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Islam et al. (822–826) tested a new method for the rapid production of pathogen-free sweetpotato [Ipomoea batatas (L.) Lam.] that uses single node leafy cuttings in a closed system supplied with artificial light. They compared yield and growth of sweetpotato plugs in multicell trays planted intact or without roots with that of conventional cuttings without roots. The yield of storage roots from plug transplants 128 d after planting in the field was 11 to 12 Mg ha-1 greater than that of the conventional cuttings. Although the incidence of coiled storage roots from plug systems was relatively higher, the high percent survival, higher yield, and more rapid and uniform growth of intact plug transplants should encourage the use of this method in commercial operations.
Poor Grain Filling in Rice
Poor grain filling is a major constraint in utilizing the heterosis of japonica/indica hybrid rice (Oryza sativa L.). Yang et al. (766–772) investigated potential causes of poor grain filling of japonica/indica hybrids (J/IHs) by examining the source-sink relations of 36 J/IHs, their parents, and two intervarietal hybrids (IVHs). The aboveground dry matter per spikelet of the J/IHs was greater than that of their parents and IVHs, indicating that source limitation was not the cause of poor grain filling of J/IHs. Low efficiency in the translocation of assimilates and remobilization of stored assimilates from the straw to the grain during the grain filling period was associated with poor grain filling of J/IHs. Understanding the mechanism of poor translocation and partitioning of assimilates into grains in J/IHs may help to improve the grain filling of J/IHs.
Seeding Rate for Old vs. New Wheat Cultivars
Historically, seeding rate was mainly studied for yield performance of winter wheat (Triticum aestivum L.) in the Great Plains. Its effect on agronomic and especially some end-use quality parameters is very limited. Geleta et al. (827–832) investigated how seeding rate influenced agronomic and end-use quality traits on both older and current cultivars. On the basis of four environments in Nebraska, lower seeding rate decreased plant population, grain yield, kernel weight, flour yield, mixing time, and caused later flowering, while it increased flour protein content and mixing tolerance. Seeding rate influenced many economic traits of winter wheat in the Great Plains. The recommended seeding rate developed for older cultivars is still appropriate for current cultivars under these four Nebraska environments.
Biodosimeter Evaluation for Turf Canopies
Measurement of ultraviolet (UV) radiation within crop canopies has been limited by instrumentation size and cost. Yuen et al. (859–868) evaluated a biodosimeter in turfgrass canopies that was comprised of a UV-sensitive bacterium enclosed in polyethylene packets. After exposure of biodosimeter packets to sunlight, bacterial mortality was related to UV-B dosage measured with a radiometer. The packets were sufficiently small to enable UV-B measurements at different canopy depths and leaf orientations with minimal disturbance to the canopy. While created for use in turfgrass, the biodosimeter has potential applications in other crop systems as well.
C2H4 Sensitivity of Wheat
Ethylene gas can become elevated in closed growth chambers, but C2H4 sensitivity is not well characterized. Klassen and Bugbee (746–753) evaluated the C2H4 sensitivity of wheat (Triticum aestivum L.) and rice (Oryza sativa L.) at levels ranging from 0 to 1000 nmol mol-1 (ppb). Yield was reduced by 36% in wheat and 63% in rice at 50 nmol mol-1, and both species failed to set seed when continuously exposed to 1000 nmol mol-1. Ethylene sensitivity was not influenced by elevated CO2, but did vary between environments and cultivars. The data suggest that genetic and environmental manipulations can be used to resolve problems with low levels of elevated C2H4 in greenhouse and growth chamber environments.
Molecular Markers for Oat Line Improvement
The production of new disease-resistant oat (Avena spp.) lines can be difficult when offspring must be evaluated at each generation for their responses to pathogen infection to determine whether the lines have retained a desired disease resistance gene. To facilitate the production of disease-resistant oat lines, Pal and coworkers (912–918) examined how frequently two classes of molecular markers detected differences among oat species and cultivars. Their results showed that information obtained from newly isolated DNA fragments and plant genome sequencing projects could be used to generate molecular markers for oat. These findings will be useful to scientists who are attempting to identify chromosomal regions that contain disease resistance genes and to develop improved oat lines.
Comparison of Early Generation Tests in Soybean
Several different plant breeding procedures have been suggested under the heading of "early generation testing," but there is little available evidence to compare these for effectiveness. St. Martin and Geraldi (705–709) compared testing of F1-, F2-, and F3-derived families in two soybean [Glycine max (L.) Merr.] populations. The three procedures resulted in similar genetic gains for yield, but testing F1-derived families resulted in undesirable changes in lodging score. The procedure based on F2-derived families provided the best combination of genetic gain and rapidity of advance.
Quantification of Diversity in Maize
Quantification of diversity among and within maize inbred lines from different sources determined by molecular markers is important but has received little attention. Gethi et al. (951–957) studied six historically important maize inbred lines from eight sources using 44 SSR markers. Among inbred gene frequency variation accounted for 87.8%, among sources within inbreds 7.6%, and within sources 4.6%. Establishing the level of homozygosity in materials appears necessary before genetic studies are initiated for meaningful data comparison across programs.
