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Gu et al. (948–955) evaluated about 40 genotypes of rice (Oryza sativa L.) for types and degrees of seed dormancy, a major adaptive characteristic in weedy and wild plants. Extremely dormant genotypes were identified from weedy rice. Three weedy genotypes of different origins were crossed and backcrossed with the same breeding line to determine the relationship between dormancy and the shattering, awn, and hull and pericarp/testa color characteristics. All of these characteristics interrelated to dormancy imposed by seed-covering tissues or interacted with each other. The interrelations and interactions reflect the importance of combined effects of weedy characteristics in the adaptation of weedy and wild populations, and suggest that domestication and breeding activities have eliminated dormancy alleles at loci near the genes for the interrelated characteristics from improved varieties.
Container- vs. Ground-Grown Soybean Responses
Questions have arisen regarding the applicability of elevated CO2 and O3 experiments that used container-grown plants for predicting future crop performance in a CO2–enriched atmosphere. In this 2-yr experiment, soybean [Glycine max (L.) Merr.] was grown by Booker et al. (883–895) in large pots and in the ground and exposed to mixtures of CO2 and O3 in open-top field chambers. Elevated CO2 increased total seed mass and O3 suppressed it to similar magnitudes in both rooting environments. Elevated CO2 also reduced the toxic effects of O3. The results indicated that planting density and rooting environment affected plant morphology, but relative responses of seed yield to elevated CO2 and O3 were not fundamentally different between soybean plants grown in large pots and in the ground in open-top chambers.
A Threshold Model for Multiyear Genebank Data
A common problem with long-term genebank rating data is that the rating scale is changed from time to time. Hartung and Piepho (1045–1051) propose a method for joint analysis of data from different rating scales, assuming a threshold model with a common latent scale for the different rating systems. The method may be used to derive mean scores on any one of the rating scales based on a long-term series of evaluation trials, as is illustrated using evaluation data on barley (Hordeum spp.).
Cool-Season Forage Grass Performance
A continuing need exists to evaluate grasses for irrigated pastures in new and marginal environments. Persistence and yield of 16 perennial cool-season forage grasses sown in the late summer of 1997 were measured by Lauriault et al. (909–915) from 1999 to 2001 at Tucumcari, NM. Soil moisture treatments included furrow irrigation during the growing season, irrigation year round, and only occasional irrigation in poorly drained, saline/sodic soil. Species responded differently to soil moisture treatments in regard to establishment, persistence, and yield.
Minimum Number of Genes for Cotton Fiber Strength
Improving cotton (Gossypium hirsutum L.) fiber strength is necessary for new high-speed textile operations and to make U.S. cottons more competitive in world markets. A germplasm release that possesses high strength was used by Meredith (1114–1119) to initiate a backcross strength selection program at Stoneville, MS. After 10 backcrosses with selection only for strength, a genetic analysis was conducted and indicated that fiber strength was conditioned by a relatively small number (1.23) of genes that increased strength 10%. This small number of genes offers a unique source of strength genes that can be selected for in small breeding populations.
Selective Phenotyping to Map QTL
Recombination events are necessary to map QTL accurately, and some progeny result from gametes with a greater number of recombination events than others. The marker genotypes of the progeny allow the number of recombination events they carry to be determined such that the most useful progeny could be phenotyped in a procedure termed selective phenotyping. Simulations of this process by Jannink (901–908) showed that selective phenotyping was most useful when marker resources were not available for dense marker mapping and when researchers also sought to accurately map QTL of small effect.
Detecting Genetic Changes in Wheatgrass
Diverse native grass populations are being developed for revegetation and land reclamation purposes, but little is known about the maintenance of the genetic diversity of these developed populations during the process of seed increase. Ferdinandez et al. (1064–1068) performed an AFLP assessment on the genetic changes across two generations of seed increase in a multisite composite population (AC Pintail) of awned slender wheatgrass [Elymus trachycaulus subsp. subsecundus (Link) Gould], and revealed that genetic shift occurred in seed increases of AC Pintail.
Quantitative Genetic Analysis of Maize Yield
Few studies have examined the inheritance and interrelationships of both grain yield and the underlying physiological processes in maize (Zea mays L.). Using the genetic effects partitioned by a Design II analysis, Lee et al. (981–987) dissected the physiological mechanisms that influenced favorable or unfavorable contributions to grain yield. Using the highest- and lowest-yielding hybrids in the study (i.e., maximum genetic variation), they attempted to dissect the physiological reasons for the difference in grain yield.
