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W.H. Gabelman, I.L. Goldman, and D. N. Breitbach

Aster yellows, an insect-vectored disease caused by a mycoplasmalike organism, limits vegetable crop production in the Midwestern U.S.A breeding effort was initiated in 1982 to develop aster yellows resistance in carrot. A synthetic population (AYSYN) composed of 5 open-pollinated varieties and 4 inbreds was assembled in 1982. Inbred lines and hybrids were extracted from AYSYN using a variety of methods. Selection in artificially-infected field sites was carried out from 1982 until 1989. Twenty-three inbreds and 3 hybrids were developed from AYSYN during the selection process. Replicated field experiments were conducted in 1990, 1991, and 1993 to determine the relative aster yellows resistance of these lines and hybrids in comparison with 6 check cultivars. Averaged over years, significant differences were detected for percent aster yellows infection among genotypes. Lines selected for resistance had a mean of 12% infected plant per plot as compared to 24% infection for standard cultivars. Results demonstrate the effectiveness of inbreeding and field selection for aster yellows resistance. This breeding effort represents the first report of aster yellows-resistant carrot germplasm.

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William J. Martin and Dennis P. Stimart

Narrow-sense heritabilities and genetic correlations of ornamental quality traits of Antirrhinum majus (snapdragon) were evaluated with special reference to cut flower postharvest longevity (PHL). Inbreds P1 (16 days PHL) and P2 (3 days PHL) were hybridized to produce an F1 (P1 × P2) that was self-pollinated to produce an F2 population. The F2 were self-pollinated to produce F3 families and advanced through single-seed descent by self-pollination to the F5 generation. P1, P2, F1, F3, F4, and F5 were evaluated for ornamental quality traits. Quality traits were found to be quantitative and normally distributed. Narrow-sense heritability (h2) estimates were high and consistent across generations examined; PHL h2 ranged from 0.79 to 0.81 ± 0.06. Phenotypic and genotypic correlations revealed underlying physiological and pleiotropic interactions relevant to breeding programs aimed at simultaneous improvement of ornamental quality traits. PHL is inversely related to cut flower strength and days to flower, -0.44 ± 0.04 and -0.43 ± 0.44. Buds at discard is positively correlated to cut flower and plant diameter, cut flower weight and days to flower, 0.77 ± 0.05, 0.58 ± 0.06, 0.71 ± 0.06, and 0.77 ± 0.07, respectively. Gain from selection for quality traits of interest can be rapid.

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M.A. Dalbó, G.N. Ye, N.F. Weeden, W.F. Wilcox, and B.I. Reisch

The efficiency of marker-assisted selection for powdery mildew (Uncinula necator (Schw.) Burr) resistance in grapes (Vitis L. sp.) was studied using molecular markers associated with a major QTL (quantitative trait loci) for this trait. Initially, genetic maps were constructed from a segregating population of the cross `Horizon' × Illinois 547-1 (a hybrid between V. rupestris Scheele and V. cinerea Engelm.). A major QTL from Ill. 547-1, the resistant parent, explained 41% of the variation. One RAPD (randomly amplified polymorphic DNA) marker and one AFLP (amplified fragment length polymorphism) marker, obtained by bulked segregant analysis, showed the highest association with powdery mildew resistance in the mapping population. Segregation of the QTL was followed in different crosses by CAPS (cleaved amplified polymorphic sequence) markers developed from these two markers. An allele-specific amplified polymorphism that segregates as present/absent was also developed from the CS25b locus. Powdery mildew resistance was evaluated visually on a 1 to 5 scale in four different seedling populations. Two populations originated from crosses using Ill. 547-1 as the resistant parent. Two other populations were from crosses with NY88.0514.03, a resistant seedling from the original `Horizon' × Ill. 547-1 mapping population. Segregation ratio distortions were observed in some crosses. In these cases, the allele associated with the QTL for powdery mildew resistance was less frequent than the alternate allele. In all crosses, the markers were closely associated with resistance. If selection were based on markers, the percentage of susceptible individuals (classes 4 and 5) would decrease from 24% to 52% to 2% to 18%. Selection efficiency was greatest in crosses where segregation distortion was most intense.

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Paul H. Jennings and Ann Fitzpatrick

Four cucumber seed lines obtained from the Inst. of Vegetables and Flowers, Chinese Academy of Agricultural Sciences, Beijing, China, were tested for chilling tolerance. Comparisons were made with `Poinsett 76', a commercially available cultivar from the United States. Seeds germinated at 25°C were exposed to 2°C for time periods up to 108 hr. Root injury was assessed by measuring subsequent root growth at 25°C at 72 hr after the chill. Electrolyte leakage measurements were taken on roots excised immediately after the chill. Total seedling root length and electrolyte leakage studies showed significant tolerance to chilling in the selections from China as compared to `Poinsett 76'. `Poinsett 76' seedling roots began to show stress after 72 hr of chill and were irreversibly damaged, with abortion of root tips, after 96 hr at 2°C. The China seed selections were more tolerant to a 96-hr chill and even at exposure times up to 108 hr only began to approximate chilling effects exhibited by `Poinsett 76' at 72 hr of treatment.

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Kenji Sakurai, Susan K. Brown, and Norman Weeden

The S-alleles of 55 apple (Malus ×domestica Borkh.) cultivars and selections were determined using an allele-specific polymerase chain reaction (PCR) amplification system for 11 different S-alleles (S2, S3, S4, S5, S7, S9, S24, S26, S27, Sd, Sf). Four cultivars had S-alleles different than those predicted by their parentage. Three commercial cultivars of unknown pedigrees had S-genotypes that suggested `Delicious' and `Golden Delicious' were the parents. S-genotyping results supported controlled pollination test results. The genotypes of the five triploid cultivars examined were consistent with the unreduced gamete being contributed by the female parent. Although a large number of S-genotypes is available in apple, artificial selection or repeated use of the same cultivars as parents appears to have significantly restricted the number of compatibility groups associated with commercial clones. In controlled reciprocal crosses between two cultivars of known S-genotypes, the segregation of S-genotypes and S-alleles was 1:1:1:1, the ratio expected for random pairing of alleles.

