Application of edible coatings that can simulate controlled atmosphere storage has become a popular concept. An experimental coating developed at the USDA Winter Haven laboratory, Nature-Seal (patent application #07/679,849), or a commercial composite coating was applied to papaya fruit at the green (immature) stage for comparison to uncoated fruit. Both types of coatings contain a polysaccharide base and therefore have different properties than most commercial “wax” coatings. The fruit were stored continuously at 21C or 3 days at 13C then ripened at 21C with 95 to 98% RH. Sample fruit from each treatment were analyzed for color, weight loss, CO2 ethylene, & % decay and softening. Results showed substantial extension of papaya shelf-life when the fruit were coated with Nature Seal while the commercial coating was less effective. This effect was due to retardation of ripening as evidenced by delayed color development, softening, and effect of coating permeability to CO2 and O2 on climacteric CO2 and ethylene production.
Abstract
Fruit of papaya (Carica papaya L.) stored at 15 mm Hg and 10°C for 21 days immediately after inoculation with Colletotrichum gloeosporioides developed less anthracnose during 5 days of ripening at room temperature than fruit stored for the same period at normal pressure. Fungistasis occurred in pure culture at low pressure and the infection process on inoculated papaya fruit was delayed. Infection did not appear to be altered otherwise and low pressure did not damage papaya fruit tissue.
Watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] is a major crop in the southern U.S., where the most important virus diseases are papaya ringspot virus (PRSV), watermelon mosaic virus-2, and zucchini yellow mosaic. The most economical control of virus diseases of watermelon is probably through genetic resistance. Watermelon has not been screened extensively for resistance to PRSV. The objective of this research was to develop a suitable method for screening watermelons for resistance to PRSV and then to screen the USDA germplasm collection. To date, we have developed an effective method and have nearly completed the screening. Several of the 1283 accessions have shown resistance to the virus. Methods tests involved 10 isolates of PRSV, several watermelon accessions and multiple inoculation procedures. Seedlings were screened in greenhouse flats with six replications per test. Tests were rated visually on a 0 to 9 scale (0 = no damage, 9 = plant dead), as well as with ELISA to detect the presence of virus. The watermelon germplasm collection was screened in four separate runs of 1283 accessions with `Charleston Gray' as the susceptible check. This research will be useful for those interested in effective screening methods, and sources of resistance for development of improved watermelon cultivars.
The disease resistance of a transgenic line expressing the coat protein (CP) gene of the mild strain of the papaya ringspot virus (PRSV) from Hawaii was further analyzed against PRSV isolates from Hawaii and other geographical regions. Line 63-1 originated from the same transformation experiment that resulted in line 55-1 from which the transgenic commercial cultivars, `Rainbow' and `SunUp', were derived. Plants of line 63-1 used in this study consisted of a population from a self pollinated R0 bisexual plant. ELISA and PCR tests provided evidence that there are at least two segregating CP loci. To allow for comparison with reactions of the previously reported line 55-1, virus isolates from Hawaii, Brazil, Thailand, and Jamaica were used to challenge seedlings of 63-1. Unlike line 55-1, a significant percentage of inoculated transgenic plants were susceptible to isolates from Hawaii. However, a proportion of plants were resistant to the non-Hawaiian isolates. In contrast, previous work showed that all plants of the hemizygous line 55-1 were susceptible to PRSV isolates from Brazil, Thailand, and Jamaica. Line 63-1, therefore, presents Hawaii with PRSV-resistant transgenic germplasm that could be used as a source of transgenes for resistance to PRSV isolates within and outside of Hawaii.
`Red Lady' papaya transplants were planted on a slope with a 30% to 35% grade and grown for 5 months. Excavation was used to determine root distribution on the uphill and downhill sides of the plants. Roots were separated into the taproot system and lateral roots on the uphill and downhill sides. The line intersect method was used to determine length of the lateral roots, and length of the taproot system was measured directly. All roots were dried at 70°C. The taproot system accounted for 2% of the total root length and 66% of the total root mass. Of the 130-m of lateral roots, 71% were located on the downhill side. Similarly, 69% of the dry mass of the lateral root system was located on the downhill side. Primary lateral roots on the uphill side of each plant developed horizontally, but some secondary lateral roots developed against gravity to maintain a portion of the root system close to the surface of the slope. Some of these lateral roots developed at angles of 55° to 60° above the horizontal.
