Isozyme markers for glutamate oxaloacetate transaminase (GOT), superoxide dismutase (SOD), peroxidase (PER), and malate dehydrogenase (MDH) were identified for Carica papaya L. and the related but sexually incompatible C. cauliflora Jacq. These markers were used to determine the nature of somatic embryos derived from papaya ovules cultured on modified Murashige and Skoog (MS) medium 65 days after controlled pollination with C. cauliflora. Zymograms of plantlets from somatic embryos contained bands specific to either C. papaya or C. cauliflora (PER, GOT) and a unique band not present in the zymogram of either species (PER). Zymograms of somatic embryo-derived plantlets were distinctively different from those of either of the Carica species for all the enzyme systems examined. Evidence from isozyme markers indicates that somatic embryos produced from cultured papaya ovules following pollination with C. cauliflora may be hybrids. The isozyme banding patterns of 60 plantlets derived from somatic embryos from the same ovule were very uniform and suggest genetic uniformity among the regenerated plantlets.
A study of reproductive barriers limiting interspecific hybridization between Carica papaya L. and C. cauliflora Jacq. was undertaken in four reciprocal interspecific crosses using two different lines of each species. Particular attention was focused on determining whether polyembryonic clusters produced in these crosses were of maternal or zygotic origin. Prezygotic barriers were unimportant; pollen tube penetration and zygote formation were similar in intra- and interspecific crosses. Substantial postzygotic disruptions were observed, including disorganized growth and abortion of hybrid embryos and lack of normal endosperm development. In most crosses, disorganized embryos aborted before differentiating into polyembryonic structures. However, crosses employing UH345 (C. cauliflora) as female parent produced some embryos that developed to maturity (6 months), and, in these crosses, embryogenic proliferation from zygotic tissue became evident as early as the beginning of the 3rd month. There was no evidence of somatic embryogenesis from maternal tissues in any cross. Embryos rescued 3 to 6 months after pollination continued embryogenic growth in vitro on basal Murashige and Skoog (MS) medium and germinated on medium containing 0.2 mg BA/liter and 0.5 mg NAA/liter. Zymograms assayed for isocitrate dehydrogenase, malate dehydrogenase, and phosphoglucomutase activity confirmed the zygotic origin of tissues taken from in vitro cultures and recovered plantlets. Vigor, viability, and fertility (< 1% stainable pollen) of hybrids recovered from embryo culture were low. Chemical names used: 6-benzylaminopurine (BA); 1-napthaleneacetic acid (NAA).
Papaya (Carica papaya L.) fruit flesh and seed growth, fruit respiration, sugar accumulation, and the activities of sucrose phosphate synthase (SPS), sucrose synthase (SS), and acid invertase (AI) were determined from anthesis for ≈150 days after anthesis (DAA), the full ripe stage. Sugar began to accumulate in the fruit flesh between 100 and 140 DAA, after seed maturation had occurred. SPS activity remained low throughout fruit development. The activity of SS was high 14 DAA and decreased to less than one-fourth within 56 DAA, then remained constant during the remainder of fruit development. AI activity was low in young fruit and began to increase 90 DAA and reached a peak more than 10-fold higher, 125 DAA, as sugar accumulated in the flesh. Results suggest that SS and AI are two major enzymes that may determine papaya fruit sink strength in the early and late fruit development phases, respectively. AI activity paralleled sugar accumulation and may be involved in phloem sugar unloading.
Suspension cultures derived from Carica papaya L. ovular callus were subcultured on modified Murashige and Skoog medium containing 60 g·liter−1 sucrose, 400 mg·liter−1 glutamine, 9 μm 2,4-D, and either 0–0.45 m sodium chloride (NaCl) or the osmotically equivalent concentrations of mannitol. After 4 successive subcultures (120 days), the suspensions from each NaCl treatment were inoculated into the entire range of salt-containing media, and were subcultured on the same media formulations for 4 months. Cultures grown in the presence of mannitol were treated in the same manner. Sodium chloride generally inhibited somatic enbryogenesis; however, somatic embryogenesis was stimulated greatly following subculture from media with 0.18 m NaCl into media containing lower concentrations of salt. Enhancement of somatic embryogenesis also occurred following preconditioning with 0.30 m and 0.45 m mannitol. The increased rate of somatic embryogenesis was lost after 2 to 3 subcultures in media having lower osmolarities. Chemical names used: (2,4-dichlorophenoxy)acetic acid (2,4-D).
Body transmittance spectroscopy and analytical measurements of chlorophyll, carotenoids, and soluble solids concentrations were used to develop a nondestructive technique for estimating the maturity of papayas (Carica papaya L.). Optical measurements were taken between 500−900 nm with a scanning monochromator and a tilting-filter, abridged monochromator. Immature and mature-green fruit which were indistinguishable by visual examination could be separated by body transmittance spectroscopy into nonripening and ripening groups.
The cocultivation method for transforming plant tissues in vitro with A. tumefaciens strains has been simplified and extended for use with leaf disks, stems, and petioles of Carica papaya L. These tissues have been transformed successfully with either pTiB6S3:pMON200 or pTiB6S3 with very high efficiency. Transformants were identified either by growth in hormone-free medium or by resistance to 300 μg·ml−1 of kanamycin. Putative transformants were confirmed on the basis of nopaline production.
