SHOOT INDUCTION COMPETENCE AND CALLUS DETERMINATION IN POPULUS-DELTOIDES

The developmental stages of shoot regeneration competence and callus determination were examined for 15 genotypes of Populus deltoides Bartr. ex Marsh. over a 1-year time period. The developmental stages were defined by transferring internodal stem explants from callus-inducing medium to shoot inducing medium after 1, 2, 4, 6, 8 and 10 days, plus immediate culture on basal and shoot-inducing medium as controls. The number of shoots regenerated per explant and the number of explants regenerating shoots were then determined after 60 days in culture. Six genotypes were recalcitrant and failed to regenerate shoots. Two genotypes were initially competent for shoot regeneration and consistently regenerated the greatest number of shoots when cultured directly onto shoot inducing medium. Shoot regeneration for the remaining seven genotypes was significantly enhanced by culture on callus-inducing medium for 1-8 days prior to transfer to shoot inducing medium. When internodal explants were cultured on callus-inducing medium for 10 days before transfer to shoot inducing medium, all 15 genotypes failed to regenerate shoots. Regeneration of the nine genotypes that were not recalcitrant was enhanced over the one year period by repeated stock plant hedging. Stock plant hedging was performed by pruning stock plants in such a manner that approximately 4 cm of current stem growth remained. Stock plants were repeatedly hedged at three to four month intervals over the one year experimental period. © 1990.