Transgenic papaya lines carrying the coat protein gene (CP) of papaya ringspot virus (PRV) strain HA 5-1 display PRV reactions ranging from complete susceptibility (39-3 & 39-4), to slight delay in onset of symptoms (39-1) and attenuation of symptoms (60-3), to high-level resistance (55-1, 63-1). Normal Mendelian segregation of transgene expression was lost in R1 of 39-3 and 39-4, and inbred R1 60-3 gave an aberrant 1:1 ratio. R0 55-1 plants were resistant in the field (Hawaii) for 2 years following manual and/or aphid inoculation, and the high-level resistance remained stable in the R1 after repeated manual inoculations in the greenhouse and graft inoculation for up to 1 year (Cornell). However, inoculation with PRV HA-Oahu strain produced symptoms in some plants at Cornell (9% after 6 weeks) and in Hawaii (50% after 1 year). Two 55-1 and one 60-3 plant subsequently underwent remission of symptoms and became ELISA-negative (Hawaii). Transmission of PRV isolates from symptomatic 55-1 plants to other CP+ 55-1 bioassay plants was unsuccessful.
Many small, pen-sized papaya side shoots were formed after injecting a BA/GA, solution into the bases of stems of one-year old plants of the 30-year old clone `Honey Gold', followed by topping the stems a day later. Standing small leafy cuttings in various fungicidal solutions for 10 or 30 minutes resulted in phytotoxic basal burn, whereas a short immersion gave good results. Benlate® and Folicur® were best, especially with the addition of paclobutrazol. There is a distinct seasonal variation in the rooting of cuttings, with those that have been exposed to cold winter conditions giving poorest results. For best results (95% rooting in 4 weeks in perlite with intermittent mist), stock plants are maintained in a protected environment, especially in cool weather. Uniform side shoots, are harvested regularly, leaving stubs to produce new shoots, while maintaining enough leaf canopy.
Papaya shoot tips, obtained either from seedlings or from in vitro plants, survived liquid nitrogen (-196°C) exposure using a vitrification procedure. Vitrification is a technically simple method but requires large concentrations of cryoprotectants. These were added in two steps, first slow addition of dimethylsulfoxide (DMSO) and PEG-8000, and subsequent fast addition of ethylene glycol (PG). The final concentration before cooling was 40% EG, 7.8% DMSO, and 10% PEG-8000. Both rapid cooling and rapid warming rates were required. Differential scanning calorimetry (DSC) was used to determine that the external solution vitrified upon cooling. It could not be demonstrated by DSC that cells within the shoot-tip vitrified, but since both DMSO and EG rapidly permeate plant cells, vitrification within the cells seems a likely explanation for retention of viability.
Trials were conducted under controlled conditions to determine the tolerance of young papaya plants (15 cm tall) to postemergence herbicides. Herbicides used were paraquat (1.68 Kg ai/Ha), MSMA (2.24 Kg ai/Ha), 2,4-D (4.26 Kg ai/Ha), bromoxynil (0.28 Kg ai/Ha), cyanazine (1.12 Kg ai/Ha), dimethenamid (1.12 Kg ai/Ha), endothal (0.56 Kg ai/Ha), imazameth (0.067 Kg ai/Ha), imazethapyr (0.028 Kg ai/Ha) lactofen (0.12 Kg ai/Ha), oxyfluorfen (0.03 Kg ai/Ha), acifluorfen (0.28 Kg ai/Ha), atrazine (2.24 Kg ai/Ha), and bentazon (1.12 Kg ai/Ha) as well as the untreated control. Atrazine, bentazon, cyanazine, imazemeth, imazethapyr, and dimethenamid did not cause phytotoxicity at the rates used and were equal to the untreated control. Other herbicides caused severe injuries followed by total death at 10 days after treatment.
Excavation of field-grown `Red Lady' and `Tainung #2' papaya plants was begun 3 months after transplanting to the field to characterize development of the papaya root system. The roots were separated into the taproot system and lateral roots within three size categories: <1, 1 to 5, and >5 mm. Length of the taproot system and the larger lateral roots was measured directly, and that of the smaller roots was determined using the line-intersect method. Mass of the various size categories was measured after drying at 70°C. A typical plant 3 months after field-planting was ≈ 60 cm tall and exhibited a root system radial spread of 34,636 cm2, total root length of 9613 cm, and total dry mass of 17.3 g. The taproot system accounted for >70% of the mass and <5% of the length of the root system. Lateral roots <1 mm in diameter accounted for <10% of the mass and >70% of the length of the root system. A typical plant during the heavy fruit set stage, about 6 months after field planting, was 175 cm tall and exhibited a root system radial spread of 101,736 cm2, total root length of 975 m, and total dry mass of 539 g. The taproot system accounted for ≈38% of the dry mass and <1% of the length of the root system. Lateral roots <1 mm in diameter accounted for ≈5% of the dry mass and 65% of the length of the root system. Plant age influenced root system characteristics more than cultivar, especially the proportional distribution of mass and length among the defined root classes.
