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field (Pomper et al. 2002b). Bottom heating of container-grown pawpaw seedlings could decrease both the time to produce a saleable plant and the cost of heating greenhouses.

Pawpaw seedlings grown outdoors appear to be sensitive to high irradiances upon emergence from the soil, and benefit from partial shading for the first year of development (Gould 1939). Seedlings produced in greenhouses do not show sensitivity to high light levels, suggesting that seedlings grown outdoors may be sensitive to ultraviolet radiation (Peterson 1991). Pawpaws are often found in the shaded forest understory. Young and Yavitt (1987) reported that proximal pawpaw leaves, which developed before the overstory forest canopy closed, were 76% smaller than distal leaves that developed after canopy closure. Growth of containerized pawpaw seedlings was positively influenced by low to moderate shading (28% or 51%) outdoors and low shading (33%) in the greenhouse, in a manner typical of that reported for other shade-preferring plants (Pomper et al. 2002a). Shading increased leaf chlorophyll a and b concentrations for pawpaw seedlings grown outdoors, while it decreased average specific leaf DW (mg leaf DW cm-2). Low to moderate shading of pawpaw seedlings grown outdoors greatly increased whole plant biomass, indicating that commercial nurseries could possibly enhance production of containerized pawpaw seedlings using this shading regime outdoors. If plants are produced on a gravel container pad, higher levels of shading (>55%) that would also reduce air temperatures could be beneficial.

Pomper et al. (2002a) reported that application of Cu(OH)₂ to the interior of Rootrainers (0.7 L) did not promote development of a more fibrous root system in pawpaw seedlings as reported for other tree species, based on failure of copper compound to increase lateral root dry weight production. In shaded plants, seedlings showed yellowing of leaves and reduced chlorophyll levels by the end of the experiment, suggesting that the plants were suffering from copper toxicity. However, the use of larger containers (8 L) with Cu(OH)₂ applied to the interior of containers does increase seedling lateral root dry weight.

Propagation by rooting of stem cuttings is difficult in pawpaw and is not currently a viable commercial practice. Experiments using cuttings taken from seedlings of various ages showed a significant impact of juvenility on pawpaw rooting ability. In a systematic study using over 1200 stem cuttings taken from mature flowering pawpaw trees at 7-day intervals

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from 17 June to 5 August only one cutting produced an adventitious root (Finneseth 1997). Cuttings taken from up to 2-month-old seedlings showed a high capacity to root. Cuttings treated with 10,000 mg L-1 IBA rooted at 75% and averaged two roots per cutting (Geneve et al. 2003). Seedlings beyond 2 months showed a reduced capacity to form roots. Cuttings taken from 7-month-old seedlings rooted at less than 10% regardless of auxin treatment. Strategies to revert stock plants to a more juvenile state (i.e., tissue culture or mound layering) may be required before a reliable method for cutting propagation can be obtained.

Pawpaw is a naturally suckering species, readily forming adventitious shoots from roots. Propagation from root cuttings can be an alternative method for multiplication of difficult-to-root species such as pawpaw. Shoots derived adventitiously from roots retain a juvenile character and could serve as a source for stem cuttings or explants for tissue culture (Hackett 1985). Finneseth (1997) collected root cuttings from a wild patch in December in Kentucky, and found no shoots formed on root pieces that were less than 5 mm in diameter, while 56% of root pieces greater than 5 mm in diameter produced one or more shoots. On average, responding roots produced 2.5 buds and 1.1 elongating shoots. Buds were visible on root pieces 12 weeks after planting and shoot elongation was evident after 16 weeks. Stooling (mound layering) has been attempted in the field as a method to propagate pawpaw selections. In a factorial experiment with two levels of girdling (girdled or not) and three levels of IBA at 0, 3000, and 6000 mg L-1 in lanolin paste, only two roots formed on one shoot out of 80 treated shoots (Pomper, unpubl. data).

Pawpaws are easily propagated by grafting and budding (e.g., whip-and-tongue, cleft, bark inlay, and chip budding) (Layne 1996). Winter collected, dormant budwood should have its chilling requirement fulfilled. The whip-and-tongue graft of a scion to an actively growing root-stock is used by several commercial nurseries to propagate pawpaw cultivars. Chip budding is most successful when the seedling rootstock is at least 0.5 cm diameter and actively growing. Bud take exceeding 90% can be obtained.

Currently, pawpaw cultivars with superior fruit characteristics are propagated by grafting and budding onto seedling rootstocks. Seedling rootstock research has used seed from open-pollinated half-sib trees at the Keedysville orchard at the Western Maryland Research and Education Center. Great variation in scion growth has been observed in orchards at KSU when these seedlings have been used as rootstock (Pomper et al. 2003d). Seedstock from various pawpaw genotypes are