Incident Irradiance and Cupric Hydroxide Container Treatment Effects on Early Growth and Development of Container-grown Pawpaw Seedlings

The North American pawpaw [Asimina triloba (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 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). 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. 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-preferReceived for publication 13 Sept. 2001. Accepted for publication 11 Oct. 2001. This research was supported by U.S. Dept. of Agriculture, Cooperative State Research, Education, and Extension Service agreement no. KYX-10-97-28P. We appreciate participation of high school summer student interns L.N. Peters, B.J. Kundu, and La’Teasa Barnes, who participated in the KSU-USDA 1890 Research and Extension Apprenticeship Program and this project. We thank R. Neal Peterson for supplying seeds used in this study. Technical assistance of E. Reed, Jr., and Terrih-Angelah Turner is also gratefully acknowledged. The cost of publishing this paper was defrayed in part by the payment of the page charges. Under postal regulation, this paper must be hereby marked advertisement solely to indicate this fact. Corresponding author and principal investigator of horticulture; curator, USDA National Clonal Germplasm Repository for Asimina sp., Kentucky State Univ. Assistant professor of pomology and extension fruit specialist, Dept. of Horticulture, Box 340375, Clemson Univ., Clemson, SC 29634-0375. Research assistant, Horticulture Program, Kentucky State University. 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 °C) through 8 (minimum of –10 °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 8901-Dev 11/12/01, 10:36 AM 13 14 J. AMER. SOC. HORT. SCI. 127(1):13–19. 2002. ring plants (e.g., rhododendron (Rhododendron L.), english boxwood (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], 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. 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 cm 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 (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 twoto 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 °C. 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. Photosynthetic Ultraviolet photon flux Red to far-red light (μmol·m·s) Shade (%) Temp (°C) light ratio (μmol·m·s) Shade (%) Outdoors Greenhouse Outdoors Greenhouse Outdoors Greenhouse Outdoors Greenhouse Outdoors Greenhouse None 1478 a 1217 a 0 0 24.2 a 30.8 1.21 1.23 131.5 a 56.5 a 3