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  • Author or Editor: Kirk W. Pomper x
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Since large ripe strawberry fruit lose water faster than smaller unripe fruit, ripening fruit may out-compete unripe fruit for water within an inflorescence and thereby reduce yields. Xylem water potential (ψ) of greenhouse-grown `Brighton' fruit, at different stages of development, was measured with a pressure chamber during mid-afternoon. Large ripe and medium sized green-white fruit within an inflorescence had a more negative ψ than small green fruit. This suggests that small green fruit have a lower priority for xylem water moving into a strawberry inflorescence. Large ripe and small green fruit within an inflorescence had positive cell turgor (about 300 KPa), whereas turgor of medium sized green-white fruit was near zero, suggesting that these latter fruit were encountering greater water stress. When competing fruit on an inflorescence were removed, the calculated cell turgor was very similar to fruit with competition. Cell turgor of strawberry fruit is greatly influenced by developmental stage; this may moderate effects caused by water loss from neighboring fruit on an inflorescence.

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Pawpaw (Asimina triloba L.), a species of the eastern United States, bears the largest edible fruit of all native trees. Relatively little is known about ripening of pawpaw, and several problems, such as short shelf life and duration of harvesting, hamper pawpaw production. While previous investigations have resulted in identifying physical properties associated with ripening, the effects on phenolic content and antioxidant capacity have not been investigated. The objectives of the study were to investigate changes in phenolic content and antioxidant capacity and to identify physical parameters of pawpaw pulp during ripening. Sample extraction of pawpaw was achieved by adding acetone (2 mL/1 g of sample) to pulp of a pawpaw cultivar, PA Golden, and then vortexing (30 s) and sonicating (15 min) the sample and solvent, prior to centrifugation (15 min) twice at 2987 × g. Folin-Ciocalteu assay and ferric reducing antioxidant power (FRAP) assay were used for the estimation of phenolic content and the antioxidant capacity, respectively. While soluble solid content increased during ripening, the hardness of the fruit decreased, confirming previous reports. The pulp of unripe fruits had the greatest phenolic content (gallic acid eq. 131.2 mg/100 g FW) and antioxidant capacity (Trolox eq. 22.7 μM/g FW), which decreased by about 20% as the fruit ripened. Of three color properties measured, chroma, an estimate of color saturation, increased with ripening, while lightness of pawpaw pulp remained the same. A high correlation was found between chroma and hardness of fruits (r = 0.62), and between phenolic content and antioxidant capacity of pawpaw pulp (r = 0.80), suggesting these parameters can be incorporated into methods to estimate the ripeness of pawpaw fruit.

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Pawpaw [Asimina triloba (L.) Dunal], a native species of the eastern United States, bears the largest edible fruit of all indigenous trees. Chemoprotective properties of fruits have been partly attributed to phenolics such as gallic acid and chlorogenic acid, and the phenolic content generally correlates with antioxidant capacity for various kinds of fruits. Despite many reports of commonly available fruits, little information is available on phenolic content or antioxidant capacity for currently underused fruits. The objectives of this study were to determine the phenolic content (PC) and antioxidant capacity (AC) in fruit of two pawpaw cultivars at different stages of ripening. Sample extraction of pawpaw was achieved by adding acetone (2 mL/1g of sample) to the pulp of ‘PA-Golden (#1)’ and advanced selection 1-23, and then vortexing (30 s) and sonicating (15 min.) the sample and solvent before centrifuging it (15 min) twice at 2987 g. Folin-Ciocalteu assay and ferric reducing/antioxidant power assay were used for the estimation of PC and AC, respectively. PC and AC tended to decrease with ripening of fruit. The highest AC was found in the semiripe ‘PA-Golden (#1)’ puree (22.06 μmol TE/g fresh weight), whereas the puree of ripe fruit contained the lowest AC (17.04 μmol TE/g fresh weight), about a 23% decrease. In contrast, the greatest PC and AC were observed in intermediate fruits for 1-23. A positive correlation was found between PC and AC of fruit of ‘PA-Golden (#1)’ (r = 0.62) and 1–23 (r = 0.82). These results suggest that phenolic components of pawpaw pulp have a major effect on AC, as reported for other fruits and vegetables. The relatively high AC found in pawpaw pulp may motivate more health-conscientious people to consume pawpaw fruit. The diversity in PC and AC between pawpaw cultivars emphasizes the need for additional screening to identify cultivars with high AC and health-promoting potential.