Brown Stem Rot Resistance in Soybean
All previously mapped brown stem rot (BSR) resistance genes in soybean [Glycine max (L.) Merr.] have been mapped to a genetic region on linkage group (LG) J. Patzoldt et al. (1092–1095), evaluated the genetic basis of BSR resistance in five plant introductions (PIs) from central China. In all five PIs, BSR resistance was mapped to the same region on LG J. This suggests that these PIs have BSR resistance genes that are likely allelic with previously reported genes.
Hybrid Bermudagrass Regeneration Protocol
Hybrid bermudagrass [Cynodon dactylon (L.) Pers. x C. transvaalensis Burtt Davy] cv. Tifton 85 is a major forage grass in the southeastern USA that suffers severe yield losses because of insect damage. Jain et al. (1069–1072) established an efficient regeneration system via somatic embryogenesis from immature inflorescences of Tifton 85 followed by the establishment of chlorophyllous, true-to-type Tifton 85 plants in the greenhouse. This facile, regeneration protocol will allow for future genetic improvement of Tifton 85 for incorporating insect resistance into this valuable forage crop.
Subdivision of a Target Regions and Breeding for Local Adaptation
Breeding for local adaptation implies subdivision of a target region into subregions and requires efficient methods of estimating genetic merit in the subregions. Piepho and Möhring (1151–1159) used Best Linear Unbiased Prediction based on a suitable mixed linear model for this purpose. From published data, the resulting estimators were shown to yield a better response to selection than alternative estimators. This finding also applied to selection for global adaptation. The method is hoped to enhance breeding efforts aimed at local or global adaptation.
Genotypic Variability in Nitrogen Remobilization
Nitrogen remobilization, considered as the main grain nitrogen source, depends on genotype and environmental conditions. Barbottin et al. (1141–1150) evaluated the effect of genotype and environment on variation of N remobilized in wheat (Triticum aestivum L.) at several locations (varying in soil and climatic conditions) under various nitrogen availabilities and various fungicide application strategies. Results show that the genotypic variability in N remobilized is mainly explained by the variability of N stored at flowering, and the strong relationship between these two variables was changed under high foliar disease pressure. These findings confirm the value of interest in disease resistant genotypes in low fungicide input and organic agricultural systems.
Performance of French and Spanish Maize Populations across Environments
Malvar et al. (1052–1057) studied the mean performance and stability of the heterotic patterns Humid Spain x Southern France and Dry Spain x Humid Spain and the influence of some environmental and genotypic covariates. The genotype x environment interaction for grain yield was studied by means of Sites Regression and factorial regression models. The crosses Humid Spain x Southern France and Dry Spain x Humid Spain had similar mean performance across environments, but the former, represented by the cross Lazcano x Millette du Lauragais, was more stable. Genotype, environment, and genotype x environment effects for grain yield were mainly due to earliness, vigor effects, and/or environmental factors related to cold stress.
Covariate-Effect Biplot
Biplots based on genotype x environment data of a trait (e.g., yield) are useful for visual evaluation of both tested genotypes and test environments. They are also useful for studying possible megaenvironment differentiations in the target environments. Biplots based on genotype x trait data are useful for understanding the interconnectedness among breeding objectives of the crop as well as the strength and weakness of tested genotypes. Yan and Tinker (1004–1016) introduce a third type of biplots called covariate-effect biplot. It is based on a trait x environment two-way table consisting of effects of a number of explanatory traits on a target trait (e.g., yield) in different environments. This type of biplot not only reveals any possible megaenvironment differentiations in the target environment but also suggests indirect selection strategies for each megaenvironment so that genotype x environment interactions can be exploited.
Diallel Analysis of Chinese and U.S. Maize Germplasm
Added genetic diversity among commercial maize (Zea mays L.) hybrids may further increase yields and safeguard against vulnerabilities. Grain yield data from the Glover et al. (1096–1102) study showed highly significant general and specific combining ability effects with 12 crosses exhibiting high-parent heterosis greater than 20%. The specific combining ability effect and grain yield mean of one cross in particular, Chinese Mix 2 x Mo17(H14)C5, indicated that Chinese Mix 2 has potential for future improvement in yield performance. This information will assist maize breeders in choosing germplasm to introgress into their programs to maintain and/or enhance genetic diversity.
