Narrow-sense heritability estimates were computed for five fruit quality characteristics and their weighted total index. Grit content and skin russeting were moderately heritable traits, while flesh texture, flavor, appearance, and the weighted total score were of relatively low heritability. Within sub-populations of crosses, defined by the species ancestry of the parents, the relative magnitudes of heritabilities for each trait varied, but were in general agreement with those for the entire population. The general combining ability variances were 4.5 to 12.0 times those for specific combining ability, although both were statistically significant for all traits and the weighted quality index. The species ancestry of a parent had no effect on its general combining ability rank. While selection of individual seedlings on the basis of their own phenotype will result in genetic improvement for grit and russet, selection based on a combination of full-sib family means and individual phenotypes is recommended for flavor, texture, appearance, and overall fruit quality.
R.L. Bell and Jules Janick
James J. Luby, Peter A. Alspach, Vincent G.M. Bus, and Nnadozie C. Oraguzie
Incidence and severity of fire blight [Erwinia amylovora (Burr.) Winslow, Broadhurst, Buchanan, Krumwiede, Rogers, and Smith] following field infection were recorded using families resulting primarily from open-pollination of Malus sylvestris (L.) Mill. var.domestica (Borkh.) Mansf. cultivars and a few other Malus Mill. sp. The families were structured as three sublines, planted in three successive years (1992 to 1994), of a diverse population of apple germplasm established at HortResearch, Hawkes Bay, New Zealand. The incidence of fire blight varied among the sublines with the oldest planting exhibiting more fire blight. Flowering trees were more likely to be infected than nonflowering trees, in terms of both incidence and severity. Furthermore, the level of fire blight was related to flowering date, with later flowering trees having higher levels. Thus, family means and narrow-sense heritability estimates were computed after first adjusting the fire blight score for flowering date by fitting a linear model. Provenance of origin of the maternal parent explained little variation except that M. sieversii Lebed. families were more resistant than M. sylvestris var. domestica families in one subline. Family means computed using all trees, and those from only flowering trees were highly correlated. Families from open-pollination of M. honanensis Rehder and M. xhartwiggii Koehne females were among the more susceptible. Those from several European M. sylvestris var. domestica cultivars as well as from M. baccata (L.) Borkh. and M. toringoides (Rehder) Hughes females were among the more resistant families. Narrow-sense heritability estimates ranged from 0.05 to 0.85 depending on the subline, with most estimates between 0.12 and 0.36. They were higher in the two older sublines that consisted primarily of open-pollinated families from M. sylvestris var. domestica, and lower in the younger subline that consisted primarily of M. sieversii, due to lower incidence and severity in the latter subline. Breeders who consider potential complications of juvenility, tree size, and flowering date in relation to infection periods should be able to exploit field epidemics to perform effective selection.
Todd C. Wehner
Variance components for low-temperature germination ability in cucumber (Cucumus sativus) were estimated for 3 germination variables (actual days to germination, days to 50% germination, and percentage of germination) at 17°C using a North Carolina Design I analysis. The estimates were made using the North Carolina Medium Base Pickle (NCMBP) population, which was developed by intercrossing adapted pickling cucumber cultivars with diverse lines for 3 cycles without selection. Estimates of additive and dominance variance for the percentage of germination and days to 50% germination were equal, but additive variance was predominant for actual days to germination. Heritabilities based on half-sib families tested in 2 replications ranged from 0.44 to 0.61 for the 3 germination variables. There were large significant genetic correlations among the 3 germination variables, ranging from 0.61 to 1.03 in absolute value. Selection for either the percentage of germination or for actual days to germination at 17° should result in significant progress in improving low-temperature germination ability of cucumbers in the NCMBP population.
John R. Stommel and Philipp W. Simon
Five cycles of phenotypic recurrent selection for total dissolved solids and sugar type (reducing vs. nonreducing) were performed on four carrot (Daucus carota L.) populations of common background. The populations contained high or low percentage of total dissolved solids (HTDS and LTDS, respectively) with high or low levels of reducing sugar (HRS and LRS, respectively). Effective selection for total dissolved solids (TDS) and sugar type was indicated by significant gains over five cycles of selection. TDS decreased in LTDS/HRS and LTDS/LRS populations by 21.9% and 15.9%, respectively. Corresponding increases of 22.4% and 28.2% were observed in HTDS/HRS and HTDS/LRS populations. Mean reducing sugar levels in HRS roots after five cycles of selection were limited to 2.0% of root fresh weight; sucrose was the primary storage carbohydrate. Reducing sugars were not detected in LRS roots. Mean total sugar levels in the HTDS and LTDS populations were 7.1% and 3.1% of root fresh weight, respectively. Realized heritability estimates ranged from 0.40 to 0.45 for the four populations. The onset of flowering was markedly delayed in plants of the two HTDS populations after five cycles of selection.
Anita N. Miller, Timothy J Ng, and Thomas H. Barksdale
A 6-parent diallel was used to study combining ability and type of gene action contributing to resistance in tomato (Lycopersicon esculentum Mill.) to anthracnose caused by Colletotrichum dematium (Pers. ex Fr.). The 6 parents, one set of F1, hybrids, and 5 selected reciprocal crosses were grown at 2 locations. Ripe fruit were harvested, puncture-inoculated with the pathogen, and subsequently evaluated for resultant lesion diameter. No reciprocal effects were found at either location for the 5 crosses studied. The analysis of variance for parent and F1 hybrid performance revealed a genotype × location interaction. Combining ability analysis based on the F1 hybrids alone indicated a significant general combining ability (GCA) effect. The specific combining ability (SCA) and GCA × location interaction mean squares were smaller than the GCA value but were still significant. Differential performance over locations of the hybrids of one line was primarily responsible for the GCA × location interaction. Analysis of variance and covariance of parental arrays indicated partial dominance in the direction of susceptibility. Narrow sense heritability for the trait was 70% over both locations.