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The pawpaw (Asimina triloba) is the largest fruit native to the U.S. and has potential as a new fruit crop. Few methods are available to clonally propagate pawpaw, with grafting or budding onto a seedling rootstock being the only currently feasible method. Developing new options for clonal propagation of pawpaw could help advance this growing industry. Layering has been used to clonally propagate other difficult to root tree species. The objective of this study was to examine trench layering as a method to clonally propagate pawpaw. A randomized factorial experiment was implemented to examine the roles of plant juvenility and auxin concentration on rooting in a greenhouse trench layering system. Seedlings were defoliated, tipped, and transplanted into trench layering beds at 3, 6, and 12 weeks after emergence. Shoots were etiolated, then girdled and treated with three levels of IBA (0, 5000, and 10,000 ppm). The main effects of age and IBA concentration significantly affected the percentage of shoots producing roots. Juvenility enhanced rooting, with 15% of the shoots of the 3-week-old pawpaw seedlings producing roots, compared to only about 5% of the 12-week-old seedlings rooting. Auxin application to shoots also promoted rooting, with 16% of IBA-treated shoots producing roots, compared to the untreated control, with only 2% of shoots producing roots. There was no significant difference in rooting percentage between the two concentrations of IBA. The treatment combination most successful at promoting root initiation was 10,000 ppm IBA applied to shoots of 3-week-old seedlings, with 31% of shoots rooting.

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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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Pawpaw [Asimina triloba (L.) Dunal] has significant potential as a new fruit crop. During ripening, loss of firmness is extremely rapid, and this trait may be the biggest obstacle to the development of a broader market. Cold storage of pawpaw fruit seems limited to 4 weeks at 4 °C, though fruit softening merely slows during storage. A study of several cultivars with commercial appeal has shown that none have superior cold storage life. Extending the cold storage beyond 4 weeks resulted in increasing loss of fruit firmness, poor poststorage ripening, and development of quality traits, and many fruit exhibiting flesh and peel discoloration. Cold storage duration affected fruit volatile production. By 4 weeks of cold storage, ethyl hexanoate and ethyl octanoate were the major volatiles produced, replacing methyl hexanoate, which was the major volatile produced by ripe fruit after harvest. By 8 weeks of cold storage, volatile ester production was generally low and ethyl hexanoate became the only major volatile. This loss of volatile production was accompanied by a decrease in alcohol acetyl transferase activity. Also, during cold storage, there was an increase in total phenolic content, lipid peroxidation products, and polyphenol oxidase activity. These changes may contribute to the black discoloration that developed in fruit cold-stored for 8 weeks or more. It is apparent that cold storage alone may not be sufficient to extend the storage life of most, if not all, current pawpaw cultivars beyond 4 weeks.

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Ripening pawpaw [Asimina triloba (L.) Dunal] fruit exhibit climacteric peaks of ethylene and CO2 production 48 to 72 hours after harvest, and thus may be considered climacteric. The development of desirable quality traits and the loss of fruit firmness during ripening is extremely rapid, and a variety of strategies to slow these processes via manipulation of ethylene production and/or response and by more direct techniques like postharvest heat treatment have been attempted. Fruit, branches with fruit, and/or whole trees have been sprayed with ethephon or aminoethoxyvinylglycine to hasten or delay ripening, respectively. After harvest, fruit have been treated with commercial and higher rates of 1-methylcyclopropene for various durations at ambient and cold storage temperatures. Fruit have also been heat-treated at various temperatures, using both brief “shock” treatments above 40 °C and longer periods at 35 °C to 40 °C. In addition, in an attempt to alleviate the loss of ripening capacity as well as the development of injury symptoms from cold storage for longer than 4 weeks, cold-stored fruit were warmed to ambient temperature intermittently and then returned to cold storage. While some effects of the treatments were noted, the responses to all of these treatment strategies have failed to appreciably alter fruit ripening, the rapid loss of firmness, or otherwise maintain fruit quality beyond that without treatment.

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Pawpaw[Asiminatriloba (L.) Dunal] is a highly perishable climacteric fruit. Generally, fruit may be stored at 4 °C for 4 weeks with minimal loss in quality or subsequent ripening capacity. However, comparisons among cultivars and advanced selections for ripening behavior and postharvest storage life have not been reported. Ideally, cultivars with superior ripening traits (higher firmness, or a slower rate of firmness loss) and longer storage life may be identified for the commercial market. To determine if differences among genotypes may exist, respiration, C2H4 production, and fruit firmness of six varieties, 8-20, 9-58, `Middletown', `PA Golden', `Taytwo', and `Taylor', were measured during ripening after harvest and after 1, 2, 3, 4, 5, and 6 weeks of 4 °C storage. No differences were observed among the cultivars regarding respiration and C2H4 production. Respiratory and ethylene peaks were detected within 48 hours after harvest or removal from cold storage. Rapid loss of firmness was measured during ripening at ambient temperature after harvest and after removal from cold storage (days 0–3), with some loss during cold storage itself (weeks 1–6). As expected, firmer fruits at harvest had a lower respiration rate. However, no obvious differences in ripening behavior or cold storage response were observed among the six genotypes. As a general guideline, only firmer fruits should be cold stored, since fruit softening did not stop at 4 °C. A broader analysis of all of the named cultivars and advanced selections of pawpaw will be needed to determine if the present results are generally representative of pawpaw.

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