg l-1 benzylaminopurine (BA). Regenerant lines we re obtained from 15 of these 18 treatments. The spectrum of variation in several morphological, physiological and leaf gas exchange traits w as evaluated in the primary regenerants in 1986, and in their secondar y vegetative propagules in the two subsequent years, in relation to di fferences in the original culture conditions. The results indicate tha t somaclonal variation was induced largely as a result of prolonged cu lture in the presence of 2,4-D, but that the terminal maintenance and regeneration phases also induced changes in the regenerants. Qualitati ve 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 suff iciently to produce lines showing general performance that was either above or below that of the original TT32 clone. An early visible indic ator of this divergence was variation in leaf shape (leaf length/width ratio), which could be related to 2,4-D-BA interactions in die final stages of culture. Graphic illustration of the independent effects of either 2,4-D or BA on stem height and gas exchange parameters suggeste d an inverse relationship with BA concentration and a complex interact ion with 2,4-D. Significant correlations were detected between gas exc hange parameters and morphological characteristics representing leaf f orm and stem development. Overall, the results indicate the presence o f somaclonal lines that offer potential for the selective improvement of growth using morphological and gas exchange parameters as screening tools.