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An efficient regeneration protocol for stem disc culture of Populus euramericana, which is important species for bioenergy resource in agroforestry, was established. The number of explants that were obtained and the number of explants that regenerated varied with the genotypes. However, in all the genotypes, stem disc culture produced more regenerated shoots than did in axillary bud culture. A comparison of the effects of cytokinin type and concentration on shoot regeneration in different explants (i.e., petiole, leaf, and root segments of P. euramericana) revealed that a concentration of 0.002 mg l-1 thidiazuron (TDZ) used on petiole segments resulted in the greatest shoot regeneration (95.83%). The hormonal requirements for the greatest shoot regeneration in the three explant types varied. Different concentrations of AgNO3 and CoCl2 were added separately to the medium to stop the yellowing and subsequent necrosis of the regenerated shoots. Lower concentrations (3 and 5 mg l-1) of these compounds improved shoot regeneration and elongation, compared with the control. The in vitro-regenerated shoots were transferred to rooting medium and subsequently acclimatized. The highly efficient regeneration system of P. euramericana reported here can be used for mass propagation of this recalcitrant for regeneration, economically important tree species.

The effect of light (blue, red, and/or green) generated by light-emitting diodes on shoot regeneration was examined in selected clones, 'I-476' and 'Dorskamp', of Populus euramericana. We found that a combination of red and blue light produced the highest percentage of shoot regeneration in comparison to monochromic or fluorescent light. Further studies of light quality and in vitro growth of ''Dorskamp'' plantlets demonstrated the suitability of combinations of red and blue light to improve most of the characteristics studied. Flow cytometry of the leaves showed that 50 % red ? 50 % blue light treatment was the most effective for inducing cell division in ''Dorskamp'', whereas monochromic red and blue were more effective in ''I-476''. Histological studies found that secondary xylem formation increased with the monochromic blue light treatment in the stem of ''I-476'', whereas in ''Dorskamp'', a combination of red and blue light (70 ? 30 %, respec- tively) was more suitable. Our study demonstrated that combinations of red and blue light increase shoot regeneration and development by stimulation of cell divi- sion, although the response may vary among genotypes of the same species. The results of this study can contribute to mass production of the selected clones for forest estab- lishment on reclaimed land and serve as an efficient pro- tocol for genetic transformation of this species.