HORTSCIENCE 37(2):329–333. 2002.

Kirk W. Pomper1, Desmond R. Layne2, Snake C. Jones3, and Michael G. Kwantes4

Kentucky State University, Land Grant Program, 129 Atwood Research Facility, Frankfort, KY 40601-2355

Additional index words. Asimina triloba, bottom heat, chlorophyll, root : shoot ratio, Kentucky banana

Abstract. Pawpaw [Asimina triloba (L.) Dunal] is a native American fruit tree that has potential as a new fruit crop or for use in landscapes, but little information is available to nurseries on the production of containerized plants. In greenhouse experiments, growth of pawpaw seedlings in Rootrainers was examined in three fertilization regimes, two root-zone temperatures, and four substrates [ProMix, 6 pine bark : 1 sand (v/v), 1 sand : 1 sphagnum peat, and 4 pine bark : 1 sand : 1 sphagnum peat medium]. A similar germination rate of 80% was obtained in all substrates. Weekly fertigation treatments were imposed when seedlings had 2 to 3 leaves, at 0, 50, and 100 mg·L-1 N as Peters 20N–8.6P–16.6K water-soluble fertilizer plus soluble trace elements. After 140 days at the highest fertilizer rate, plant height, leaf number, and dry weight (roots, shoots, and total plant) were greater in ProMix and 1 sand :1 sphagnum peat than in 6 pine bark :1 sand (v/v) or 4 pine bark :1 sand :1 sphagnum peat. Also, the root : shoot ratio was lower in ProMix and 6 pine bark :1 sand (v/v). Overall, plant biomass production was greater in ProMix than in 6 pine bark :1 sand (v/v). In a separate experiment, bottom heat (32 ± 0.3 °C) hastened seedling emergence from ProMix by 9 days compared to ambient root-zone conditions (24 ± 0.2 °C). An average seedling height of 10 cm was attained by ambient plants 79 days after sowing, whereas seedlings with bottom heat reached this height after 69 days. Seedlings subjected to bottom heat had increased leaf number (30%), plant height (32%), whole plant leaf area (94%), shoot dry weight (104%), root dry weight (50%), lateral root dry weight (125%), and total plant dry weight (87%). Seedlings with bottom heat had a reduction in root : shoot ratio of 25% and in specific leaf dry weight of 16% compared to ambient plants. Seedlings subjected to bottom heat had a higher leaf chlorophyll (chl) concentration of chl a (39%), chl b (33%), chl p (43%), total chl (38%), and chl a : b ratio (8%) than seedlings grown without bottom heat. Pawpaw seedling growth was best using ProMix with 100 mg·L-1 N Peters applied once weekly, or using ProMix with bottom heat and 50 mg·L-1 N Peters applied twice per week.

Pawpaw is a native American fruit tree that has potential for use as a new fruit crop or in landscapes (Darrow, 1975; Layne, 1996). Possessing a unique, almost tropical flavor, resembling a blend of banana, mango and

pineapple, pawpaw fruit have excellent fresh market and processing potential. Difficulties in nursery propagation have been a major factor in limiting development of a commercial pawpaw fruit industry. Recently, improved seedling and clonal propagation techniques for pawpaw have been demonstrated (Finneseth et al., 1998a, 1998b, 2000; Layne, 1996; Pomper et al., 2002). However, further improvements in cultural methods for the production of robust pawpaw seedlings for grafting or planting directly into orchards or landscapes would be desirable.