Papaya (Carica papaya L., cv. Sunrise) fruits were exposed to a continuous flow of an atmosphere containing <0.4% 02 (the balance being N2) for 0 to 5 days at 20C. Decay was a major problem, and some fruit had developed off-flavors after 3 days in low O2 plus 3 days in air at 20C. The intolerance of the fruit to low O2 correlates with an increase in the activity of pyruvate decarboxylase and lactate dehydrogenase but not with the activity of alcohol dehydrogenase. Insecticidal O2 (< 0.4%) atmospheres can be used as a quarantine insect control treatment in papaya for periods <3 days at 20C without the risk of significant fruit injury.
Interspecific hybridizations were attempted between papaya (Carica papaya L.) and six Carica taxa, including C. monoica Desf., C. parviflora (A. DC.) Solms, C. pubescens Lenne et Koch, C. quercifolia (St. Hil.) Hieron., stipulata Badillo, and C. × heilbornii Badillo nm. pentagona (Heilborn). Prezygotic barriers were minimal; pollen tubes of wild species freely penetrated into the seed cavity of papaya, and papaya pollen tubes were similarly unhindered in reciprocal pollinations on C. pubescens. Postzygotic barriers were formidable due to ovule abortion and endosperm failure. However, dissection of more than 150 C. papaya fruits 90 to 180 days after interspecific pollination yielded at least a few hybrid embryos of each species combination. All crosses in which C. papaya was the male parent failed, with the exception of C. pubescens × C. papaya, which succeeded only after young ovules were cultured 30 to 45 days after pollination. Multiple embryos were common in all successful crosses, and these were shown to be of zygotic origin by analyses of isocitrate dehydrogenase, malate dehydrogenase, and phosphoglucomutase isozymes in parental and hybrid tissues. Hybrids successfully recovered from in vitro cultures included C. papaya × C. pubescens and reciprocal, C. papaya × C. quercifolia, and C. papaya × C. stipulata.
The uptake of Ca by `Sunset' papaya (Carica papaya L.) fruit and its role in ripening was studied. The highest mesocarp Ca uptake rate occurred in fruit that were <40 days postanthesis when fruit transpiration was probably highest. Ca uptake rate by the mesocarp was low, from 60 to 80 days postanthesis when fruit fresh and dry weight increased. Mesocarp Ca uptake rate increased again from 100 to 140 days postanthesis when fruit fresh weight growth rate declined and dry weight growth rate increased. Mesocarp Ca concentration did not significantly differ from the peduncle to the blossom end. although Ca was significantly higher in the outer than inner mesocarp at the fruit equator. Mesocarp Ca concentration fluctuated significantly throughout the year ranging from 68 to 204 μg·g-1 fresh weight (FW). Soil Ca application did not always increase fruit mesocarp Ca concentration, while K and N fertilization decreased mesocarp Ca concentration. Attempts to increase mesocarp Ca concentration by spraying CaCl2 onto papaya fruit during growth and by postharvest vacuum infiltration and dipping of the cut peduncle into CaCl2 were unsuccessful. Mesocarp Ca concentration was positively correlated to the firmness of ripe papaya fruit and the rate of softening of mesocarp plugs. Less correlation was found between fruit firmness and the ratio of Ca concentration to K or Mg concentration, or to Mg plus K concentrations. Mesocarp Ca concentration of 130 μg·g-1 FW or above was associated with slower fruit softening rate than fruit with a lower concentration.
The temperature and ethylene response of ripening papaya fruit (Carica papaya L. cv. Sunset) was determined with and without 14 days of storage at 10C. Temperatures at or higher than 30C adversely affected the quality of the ripe papaya. Papayas held at 32.5C for 10 days failed to ripen normally, as evidenced by poor color development, abnormal softening, surface pitting, and an occasional off-flavor. Skin yellowing, fruit softening, and flesh color of papayas exhibited a quadratic response to ripening time within the temperature range of 22.5 to 27.5C. Flesh color development of nonstored fruit did not change significantly during the first 6 days at ripening temperatures, then rapidly increased. Fruit stored for 14 days at 10C exhibited faster ripening rates (e.g., degreening and softening and no delay in flesh color development) than nonstored fruit when removed to other ripening temperatures (17.5 to 32.5 C). Problems of weight loss and development of external abnormalities were more significant at temperatures higher than 27.5C. The optimal temperature range was found to be between 22.5 and 27.5C, with fruit taking 10 to 18 days to reach full skin yellowing from color break, whether or not fruit was stored at 10C. Exogenously applied ethylene (=100 μl·liter-1) stimulated the rate of fruit ripening, as measured by more uniform skin yellowing and rate of flesh softening whether or not the fruit were stored for 14 days at 10C. Ethylene did not ripen immature papayas completely in terms of skin and flesh color development. The outer portion of the flesh of ethylene-treated fruit had a faster rate of ripening, as indicated by carotenoid development and softening rate, while the same area of the flesh was still pale white in nonethylene-treated fruit. Ethylene reduced the coefficient of variation for skin color, softening rate, and flesh color development in treated fruit. Ethylene increased the rate of skin degreening and hastened the rate of carotenoid development and softening in the outer mesocarp, while having little effect on the inner mesocarp.