Can explant choice help resolve recalcitrance problems in in vitro propagation, a problem still acute especially for adult conifers?

For many conifer species, regeneration by organogenesis or somatic embryogenesis (SE) is still difficult and is often restricted to explants taken from juvenile donors. This review is based on the premise that there are tissues in the plant that are not normally used as explant, mostly because excising them in a viable state is difficult. Nevertheless, in cases where recalcitrance is a major problem, it may be worthwhile to pay closer attention to these tissues. Recalcitrance is a general problem, and discussion of it requires examples from the general literature. However, to restrict the scope of this review, preference will be given to conifer examples whenever possible.