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  • Author or Editor: Kirk W. Pomper x
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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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Pawpaw (Asimina triloba) fruit stored longer than 4 weeks at 4 °C fail to ripen normally and may develop internal discoloration, indicative of chilling injury (CI). To determine if loss of antioxidant protection in the fruit tissue during cold storage could be the cause of these problems, the levels of total, reduced, and oxidized glutathione and ascorbate and the key enzymes glutathione reductase (GR) and ascorbate peroxidase (APX) of the ascorbate-glutathione cycle were studied in fruit at 4 and 72 h after harvest and after 2, 4, 6, and 8 weeks of 4 °C storage. The total phenolic level was also studied due to its potential antioxidant role, and the activity of polyphenoloxidase (PPO) was assayed, as it may contribute to phenolic oxidation and tissue browning. Fruit ethylene production and respiration rates were in typical climacteric patterns during ripening after harvest and after up to 4 weeks of cold storage, increasing from 4 to 72 h after removal from cold storage, though maximum ethylene production declined after 2 weeks of cold storage. However, fruit showed higher respiration rates at 4 versus 72 h of ripening at 6 or 8 weeks of cold storage, opposite to that at earlier storage dates, possible evidence of CI. Ripening after harvest generally resulted in an increase in total and reduced glutathione, reduced ascorbate, and total phenolics. However, levels of total and reduced glutathione, total ascorbate, and total phenolics declined as storage time progressed. Neither GR nor APX exhibited changes during ripening or trends over the cold storage period. PPO activity increased as the storage period lengthened. Thus, the declining ability of these components of the protective antioxidant systems during the prolonged stress of low temperature storage may be one of the major causes of pawpaw CI limiting it to 4 weeks or less of cold storage. An increase in reactive oxygen species with prolonged storage, coupled with the increase in PPO activity, may have led to greater oxidative damage and been a major cause of the loss of ripening potential and the tissue browning that occurs in fruit stored for more than 4 weeks.

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Pawpaw fruit ethylene production, 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) and ACC oxidase (ACO) activities, and tissue content of the ethylene precursor ACC and conjugate malonyl-ACC (MACC) were measured during postharvest ripening. Fruit were harvested near the advent of the ripening process and were ripened at room temperature. The fruit displayed increases in ethylene production and respiration rate during ripening with maxima for both 3 days after harvest. Mean ethylene maxima on a fresh weight basis were 4.7 and 7.6 μg·kg-1·h-1 and mean respiratory (CO2 production) maxima on a fresh weight basis were 220 and 239 mg·kg-1·h-1 in 1999 and 2001, respectively. The increase in ethylene evolution coincided with an increase in respiration and a rapid decline in fruit firmness. Internal and external fruit firmness declined in a parallel manner. The ethylene climacteric peak occurred after the greatest decline in fruit firmness, indicating that low levels of ethylene may be sufficient to initiate the ripening process. The ethylene climacteric peak also coincided with the highest activities of both ACS and ACO as well as the maximum tissue ACC content. As ACC content increased, MACC content declined, suggesting a regulation of ethylene production via free ACC levels by malonylation of ACC. Thus, the climacteric development of ethylene production may be regulated by an increase of ACS activity and a decrease in ACC malonyltransferase activity, making more free ACC available for the production of ethylene by increased activity of ACO.

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Pawpaw fruit were harvested at the advent of the ripening process and were ripened at room temperature. Based on fruit firmness and respiration and ethylene production rates at harvest and during ripening, fruit were classified into one of four categories: preripening (no to very slight loss of firmness; preclimacteric), early ripening (some softening; increasing rates of ethylene and CO2 production), mid-ripening (soft; at or just past climacteric), and late ripening (very soft; postclimacteric). The activities of the cell-wall degrading enzymes polygalacturonase (PG), endo-(1→4)ß-D-glucanase (EGase), and endo-ß-1,4-mannanase (MAN) were low in the preripening and early ripening stages, increased dramatically by mid-ripening coincident with the respiratory and ethylene climacterics, and decreased at late ripening. However, pectin methylesterase (PME) activity per milligram protein was highest at the green stage when the fruit firmness was high and decreased as ripening progressed. Tissue prints indicated both EGase and MAN increased as ripening proceeded. The EGase activity was evident near the seeds and the surface of the fruit at preripening and eventually spread throughout, while MAN activity was evident near the fruit surface at preripening and was progressively expressed throughout the flesh as fruit ripened. The greatest decline in fruit firmness occurred between pre- and early ripening, before the peak activities of PG, EGase, and MAN, although MAN exhibited the greatest relative increase of the three enzymes in this period. The data suggest that PME may act first to demethylate polygalacturonate and may be followed by the action of the other enzymes resulting in cell wall disassembly and fruit softening in pawpaw.

