J. AMER. SOC. HORT. SCI. 127(1):13–19. 2002.

Kirk W. Pomper,1 Desmond R. Layne,2 and Snake C. Jones3 Kentucky State University, Land Grant Program, Atwood Research Facility, Frankfort, KY 40601-2355

ADDITIONAL INDEX WORDS. Asimina triloba, biomass partitioning, chlorophyll, root to shoot ratio, shade, Kentucky banana, Cu(OH) 2

ABSTRACT. The North American pawpaw [Asimnatriloba (L.) Dunal] has great potential as a fruit crop or as a landscape plant. The influence of incident irradiance on pawpaw seedling growth and development in containers was examined in the greenhouse and outdoors. Root spiraling can be a problem for container-grown pawpaw seedlings; therefore, the influence of paint containing cupric hydroxide [Cu(OH) 2] at 100 g·L-1 applied to the interior of containers on plant growth was also examined in a greenhouse environment. In pawpaw seedlings grown outdoors for 11 weeks, low to moderate shading levels of 28%, 51%, or 81% increased leaf number, total leaf area, and total plant dry weight (DW) compared to nonshaded seedlings. A shading level of 81% decreased the root to shoot ratio by half compared to nonshaded plants. Shading of 98% reduced leaf number, leaf size, and shoot, root, and total plant DW. Shading increased leaf chlorophyll a and b concentrations for pawpaw seedlings grown outdoors, while it decreased average specific leaf DW (mg·cm-2). In a separate greenhouse experiment, pawpaw seedlings subjected to shade treatments of 0%, 33%, 56%, 81%, or 98% did not respond as greatly to shading as plants grown outdoors. Greenhouse-grown plants had greater total and average leaf area under 33% or 56% shading than nonshaded plants; however, shading >56% reduced root, shoot, and total plant DW. Total shoot DW was greater in greenhouse grown plants with 33% shading compared to nonshaded plants. Pawpaw seedlings in control and most shade treatments (33% to 81%) in the greenhouse environment had more leaves and greater leaf area, as well as larger shoot, root, and total plant DW than seedlings in similar treatments grown outdoors. The greenhouse environment had a 10% lower irradiance, a 60% lower ultraviolet irradiance, and a significantly higher (1.23 vs. 1.20) red to far-red light ratio than the outdoors environment. Treatment of container interiors with Cu(OH) 2 decreased total and lateral root DW in nonshaded seedlings, and it adversely affected plant quality by causing a yellowing of leaves and reduction of chlorophyll levels by the end of the experiment in shaded plants. Growth characteristics of pawpaw seedlings were positively influenced by low to moderate shading (28% or 51%) outdoors and low shading (33%) in the greenhouse. Seedlings did not benefit from application of Cu(OH) 2 to containers at the concentration used in this study. Commercial nurseries can further improve production of pawpaw seedlings using low to moderate shading outdoors.

The pawpaw (Asimina triloba) is the largest edible tree fruit native to the United States (Darrow, 1975). With a unique, almost tropical flavored fruit, the pawpaw has great potential as a new high-value fruit crop or for landscape use (Layne, 1996). This plant can be grown successfully in USDA plant hardiness zones 5 (minimum of -25 circ C) through 8 (minimum of -10 circ C), and its native range covers 26 states from northern Florida to southern Ontario, Canada and as far west as eastern Nebraska and Texas (Kral, 1960). More than 40 pawpaw cultivars are commercially available, either having been selected in the wild or the result of breeding efforts of hobbyists (Jones et al., 1998).

Most commercial nurseries in the United States grow pawpaw trees in containers (1998 Kentucky State University Nursery Survey, unpublished). Although this results in a significantly greater cost to the purchaser, trees usually have a stronger, healthier root system than field-dug seedlings. Recent studies have allowed development of recommendations (e.g., potting medium type, fertilizer regime, and use of tall containers to accommodate the strong taproot of seedlings) toward successful container production of pawpaw, but further improvements in cultural conditions would be advantageous to growers (Finneseth et al., 1998a, 1998b; Jones et al., 1998; Layne, 1996). To allow production of high-value grafted trees, it would be desirable to develop a protocol for rapid container production of seedling rootstocks of pawpaw that would result in a large graftable plant (pencil diameter or 1 cm diameter stem) in 1 year.

Survey, unpublished). Although this results in a significantly greater cost to the purchaser, trees usually have a stronger, healthier root system than field-dug seedlings. Recent studies have allowed development of recommendations (e.g., potting medium type, fertilizer regime, and use of tall containers to accommodate the strong taproot of seedlings) toward successful container production of pawpaw, but further improvements in cultural conditions would be advantageous to growers (Finneseth et al., 1998a, 1998b; Jones et al., 1998; Layne, 1996). To allow production of high-value grafted trees, it would be desirable to develop a protocol for rapid container production of seedling rootstocks of pawpaw that would result in a large graftable plant (pencil diameter or 1 cm diameter stem) in 1 year.

Pawpaw seedlings grown outdoors have been reported 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 (UV) radiation (Peterson, 1991). After the first year of growth, pawpaw seedlings are tolerant of full sun, and mature trees will bear large quantities of fruit in an open exposure if properly pollinated.

Plant biomass production and partitioning can be significantly influenced by incident irradiance. Moderate shading of developing sun-preferring plants [e.g., chinese juniper (Juniperus chinensis L. Lindl), japanese euonymous (Euonymus japonica Thunb.), and elaeagnus (Elaeagnus macrophylla Thunb.) decreases root and shoot weight (Whitcomb, 1988). In contrast, shade-prefer-

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