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  • Author or Editor: Michael G. Kwantes x
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The role of incident light intensity (light) and fertilization regime (Fert) on early growth and development of pawpaw seedlings in the greenhouse was investigated. Three-hundred and fifty stratified seeds were sown in 45 cubic inches. Rootrainer cells in Promix BX potting medium. A two-factor factorial design was used. Light was either ambient (Amb) or 80% shaded (Shade) Fert was 500 ppm of Peter's 20N–20P–20K plus soluble trace elements (+ Fert) or water (–Fert) applied twice weekly to runoff. Thirty uniform seedlings were selected at the two- to three-leaf stage for each of the four treatments. Treatment imposition was for 5 weeks until the 10- to 11-leaf stage (when seedlings would be transplanted). Plant height and leaf number was recorded three times weekly. After 5 weeks, 15 uniform plants per treatment were destructively harvested and leaf chlorophyll (chl) analysis was performed. Amb/+ Fert plants were 25% taller and had two more unfolded leaves, on average, than the other treatments.–Fert plants (Amb or Shade) had set terminal buds. The effect of Fert on chl was much greater under Amb than Shade conditions. For–Fert plants, total chl was reduced by 52% under Amb but only 30% under Shade compared to +Fert plants. Total biomass for Amb/–Fert seedling was double that in either Shade treatment and 40% greater than for Amb/–Fert seedlings.

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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.

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