Pawpaw is often difficult to propagate by seed in the field. Germination rate in the field declines significantly if pawpaw seed becomes dry. Reducing seed moisture content from 37% to 25% reduces viability by 50% (Finneseth et al., 1998a). Damage to the delicate root system of field-dug pawpaw seedlings can also lead to poor seedling establishment (Layne, 1996). Due to the problems encountered by field propagation, most commercial nurseries in the United States prop Elevated soil temperatures have been reported to significantly increase nutrient uptake and dry weight gain in passionfruit (Passiflora edulis f. edulis and Passiflora edulis f. flavicarpa; Menzel et al., 1994), flowering and stem length for hydroponically grown roses (Rosa ×hybrida L.; Moss, 1984), and nitrogen accumulation by apple trees [Malus ×sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.; Toselli et al., 1999]. Hurewitz and Janes (1983) reported that tomato (Lycopersicon esculentum Mill.) leaf carbon exchange rate and leaf area increased as soil temperature was increased to 30 °C. However, root-zone temperatures > 30 °C have been reported to restrict plant growth in some vegetable crops (Bode-Stoltzfus et al., 1998; Hurewitz and Janes, 1983; Klock et al., 1996) and several temperate tree species (Graves and Aiello, 1997; Graves et al., 1989, 1991). In

gate pawpaw trees in containers (1998 KSU Nursery Survey, unpublished). This results in a significantly greater cost to the purchaser; however, these trees usually have strong, healthy root systems at transplanting. Our long-term goal is to enhance container production of robust pawpaw seedlings that would be well suited either for grafting or for transplanting to orchards or home landscapes.

Container medium plus form and application rate of N can significantly affect growth of woody species (Aiello and Graves, 1997; Cruz et al., 1993; Sartain and Ingram, 1984). Because of its relatively low cost, light weight, and availability, pine bark alone or blended with sand, sphagnum peat, or perlite is a common substrate used by nurseries in the south-eastern United States for container grown plants (Davidson et al., 2000; Hartmann et al., 1997). Japanese maple (Acer palmatum Thunb.), sugar maple (Acer saccharum Marsh.), redbud (Cercis canadensis L.), flowering dogwood (Cornus florida L.), kousa dogwood (Cornus kousa), golden-rain tree (Koelreuteria paniculata Laxm.), saucer magnolia (Magnolia ×soulangiana Soul.-Bod.), blackgum (Nyssa sylvatica Marsh.), and pin oak (Quercus palustris Muench.) have been grown successfully with pine bark that is supplemented with micronutrients (Wright et al., 1999). Chinese chestnut (Castanea mollissima Bl.), American chestnut [C. dentata (Marsh.)], and Japanese chestnut (C. crenata Sieb.) seedlings have been grown successfully with a 6 pine bark: 1 sand (v/v) substrate (Huang, et al., 1994). Green ash (Fraxinus pennsylvanica Marsh.) and red oak (Quercus rubra L.) seedlings have been propagated successfully with 3 pine bark: 1 peat moss: 1 sand (v/v) substrate (Arnold and Struve, 1989). Pawpaw seedlings have been grown successfully in containers with a 1 peat moss: 1 pine bark: 1 vermiculite (v/v) medium (Nash and Graves, 1993) and also using well-aerated growing substrate such as ProMix 'BX', which contains sphagnum peat (75% to 85% by volume), perlite, vermiculite, macronutrients, and micronutrients (Layne, 1996). However, no studies have compared growth substrates or developed a fertilization regime for enhanced container production of pawpaw seedlings.

Elevated soil temperatures have been reported to significantly increase nutrient uptake and dry weight gain in passionfruit (Passiflora edulis f. edulis and Passiflora edulis f. flavicarpa; Menzel et al., 1994), flowering and stem length for hydroponically grown roses (Rosa ×hybrida L.; Moss, 1984), and nitrogen accumulation by apple trees [Malus ×sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.; Toselli et al., 1999]. Hurewitz and Janes (1983) reported that tomato (Lycopersicon esculentum Mill.) leaf carbon exchange rate and leaf area increased as soil temperature was increased to 30 circC. However, root-zone temperatures > 30 circC have been reported to restrict plant growth in some vegetable crops (Bode-Stoltzfus et al., 1998; Hurewitz and Janes, 1983; Klock et al., 1996) and several temperate tree species (Graves and Aiello, 1997; Graves et al., 1989, 1991). In

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