Over ripe and abnormally soft fruits occur often during papaya shipments to the mainland U.S.A. Calcium fertilization to the soil did not always increased Ca concentration in the mesocarp. Calcium plus K treatment was more effective at increasing the Ca concentration in the mesocarp than Ca treatment alone. Calcium and K fertilization did not affect the fruit color development. There was a positive correlation between mesocarp Ca concentration and ripe fruit firmness, with no relationship between K or Mg concentration and ripe fruit firmness. Vacuum infiltration with CaCl2, MgCl2, KCl to mesocarp plugs in vitro showed that Ca significantly delayed softening and reduced C2H4 production, and that MgCl2 and KCl also slowed the softening. Use of the chelating agent sodium citrate increased the rate of softening, probably, by removing Ca from the cell wall. We conclude that Ca is an important factor in fruit firmness and that the increase of Mg and K by infiltration has different effects on fruit firmness from that by soil fertilization.
Abstract
Sexual changes in papaya (Carica papaya L.) from the normal hermaphroditic elongata flower toward femaleness by means of stamen carpellody are discussed in relation to the transition from the bisexual elongata flower toward maleness. A series of changes occurs that involves reduction in ovary size and shape, and in numbers of stigmatic rays, dorsal vascular bundles, carpels, and placenta. Placentation is normally parietal in papaya but there is evidence of ancestral axil placentation.
Demand for locally produced papaya fruit (Carica papaya) far outweighs the supply in the U.S. Virgin Islands. Due to the high incidence of papaya ringspot virus (PRSV), papayas are grown as an annual crop. The need exists in the Virgin Islands for papayas with early production to ensure a marketable crop before being devastated by PRSV. Breeding and selection has been ongoing for 5 years to develop papayas with tolerance to PRSV and fruit production starting at or less than 60 cm from the ground. The height at first fruit set, of 15 papaya cultivars recommended for the Virgin Islands, ranges from 58 cm to 253 cm. Generally, female plants started setting fruit lower on the stem than hermaphroditic plants. Through breeding and selection, three papaya lines have been developed that set the first fruit between 40 and 60 cm from the ground and exhibit tolerance to PRSV. These low-bearing papaya lines produce fruit that are marketable 1 month earlier than other cultivars.
Seedlings of Carica papaya L. `Waimanalo' (papaya) were transplanted into 27-L containers filled with nonsterile composted landscape yard trimmings passed through a 1.3-cm screen. At transplanting, papaya plants were inoculated with either one of three different AMF communities or were not inoculated as control plants. Two of the AMF communities were from Arizona citrus orchards, and one AMF community was from an undisturbed western Chihuahuan Desert soil. After transplanting, papaya plants were grown for 4 months under well-watered conditions in a temperature-controlled (32 °C day/24 °C night) glasshouse (45% light exclusion). Control plants remained non-mycorrhizal. Total colonization of papaya roots by AMF communities ranged from 56% to 94%. Depending on mycorrhizal treatment, AMF arbuscules and internal hyphae were present in 30% to 60% and 20% to 24% of roots, respectively. Noticeably absent in papaya roots were AMF vesicles. Papaya height, trunk diameter, and leaf phosphorus concentration were similar for inoculated and control plants. Compared with control plants, papayas inoculated with AMF communities had about 20% less shoot dry weight and about 50% less root dry weight. Under nonlimiting conditions in an organic substrate, AMF communities did not stimulate papaya growth but rather appeared to function as a carbon sink.
`Red Lady' and `Tainung #1' papaya plants were grown in nursery trays with cells 5.1 cm in diameter. After 10 weeks, mean height of the `Red Lady' plants was 10.1 cm and that of the `Tainung #1' plants was 9.3 cm. Each of five plants per cultivar was planted between two root observation windows, one at 45 cm and the other at 95 cm. Roots reached the 45-cm observation window in 30 days, when mean height of the `Red Lady' plants was 18.7 cm and that of the `Tainung #1' plants was 13.0 cm. Roots reached the 95-cm observation window in 55 days, when mean height of the `Red Lady' plants was 55.4 cm and that of the `Tainung #1' plants was 40.6 cm. Thus, root extension during these initial 55 days was 17 to 18 mm per day for both cultivars, and stem extension during this period was 8.7 mm·d–1 for `Red Lady' and 5.5 mm per day for `Tainung #1'. Root extension declined for both cultivars to ≈12 mm·d–1 by the initial bloom period, then further declined to ≈4 mm·d–1 during and after the initial fruit set stage. Stem extension increased to about 19 mm·d–1 after the plants were established and remained at this rate until well into the stage of heavy fruit set and growth, when it declined to about 8 mm·d–1. The amount of fruit set influenced root characteristics more than cultivar.