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  • Author or Editor: B. B. Rhodes x
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Watermelon (Citrullus Lanatus (Thunb.) Matsum and Nakai) and muskmelon (Cucumis melo) were regenerated from immature cotyledons cultured on MS medium containing 10 μM BA. Small population of watermelon and muskmelon regenerants contained tetraploids as variants. The tetraploid individuals were recognized by morphological features including enlarged leaves, tendrils, male flowers, and variable pollen grains. After self-pollination, seed lots reflected differences in size expected from tetraploid parents.. Cytological data from root tips of R1 populations will be presented.

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Abstract

Significant differences in oxalate levels were obtained with hybrid progeny of Dieffenbachia picta (Schott) ‘Exotica’ which indicate a potential for selection for low oxalate levels.

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Abstract

In leaves of carrot (Daucus carota L.) treated with 120 ppm 2-(4-chlorophenylthio)-triethylamine hydrochloride (CPTA), increased lycopene levels were found. Significant amounts of carotenes tentatively identified as gamma- and delta- were also found. Alpha- and beta-carotene levels were reduced. The effect of CPTA was modified by temperature and genotype. The data suggest that lycopene is a precursor of the carotenes mentioned and that alpha-ionone cyclase has a higher optimum temperature than beta-ionone cyclase.

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Vitrification, a physiological disorder characteristic of in vitro grown plants, was observed in single-node cultures of sweet potato in mannitol-enriched medium during their second year of storage. Vitrified or vitreous sweet potato plantlets were watersoaked, translucent or glassy in appearance, with thick, swollen, leaves and stems, stunted shoot growth and poor root growth. These plantlets were not able to regenerate normal plants when transferred into fresh regeneration medium nor were they able to survive outside culture conditions.

Electron microscopy revealed changes in the ultrastructures of vitrified sweet potato plantlets. Vitrified plants had defective stomatal complex, starch grain-filled chloroplasts, disrupted cell wall, big air spaces (lacunae), high frequency of cell membrane separation from the cell wall, nuclear disintegration, and cytoplasmic disorganization. These changes in the internal structures of vitrified plants were reflected in their abnormal morphology and physiology.

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Vitrification, a physiological disorder characteristic of in vitro grown plants, was observed in single-node cultures of sweet potato in mannitol-enriched medium during their second year of storage. Vitrified or vitreous sweet potato plantlets were watersoaked, translucent or glassy in appearance, with thick, swollen, leaves and stems, stunted shoot growth and poor root growth. These plantlets were not able to regenerate normal plants when transferred into fresh regeneration medium nor were they able to survive outside culture conditions.

Electron microscopy revealed changes in the ultrastructures of vitrified sweet potato plantlets. Vitrified plants had defective stomatal complex, starch grain-filled chloroplasts, disrupted cell wall, big air spaces (lacunae), high frequency of cell membrane separation from the cell wall, nuclear disintegration, and cytoplasmic disorganization. These changes in the internal structures of vitrified plants were reflected in their abnormal morphology and physiology.

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Tetraploids are needed to synthesize triploid watermelons, which produce “seedless” fruit with improved quality. Traditionally, the tetraploids are induced by applying colchicine to the growing apex of seedlings or soaking the seeds with colchicine solution. This method often produces low frequency of tetraploids and high frequency of chimeras. Breeding tetraploids takes much longer time than breeding diploids because of the low female fertility. We developed a tissue culture approach that allows breeders to develop desirable tetraploids with commercially acceptable volume of seed in 2 years. This tissue culture approach includes: 1) regenerate plants via shoot organogenesis from cotyledon tissue; 2) screen tetraploids based on leaf morphology (more serrated leaf margin and wider leaf shape) before transplanting, and confirm tetraploids based on pollen morphology (larger pollen with four copi) and/or seed characteristics; 3) self-pollinate tetraploids or cross the tetraploids with diploids to accurately estimate the female fertility; 4) micropropagate the best tetraploid(s) using axillary buds during the off-season; and 5) produce tetraploid seed from the cloned tetraploids in an isolation plot and evaluate the triploids derived from the tetraploid(s) in the following season. This approach has been practiced on more than 20 genotypes over the past 4 years.