RAPD Assessment of a Flax Collection
Fu (1084–1091) made the first attempt to assess the range and structure of RAPD variation in an entire flax (Linum usitatissimum L.) collection that consisted of 2727 accessions and represented 63 countries. The collection displayed a large amount of genetic variation, with 84.2% of the RAPD variation residing within accessions of each country and 8.2% among the accessions of different regions. Accessions from the East Asian and European regions were genetically most diverse, but accessions from the Indian Subcontinent and African regions were most distinct. These findings provide additional insights to flax domestication and are significant for the management and utilization of flax germplasm for flax improvement.
Wild–Weedy–Domesticated Complexes of Common Bean
Zizumbo-Villarreal et al. (1073–1083) analyzed the role played by traditional farmers in the present population structure and evolutionary dynamics of wild–weedy–domesticated complexes of common bean (Phaseolus vulgaris L.) within a region of its Mesoamerican center of domestication using a morphological marker and 37 polymorphic ISSR loci. Gene flow among populations within the complexes was close to one, theoretically sufficient to counteract genetic drift and/or autogamy. Results suggest that negative farmer selection of cultivated plants with morphological characters implying introgression is the most important evolutionary mechanism for maintaining the high wild–domesticated differentiation found. New conservation and breeding strategies are suggested to maintain and use the gene pools from these complexes.
Wheat Dwarfing Alleles under Heat and Drought Stress
Dwarfing alleles at the Rht-B1 and Rht-D1 loci are widely used in wheat breeding programs, but their effects under heat and drought stress have been variable. Butler et al. (939–947) evaluated the effects of those alleles in a spring wheat (Triticum aestivum L.) recombinant inbred line population under a range of soil moisture conditions. Lines with both dwarfing alleles were shorter, lower yielding, and later heading in all environments compared with lines with one or no dwarfing alleles. Desirable values for most traits occurred across a relatively wide range of plant heights, with the best performing lines either being shorter lines in the tall class or taller lines in the semidwarf classes.
Soybean SCN resistance and Yield
Soybean [Glycine max (L.) Merr.] breeders have found it difficult to develop soybean cyst nematode (Heterodera glycines Ichinohe; SCN) resistant cultivars that yield as well as susceptible cultivars in nonSCN infested environments. Kopisch-Obuch et al. (956–965) compared the yields of near isogenic lines that have the resistance or susceptibility allele at the major SCN resistance gene rhg1 and the minor gene cqSCN-003. They found that lines with the susceptibility allele had significantly greater yield than lines with the resistance allele in one out of five populations segregating for rhg1 and one out of two populations segregating for cqSCN-003. These results should help breeders use marker-assisted breeding to select for SCN resistance without sacrificing yield.
Nitrogen Rates and Yield in Double-Cropped Southern U.S. Soybean
Much of the soybean [Glycine max (L.) Merr.] crop in the southeastern USA is planted late in a double-cropping system following small grains or maize (Zea mays L.). As a result of late planting, early vegetative growth and yield is often reduced. Previous research has shown that nitrogen (N) applied at planting can improve early vegetative growth and yield. Taylor et al. (854–858) studied five N rates in several environments to determine the optimal economic rate. An N rate of 75 kg ha–1 was best at promoting early growth and yield.
Forage Allowance in Grazing Experiments
Forage allowance is a function of both forage mass and stocking rate and can be a powerful tool for explaining differences in animal performance. Some methods used to express forage allowance do not allow useful comparisons across grazing methods or management strategies. A meaningful method of reporting forage allowance is needed that applies across a wide range of pasture management treatments. Sollenberger et al. (896–900) suggests a method that does not include a unit of time, has application across continuous and rotational stocking methods, and, within the rotational stocking method, applies to any size or number of pasture subunits.
Accurate Determination of Crop Lignin Content
Accurately determining the lignin content of crop plants is problematic, producing variable results dependent on the specific method used for analysis. Hatfield and Fukushima (832–839) compares a wide range of methods that have been developed and highlights advantages and disadvantages of each. Particular attention is given to acid detergent lignin, Klason lignin, permanganate lignin, and acetyl bromide lignin methods that are commonly used in the analyses of forages. When used to evaluate a wide range of forages, there was a wide variation in the lignin values for each type of plant across the different lignin methods. Particular caution must be used when the acid detergent lignin method as is applied to grass samples, as these types of lignin are soluble in the hot acid detergent solution. The acetyl bromide lignin method is perhaps the most reliable for a wide range of plant types when collaborated with an appropriate lignin standard.
