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Michael Marcotrigiano, Thomas H. Boyle, Pamela A. Morgan, and Karen L. Ambach

Nuclear-controlled leaf variegation was studied among Coleus × hybridus Voss (formerly C. blumei Benth.) cultivars propagated by seed and as shoot cultures on Murashige and Skoog (MS) medium + 1 to 3 mg BA/liter. Cultivars tested possessed pattern chlorophyll variegation and either pattern or nonpattern anthocyanin variegation. The gene controlling an albino midrib region appears to be fairly stable, with only 2% of the micropropagated plantlets having a solid-green leaf characteristic, a characteristic that was always inherited following selfing. Pattern anthocyanin variegation (PAV) was fairly stable, while nonpattern anthocyanin variegation (NAV) was very unstable. In addition, variants from pattern-variegated phenotypes produced offspring identical to their parent following selfing. In contrast, variants of nonpattern cultivars, when selfed, yielded offspring identical to the original cultivar, identical to the variant, or novel phenotypes. When variants were returned to culture, those derived from cultivars with PAV were more stable than those derived from nonpattern cultivars. In Coleus, micropropagation may induce epigenetic and/or heritable changes in leaf variegation. Cultivars with NAV are less stable than cultivars with PAV. Chemical names used; N-(phenylmethyl)-lH-purine-6-amine [benzyladenine (BA)].

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Kee-Yoeup Paek and Eun-Joo Hahn

Adventitious buds regenerated from homogenized leaf tissue of Saintpaulia ionantha Wendl. `Crimson Frost' were micropropagated to determine types and frequencies of the variants obtained. Plants grown for one year in a greenhouse showed 67% variation and 33% normality. A higher rate of variation was observed in leaf color rather than in leaf shape. The variations in leaf color and leaf shape were 67% and 19%, respectively. In regard to flower type, greater numbers of semidouble and double types were obtained as compared to single types. Both flower types showed a much higher rate of normal (mixed) color (81%) as compared to pink, red or white (19%). An sodium dodecylsulfate polyacrylamide gel electrophoresis profile of protein extracted from leaves of the stock plants and the variants indicated no difference between them and did not reflect the variation in phenotype.

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Dirk R. Vuylsteke, Rony L. Swennen, and Edmond A. De Langhe

Four types of morphologically distinct somaclonal variants were identified in a population of False Horn plantain (Musa spp., AAB group) plants produced by in vitro shoot-tip culture. Field performance of these variants was compared with true-to-type plantain to evaluate their horticultural traits. Significant variation was observed for plant and fruit maturity, leaf size, yield and its components, but not for leaf number, plant height, or suckering. Three of the four somaclonal variants were horticulturally inferior to the original clone from which they were derived. Yields of these variants were very poor due to inflorescence degeneration or abnormal foliage. Only the `French reversion' variant, which resembled an existing cultivar, outyielded the true-to-type clone. However, its fruit weight and size were lower. Somaclonal variation through micropropagation is of limited use in plantain improvement as it mostly mimics naturally occurring variation along with the observed poor horticultural performance of somaclonal variants.

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Stephen L. Love, Asunta Thompson-Johns, and Timothy P. Baker

Eight hundred and fifty-three clones of Russet Burbank and 1012 clones of Lemhi Russet were obtained from Native Plants, Inc. in 1988. The clones were produced via a tissue culture system designed to produce somoclonal variants. Four cycles of selection were completed from 1988-1991. Selection was based on resistance to blackspot bruise, a tuber flesh discoloration caused by condensation of free tyrosine; or the ability to produce light french fry color following cold storage. At the end of the four selection cycles all but six Russet Burbank clones and seven Lemhi Russet clones were eliminated. ANOVA across years was completed for the eleven somaclonal variants and Russet Burbank and Lemhi Russet checks.

Of the Russet Burbank clones, three were significantly (p = .05) more resistant to blackspot bruise and one had significantly better fry color after cold storage. All four clones had significantly reduced yield in comparison to the check clones. Of the Lemhi Russet clones, three were significantly more resistant to blackspot bruise, and four had significantly better fry color than the check clone. Only one of the seven clones (one with superior fry color designated L1908) did not show a significantly lower yield potential.

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Josefina Alcalá, Leonard M. Pike, and James J. Giovannoni

The relatively low evolution rate of the chloroplast DNA has made it an ideal tool to study evolutionary processes in plants above the species levels. However, recent studies have demonstrated that intraspecific variation in the chloroplast DNA is also common. This variation has provided useful insights into population level evolutionary processes. The polymerase chain reaction and sequencing of a noncoding chloroplast region used to classify onion lines for cytoplasmic type facilitated the identification of one sterile and two normal plastome variants in onion (Allium cepa L.). Sequence comparison revealed that differences between plastome variants included the presence of single-nucleotide polymorphisms associated with cytoplasmic type and variable numbers of tandem repeats, possibly resulting from slipped-strand mispairing. Our observations support the use of chloroplast-specific markers to assist in the selection of specific cytoplasmic types, suggest the potential to facilitate genotype determination, and demonstrate the presence of additional variation within cytoplasm type which gives insight into plastome evolution and may facilitate more accurate genotyping and selection.

