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U.L. Yadava

A planting of 48 trees of `Redhaven' scion on Lovell, Nemaguard, and Wildpeach rootstocks (RS) was established in 1990, with four replications in randomized complete-block design. Cultural practices common in Georgia were used to maintain the planting. Orchard performance for peach tree short life (PTSL) related tree survival, RS suckering, fungal gummosis, and tree stresses from cold injury and Pseudomonas canker, was investigated to examine RS potential of Wildpeach compared with Lovell and Nemaguard. Trees on all RS showed 100% survival for the first 5 years in the orchard. Although canker became more prevalent in later years, trees had significantly higher ratings on Nemaguard (2.88) and Lovell (2.50) RS than on Wildpeach (1.44). However, PTSL stress enraged by Pseudomonas killed one tree each on Lovell and Wildpeach RS during 1995. Trunk cambial browning that estimated cold injury was trivial due to mild winters; however, trees on Nemaguard had higher TCB ratings (1.25) than on other RS. Trees on Wildpeach had fewer suckers than on Nemaguard or Lovell. Gummosis ratings were higher on Nemaguard RS than on Lovell and Wildpeach. The results showed that Wildpeach has good potential for a peach RS.

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U. L. Yadava

Three exotic lines (Dwarf, L-45, and L-50) of precocious papaya (Carica papaya L.) from India, were grown in nursery rows at the Fort Valley State College Agricultural Research Farm during 1986-1990. Performance of these lines was evaluated for their adaptation and production feasibility under the growing conditions of Middle Georgia. Two lines (L-50 and Dwarf papaya) showed a less satisfactory overall performance than did L-45, which had the highest female to male ratio (7:3) and abundantly produced tree-ripened fruits under cold protection frames during 1989 and 1990. Tree growth and survival for L-45 were greater than those for L-50 and Dwarf papaya lines. Two-month-old greenhouse-grown seedlings when established in the field in April, flowered in 60 to 65 days following transplanting. Under Georgia conditions, fruits ripened on trees in approximately 150 days after fruit set. During 1989-90, the fruit size on L-45 trees varied from 574 g to 2,286 g (mean 1,530 g) with an average of 22.5 fruits per tree. Four years data suggest that papaya can be a successful annual crop if shelter is provided during late fall to protect ripening fruits and trees from frost/cold.

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U.L. Yadava

Two-year-old trees of `Red Flesh' (RDF) and `Lucknow-49' (L49) guavas from India and `Beaumont' (BMT) guava from Hawaii were established in the field during Spring 1995, inside an open wooden structure equipped with electric heaters and fans. Trees were cold-protected from November to the middle of April by covering the wooden structure with 6-mil clear polyethylene and using heaters and fans. Trees of RDF grew compact, while those of L49 and BMT were open, upright, and grew taller. Other than blossom-end rot on few fruits, no incidents of insect-pest and diseases were observed on trees or fruits. All cultivars bloomed from March to June 1996. Fruit set was heavier on BMT and L49 than on RDF trees. Fruit harvest extended from Sept. 1996 to Jan. 1997. Cultivar significantly influenced harvest and fruit weight. Peak harvest date was earlier for BMT, followed by RDF and then L49. Mean fresh weight (g/fruit) was 535.7, 284.2, and 150.7 for RDF, L49, and BMT, respectively. Fully developed RDF fruits were round, sometimes flat vertically, with blush on green skin when ripe, and had a small core in red flesh. Fruits of BMT were round to elliptical, yellow when ripe, and had numerous seeds in red flesh. Fruits of L49 varied from round to elliptical to pyriform with yellow to light green skin color and cream flesh with fewer seeds in a large core. The fruit flavor was strong and astringent for both BMT and L49, whereas RDF had a mild fruit flavor.

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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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S.K. Dhir and U.L. Yadava

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.

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U.L. Yadava and S.K. Dhir

The morphogenetic potential of parval or pointed gourd (Trichosanthes dioica Roxb.) shoot-tip explants was investigated to establish this species as a model tissue culture system. An effective multiple-shoot propagation method is described. Ten-millimeter shoot tips from young branches of greehouse-grown plants served as explants. They were initiated on a MS basal medium. Multiple shoots were encouraged by transferring established explants to a proliferation medium consisting of MSB + 1 mg BAP/liter, because lower concentrations of BAP (0.1 to 0.5 mgliter–1) inhibited multiple shoot formation; however, the same concentrations promoted rooting in explants. Medium supplemented with 1 mg BAP/liter and 100 mg PVP/liter caused the best proliferation of shoot tips. Upon transferring to fresh medium of the same composition, these shoot tips elongated 24 cm with three to five nodes in 4 weeks of culturing. Shoot multiplication cultures were maintained by transferring segments of multiple-shoot clusters to medium containing 1 mg BAP/liter and 0.5 mg GA3/liter. Medium supplemented with TDZ inhibited the number of regenerating explants but enhanced the number of shoot buds. Eighty percent of these plantlets were successfully rooted on MS medium supplemented with 1 mg NAA/liter. Plantlets survived in potting soil and exhibited normal growth under mist in the greenhouse.

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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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F.A. Hammerschlag, R.H. Zimmerman, U.L. Yadava, S. Hunsucker, and P. Gercheva

A range of antibiotics was evaluated for their effect on eliminating Agrobacterium tumefaciens supervirulent strain EHA101(pEHA101) from leaf explants of `Royal Gala' apple (Malus domestica Borkh) and on regeneration. After long-term (38 days) exposure to 100-μgml–1 concentrations of either cefotaxime (cef), carbenicillin (carb), mefoxin (mef), or combinations of these antibiotics, only on carb or carb with mef was regeneration not inhibited. None of the above antibiotics or antibiotic combinations eliminated A. tumefaciens from leaf explants. Short-term (1-18 hours), vacuum infiltration with 500- to 1000-μgml–1 concentrations of either of the above antibiotics did not inhibit regeneration, but did not eliminate A. tumefaciens from leaf explants. After a 30-min vacuum infiltration with a 2000-μgml–1 concentration of either cef, carb, or mef, only cef reduced the number of leaf explants with A. tumefaciens.