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We examined the RAPD (random amplified polymorphic DNA) technique in our efforts toward mapping the watermelon genome. Three cultigens: Dixielee, G17AB and New Hampshire Midget (NHM), and the primitive watermelon PI 296341 and the hybrid NHM × PI 296341 were tested for polymorphic RAPD markers with 53 10-mer primers. Among total of 159 readable bands, 89 (62.3%) of the bands were polymorphic among the four accessions and 82 (51.6%) of the bands were polymorphic between NHM and PI 296341, but only 16 (10.1%) were polymorphic within the three cultigens. Results of cluster analysis based on the RAPD data were correlated with agronomic characters. The watermelon genome size was estimated by flow cytometer analysis to be relatively small, ca. 0.98 pg. The large number of polymorphic loci and the relatively small genome will enable us to develop a high density linkage map. Cosegregation analysis is under way to establish linkage relationships between the RAPD markers and estimate recombination distances between agronomic traits and molecular markers.

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Cytological studies on a male-sterile, female-fertile mutant ms in watermelon were done to determine differences in gene expression and stage of microsporangium development between male-sterile and male-fertile genotypes. Anthers of male-sterile (msms) and normal (Ms) plants of a line G17AB were compared using a single-staining procedure for sporopollenin, a double-staining procedure for nucleic acid and polysaccharides and a triple-staining procedure for DNA, protein and polysaccharides with bright field and fluorescent microscopy. No distinguishable features were observed between sterile and fertile plants at the microsporogenous stage. By microsporocyte stage, tapetum surrounding the sporocytes was differentiated and enlarged in normal plant anthers. There was no tapetum differentiation in ms mutant, and sporangium wall consisted of 5-7 layers of small cells. Ontogeny of microsporocytes of msms stopped at telophase II. The tetrad stage was not observed in the male-sterile plant. As maturation of msms anthers progressed, sporangium locules collapsed, and the telophase II meiocytes degenerated. Degenerated meiocytes in the collapsed locules gave low density primuline-induced fluorescence. All of these features suggest that the ms gene is expressed at a very early stage of microsporangium development (before meiosis) and results in failure of the tapetum to differentiate. Absence of tapetum results in abortion of meiocytes. Developmental bases for this male-sterility may be attributed to mutation in sporophytic anther tissue.

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Hybrid seed production can be facilitated by using male sterility coupled with a seedling marker. This research was initiated to combine the ms male sterility and dg delayed-green seedling marker into watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai] lines. Male-sterile plants of the male-sterile line G17AB were crossed with plants of delayed-green breeding line Pale90, which has yellow cotyledons and pale-green, newly developed, true leaves. The double-recessive recombinants, male sterile and delayed green, from the F2 population were backcrossed to the male-fertile plants of G17AB. The pedigree method was used for selection in the progenies. The segregation ratios obtained from F2 and BC1F2 populations suggest that the male-sterile and delayed-green traits are inherited independently and that delayed green is inherited as a single recessive nuclear gene. Two male-sterile watermelon lines with delayed-green seedling marker have been developed. These lines will provide a convenient way to introduce male sterility and the delayed-green seedling marker into various genetic backgrounds. These two lines can be used for testing the efficiency of a new, hybrid, watermelon, seed production system.

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This research was conducted to develop genic male-sterile lines of watermelon (Citrullus lanatus Matsum & Nakai) homozygous for the juvenile albino (ja) seedling marker. Male-sterile plants (msms) of the genic male-sterile line G17AB were crossed with a Dixielee plant that was heterozygous for the ja locus. Male-fertile, juvenile albino recombinants of the F2 progeny were self-pollinated, resulting in F3 progeny. The male-sterile normal green recombinants of the F2 progeny were crossed with an F1 hybrid plant with genotype MsmsJaja, and three populations (93JMSB-1, -2, and -3) were obtained from these crosses. Juvenile albino recombinants were confined to 93JMSB-1. Of the juvenile albino plants of 93JMSB-1, male-sterile plants were sib-crossed with male-fertile plants, resulting in 93JMSB-1-1. Progeny of 93JMSB-1-1 was homozygous for ja and segregated for ms in a 127 male-sterile: 128 male-fertile ratio, fitting a 1:1 ratio. The male-sterile juvenile albino plants of F3 were crossed with male-fertile juvenile albino plants of 93JMSB-1, resulting in 93JMSF3-1 and -2. Plants 93JMSF3-1 and -2 were homozygous for ja but segregated for ms at 10 male-sterile: 13 male-fertile and 15 male-sterile: 19 male-fertile for 93JMSF3-1 and 93JMSF3-2, respectively, fitting the 1:1 ratio. These three genic male-sterile lines with the ja seedling marker provide valuable germplasm for introducing ms and ja genes into diverse genetic backgrounds and for studying cross-pollination and gene flow in watermelon populations.

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