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  • Author or Editor: Snake C. Jones x
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Kentucky State Univ. (KYSU) emphasizes research on developing alternative, high-value crops and sustainable agriculture methods for use by limited-resource farmers. Since 1990, KYSU has maintained a research program to develop pawpaw into a new high-value tree fruit crop. With its high tolerance for many native pests and diseases, pawpaw shows great potential as a crop for organic and sustainable production. The objectives of KYSU's pawpaw research program include: 1) variety trials; 2) development of new or improved methods of propagation; 3) collection, evaluation, preservation, and dissemination of germplasm; and 4) sharing of information on pawpaw with scientists, commercial growers and marketers, and the general public. To aid in dissemination of information on pawpaw, a web site has been developed (http://www.pawpaw.kysu.edu) that includes information on current and past pawpaw research at KYSU and information on the PawPaw Foundation. On this site, there are a selected bibliography of publications on pawpaw and related species; pawpaw recipes and nutritional information; a guide to buying and growing pawpaws; photos of pawpaw trees, flowers and fruit; and links to other web sites with pawpaw information. In the future, the site will include results from the pawpaw regional variety trials and the database for the National Clonal Germplasm Repository for Asimina spp., located at KYSU. The pawpaw information web site will be an increasingly useful aid in the introduction of pawpaw as a new, potentially high-value, tree fruit crop.

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The pawpaw [Asimina triloba (L.) Dunal] is a native American tree fruit with potential in edible landscapes and as a new fruit crop. A split-plot experiment (main plot: fertilizer level and subplot potting medium) was conducted in the greenhouse to identify the best growing medium for production of pawpaw seedlings. Seeds were sown in rootrainers containing one of the following media: 1) Promix (control); 2) 6 pine bark:1 mason sand (v/v); 3) 1 mason sand: 1 sphagnum peat; and 4) 4 pine bark:1 mason sand:1 sphagnum peat. When seedlings had at least two to three leaves, weekly fertigation of seedlings began, using 0, 250, or 500 ppm Peters 20N-20P-20K. Germination rate at 10 weeks was similar in all media, at about 80%. The plants were destructively harvested 10 weeks after imposition of fertigation treatments. Both potting media and fertigation influenced leaf number and height; however, there was a significant interaction between these main effects. Leaf number and height for plants in medium 3 were similar to those of the control (medium 1), at about 11 leaves and 18-cm plant height, respectively, at 500 ppm fertigation. Plants in media 2 and 4 were about half as tall and had about half as many leaves as control medium plants at 500 ppm fertigation. Plant leaf area and biomass data will be discussed.

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Pawpaw is a native American tree fruit that has great potential as a new commercial crop. The USDA National Clonal Germplasm Repository for Asimina sp. is located at Kentucky State Univ. (KSU); therefore, germplasm collection and storage are important components of the research program. Recalcitrant seeds do not tolerate desiccation, have a relatively short period of viability, and tend not to tolerate subfreezing temperatures. Since pawpaw seed shows a moderate level of recalcitrance, the objectives of this experiment were to determine which storage temperatures (20, 5, -15, and -70 °C for 8.5 weeks) would maintain viable seed, and whether prior seed stratification (5 °C for 17.5 weeks) would influence survival at the various storage temperatures. Seeds were placed in ziplock bags in moist peat moss and subjected to the range of storage temperatures either before or after stratification. After storage and stratification treatments, seed germination rate was examined for 10 weeks at 25 °C on moist filter paper in petri dishes. Both stratification and storage temperature significantly affected seed germination rate. Seeds did not germinate after storage at subfreezing temperatures, regardless of stratification treatment. The best germination rate, ≈70%, was obtained with stratification followed by storage at 20 °C. However, for long-term storage of viable nongerminating pawpaw seed, stratification followed by storage at 5 °C would be most appropriate. Subfreezing storage temperatures were found to be lethal to pawpaw seeds.

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In an effort to determine the optimal light level for growing pawpaw [Asimina triloba (L.) Dunal] seedlings outside, seedlings were germinated in a greenhouse until the two- to three-leaf stage, at which time they were placed outside and shade treatments were imposed that reduced incident light intensity by 30%, 55%, 80%, and 95%. Control seedlings were left unshaded outside. A randomized block design was used, with 20 replicate seedlings in each experimental treatment per block. Plants were destructively harvested 11 weeks after the start of the experiment. After 11 weeks, the height and number of leaves per seedling were about 35% higher with light to moderate shading (30%, 55%, and 80%) than in control (unshaded) seedlings. Shoot and leaf dry weights of seedlings grown in 30%, 55%, and 80% shade were almost 2-fold greater than control plants. Root dry weight of seedlings in 30% and 55% shade was 2-fold higher than in control plants. Total plant biomass was greatest in the 30%, 55%, and 80% shade treatments, about 2-fold higher than control plants. Total leaf area per seedling increased significantly with up to 80% shading. Seedlings growing under 95% shading had fewer and smaller leaves and reduced biomass production compared to control plants. There was a trend for shaded plants to display a higher leaf chlorophyll content than control plants. Overall, the best seedling growth was achieved in the 30% and 55% shade treatments outdoors.

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The pawpaw [Asimina triloba (L.) Dunal] is a native plant found mainly in the southeastern and eastern United States, and its fruit has great potential as a new high-value crop in these regions. Although there are ≈45 named pawpaw cultivars, breeding for improvement of specific traits, such as fruit size and quality, is desirable. Our long-term goal is to utilize molecular marker systems to identify markers that can be used for germplasm diversity analyses and for the construction of a molecular genetic map, where markers are correlated with desirable pawpaw traits. The objective of this study was to identify random amplified polymorphic DNA (RAPD) markers that segregate in a simple Mendelian fashion in a controlled A. triloba cross. DNA was extracted from young leaves collected from field-planted parents and 20 progeny of the cross 1-7 × 2-54. The DNA extraction method used gave acceptable yields of ≈7 μg·g-1 of leaf tissue. Additionally, sample 260/280 ratios were ≈1.4, which indicated that the DNA was of high enough purity to be subjected to the RAPD methodology. Screening of 10-base oligonucleotide RAPD primers with template DNA from the parents and progeny of the cross has begun. We have identified two markers using Operon primer B-07 at 1.1 and 0.9 kb that segregate in a simple Mendelian fashion in progeny of the 1-7 × 2-54 cross. Other primers and controlled crosses will also be screened.

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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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