Wiley

Breeding programs in major crops normally restrict the use of parents to those improved for a variety of traits. Gain from utilizing these good × good crosses appears to be high, and improvements are sufficient to encourage continued breeding within narrow gene pools even though each cycle is expected to lead to reduced genetic variability. These finely tuned programs have gradually limited the amount of new diversity introduced into the breeding gene pool. This breeding strategy has led to a genetic gap where there is a large difference in the favorable gene frequency between the improved and unimproved lines and to a narrowing of genetic diversity within elite gene pools. At the same time, evidence has accumulated in plant breeding programs and long‐term selection experiments in several organisms that the genome is more plastic and amenable to selection than previously assumed. In the barley (Hordeum vulgare L.) case study reported here, incremental genetic gains were made for several traits in what appears, based on pedigree analysis, to be a narrow gene pool. Given this situation, we call for an examination of the generally held belief that the variation on which selection is based in elite gene pools is provided almost exclusively from the original parents. Classical and molecular genetic analyses have shown that many mechanisms exist to generate variation de novo, such as gene amplification and transposable elements. Accordingly, we put forward the hypothesis that newly generated variation makes an important contribution. We also hypothesize that gene interaction, epistasis, is more important than commonly viewed and that it arises from de novo generated diversity as well as the original diversity.

Critical issues in the use of diallel analysis are reviewed. From a statistical point of view the critical issue concerns the choice of a model with fixed or random genotypic effects. From a genetical point of view, two assumptions are critical in attempts to interpret the resuits of diallel analyses. The assumption concerning the independent distribution of genees in the parents is most critical to proper interpretation and seems to be least acceptable in actual practice. The second assumption, that there is no epistasis, may frequently be incorrect. Epistasis affects estimates of general and specific combining ability mean squares, variances, and effects in an unpredictable manner. As an alternative to genetic interpretation, the statistical description provided by dialiel analysis can be used to help answer questions concerning the importance of specific combining ability and the predictability of hybrid performance using general combining ability or parental performance.

To date, only one gene conferring resistance to Wheat streak mosaic virus (WSMV) designated as Wsm1 was transferred from Thinopyrum intermedium (Host) Barkworth and Dewey to wheat (Triticum aestivum L.) in the form of a compensating Robertsonian translocation T4DL·4J^sS. Wsm1 confers high levels of resistance to WSMV under field conditions; however, in certain genetic backgrounds and environments, the presence of the T4DL·4J^sS translocation reduces agronomic performance. The objective of this study was to shorten the Th. intermedium segment in the T4DL·4J^sS translocation. We recovered one proximal (rec36) and four distal (rec45, rec64, rec87, rec213) primary recombinants. Genomic in situ hybridization and molecular marker analyses determined the size of the Th. intermedium segments in the distal recombinants to be about 20% of the 4DS‐4J^sS arm. All primary recombinant stocks, together with appropriate controls, were evaluated for their resistance to WSMV and Triticum mosaic virus (TriMV) in greenhouse tests. Whereas the distal recombinants rec45, rec64, rec87, and rec213 were resistant to both WSMV and TriMV at low temperatures of 18°C, the proximal recombinant rec36 reacted susceptible, which mapped the Wsm1 gene to the distal 20% of the 4DS‐4J^sS arm. We successfully shortened the Th. intermedium segment while still retaining the Wsm1 gene. The T4DL·4DS‐4J^sS recombinant chromosome of the rec213 stock was transferred to adapted Kansas hard red winter wheat cultivars.

