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Salvador Guzmán-González, Pedro Valadez-Ramírez, Rosa-Edith Robles-Berber, Laura Silva-Rosales and José-Luis Cabrera-Ponce

Biolistic genetic transformation of plants with viral genes is a method for controlling plant virus diseases; however, optimization of the particle bombardment parameters according to the transformation system is a key factor for an appropiate transgene expression and, therefore, a stronger resistance mechanism in transgenic plants. In order to optimize biolistic parameters, somatic papaya (Carica papaya L.) cv. Maradol embryo masses were bombarded with the CAMBIA 1301 plasmid construction that contains the coat protein gene (CP) of the papaya ringspot virus isolate of Colima, Mexico, driven by the double constitutively CaMV 35S promoter and flanked for the GUS and hygromycin (hpt) resistance genes. Particle bombardment protocol was carried out using the Helios™ Gene Gun device (BioRad) and the manufacturer's instruction manual. Helium pressure (50, 100, and 150 psi) and gold particle size (0.6, 1.0, and 1.6 μm) were evaluated. Five days after bombardment, somatic embryo clusters were used for GUS transient expression and, during 2 months, were selected into 50, 75, and 150 mg·L-1 hygromycin-containing media to its later CP-PCR detection. Results showed that 50 psi and 1.0 μm were the two optimal values for the assayed analyses. This is the first report of genetic transformation of papaya using the Helios™ Gene Gun device as a new tool compared to conventional PDS-1000/He.

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Tracie K. Matsumoto, Francis T.P. Zee, Jon Y. Suzuki, Savarni Tripathi, James Carr and Bruce Mackey

The Caricaceae family consists of six genera, including Vasconcellea , which contains 21 of the 35 Caricaceae species, and Carica papaya L., which is the most economically important species attributable largely to its cultivation in the tropics

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B. Castillo, M.A.L. Smith, D.L. Madhavi and U.L. Yadava

Interactions between irradiance levels (5–40 μmol·m-2·s-1) and iron chelate sources (FeEDTA and FeEDDHA) were observed for Carica papaya shoot tip cultures during both the establishment and proliferation stages of microculture. Reduced levels of irradiance (5 μmol·m-2·s-1) favored shoot tip establishment regardless of the source or level of iron. However, the highest percentage of successful explant establishment (100%), and significantly greater leaf length (1.16 cm; over double the size attained in any other treatment), resulted when a low concentration of FeEDTA alone was used at low irradiance. During the subsequent shoot proliferation stage, however, higher irradiance levels (30 and 40 μmol·m-2·s-1) were required, and FeEDTA failed to support culture growth when used as the sole iron source. The highest multiplication rates (3.6 shoots per explant) and leaf chlorophyll concentrations (0.22 mg/g fresh mass), and significantly improved shoot quality were achieved at 30 μmol·m-2·s-1 irradiance when both iron chelate formulations were combined (each at a 100 μM concentration) in the proliferation medium. Chemical names used: benzylamino purine (BA); ferric disodium ethylenediamine tetraacetate or FeNa2EDTA (FeEDTA); ferric monosodium ethylenediamine di(o-hydroxyphenylacetate), (FeNaEDDHA) or Sequestrene 138Fe (FeEDDHA); indoleacetic acid (IAA); 1-naphthaleneacetic acid (NAA).

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S. Jayasankar and U.L Yadava

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.

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Rajeswari Srinivasan* and Richard Manshardt

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.

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B. Castillo, D.L. Madhavi and M.A.L. Smith

Interaction between irradiance levels (5–40 mMm–2–s–1) and iron chelate sources (FeNa2EDTA and FeNaDTPA) on the establishment, growth, and proliferation of shoot tips of Carica papaya were tested. Reduced irradiance level (5 mMm–2–s–1) enhanced the establishment of shoot tips regardless of the source of iron chelate tested. At higher irradiance levels (30 and 40 mMm–2–s–1), presence of FeNaDTPA in the medium enhanced establishment of shoot tips. Continuous or alternating light/dark (16/8 h) photoperiods at high irradiance levels had no effect on the establishment or growth of the culture. At higher irradiance levels, the cultures produced smaller leaves as compared to lower irradiance levels. Low irradiance and FeNa2EDTA was preferred during the proliferation stage.