Alfred Jones, J. M. Schalk, and P. D. Dukes
Twenty-two sweet potato (Ipomoea batatas (L.) Lam.) breeding lines and 19 open-pollinated offspring from each were used to estimate the heritabilities of 7 measures of soil insect injury. Four measures of injury by the wireworm, Diabrotica spp., and Systena spp. (WDS) complex and h2 (± SE) were: percentage of roots injured, 0.45 ± 0.12; holes per root, 0.32 ± 0.09; severity index, 0.37 ±0.11; and damage score, 0.39 ± 0.17. Two measures of injury by the sweetpotato flea beetle, Chaetocnema confinis Crotch, and h2 were: percentage of roots injured, 0.40 ± 0.07, and tunnels per root, 0.25 ± 0.08. The h2 of percentage of roots injured by all insects was 0.51 ± 0.12. The percentage measures were more easily obtained and were as effective as the other measures under the conditions of natural infestation that occurred in this test. Further advances in selection for high levels of resistance to soil insects are possible within the breeding materials tested.
William J. Martin and Dennis P. Stimart
Stomatal density during plant development and inheritance of the trait were investigated with the goal of utilizing stomatal density as a correlated trait to cutflower postharvest longevity in Antirrhinum majus L. Inbred P1 (stomatal index = 0.2) was hybridized to inbred P2 (stomatal index = 0.3) to produce F1 (P1 × P2), which was backcrossed to each parent producing BCP1 (F1 × P1) and BCP2 (F1 × P2). P1, P2, F1, BCP1, and BCP2 were used to examine changes in stomatal density with plant development and early generation inheritance. An F2 (F1 self-pollinated), and F3, F4, and F5 families, derived by self-pollination and single seed descent, were used to obtain information on advanced generation inheritance. Stomatal density was stable over time and with development of leaves at individual nodes after seedlings reached two weeks of age. Therefore, stomatal density can be evaluated after two weeks of plant development from a leaf at any node. Stomatal density is quantitatively inherited with narrow sense heritabilities of h2 F2:F3 = 0.47 to 0.49, h2 F3:F4 = 0.37 ± 0.06 to 0.60 ± 0.07, and h2 F4:F5 = 0.47 ± 0.07 to 0.50 ± 0.07.
Douglas V. Shaw
The heritabilities of, and genetic correlations among, variables that describe internal and external color in fresh strawberry (Fragaria × anarrassa) fruit were estimated using factorial analyses of seedlings from 20 controlled crosses. Hunter L and a values, and a subjective score generated by comparison with color plates were obtained for seedling genotypes and their parents at two locations. Genetic effects were responsible for 33% to 61% of the phenotypic variance for color traits, after correction for location effects. Means for objective color variables differed significantly between locations, but means for subjective color scores did not. Genetic × location interaction variances were usually nonsignificant, and were < 12% of the phenotypic variance for all variables. Phenotypic and genetic correlations between objective and subjective color scores were significant and large (absolute values of r = 0.42-0.69; rg = 0.84-1.00). Multiple regression of subjective scores on L and a explained 69% and 59% of the phenotypic variation for external and internal color, respectively. Genetic correlations between measures of internal and external color were small and mostly nonsignificant, suggesting that separate sets of genes condition these traits.
Leon A. Hansen, James R. Baggett, and Kenneth E. Rowe
Tassel date, silk date, plant height, ear height, shank length, husk extension, tip blanking, row number, ear length, and first ear weight were studied in a diallel involving 7 inbred sweet corn parents. Both general combining ability (GCA) and specific combining ability (SCA) were involved in the inheritance of all 10 characters. This was consistent for F1 crosses in 2 years and for F2 families. SCA variance (VSCA) was larger than GCA variance (VGCA) for ear length and first ear weight in the F1 and ratios of VSCA/VGCA for these characters were slightly larger than 1.0. VGCA was larger than VSCA all other characters in the F1. Ratios of VSCA/VGCA ranged from .05 for row no. to .57 for plant height.
Variance ratios for most characters decreased in the F2. The failure of some ratios to decrease in the F2 was attributed to either differential interactions of GCA and SCA with environment or inadequate sampling of F2 families. Genotype × year interactions influenced the expression of most characters. A greater portion of the genotype × year interaction was contained in estimates of SCA than in estimates of GCA. Heritability estimates from parent-progeny regression were generally larger than those from variance components, although these estimates were generally in close agreement.
Garald L. Horst and Nancy B. Dunning
A laboratory experiment was conducted with seeds of perennial ryegrass (Lolium perenne L.) cultivars germinating and growing on floating mats in saline hydroponic solutions. This study was done to determine the relative intraspecific salt resistances of 48 perennial ryegrasses during germination and seedling growth in saline solutions. Total germination, germination rate, leaf blade length, root length, and total seedling fresh and dry weight were measured after 21 days. Test solutions prepared from deionized water and equal quantities of NaCl and CaCl2 by weight consisted of 11.6 (low), 19.5 (medium), and 23.5 dS·m−1 (high) salinity. Cultivars had highly significant total germination and germination rate responses to salt stress. Seedling growth responses as measured by blade and root length and weights were also significant. A hydroponic medium with a salt concentration of 23.4 dS·m−1 should provide a suitable stress level for screening ryegrass genotypes for improved germination and seedling salt resistance. At the high salinity level, cultivars that average less than a 50% reduction in growth parameters relative to high-yielding cultivars should be considered. Broad-sense heritability estimates indicate that seedling dry and fresh weight and germination rate would be valuable criteria for use in selection of perennial ryegrasses for salt resistance.