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Growth of pawpaw (Asimina triloba) seedlings in containers was examined in a factorial greenhouse experiment with four treatment levels of the slow-release fertilizer, Osmocote 14-14-14 (14N- 6.1P-11.6K), incorporated in Pro-Mix BX potting substrate at 0, 0.13, 0.26 or 0.81 kg·m-3 (0, 0.22, 0.44, or 1.37 lb/yard3) and three treatment levels of liquid-feed fertilizer of Peters 20-20-20 (20N-8.7P-16.6K) water-soluble fertilizer at 0, 250, or 500 mg·L-1 (ppm). When plants were harvested 18 weeks after sowing, seedlings subjected to the highest rate of Osmocote 14-14-14 at 0.81 kg·m-3 and liquid-feed at 500 mg·L-1 had the greatest total biomass, about 3-fold greater than nonfertilized plants. In a separate greenhouse experiment, growth of seedlings was examined with Osmocote 14-14-14 as the sole fertilizer source at six treatment levels of: 0, 0.81, 2.22, 4.43, 8.86, or 17.7 kg·m-3 (0, 1.37, 3.74, 7.47, 14.9, or 29.9 lb/yard3). Early seedling growth was hastened in the 2.22 kg·m-3 treatment rate, but delayed in 17.7 kg·m-3 treatment rate, when compared to nonfertilized control plants. When seedlings were harvested 17 weeks after sowing, plants had the greatest shoot, root, and total dry weight with Osmocote 14-14-14 at a rate of 2.22 kg·m-3. Root:shoot ratio decreased from about 1.5 without Osmocote 14-14-14, to about 0.65 at rates of 2.22 kg·m-3 or greater. Based on the results of this study, the slow-release fertilizer, Osmocote 14-14-14, can be used effectively as a sole fertilizer source when incorporated into potting substrate at a rate of 2.22 kg·m-3 or at a reduced rate of 0.81 kg·m-3 when supplemented with weekly applications of liquid-feed fertilizer at a rate of 500 mg·L-1 of Peters 20-20-20, to enhance production of container-grown pawpaw seedlings.

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A pawpaw regional variety trial (PRVT) was established at Cornell University, Ithaca, N.Y. in Apr. 1999 consisting of 28 commercially available pawpaw (Asimina triloba) varieties or advanced selections from the PawPaw Foundation (PPF; Frankfort, Ky.). Eight replicate trees of each selection, grafted onto seedling rootstocks, were planted in a randomized block design. The first two winters at the test planting site were unusually mild for the Finger Lakes region, with the lowest recorded temperatures above -16 °C (3.2 °F). Despite these mild winters, there was extensive winter mortality of some pawpaw varieties. Survival rates were >75% for 11 varieties, and were <40% for five other varieties. Poor establishment of grafted clonal pawpaws and insufficient pollination or fertilization of established pawpaws were important limitations of successful commercialization of this new fruit crop under conditions typical of upstate New York. Open mesh black plastic trunk guards provided adequate shade and protection for newly planted pawpaws, whereas translucent plastic tree-tubes caused heat stress and scorching of the young trees.