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Monica Ozores-Hampton and Herbert H Bryan

Municipal solid waste compost was applied with a side delivery applicator on top of the bed as a mulch in May 1993, 6 months after transplanting at Homestead, Fla. Papaya (`Know You No 1') was grown with and without compost mulch. Compost was distributed on the surface of the bed ≈90 cm wide and 5 cm thick. There were no mulch effects on trunk diameter nor plant height. Plant height was affected by papaya sex 4 and 6 months after transplanting. Hermaphroditic plants were taller than female plants. There were no mulch effects on marketable yield per plant, marketable size, or number of cull fruit. Sex, however, influenced papaya size and total cull number. Hermaphroditic plants produced larger marketable fruit and more cull fruits than female plants. Lower plant mortality rates were found after 1.5 years in the mulched plants compared to unmulched plants. Soil and tissue analysis showed no differences in N, P, K, Mg, S, Mn, Fe, Cu, and B, except for Zn. Zinc contents in soil and tissue were higher in the mulched areas than unmulched areas.

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Maureen M.M. Fitch, Paul H. Moore, Terryl C.W. Leong, Leslie Ann Y. Akashi, Aileen K.F. Yeh, Susan A. White, Amy S. Dela Cruz, Lance T. Santo, Stephen A. Ferreira and Leslie J. Poland

Gynodioecious papaya (Carica papaya L.) seedlings in commercial cropping systems in Hawaii are typically multiple-planted and thinned upon flowering to a single hermaphrodite because seedlings segregate for sex expression. Use of clonally propagated hermaphrodites would eliminate the over-planting practice and may provide other advantages. Yields of clonally propagated hermaphrodites were compared with single- and multiple-planted seedlings in three fields on two islands in Hawaii. Cloned hermaphrodites were either rooted cuttings or in vitro micropropagated plants. Clonally propagated plants bore ripe fruit 1 to 3 months earlier than thinned seedlings and had significantly higher early and cumulative yields. At each site, cumulative yields of thinned seedlings never reached the same level as those of clonally propagated plants. The yield benefit from clonally propagated plants was greatest at Keaau, the lowest sunlight and least productive test site.

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Jennifer Han, Jan E. Murray, Qingyi Yu, Paul H. Moore and Ray Ming

Papaya ( Carica papaya L.) is a major fruit crop in tropical and subtropical regions. In a comparison against 34 commonly consumed fruits, papaya ranks number one in many categories including vitamin A and C, potassium, folate, niacin, thiamine

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Maureen M.M. Fitch, Paul H. Moore, Terryl C.W. Leong, Leslie Ann Y. Akashi, Aileen K.F. Yeh, Susan A. White, Amy S. Dela Cruz, Lance T. Santo, Stephen A. Ferreira and Leslie J. Poland

Papaya seedlings segregate for sex expression as females or hermaphrodites. Typically only hermaphrodite fruit are marketed in Hawaii. The agronomic practice of growing multiple seedlings that are later thinned to a single hermaphrodite tree is wasteful of seed, labor, and resources, especially when seed is costly. We compared growth of plants propagated by the clonal methods of micropropagation or rooting vegetative cuttings versus plants initiated as seedlings and transplanted. The seedlings were either single-planted hermaphrodites as identified by the polymerase chain reaction (PCR) or multiple-planted, thinned seedlings. The experiments were carried out in three different locations on two islands in Hawaii. Clonally propagated plants were significantly shorter than seedlings and bore flowers earlier and lower on the trunk at all locations. Stem diameter differences were not significant even though plant size was different at planting time. Percentage of trees in bud varied significantly in the third month after transplanting when about 90% of the rooted cuttings and large micropropagated plants had formed flower buds while only one multiple-planted seedling developed a bud. Overall, the clonally propagated plants were more vigorous and earlier bearing than were the seedling plants. There is good potential for adoption of clonal propagation when production becomes efficient enough to compete in price with the current practice of over planting and thinning.