Establishment of north American pawpaw [Asimina triloba (L.) Dunal] shoots In vitro from mature or juvenile explants

North American pawpaw [Asimina triloba (L.) Dunal] is the largest tree fruit native to temperate North America. It is an under-utilized plant that has potential as a landscape tree, fruit crop and as a source of pharmaceutical, secondary products. Clonal propagation of this species is difficult from conventional methods and selection and distribution of superior pawpaw clones would benefit from a micropropagation system. Initial experiments showed that pawpaw was difficult to establish in tissue culture. A comparison was made between juvenile and mature sources of expiants for establishment in vitro. Juvenile expiants were taken from seedlings and shoots developed from roots of mature trees. Mature expiants (flowering-age plants) were taken from a population of trees with a diverse genetic background. After four weeks in culture, only seedling expiants showed axillary bud growth. It was not until 8 weeks in culture that bud growth started for expiants taken from rejuvenated shoots developed from roots. Tissue from mature plants failed to initiate growth in culture in 96% of the expiants. This was associated with excessive browning of the expiant and medium even though expiants were transferred to fresh medium every two weeks. These results suggest that shoots developed from root cuttings could be a promising source of expiants for micropropagation of clonal selections of pawpaw.