Information on gas exchange of crop stands grown in controlled environments is limited, but is vital for assessing the use of crops for human life‐support in closed habitats envisioned for space. To studies were conducted to measure gas exchange of wheat stands (Triticum aestivum L. cv. Yecora Rojo) grown from planting to maturity in a large (20 m² canopy area), closed growth chamber. Daily rates of dark‐period respiration and net photosynthesis of the stand subsequent CO₂ drawdown in the light (i.e., a closed‐system approach). Lighting was provided as a 20‐h photoperiod by high‐pressure sodium lamps, with canopy‐level photosynthetic photon flux density (PPFD) ranging from 500 to 800 μmol m^‐2 s^‐1 as canopy height increased. Net Photosynthesis rates peaked near 27 μmol CO₂ m^‐2 s^‐1 at 25 d and then gradually declined with age. Responses to short‐term changes in irradiance after canopy closure indicated the stand light compensation point for photyosynthesis to be near 200 μmol m^‐2 s^‐1 PPFD. Tests in which CO₂ concentration was raised to ≈2000 μmol mol^‐1 and then allowed to draw down to a compensation point showed that net photosynthesis rates dropped sharply with decreasing CO₂. The CO₂ compensation point for photosynthesis occurred near 50 μmol mol^‐1 Short‐term (24 h) temperature tests showed net photosynthesis at 20 °C ≥ 16 °C > 24 °C, while dark‐period respiration at 24 °C > 20 °C > 16 °C. Rates of stand evapotranspiration peaked near Day 25 and remained relatively constant until about Day 75, after which rates declined slowly. Results from these tests will be used to model the use of plants for CO₂ removal, 0₂ production, and water evaporation for controlled ecological life support systems propsed for extraterrestrial environments.

Vitamin A deficiency affects approximately 250 million people in semiarid regions of Africa and Asia, where sorghum (Sorghum bicolor Moench) is a major staple crop. Yellow endosperm sorghums contain carotenoids, some of which can be transformed by humans into vitamin A. Our objective was to study the genetic basis of variation in carotenoid levels in sorghum endosperm by mapping quantitative trait loci (QTL) associated with carotenoid content and endosperm color, as a putative predictor of carotenoid concentration. A recombinant inbred line population developed from a yellow (‘KS115’) by a white endosperm (‘Macia’) parental cross was evaluated in two locations in 2005. A genetic map was generated using 112 molecular markers including nine carotenoid candidate genes. Lutein, zeaxanthin, and β‐carotene were the major carotenoids identified. Several QTL were detected for each compound as well as for color and total carotenoids. Color was significantly correlated with the levels of all compounds, and color QTL co‐localized with carotenoid QTL. For β‐carotene (provitamin A), five QTL were localized on chromosomes 1, 2, and 10. One of them, on chromosome 2, was stable across both environments, had positive additive effects (1.179 and 1.379), explained large proportions of the phenotypic variance (11.6% and 15.15%), and was associated with a new phytoene synthase gene (Psy3). This first report of QTL for carotenoid content in sorghum grain provides a starting point for breeding high‐provitamin A sorghums.

High dark respiration rate (R_D) may be a physiological process associated with low herbage yield of tall fescue (Festuca arundinacea Schreb.) during summer. Our objectives were to 1) examine genotypic variability and determine genetic mechanisms controlling R_D and 2) examine interrelationships between R_D, concentrations of Kjeldahl N and water‐soluble carbohydrate (WSC) of leaf blades, and yield components. Progeny of a seven‐parent diallel were grown and sampled in controlled‐environment chambers at 20°C and in the field during August and September. The R_D was estimated manometrically on center‐sections of recently‐collared leaf blades. Concentrations of N and WSC were estimated on adjacent leaf blade tissue. Herbage was harvested, tillers per plant counted, and yield per tiller (YPT) calculated. Reciprocal effects evaluated in controlled‐environment chambers were not significant for any characteristic. Genetic variation among single‐cross progenies was significant for R_D, leaf N, tillers per plant, and YPT largely due to general combining ability. Narrow‐sense heritabilities ranged from 0.60 to 0.90 which suggested that additive gene action predominated for these characteristics in this population. The R_D was not correlated consistently with any characteristic measured. Concentrations of N and WSC of leaf blades were negatively correlated (r = −0.61). Positive rank correlations (P < 0.05) were obtained among progeny responses from controlled‐environment chambers and that of both field harvests for R_D, tillers per plant, and YPT, and between field harvests for all variables (P < 0.01). Interrelationships among characteristics for parental plants were similar to those of the progeny. The similarity in ranking of R_D of genotypes among controlled environment chambers and the field during summer and autumn suggests efficient identification of desired genotypes is probable over a range of environments.

