QTL mapping gives an insight into the number, position and effect of loci controlling quantitative traits. Although a few linkage maps already exist for papaya, not many economically important traits have been studied. An investigation was undertaken to map two qualitative traits: 1) fruit flesh color and 2) an isozyme locus, phosphoglucomutase (PGM); as well as two quantitative traits: 1) number of nodes to first flowering and 2) stamen carpellody. An F2 population consisting of 281 plants derived from the parents Kapoho X Saipan Red was used for this study. Field observations suggested that there may be a linkage between PGM locus and one of the major QTLs controlling number of nodes to first flowering. Also, phenotypic data suggested that there may be a linkage between flesh color and carpellody. Marker genotyping was performed on a subset of 84 plants chosen from the phenotypic extremes of the population for node number and carpellody. Using AFLP (Amplified fragment length polymorphism) method, 510 markers were generated with 161 primer pairs. Although papaya has a haploid chromosome number of 9, at LOD score 5.0 and a maximum recombination frequency of 0.25, 25 linkage groups with number of markers ranging from 2 to 109 were generated using the software Mapmaker\EXP. Linkage and QTL maps are being constructed to reveal the molecular markers linked with the traits of interest and the nature of QTLs controlling the quantitative traits.
Petiole discs from young leaves of female papaya (L-45) plants were cultured in MS or B5-based media containing 0, 2.25, 4.5, 11.25, and 22.5 μm 2,4-D. Compact embryogenic callus emerged from vascular tissue of petiole discs in about 3 weeks. In MS medium, 66% and 51% explants formed embryogenic callus with 11.25 and 22.5 μm 2,4-D, respectively. On the other hand, 79% explants formed embryogenic callus in B5-based medium with 4.50 μm 2,4-D. However, explants became necrotic in B5-based medium with 22.5 μm 2,4-D. Subculturing callus in auxin-free medium resulted in the development of roots or somatic embryos. Microscopic observations revealed that the roots were produced only by the callus that had retained its continuity with the vascular tissue. This investigation revealed that petioles from field grown papaya plants are potential explants for somatic embryogenesis and 2-week exposure to 2,4-D is adequate for inducing morphogenesis. Additionally, an interaction between 2,4-D and the components in the MS and B5-based media was observed.
Nursery experiments were conducted in Santo Domingo, Dominican Republic, to determine the effect of increasing population densities of purple nutsedge (Cyperus rotundus) on the growth of papaya (Carica papaya) transplants. Seeds of `Sunrise Solo', `Red Lady', and `Cartagena Ombligua' were separately sown in plastic 12 × 15-cm containers filled with a 1:1 mixture of sand and loamy soil. Viable purple nutsedge tubers were planted 5 cm apart from the papaya seeds. The purple nutsedge initial population densities were 0, 1, 2, 4, and 6 tubers per container. The crop and the weed were sown the same day and allowed to interfere during 6 weeks. Purple nutsedge density had a significant effect on the height, leaf area, and shoot dry weight of the three papaya cultivars. There was no significant difference in the response of the three papaya cultivars to purple nutsedge densities. In general, as purple nutsedge density increased, papaya growth decreased. Nutsedge interference caused papaya shoot dry weight losses of 15% at the density of one plant per container and 73% at six plants per container.
Abstract
Sugar composition of papaya (Carica papaya L.) during fruit development was determined by gas-liquid chromatography. Sucrose made up less than 18% of the total sugar content 110 days after anthesis and increased rapidly to make up 80% of the sugars about 135 days after anthesis. The dramatic changes in sugar content and composition at 110 days after anthesis corresponded with the commencement of color changes of seed and pulp.
Nutrient field studies were conducted on papaya, Carica papaya `Kapoho Solo' under rocky soil conditions in Puna, Hawaii. The objective of the study was to establish the relationship between boron (B) petiole tissue levels of trees with deformed and normal fruits. Field surveys indicated that trees with deformed fruits had 25% less B than normal trees. A 4.05 hectare field was treated with rates of 0, 1.12, 3.92 and 5.04 kilograms of B per hectare. By-weekly tissue results showed that B levels increased from 4 to 10 weeks after application with the 16 week levels substantially higher than the control trees. The results showed that maintaining the tissue B levels over 25 ppm B corrected the deformed fruit condition of papaya. Commercial fertilizer blends have been formulated with 0.3% B which is now used semi-annually on all commercial plantings.
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.
Abstract
Five inbred lines of papaya (Carica papaya L.) ranging from susceptible to resistant to phytophthora root rot (Phytophthora palmivora Butl.) were crossed in a diallel, and inbred parents and F, progenies were inoculated with P. palmivora isolate P170. The diallel analysis of mortality from phytophthora infection indicated significant general combining ability and specific combining ability; the GCA: SC A mean square ratio of 15.7 indicated GCA to be more important than SCA. Parental means correlated well with hybrid array means (r=0.89).
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.
Abstract
Papaya (Carica papaya L.), a climacteric fruit, became progressively less susceptible to chilling injury as it ripened. Symptoms of chilling injury included skin scald, hard areas in the pulp around the vascular bundles, and water soaking of tissue. Mature green fruit were most sensitive to chilling and began showing injury after 10 days of storage at 2°C. Chilling injury symptoms began to occur after 20 days at 7.5°. Fruit that showed 60% yellowing could be kept at 2° for 17 days without developing injury. Preconditioning papaya fruit for 4 days at 12.5° before storage for 12 or 14 days at 20° reduced chilling sensitivity. The decrease in chilling sensitivity with preconditioning was associated with partial fruit ripening. Waxing and wrapping papaya with polyethylene reduced chilling injury, but the fruit had an off-flavor. Controlled atmospheres of low oxygen (1.5% to 5%) with or without high C 02 (2% or 10%) delayed ripening, but did not reduce chilling injury symptom development. Calcium treatment led to increased chilling injury of papaya fruit. Delaying storage until the fruit ripened decreased chilling susceptibility and increased storage life at chilling temperatures. Shipping 60% yellow fruit at 2° could provide a procedure for achieving fruit fly disinfestation. Differences in cultivar response to chilling injury were noted.
A series of container studies was conducted from Sept. 1991 through July 1992 in which papaya plants were subjected to drying and re-wetting cycles to determine short term leaf gas-exchange and water relations responses to drought. The first response of papaya plants to a decline in substrate matric potential was a rapid reduction in stomatal conductance and net photosynthesis. Apparent quantum yield was reduced and light compensation point was increased shortly thereafter as stress continued to develop. However, leaf gas-exchange rapidly returned to the level of control plants after re-wetting. There was minimal or no effect of drought stress on relative leaf water content, pre-dawn xylem potential, dark respiration, the ratio of variable to maximum chlorophyll fluorescence, osmotic potential of leaf or root tissue, or root hydraulic conductivity of papaya plants. Chlorosis and shedding of the oldest leaves occurred following rewatering. These results indicate that papaya plants respond to short term drought by dehydration postponement via maintenance of water uptake and reduction of water loss.