Morphological development of the North American pawpaw during germination and seedling emergence

The North American pawpaw [Asimina triloba (L.) Dunal], a temperate member of the Annonaceae, is a deciduous woody tree with ornamental value and has merit as a fruit crop. Anatomical studies of pawpaw seed revealed a small, linear embryo that does not change in length during cold or warm stratification. Radicle and cotyledon growth from planting until radicle protrusion was concurrent and at about the same rate. Cotyledons grew through a specialized channel of cells extending above the cotyledon tips, but never emerged from the seed. The extended period of time required for the development of the cotyledons delayed seedling emergence more than 50 days. The cotyledons appear to be haustorial and translocate storage material from the endosperm to the growing embryo. At the time of epicotyl elongation, the radicle and developing root system were well developed and comprised 81% of the seedling biomass. Seedling development could be divided into four distinct stages, including radicle protrusion, hypocotyl emergence, epicotyl elongation, and seedcoat abscission.