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Table 7.3. Comparison of genetic variation of pawpaw (Asimina triloba) with plant species having the same characteristics.
| Species characteristics | Polymorphic locus (%) (P)y | Expected heterozygosity (He) |
|---|---|---|
| Life form: long-lived woody perennialz | 64.7 \pm 2.7 | 0.177 \pm 0.010 |
| Regional distribution: widespreadz | 58.9 \pm 3.1 | 0.202 \pm 0.015 |
| Geographic range: temperatez | 48.5 \pm 1.5 | 0.146 \pm 0.000 |
| Breeding system: outcrossing-animalz | 50.1 \pm 2.0 | 0.167 \pm 0.010 |
| Seed dispersal: animal ingestedz | 45.7 \pm 3.9 | 0.176 \pm 0.019 |
| Mode of reproduction: sexual and asexualz | 43.8 \pm 3.7 | 0.138 \pm 0.016 |
| Average of all characteristicsz | 51.2 \pm 8.2 | 0.168 \pm 0.023 |
| California cherimoyax | 73.3 | 0.330 \pm 0.064 |
| California and Spanish cherimoyaw | 44.8 | 0.183 \pm 0.044 |
| Cultivated pawpaw (Isozymes)v | 44.4 | 0.166 \pm 0.048 |
| Pawpaw wild accessions (Isozymes)u | 43.5 | 0.172 \pm 0.013 |
| Pawpaw wild accessions (RAPDs)t | 64 | 0.249 \pm 0.022 |
| Cultivated pawpaw (RAPDs)s | — | 0.285 \pm 0.160 |
| Cultivated pawpaw (ISSRs)r | 80 | 0.358 \pm 0.205 |
2Data from Hamrick and Godt, 1989 derived from isozyme studies +/- of standard deviation.
yP is the percent polymorphic loci and He the mean gene heterozygosity, respectively.
xCalculated by Huang et al. (1997) from the data published by Elstrand and Lee (1987) including three other monomorphic loci.
wCalculated by Huang et al. (1997) from data published by Pascual et al. (1993) including 16 other monomorphic loci.
vFrom Huang et al. (1997).
uFrom Huang et al. (1998).
tFrom Huang et al. (2000).
sFrom Huang et al. (2003).
rFrom Pomper et al. (2003b).
al. (2000). Pomper et al. (2003b) used 10 inter-simple sequence repeat (ISSR) markers, and determined estimates of genetic diversity (P=80% and He=0.358) that were higher than those based on isozymes (P=44% and He=0.172) for cultivated pawpaw, for RAPDs for wild pawpaw accessions (P=64% and He=0.249) and cultivated pawpaw (He=0.285) by Huang et al. (1997, 2000, 2003). These higher diversity values estimated for cultivated pawpaw by the ISSR marker system indicate that this marker methodology has a higher level of discrimination in evaluating genetic diversity in pawpaw than the isozyme or RAPD marker systems and/or that pawpaw has greater levels of genetic diversity than previously found.
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Pawpaw orchards should be located at a site with characteristics that would be well suited for production of other temperate tree fruit species. The planting site should have good air drainage to reduce the risk of damage from late spring frosts that can damage both foliage and flowers. In mid-April in Kentucky, frost damage to flowers and leaves has occurred when early morning temperatures drop to -2.2circC or lower. During the first year in the ground, trees benefit from partial shading (Gould 1939). After the first year of growth, if trees are over about 45 cm in height, pawpaw trees are tolerant of full sun, and mature trees will bear large quantities of fruit in an open exposure if properly pollinated. The soil should be slightly acid (pH 5.4–7.0), deep, fertile, and well-drained. Bonney (2002) found that the soil pH ranged from neutral (7.05) to acidic (5.38) in eight wild patches in Kentucky. Pawpaw trees will not thrive if frequently flooded (Nash and Graves 1993) or if they are grown in heavy soil or waterlogged soil (Lagrange and Tramer 1985). Lagrange and Tramer (1985) reported that soils in wild pawpaw patches varied considerably in sand/silt/clay ratios, although clay contents were generally low. Pawpaw trees attained their greatest height in moist sites that were characterized by occasional flooding, but with soils with a high percentage of sand and good drainage. Pawpaws are sensitive to both flooding and drought (Nash and Graves 1993). Adequate soil moisture is critical during the first two years of establishment. Mulching pawpaws with straw helps to preserve soil moisture and enhances survival. Straw mulch also produces a soft cushion to prevent bruising of fruits that may fall to the ground before hand-harvesting.
The pawpaw produces a relatively large seed averaging 2.8 ± 0.1 cm in length and 1.5 ± 0.1 cm in width (Geneve et al. 2003). The flat, spatulate seed has a dark-brown fibrous seed coat 16 cell layers thick (Corner 1948; Mohana Rao 1982). A ruminant endosperm occupies most of the seed cavity, characteristic of seeds from annonaceous species (Hayat 1963; Rizzini 1973). A small (1 mm long) embryo is located at the hilar end of the seed (Finneseth et al. 1998a).
Seed can be collected from fruit when the flesh is soft or over-ripe (Layne 1996; Hartmann et al. 2002). Pawpaw has moderately recalcitrant seed that does not tolerate desiccation, and it only has a relatively short period of viability (Bonner and Halls 1974; Finneseth et al. 1998b). Des