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J.A. Plummer, J. Wann, J.A. Considine, and Z. Spadek

Boronia megastigma is cultivated or picked from natural stands in Western Australia for the production of essential oil. Boronia absolute is extracted from the highly perfumed flowers. It is currently valued at between US$4000 and US$7000 per kilogram, and world consumption for perfumery is about 1 tonne. The variation in essential oil composition within and between populations has indicated considerable variation in oil components. Some individuals have high β-ionone and low levels of pinenes. Principle components analysis indicated that the content of β-ionone and dodecyl acetate were tightly linked, as were the monoterpenes, α-pinene, β-pinene, and, to a lesser extent, limonene. Separate linkages between the desirable oil components (β-ionone and dodecyl acetate) and the undesirable components (α-pinene, β-pinene, and limonene) will facilitate selection of plants to be used in oil production.

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R.B. Hardin, D J. Eakes, C.H. Gilliam, and G.J. Keever

In a full-sun Auburn, Ala., field study, 23 cultivars and 1 forma of Cornus florida L. were evaluated for growth from 1994 to 1996 and bract characteristics in Spring 1996. The selections were divided into three groups for analyses: 1) white bracted with green foliage, 2) red or pink bracted with green foliage, and 3) variegated foliage. Among the white bracted cultivars with green foliage, `Weaver' and `Welch Bay Beauty' had the greatest height and stem diameter increases, `Autumn Gold' the least. `Cloud 9' had the largest bract size. `Welch's Junior Miss' had the greatest height increase, while `Stokes' Pink' had the greatest stem diameter increase for the red or pink bracted cultivars with green foliage, and f. rubra the least. `Red Beauty' had the largest bract size. There were no differences among the variegated cultivars in height increase or bract size; however, `First Lady' had the greatest stem diameter increase.

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M.H. Dickson, A.M. Shelton, S.D. Eigenbrode, Margaret L. Vamosy, and Marvin Mora

Inbreds and hybrid cabbage (Brassica oleracea var. capitata) selected for resistance to diamondback moth (DBM, Plutella xylostella L.) were tested in the field in New York and Honduras for resistance. In New York, plants were inocrdated with up to 400 eggs per plant to enhance the severity of the tests. In Honduras, where natural populations of DBM were high, especially in the dry season, there were three distinct classes of susceptibility to DBM: the very susceptible controls or standard cultivars, the highly resistant glossy-leaved lines, and the intermediate selections with normal leaf bloom. Some normal-leaf hybrids were more resistant than either of their parents, which indicates the need to select for specific combining ability for high resistance levels. At maturity, the glossy-leaved hybrids produced marketable crops with: out aid of chemical sprays.

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Melvin R. Hall

In 1983-1987, a Gardner color difference meter standardized to a pink tile (L=70.5, a=+23.9, b=+9.3) and equipped with an aperture of 3.8 cm (1983-1986), 1.9 cm (1987), or 1.0 cm (1988-1989) was used to measure lightness (L) and intensity (chroma) of `Georgia Red' sweetpotato [Ipomoea batatas (L.) Lam.] seed roots cut into longitudinal sections. Individual roots were selected with good color when L<68 and chroma≥39 and fair color when L≥72 and chroma <35 (1983-1985), L<65 and chroma≥42 for good color and L≥80 and chroma <25 for fair color (1986), L≤66 and chroma≥41 for good color and L≥85 and chroma≤20 for fair color (1987). In each year, roots falling between the defined selection values were discarded. In 1988, root sections from a common 1983-ancestor parent root were bulked for plant propagation if L and chroma values were similar. Subsequent measurements of these bulk populations were made in 1989. Measurements by a color difference meter were helpful in making objective judgements in selecting for internal color of sweetpotato. Also, these measurements were helpful in following changes in internal color through several generations of vegetative propagation.

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Yan Wang and Stanley J. Kays

Flavor quality is one of the most difficult traits to select in plant breeding programs due to the large number of sensory panelists required, the small number of samples that can be evaluated per day, and the subjectivity of the results. Using sweetpotato [Ipomoea batatas (L.) Lam.] as a model, clones exhibiting distinctly different flavors were analyzed for sugars, nonvolatile acids, and aroma chemistry to identify the critical flavor components. Differences in sugars, sucrose equivalents, nonvolatile acids, and 19 odor-active compounds were identified that accounted for differences in flavor among the clones. Using the intensity of the aroma per microliter for each of the 17 most important aroma-active compounds (maltol, 5-methyl-2-furfural, 2-acetyl furan, 3-furaldehyde, 2-furmethanol, benzaldehyde, phenylacetaldehyde, β-ionone, 1,2,4-trimethyl benzene, 2-pentyl furan, 2,4-decadienal, 2,4-nonadienal, linalool, geraniol, cyperene, α-copane and a sesquiterpene) and the relative sweetness of individual sugars × their respective concentrations, multivariate (principal component and cluster) analysis allowed accurate classification of the clones according to flavor type without sensory analysis. The level of precision was such that sweetness, starch hydrolysis potential, and the concentration of β-carotene could be accurately predicted by quantifying specific volatiles. Analytical assessment of flavor would greatly facilitate the accurate evaluation of large numbers of progeny, the simultaneous selection of multiple flavor types, and the development of superior new cultivars for a wide cross-section of food crops.