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Christopher D. Gussman, Joseph C. Goffreda, and Thomas J. Gianfagna

Ethylene production and fruit softening during postharvest storage of several apple (Malus domestica Borkh.) ripening variants were compared with two standard cultivars. PA14-238 and D101-110 produced only low levels of ethylene (<10 μl·kg–1·hour–1) at harvest and throughout most of 86 days of storage at 4C, whereas `Red Chief Delicious' and `Golden Delicious' fruit produced >100 μl ethylene/kg per hour during the same time period. PA14-238 and D101-110 flesh disks converted aminocyclopropane-1-carboxylic acid (ACC) but not methionine (MET) to ethylene. `Red Chief Delicious' readily converted both MET and ACC to ethylene at the end of cold storage. PA14-238 fruit were the firmest and did not soften during postharvest storage; however, D101-110 softened appreciably. NJ55 did not produce ethylene at harvest, but produced a significant amount of ethylene (90 μl·kg–1·hour–1) during storage. Despite its high capacity to produce ethylene, NJ55 remained nearly as firm as PA14-238 at the end of cold storage.

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Stan C. Hokanson, Kelvin G. Grant, Elizabeth L. Ogden, and Lisa J. Rowland

Commercial strawberry plantings in the mid-Atlantic region are often quickly infected with one or more aphid-transmitted viruses, resulting in the loss of plant vigor, stunting, lowered yields, etc. To produce virus-free plant material for the strawberry industry and for cultivar development programs, heat therapy and/or meristem tip culture protocols are generally employed. One of the problems associated with meristem culturing is the potential for somaclonal mutations to occur in the meristem or surrounding proliferating tissue. As a result, distinct “bud lines” displaying functionally insignificant to distressingly high levels of phenotypic variation can arise from individual meristems. It would be desirable to differentiate these off-types by genetic fingerprinting to maintain trueness-to-type. Randomly amplified polymorphic DNA (RAPD) markers were evaluated for the potential to differentiate six pairs of strawberry bud lines that exhibit slight to fairly extreme levels of phenotypic variation. Reproducible RAPD marker profiles were generated using 10 primers in amplification reactions with genomic DNA obtained from multiple extractions. While five of the bud line pairs remained indistinguishable, three primers distinguished two variants of the Mohawk cultivar that are now in existence in the strawberry industry. Results suggest that typical somaclonal variation produced in the meristem culture process is of a magnitude that is not readily detectable with the RAPD protocol. The two Mohawk lines were probably produced by a higher magnitude mutation event than generally occurs or a cultivar mix-up.

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Anna L. Hale, J. Creighton Miller Jr., K. Renganayaki, Alan K. Fritz, J.J. Coombs, L.M. Frank, and D.S. Douches

The objective of this study was to differentiate six intraclonal variants of the potato (Solanum tuberosum L.) cultivar Russet Norkotah. One-hundred-twelve AFLP primer combinations producing 3755 bands and 79 microsatellite primers producing over 400 bands failed to identify any reproducible polymorphisms among the intraclonal variants and `Russet Norkotah'. The inability to detect differences between clones underscores the degree of genetic similarity between them, despite differences in phenotypic expression. This inability could be due to the tetraploid nature of the clones and/or to epigenetic differences not detected by the utilized procedures.

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Sumonthip Bunnag, Ramon Dolcet-Sanjuan, David W.S. Mok, and Machteld C. Mok

Somaclonal quince (Cydonia oblonga Mill.) variants IE-1 and IE-2 (Dolcet-Sanjuan et al., 1992) were more tolerant to Fe-deficient conditions in vitro than the original clone Quince A. The tolerance was evidenced by higher chlorophyll concentrations in leaves, higher Fe(III) reduction in roots, and increased acidification of the medium. In Summers 1993 and 1994, the two variants and Quince A were compared in the greenhouse, grown in normal potting soil (pH 5) and in soil obtained from an Fe-deficient orchard, with and without the addition of lime and KHCO3 solution to increase the pH (up to pH 8.3) and intensify Fe stress. In both years, the variants had significantly higher leaf chlorophyll concentrations than Quince A when grown in high-pH soils. The Fe+2 concentrations in leaves were also higher in the variants. Under field conditions at IRTA Reus, Spain, IE-1 and IE-2 had higher chlorophyll concentrations than the controls at some, but not all, analysis dates. These results suggest that IE-1 and IE-2, when grown as young plants in the greenhouse, have higher tolerance to Fe deficiency than Quince A, as observed in vitro, but that the tolerance is more variable under uncontrolled conditions in the field.

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Kalavathy Padmanabhan, Daniel J. Cantliffe, Roy C. Harrell, and Dennis B. McConnell

A comparison of external morphology captured via a computer vision system and a study of internal anatomy of sweetpotato somatic embryos identified five different major morphological variants among torpedo and cotyledonary stage embryos. These included 1) Perfect Type, 2) Near Perfect Type, 3) Limited/No Meristematic Activity Type, 4) Disrupted Internal Anatomy Type, 5) Proliferating Type. Perfect and Near Perfect types of somatic embryos were categorized as competent, while Limited/No Meristematic activity, Disrupted Internal Anatomy, and Proliferating types were categorized as noncompetent with respect to their conversion ability. Lack of organized shoot development in somatic embryos of sweetpotato was attributed to the following abnormalities: 1) lack of an organized apical meristem, 2) sparsity of dividing cells in the apical region, 3) flattened apical meristem, 4) multiple meristemoids and/or diffuse meristematic activity throughout the embryo. A morphological fate map of most of the torpedo and cotyledonary embryo variants was identified, which will be beneficial in synthetic seeding and transgenic research and development of sweetpotato.