Mechanisms by Which Lignification Limits Fiber Degradability
Lignin in plant cell walls plays a vital role in plant growth and development, but it also restricts the enzymatic degradation of fiber. Grabber (820–831) describes the development and use of a biomimetic cell wall model system to identify mechanisms by which lignification limits fiber degradability. Overall, these studies suggest that reductions in lignin concentration, hydrophobicity, and cross-linking will improve the enzymatic hydrolysis and utilization of structural polysaccharides for nutritional and industrial purposes.
Corn Rootworm in Transgenic Maize Hybrids
Maize (Zea mays L.) yields are negatively affected by a number of insect pests. One of the most pernicious in the U.S. Corn Belt is the corn rootworm (Diabrotica spp.). Corn rootworm larvae damage maize by feeding on the roots, reducing the ability of the plant to absorb water and nutrients from soil, and causing harvesting difficulties because of plant lodging. Using modern molecular techniques, Vaughn et al. (931–938) have engineered a variant of the wild-type cry3Bb1 gene that encodes a protein with enhanced insecticidal activity against corn rootworms. The result of this research was the identification of corn event MON 863, which controls corn rootworm larval feeding and provides growers with another tool to protect crop yields.
Classifying Bentgrass Clones by Molecular Markers
Bentgrasses (Agrostis spp.) are extensively used on golf courses in temperate regions for putting greens, tees, and fairways. The three most commonly used bentgrasses, creeping bentgrass (Agrostis stolonifera L.), colonial bentgrass (Agrostis capillaris L.), and velvet bentgrass (Agrostis canina L.), are often difficult to identify based on morphological features. Hollman et al. (923–930) evaluated known cultivars of these species and a large number of unknown clones, using molecular markers to classify the unknown clones. They showed that some native Azorean collections contained a mixture of species and possible interspecific hybrids. Clones collected from old golf courses at Milwaukee largely contained fine-textured creeping bentgrass with one clone of velvet bentgrass. Molecular markers provided a highly reliable mechanism for classifying bentgrass clones according to species.
Gas Exchange and Ultrastructural Responses in Drought-Acclimated Senna
Choice of crops is limited in agro-ecosystems faced with drought and nitrogen stress. Ratnayaka and Kincaid (840–847) investigated gas exchange and leaf surface ultrastructural responses to drought and foliar-applied nitrogen in senna (Cassia angustifolia Vahl), a tropical medicinal plant with potential as a dryland crop. Drought-acclimated plants had about 25% less stomatal counts per leaflet but maintained the same net photosynthesis (Pnet) per unit surface area as did well-watered plants until the leaf water potential dropped below about –0.8 MPa. Foliar nitrogen treatment increased Pnet by 30% and differentially influenced adaxial vs. abaxial stomatal count, regardless of drought treatment. Results showed that physiological, developmental, and morphological characteristics collectively confer tolerance to drought and nitrogen stress in senna, and foliar nitrogen application is a promising way to supplement nitrogen.
Trends among Cotton Cultivars
Comparisons among cultivars released over time can be used to estimate selection progress within a breeding program. Bayles et al. (966–980) evaluated 12 cotton (Gossypium hirsutum L.) cultivars, released by the Oklahoma Agricultural Experiment Station between 1918 and 1982 inclusive, to estimate selection progress for lint yield, six fiber properties, eight agronomic characters, and three diseases. Increases over time were observed for lint yield, two measures of fiber length, two measures of fiber strength, picked and pulled lint percentages, boll size, bur size, lint weight per boll, lint index, seed index, lock tenacity, resistance to bacterial blight [caused by Xanthomonas campestris pv. malvacearum (Smith) Dye], and resistance to the Fusarium wilt [caused by Fusarium oxysporum Schlect. f. sp. vasinfectum (Atk.) Snyd. & Hans.]–root-knot nematode [Meloidogyne incognita (Kofoid & White) Chitwood] complex. Uniformity index and micronaire displayed significant differences among cultivars but no trends over time. Tolerance to Verticillium wilt (caused by Verticillium dahliae Kleb.) was not significant different among cultivars.
Genetic Variation within Cynodon transvaalensis
Cynodon transvaalensis Burtt Davy is used as turf and has been hybridized with C. dactylon (L.) Pers. var. dactylon to produce high quality turf bermudagrass cultivars. Little information exists on the magnitude of genetic variation within the South Africa indigenous species. Wu et al. (848–853) demonstrated variation among 14 C. transvaalensis accessions for AFLP DNA markers and characterized their relatedness to two C. dactylon var. dactylon accessions and three derivative interspecific hybrids. The results suggest that there is sufficient genetic variation within C. transvaalensis for genetic improvement.
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