White Mold Resistance in Common Bean
White mold [caused by Sclerotinia sclerotiorum (Lib.) de Bary] is a devastating disease in common bean (Phaseolus vulgaris L.). Resistance to white mold is complexly inherited. Kolkman and Kelly (693–699) described moderate to high estimates of heritablilty for resistance to white mold in navy bean, and great variation for agronomic traits affecting white mold levels. The indeterminate growth habit was the most important avoidance mechanism in navy bean.
Alfalfa Yield Improvement
Alfalfa (Medicago sativa L.) yields have stagnated in recent years, yet little effort has been devoted to capture heterosis during cultivar development. Riday and Brummer (716–723) attempted to determine if heterosis could be identified in crosses between the two major subspecies of cultivated alfalfa, Medicago sativa subsp. sativa and subsp. falcata. They crossed genotypes between and within the subspecies and measured forage dry matter yield in field experiments. Sativa–falcata heterosis was calculated in several ways, and on average, the intersubspecies crosses averaged 18% more yield than the average yield of intrasubspecies crosses. This heterotic pattern could potentially be exploited to improve yield in alfalfa.
Germplasm Characterization for Sugars in Sugarcane
The lack of germplasm characterization of the World Collection of Sugarcane impedes its effective preservation and use. Tai and Miller (958–964) evaluated sugar composition of the plant cane of four Saccharum species plus four commercial cultivars. Commercial cultivars and S. officinarum L. had the highest mean sucrose content and the lowest mean glucose and fructose content, whereas S. barberi Jesw. had the lowest mean sucrose content and the highest mean glucose and fructose content. Principal component and cluster analyses showed no clear-cut separations among the four species. Information on sugar composition should assist curators in classifying clones in their collections and breeders in selecting superior clones for their breeding programs.
Root Distribution and Water Uptake in Maize
Root clustering is a possible cause of reduced water uptake and for the occurrence of strong gradients in water potential between root clusters and the bulk soil. Amato and Ritchie (773–780) investigated the relations of different degrees and spatial patterns of soil compaction with the spatial distribution of roots and soil moisture in maize (Zea mays L.) grown on stored water in containers. Clods of locally compacted soils were penetrated by roots only in the outer 2 cm, and showed incomplete water extraction compared with noncompacted soil. Uniformly high compaction resulted in limited but stable plant size with low water uptake. Nonuniform soil structure may result in reduced water uptake due to the spatial variability of root distribution.
Improved Insect Resistance in Maize
In Mediterranean countries, the principal corn pest is the pink stem borer (PSB) (Sesamia nonagrioides Lef.), followed in importance by the European corn borer (ECB) (Ostrinia nubilalis Hübner). The objective of Velasco et al. (724–729) was to quantify injury by these pests in a set of sweet inbreds and in their hybrids. The inbred EP61 showed negative and significant GCA effects for ear damage under infestation by both species. Inbreds EP59 and V7726 showed negative and significant GCA effects for ear damage by European corn borer. These inbreds could be included in a sweet corn synthetic population that would be improved to decrease ear damage by corn borers.
Estimation of Forage Intake by Ruminants on Pasture
Accurate estimates of forage intake are extremely important to an understanding of variation in performance of ruminants on pasture. In this review, Lippke (869–872) discusses the basis, advantages, and disadvantages of techniques of intake estimation currently used. Most techniques rely on a combination of markers, one intrinsic to the forage plant and the other administered in known amounts, as the basis for calculated estimates of intake. Failure to achieve consistent recovery of these markers is a persistent problem. Recently, techniques using artificial and naturally occurring alkane markers have offered levels of precision and accuracy never before available.
Selection for Greater ß-Glucan Content in Oat
ß-Glucan is the active ingredient in oatmeal that lowers cholesterol in humans. ß-Glucan content in oat can be increased by traditional selection methods. To be acceptable to farmers, oat cultivars with elevated ß-glucan content must also have good agronomic performance. Lines with elevated ß-glucan contents tend to have improved nutritional quality for food (lower oil content and greater protein content) but reduced agronomic performance (lower grain yields and test weights). Development of oat cultivars with both elevated ß-glucan content and acceptable agronomic performance will be challenging, but can be accomplished with appropriate selection methods (see Cervantes-Martinez et al., 730–738).
Mapping Genes for Protein Variation in Barley
Managing grain protein percentage is key to attaining grain quality objectives. Genes with alternative high grain protein and low grain protein alleles have been mapped in both wheat and barley. See et al. (680–685) detail the location of what appears to be a single gene on barley Chromosome 6 that has a direct impact on grain protein percentage. Depending on the allele state at this locus, grain protein percentage may be varied by more than two percentage points, a difference that could determine whether barley is sold for malt or cattle feed. Marker assisted backcrossing is now being done to move this gene into high yielding, high quality genetic backgrounds.
Poly-ß-Hydroxybutyrate in Transgenic Alfalfa
Many organisms produce poly-ß-hydroxybutyrate (PHB), a compound that can be formed into a biodegradable plastic. Bacterial fermentation can produce PHB, however this is not cost competitive with petroleum-based plastics. Saruul et al. (919–927) introduced three genes needed for plastic production into alfalfa plants, which would be economical to grow for plastic production. The original transformed plants and their offspring accumulated up to 2 g PHB kg-1 dry alfalfa leaves. These alfalfa plants could be integrated into an alternative crop use system in which leaves are extracted for PHB and stems are used for energy production through synthesis of ethanol or burning to produce electricity.
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