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Pawpaw (Asimina triloba) is a native North American tree that has potential as a new fruit crop or for use in landscapes, but until recently, little information has been available to nurseries on containerized production of this species. Pawpaw seedlings develop a strong taproot with a fragile root system, which can be easily damaged upon digging; therefore, most nurseries propagate trees in containers. Pawpaw seed requires stratification for optimal germination and seed is sensitive to desiccation. The seed also cannot tolerate freezing temperatures [<-15 °C (5.0 °F)]. A well-aerated potting substrate with a high sphagnum peat moss component (>75% by volume), cation exchange capacity, and water holding capacity can be used effectively in container production. Tall containers should be used to accommodate the developing taproot of seedlings. The slow-release fertilizer Osmocote 14-14-14 (14N-6.1P-11.6K) incorporated into Pro-Mix BX potting substrate can be used effectively as the sole fertilizer source at a treatment rate of 2.22 kg·m-3 (3.742 lb/yard3) in containerized pawpaw production. It can also be used at a lower rate of 0.81 kg·m-3 (1.365 lb/yard3) when supplemented with weekly applications of 500 mg·L-1 (ppm) of Peters 20-20-20 (20N-8.78P-16.6K) liquid-feed fertilizer. Bottom heating [32 °C (89.6 °F)] of container-grown pawpaw seedlings results in greater lateral and total root dry weight than in seedlings grown at ambient temperature [24 °C (75.2 °F)], which could increase the rate of establishment of seedlings in the field. Bottom heating of container-grown pawpaw seedlings could decrease both the time to produce a saleable plant and the cost of heating greenhouses. Growth of containerized pawpaw seedlings is enhanced by low to moderate shading with polypropylene shade fabric (28% or 51%) outdoors and low shading (33%) in the greenhouse, in a manner typical of that reported for other shade-preferring plants. Low to moderate shading of pawpaw seedlings grown outdoors greatly increases leaf number, total leaf area, and total plant dry weight compared to nonshaded seedlings, suggesting that commercial nurseries can improve production of containerized pawpaw seedlings using a shading regime outdoors.

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To facilitate the growth of a commercial pawpaw (Asimina triloba) industry, several problems with harvest and postharvest handling of fruit need to be resolved. Pawpaw fruit ripening is characterized by an increase in soluble solids content, fl esh softening, increased volatile production, and a loss of green color intensity. Within 3 days after harvest, ethylene and respiratory climacteric peaks are clearly evident. Softening of fruit is due to the action of at least four enzymes, with the softening proceeding from the surface to the interior tissue. Fruit on a single tree can ripen over a 2-week period, creating labor problems. When immature fruit is harvested it does not ripen, even if treated with ethephon at 1000 mg·L-1 (ppm), but the use of commercially available growth regulators both pre- and postharvest warrants further study. Fruit soften very rapidly at room temperature after harvest and have a 2-to 4-day shelf life. However, we have stored pawpaw fruit for 1 month at 4 °C (39.2 °F) with little change in fruit firmness and fruit apparently continue normal ripening upon removal to ambient temperature. The optimum temperature and duration for holding fruit will need to be determined. Further extension in pawpaw storage life may be feasible with controlled or modified atmosphere storage. Although there are a number of practical problems with pawpaw harvest and postharvest storage that need to be addressed, we hope to develop recommendations for harvest and handling of fruit in the near future.

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A pawpaw (Asimina triloba) regional variety trial (PRVT) was established at the U.S. Department of Agriculture, Agricultural Research Service, National Clonal Germplasm Repository (NCGR), Corvallis, Ore., in Fall 1995. This orchard was a replicated planting of 28 commercially available varieties or advanced selections from the PawPaw Foundation (PPF; Frankfort, Ky.), with eight replicate trees of each selection grafted onto seedling rootstocks and planted in a randomized block design. Two years after planting, 32 trees had either failed to establish or had died after an initial healthy start. By July 1999, 25% of grafted trees had died due to a vascular wilt-like disease, and 2 years later mortality exceeded 50%. Grafted selections with the lowest symptom severity include 1-7-2, 2-54, 7-90, 8-58, 9-58, `Mitchell', `PA-Golden #1', `Taylor' and `Wilson'. Seedling guard trees were unaffected until July 2000, when six guard trees of 76 died and 10 more were declining. By July 2001, 14 guard trees were dead. No fungi were consistently isolated from declining trees. A number of bacteria were isolated from infected trees, but no specific pathogen has been confirmed as the causal agent. Polymerase chain reaction (PCR) tests for phytoplasmas and for the bacterium Xylella fastidiosa were also negative. Research is ongoing to determine if a bacterial pathogen was the cause of the pawpaw decline in the Oregon PRVT.

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