ring plants (e.g., rhododendron (Rhododendron L.), english box-wood (Buxus sempervirens L.), and chinese holly (Ilex cornuta Lindl. & Paxt.)) show increases in root and shoot weight with moderate shading (Whitcomb, 1988). With the shade-preferring plant belgian evergreen (Dracaena sanderana hort Sander ex Mast.), moderate shading decreased the root to shoot ratio and increased plant biomass and leaf area (Vladimirova, 1997). In peach [Prunus persica (L.) Batsch (Peach Group)], shading increased average leaf area, decreased specific leaf weight [SLW; mg·cm⁻²], and increased the chlorophyll concentration in leaves (Kappel and Flore, 1983). In apple [Malus sylvestris (L.) Mill var. domestica (Borkh.) Mansf.], shading increased individual leaf area, decreased specific leaf weight, and reduced leaf number (Maggs, 1960). Young and Yavitt (1987) noted that solar irradiance in the understory of eastern deciduous forests, where wild pawpaw trees are usually found, decreased by 85% when the forest canopy had developed fully. Although pawpaw is native to the shaded environment of the forest understory, optimal light conditions for seedling production in containers have not yet been determined.

A serious problem with transplanting container-grown trees is poor root development (i.e., circling, 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 [e.g., river birch (Betula nigra L.), baldcypress (Taxodium distichum (L.) Rich), american plane tree (Platanus occidentalis L.), and weeping willow (Salix babylonica L.)] (Ruter, 1994). Root elongation is inhibited when root tips encounter copper compounds in paint applied to container walls, thus causing lateral branching further back on the root and the formation of a more fibrous root system and a lower root to shoot ratio (Arnold and Struve, 1989, 1993; Arnold and Young, 1991). The lower root to shoot ratio with Cu(OH)2 treatment of containers is the result of a change in dry-matter allocation due to reduced growth of taproots after they contact the bottom of the containers and the subsequent more even vertical distribution of the root systems (Arnold and Struve, 1993). Cupric hydroxide treatment of the interior of containers decreased the root to shoot ratio in West Indian mahogany (Swietinia mahagoni) (Svenson et al., 1995). The root to shoot ratio of green ash (Fraxinus pennsylvanica Marsh.) and red oak (Quercus rubra L.) decreased with use of CuCO3 on the interior surface of containers (Arnold and Struve, 1989).

Since pawpaws are propagated primarily in containers both in the greenhouse and outdoors, the first objective of this study was to determine the influence of incident irradiance on seedling growth and development in both greenhouse and outdoor environments. A second objective was to determine the influence of root-zone modification with Cu(OH) 2-treated containers on pawpaw seedling growth and development.

and development in both greenhouse and outdoor environments. A second objective was to determine the influence of root-zone modification with Cu(OH)2-treated containers on pawpaw seedling growth and development.

Materials and Methods

PLANT MATERIAL. For all experiments, pawpaw seeds were harvested in Fall 1995 from open-pollinated half-sib trees at an orchard at the Western Maryland Research and Education Center, Keedysville, Md. Seeds were sown to a 3 cm depth in moist Pro-Mix ‘BX’ potting medium (Premier Hort. Inc., Red Hill, Pa.) in 740 cm³ Rootrainers (Hummert Intl., Earth City, Mo.). All seeds were stratified (moist-prechilled) at 5 °C for at least 3 months prior to sowing. A day/night regime of 25/20 °C was maintained in a 6 × 9 m glass covered greenhouse during seed germination. Seedlings were watered weekly to runoff and were grown in a greenhouse under a 16 h photoperiod supplemented by high pressure sodium lamps.

GREENHOUSE STUDY (EXPT. 1). A factorial greenhouse experiment was conducted with treatments that included five levels of shade (0%, 33%, 56%, 81%, and 98%) and two levels of root-zone treatment [with or without Cu(OH)2 treatment of the interior of containers]. Actual photosynthetic photon flux (PPF) values for shade treatments are found in Table 1. The treatments were arranged in a split-plot design in two replicated blocks, where the main plot effect was shade, and subplot effect was Cu(OH)2. Experimental treatment combinations had 10 seedlings per replication. Blocks were arranged along the long, north–south axis in the greenhouse.

Before seed sowing, as described above, half of the 40 Rootrainer trays used in this study were treated with a water based latex paint with Cu(OH) 2 at 100 g · L -1 (Spin Out, Griffin Corp., Valdosta, Ga.). Each tray was used to produce 27 seedlings (nine books with three seedling cells per book). Individual books were unfolded and immersed in the latex paint with Cu(OH) 2 and allowed to dry completely before moist medium was added. On 11 Apr. 1996, seeds were sown in each cell of the Cu(OH) 2-treated and nontreated (control) trays. Seedlings were grown in the greenhouse until they reached the two- to three-leaf stage on 16 June 1996, when seedling number per 27-cell tray was reduced to the most uniform 10 by excising the remaining plants at the soil line. Following roguing, all seedlings were moved to a different 6 × 9 m greenhouse with days/nights of 29 ± 3 / 24 ± 2 circC. This greenhouse had clean, clear glass walls and a roof (not whitewashed), and no supplemental irradiation was supplied in it or the adjacent houses during the duration of the study. Four trays (two from each root-zone modification treatment)

Table 1. Environmental conditions for pawpaw seedlings grown outdoors or in the greenhouse under selected shading treatments.

2Mean separation within columns by LSD at P < 0.05.

NS, ***Nonsignificant at P > 0.05 or significant at P < 0.0001, respectively.

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

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