The development of forage grass cultivars which maintain a high level of performance over a wide range of environments is a goal of most breeding programs. Twenty‐five synthetics and two cultivars of tall fescue (Festuca arundinacea Schreb.) were evaluated for 3 years at two locations. Entries differed significantly for reproductive, vegetative, and total herbage dry matter yields. Significant entry ✕ environment (linear) interactions occurred for reproductive and vegetative regrowth, indicating that there were significantly different environmental responses among the 27 entries. Entry mean yields were linearly regressed on the average of all entries La each environment. Regression coefficients of the synthetics ranged from 0.71 to 1.24, 0.76 to 1.17, and 0.84 to 1.15 for reproductive, vegetative, and total herbage dry matter yield, respectively. Linear regression accounted for 95 to 99%, 78 to 99%, and 89 to 99% of the variation in yields of the 27 entnes for reproductive, vegetative, and total herbage dry matter yield, respectively. Mean square deviations from regression were found to be homogeneous, using the Bartlett test, for the three yield measurements. In the regression analysis of variance, the pooled deviations were not significant for all three yield measurements. The mean square deviations, coefficient of determination, and ecovalence stability indexes were highly and significantly correlated. We concluded that the mean herbage yield was a sufficient statistic for the selection of synthetics which have high and stable total yield. The regression coefficient was a useful statistic, in addition to mean herbage yield, if the goal was selection for desirable stable herbage regrowth yield.

The importance of crop germplasm found in landraces is well established, and a comprehensive international program exists to conserve this resource ex situ in gene banks and botanical gardens. Landraces are still cultivated in regions of crop domestication and diversity. In situ maintenance has been neglected by genetic resource conservation programs in part because of misconceptions about farming systems that produce landraces. This paper presents three cases of on‐going maintenance of landraces by farmers who have also adopted high‐input technology, including high yielding crop cultivars. These cases are potatoes (Solanum spp.) in the Andes of Peru, maize (Zea mays L.) in southern Mexico, and wheat (Triticum spp.) in western Turkey. These cases suggest that on‐farm conservation of landraces can be decoupled from traditional farming practices. Factors that promote in situ conservation are the fragmentation of land holdings, marginal agricultural conditions associated with hill lands and heterogeneous soils, economic isolation, and cultural values and preference for diversity. Landraces are likely to persist in patches and islands of farming systems in regions of crop domestication and diversity, and these patches provide potential sites for conservation programs. In situ conservation may be a valuable complement to ex situ methods because it can preserve the biological and social processes of crop evolution. Research is needed on the biogeography and conservation biology of remaining landrace populations in order to plan in situ conservation.

Information on the distribution of vernalization genes and their association with growth habit is crucial to understanding the adaptability of wheat (Triticum aestivum L.) cultivars to different environments. In this study, 278 Chinese wheat cultivars were characterized with molecular markers for the vernalization genes Vrn‐A1, ‐B1, ‐D1, and ‐B3 Heading time was evaluated in a greenhouse under long days without vernalizaton. The dominant Vrn‐D1 allele showed the highest frequency in the Chinese wheat cultivars (37.8%), followed by the dominant Vrn‐A1, ‐B1, and ‐B3 alleles. Ninety‐two winter cultivars carried recessive alleles of all four vernalization loci, whereas 172 spring genotypes contained at least one dominant Vrn allele. All cultivars released in the North China Plain Winter Wheat Zone were winter type. Winter (53.0%), spring (36.1%), and early‐heading (10.9%) cultivars were grown in the Yellow and Huai River Valley Winter Zone. Most of the spring genotypes from this zone carried only the dominant Vrn‐D1 allele, which was also predominant (64.1%) in the Middle and Lower Yangtze Valley Winter Zone and Southwestern Winter Wheat Zone. In three spring‐sown wheat zones, all cultivars were early‐heading spring types that frequently possessed the strongest dominant Vrn‐A1a allele and combinations with other dominant Vrn gene(s). The Vrn‐D1 allele is associated with the latest heading time, Vrn‐A1 the earliest, and Vrn‐B1 intermediate values. The information is important for breeding programs in countries interested in using Chinese wheats.