Factors influencing direct shoot regeneration from leaves,petioles, and plantlet roots of triploid hybrid Populus sect. Tacamahaca

Since the generation of full-sib artificial triploid families, rapid clone establishment and genetic improvements have been needed. Here, we report an in vitro method of direct shoot regeneration of a triploid hybrid poplar [(Populus simonii × P. nigra ‘Italica’) × (P. × ‘popularis’)]. Using different randomized block designs, we selected one triploid to evaluate the explant type, optimal concentrations of plant growth regulators and agar, and culture time under light or dark conditions over 60 days. The highest rate of shoot induction, 80.0%, was obtained using Murashige and Skoog (MS) medium supplemented with 0.2 mg/L benzyladenine, 0.04 mg/L naphthaleneacetic acid (NAA), and 5.5 g/L agar for the first 30 days in the dark, then 3 g/L agar for the next 30 days in light. This last medium yielded the best rate of shoot induction (6.32 shoots/explant). These three media were also used to evaluate the influence of the genotypes of the parents and hybrid triploids on regeneration. Two parents and three of the four full-sib triploids were regenerated successfully; different genotypes and explant types significantly affected the rate of shoot induction and average number of shoots. Leaves but not petioles were a suitable explant. One genotype produced the highest rate of shoot induction of 96.67%. Half-strength MS medium supplemented with 0.2 mg/L indole butyric acid and 0.04 mg/L NAA was the most effective for rooting; rooting rate was 96.67%, survival rate of transplants was 73.33%, and rooting frequency surpassed 85% for each genotype. Overall, this in vitro regeneration system will be useful for the propagation and genetic modification of triploid poplars.