In vitro shoot regeneration in Populus deltoides Bartr. ex Marsh.

An adventitious shoot regeneration system was developed for Populus deltoides Bartr. ex Marsh. using internodal stem explants. Both the genotype of the explant tissue and the type and concentration of cytokinin used for shoot regeneration significantly influenced the regeneration response. The cytokinin zeatin consistently stimulated regeneration of the greatest number of shoots compared to 2-isopentyladenine and 6-benzyladenine. Six of 16 genotypes were highly recalcitrant and failed to regenerate shoots. Stabilized shoot cultures were established from adventitious shoots of four genotypes and were maintained for over one year. The shoot cultures were used to determine the effect of zeatin concentration and genotype on axillary shoot production and growth. Genotypic differences were significant for axillary shoot production and growth. Zeatin concentration also significantly affected shoot production and growth with 1.0 mgL$\sp{-1}$ zeatin stimulating the greatest number of shoots while growth was best at 0.25 mgL$\sp{-1}$ zeatin. The in vitro developmental stage of shoot regeneration competence was determined for 15 genotypes. Two genotypes were highly competent and regenerated the greatest number of shoots when cultured directly onto shoot inducing medium. Competence was stimulated in seven genotypes as determined by a significant increase in shoot regeneration when explants were cultured for eight days or less on callus inducing medium before transfer to shoot inducing medium. Six genotypes were found to be recalcitrant and competence could not be induced. The ability to manipulate the competence response by stock plant hedging was demonstrated. Protein differences associated with competence and shoot regeneration were studied for one genotype. Two proteins (51 kD and 26 kD) were associated with the acquisition of shoot regeneration competence. Three proteins (34 kD, 31 kD and 25.5 kD) were associated with callus determination. A 31.5 kD protein was found to be associated with shoot organogenesis while a 35 kD protein was associated with callus proliferation