Multiple adventitious shoot formation in Spanish Red Cedar ( Cedreto odorata L.) cultured in vitro using juvenile and mature tissues: an improved micropropagation protocol for a highly valuable tropical tree species

Cedrela odorata L. is a valuable tropical tree widely appreciated for its wood. This species confronts serious problems due to both overexploitation of its natural populations and its susceptibility to the Meliaceae borer Hypsipyla grandella, which destroys the apical meristems and produces structural deformations. The rapid introduction of new varieties through clonal forestry has been demonstrated to be the most effective way to improve the production of perennial plantation species. In this work, we report both a protocol for the rejuvenation of elite mature trees of C. odorata and the optimization of an in vitro culture system to scale up micropropagation. Several media formulations and the use of temporary immersion culture in bioreactors were evaluated. The addition of 20% coconut water to TY17 medium increased the number of adventitious shoots from hypocotyl segments to an average number of 4.68 shoots per expiant. To replace coconut water and to define the culture medium, several cytokinins were tested at various concentrations; however, none of them produced the effect of coconut water. Rejuvenation of elite mature individuals was investigated by ex vitro grafting of mature tree twigs onto 3-mo-old juvenile trees. Although the grafting had a positive effect on the micropropagation of mature material, the multiplication rate of 1.5 new shoots per expiant did not compare to the organogenic capacity of younger materials. Shoot and root elongation as well as acclimatization to ex vitro conditions were carried out in a temporary immersion culture of juvenile material using BioMINT® bioreactors. A 3.5-fold increase in shoot elongation and a 4-fold increase in root elongation were achieved compared to material cultured on semisolid media. Furthermore, this culture system allowed for 98% effectiveness in the soil adaptation of the in vitro grown plants. The scaled-up multiplication capacity over a period of 6 mo calculated for the system is above 16,000 plants per mother plant with young materials but is only 125 with mature materials.