Pawpaw produces a relatively large seed that averages 2.8 ± 0.1 cm (1.10 ± 0.04 inch) in length and 1.5 ± 0.1 cm (0.59 ± 0.04 inch) in width. It is a flat, spatulate seed with a dark brown seed coat that is fibrous and 16 cell layers thick (Corner, 1948; Mohana Rao, 1982). The ruminant endosperm occupies most of the seed cavity and is living at seed maturity. A small [about 1 mm (0.04 inch) long] rudimentary embryo is located at the hilar end of the seed and although small, it has easily distinguishable cotyledons and radicle (Finneseth et al., 1998a) (Fig. 1). Ruminant endosperm surrounding a small embryo is characteristic of seeds from annonaceous species (Hayat, 1963; Rizzini, 1973).

Seeds are usually extracted from the fruit after several days of fermentation in water (Hartmann et al., 2002). Care must be exercised in handling pawpaw seeds before germination because they are recalcitrant and sensitive to drying in storage (Bonner and Halls, 1974). As little as 5 d under open air conditions can reduce the moisture content of pawpaw seeds to 5% and result in total loss of viability (Finneseth et al., 1998a). Recalcitrant seeds can be classified as either highly, moderately or minimally recalcitrant based on their ability to withstand desiccation (Farrant et al., 1988). Pawpaw seeds display a moderate form of recalcitrance (Finneseth et al., 1998b). Seed moisture in mature seeds immediately removed from fleshy fruits is about 37%. This value does not rise significantly if seeds are imbibed. Seeds lose 50% viability when dried from their initial 37% to 25% moisture. Total loss in viability occurs between 15 and 5% moisture.

Pawpaw seeds must be stored moist at chilling [5 °C (41.0 °F)] temperature to retain viability in long term storage (Finneseth et al., 1998b). Under these conditions, seeds retained between 52% and 74% viability depending on the seed lot. However, germination was reduced to less than 25% after a third season of storage. In contrast, seeds stored moist at warm [25 °C (77.0 °F)] temperature lost between 40 and 65% viability after only 28 weeks in storage and no seeds remained alive after 52 weeks of storage.

In addition to being recalcitrant, pawpaw seeds display combinational (morphophysiological) dormancy (Nikolaeva, 1977) requiring a period of chilling stratification to satisfy in- termediate physiological endogenous dormancy followed by a moist warm pe- riod to satisfy morphological dormancy (Finneseth et al., 1998b). Several refer-

(morphophysiological) dormancy (Nikolaeva, 1977) requiring a period of chilling stratification to satisfy intermediate physiological endogenous dormancy followed by a moist warm period to satisfy morphological dormancy (Finneseth et al., 1998b). Several refer-

ences suggest that pawpaw seeds need to be stratified between 60 and 120 d (Dirr and Heuser, 1987; Reich, 1991; Young and Young, 1992). Pawpaw has a wide geographic distribution (Florida to Maine) and the varied times cited for stratification could reflect inherent variation found between provenances native to different locations. Using pawpaw seeds collected from six locations within Kentucky, Finneseth et al. (1998b) determined that about 7 weeks of chilling (5 °C) stratification was required to reach 50% germination and that the greatest germination percentage (84% to 90%) occurred after about 100 d of stratification.

Once endogenous dormancy is relieved, pawpaw seeds still exhibit morphological dormancy. Morphological dormancy is described as seeds containing either a rudimentary or linear embryo that is not fully developed at the time the seed is mature and occupies less than one-half of the seed cavity (Baskin and Baskin, 1998; Nikolaeva, 1977). Pawpaw has a small linear embryo that is about 1 mm in length at maturity (Finneseth et al., 1998a). The size of the embryo does not increase during chilling stratification. However, when stratified seeds are moved to warm conditions the cotyledons and radicle begin simultaneous growth at comparable rates (Finneseth et al., 1998a). The cotyledons grow through two specialized channels that are distinct from the rest of the ruminant endosperm (Fig. 1B), while the hypocotyl and radicle emerge from the seed coat. The expanding cotyledons eventually extend to the tip of the seed and it is postulated that the cotyledons act as haustorial structures transferring digested materials from the endosperm to the developing radicle (Finneseth et al., 1998a). The hypocotyl and radical continues to thicken to form a tap root.