In vitro rooting of cloned shoots in Pinus pinaster

In vitro plantlet regeneration will be a sueful tool in reforestation only when rooting problems are overcome. Our study was therefore focused on Pinus pinaster rooting conditions. Methods were used which permit multiple axillary (instead of adventitious) bud production in order to obtain genetically identical explants and hence to procure more homogeneous resutls. The hormonal treatment efficiency was then tested on several clones for rooting. Different substrates were compared. A mixture of peat and perlite (1/3, v/v) permitted rooting and the further development of the root system if humidity was properly controlled. The use of an agar medium avoided this difficulty; it supported root induction but resulted in limited growth. For hormonal treatment, explants, submitted to a 12‐day‐period with NAA (10 −6 M ) and then subcultured in an auxin‐free medium, gave rooting percentages ranging from 80 to 100%. In terms of rooting quality (intensity, growth rate and branching ability), a 15‐to 19‐day‐period of auxin treatment was more effective. Application of this optimal treatment to several clones revealed slight differences within clones and between clones (80 to 100% of rooting according to the clone). Regenerated plantlets were able to undergo an autotrophic growth after transplantation under greenhouse conditions.