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.
Field experiments were conducted in the Dominican Republic to determine the effects of different rates of the biostimulants folcysteine and kinetin on fruit yield of `Sunrise' papaya. Aqueous solutions of either 50, 70, 90, 110, or 130 ppm. Four applications were made at 3-week intervals. Fruit number, size, and weight were recorded weekly during 15 weeks after application. Yields for the control and kinetin-treated plants were not significantly different. Significant yield increase was found in plants treated with 70 and 90 ppm of folcysteine solution. Fruit yield in plants treated with 30, 50, 110, or 130 ppm of folcysteine did not differ significantly from that of the control. These results indicate that folcysteine treatment at 70 and 90 ppm at flowering can significantly increase fruit yield in `Sunrise' papaya.
Field and container experiments were conducted in the Dominican Republic to determine the effect of gibberellic acid 3 (GA3) rates on papaya ringspot virus (PRSV)-infected seedlings and adult plants of `Cartagena Ombligua' papaya. The apical region of PRSV-infected and PRSV-uninfected plants was sprayed with GA3 aqueous solutions at rates 0, 25, 50, 75, and 100 ppm. PRSV-uninfected adult plants and seedlings produced longer internodes as GA3 rates increased. Adult PRSV-uninfected plants flowered normally at any GA3 rate. PRSV-infected seedlings and adult plants also responded to GA3 sprays, but to a lower extent. Typical symptoms of the disease were present in all the infected plants regardless of the GA3 rate applied, and adult plants did not flower at any rate. Results indicate that PRSV-infected `Cartagena Ombligua' papaya plants are responsive to exogenous GA3, although in a lesser degree than PRSV-uninfected plants. Linear regression equations described the effect of GA3 on the stem elongation of PRSV-infected and uninfected `Cartagena Ombligua' seedlings and adult plants.
An efficient protocol has been developed for the in vitro multiplication of papaya (Carica papaya L.) through somatic embryogenesis utilizing immature zgotic embryos. Somatic embryos were initiated on MS basel media supplemented with 5 mg·liter–1 2,4-D, 400 mg·liter–1 glutamine, and 6% sucrose. After culturing for 2 months, 65% of the explants became highly embryogenic. Each explant produced 50 to 80 embryos in 4 months on culture induction medium. Frequency of embryogenesis was increased (75 to 150 somatic embryos on 80% explants) upon supplementing medium with 4% maltose as a carbon source and 100 mg·liter–1 L-asparagine. The embryogenic callus appeared yellow and embryos at different stages of development were well-organized. On regular subculturing, these cultures continued to produce secondary embryos. Following their transfer to the hormone-free medium supplemented with 4% maltose, these embryos germinated. The somatic embryogenesis system is rapid, repetitive, and highly proliferative. Thus, this system may have a potential use in the development of synthetic seed and transgenic papaya plants. Details of important factors affecting somatic embryogenesis will be discussed.
`Honey Jean #3' sweet corn was planted in one-half of a split-root culture system containing `Tainung 1' or `Known You 1' papaya seedlings to determine if papaya roots could transfer water to the corn seedlings. After the corn seedlings were established, water was withheld from both compartments (2/2) or only the compartment containing the corn seedlings (1/2). Control plants were grown with both halves well-watered. Pre-dawn relative water content (RWC) of corn leaves was measured as an indicator of drought stress. Following 11 days, root competition was relieved in half of the 1/2 plants by cutting the papaya root connection between the half with corn from the rest of the papaya culture system. RWC of 1/2 corn plants was maintained above that of 2/2 plants, but below that of control plants. After relieving root competition, the 1/2 plants in competition with papaya roots maintained higher RWC than the 1/2 plants relieved of competition. Leaf tissue of all corn plants except the control plants was necrotic by 30 days. The results indicate that development of drought stress in corn using this culture system was retarded by watering a portion of the papaya roots not associated with the corn roots. Drought stress was accelerated by relief of competition with papaya, which is evidence that water was being supplied by the papaya roots within the papaya/corn system.