duces greater lateral and total root dry weight than in pawpaw seedlings grown at ambient temperature; this could result in a higher rate of establishment of bottom heated seedlings in the field.

Plant biomass production and partitioning can be significantly influenced by incident irradiance upon the plant (Buisson and Lee, 1993; Givinish, 1988; Rajapakse et al., 1999). Shade-preferring plants [e.g., rhododendron (Rhododendron), english boxwood (Buxus sempervirens), and chinese holly (Ilex cornuta) showed increased root and shoot weight with moderate shading (Whitcomb, 1988). With the shade-preferring plant belgian evergreen (Dracaena sanderana), moderate shading decreased the root:shoot ratio and increases plant biomass and leaf area (Vladimirova et al., 1997).

Pawpaw seedlings grown outdoors have been reported to be sensitive to high irradiance upon emergence from the soil, and benefit from partial shading during the first year of development (Gould, 1939; Peterson, 1991). Seedlings produced in greenhouses do not show sensitivity to high light levels, suggesting that seedlings grown outdoors may be sensitive to ultraviolet (UV) radiation (Peterson, 1991).

Pomper et al. (2002a) found that growth of containerized pawpaw seedlings was positively influenced by low to moderate shading with polypropylene shade fabric (28% or 51%, respectively) outdoors and low shading (33%) in the greenhouse, in a manner typical of that reported for other shade-preferring plants. Low to moderate shading of pawpaw seedlings grown outdoors greatly increased leaf number, total leaf area, and total plant dry weight compared to nonshaded seedlings, indicating that commercial nurseries can improve production of containerized pawpaw seedlings by using a 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. Shading of field-planted pawpaw trees has been suggested to assist in the establishment of young trees (Layne 1996). Trees that are taller than 0.46 m (1.5 ft) do not require shading the first year, but trees should be provided irrigation to enhance establishment (author, unpublished).

A serious problem with transplanting container-grown trees is poor root development (i.e., circled, matted, and kinked roots), which is associated with increased tree mortality (Nichols and Alm, 1983). A water based latex paint containing cupric hydroxide [Cu(OH)2] has been used in commercial nurseries to treat the interior surface of containers and successfully reduce root spiraling in several tree species (Ruter, 1994). Root elongation is inhibited when root tips encounter copper compounds in paint applied to container walls, causing lateral branching from older root regions more distant from the root tip and the formation of a more fibrous root system and a lower root:shoot ratio (Arnold and Struve, 1989, 1993; Arnold and Young, 1991). Pomper et al. (2002a) reported that application of Cu(OH)2 to the interior of Roottrainers did not promote development of a more fibrous root system in pawpaw seedlings, based on failure of copper compound to increase lateral root dry weight production (Pomper 2002a). Seedlings also showed yellowing of leaves and reduced chlorophyll levels by the end of the experiment in shaded plants, suggesting that the plants were suffering from copper toxicity. The use of larger containers and lower concentrations of Cu(OH)2 compared to those used by Pomper (2002a) might reduce plant copper toxicity problems and stimulate lateral branching of seedling root systems.

In conclusion, recent studies have allowed the development of recommendations (e.g., potting substrate type, fertilizer regime, the use of tall containers to accommodate the seedling’s strong tap root, bottom heating, and shading regime) toward successful container production of pawpaw. However, further improvements in cultural methods for container production of robust pawpaw seedlings for grafting, transplanting to orchards, or home landscapes would be desirable.

Arnold, M.A. and D.K. Struve. 1989. Growing green ash and red oak in CuCO3-treated containers increase root regeneration and shoot growth following transplant. J. Amer. Soc. Hort. Sci. 114:402–406.

Arnold, M.A. and D.K. Struve. 1993.

Root distribution and mineral uptake of coarse-rooted trees grown in cupric hydroxide-treated containers. HortScience 28:988–992.

Arnold, M.A. and E. Young. 1991. CuCO3-painted containers and root pruning affect apple and green ash root growth and cyto-kinin levels. HortScience 26:242–244.

Buisson, D. and D.W. Lee. 1993. The developmental response of papaya leaves to simulated canopy shade. Amer. J. Bot. 80:947–952.

Finneseth, C.L.H., D.R. Layne, and R.L. Geneve. 1998a. Requirements for seed germination in North American pawpaw [Asimina triloba (L.) Dunal.]. Seed Sci. Technol. 26:471–480.

Finneseth, C.L.H., D.R. Layne, and R.L. Geneve. 1998b. Morphological development of pawpaw [Asimina triloba (L.) Dunal.] during seed germination and seedling emergence. HortScience 33:802–805.

Gould, H. 1939. The native pawpaw. USDA Lft. 179.

Layne, D.R. 1996. The pawpaw [Asimina triloba (L.) Dunal.]: A new fruit crop for Kentucky and the United States. HortScience 31:777–784.

Mandujano, M. and D.W. Fulbright. 1998. Meristem dieback of greenhouse-grown pawpaw is associated with soil pH. Annu. Rpt. N. Nut Growers Assn. 89:103–107.

Menzel, D.M., D.W. Turner, V.J. Doogan, and D.R. Simpson. 1994. Root shoot interactions in passionfruit (Passiflora sp.) under the influence of changing root volumes and soil temperatures. J. Hort. Sci. 69:552–564.

Moss, G.I. 1984. The effects of root-zone warming on the yield and quality of roses grown in a hydroponic system. J. Hort. Sci. 59:549–558.

Nelson, P.V. 1998. Greenhouse operation and management. Prentice-Hall, Upper Saddle River, N.J.

Nichols, T.J. and A.A. Alm. 1983. Root development of container-reared, nursery-grown, and naturally regenerated pine seedlings. Can. J. For. Res. 13:239–245.

Peterson, R.N. 1991. Pawpaw (Asimina). In: J.N. Moore and J.R. Ballington (eds.). Genetic resources of temperate fruit and nut trees. Acta Hort. 290:567–600.