Major gene control of tolerance of bread wheat (Triticum aestivum L.) to high concentrations of soil boron

Summary The genetic control of tolerance of wheat to high concentrations of soil boron was studied for five genotypes. Each genotype represented one of five categories of response to high levels of boron, ranging from very sensitive to tolerant. Tolerance to boron was expressed as a partially dominant character, although the response of an F₁ hybrid, relative to the parents, varied with the level of boron applied. The F₁ hybrids responded similarly to the more tolerant parent at low B treatments and intermediate to the parents at higher treatments. Ratios consistent with monogenic segregation were observed for the F₂ and F₃ generations for the combinations (WI^*MMC) × Kenya Farmer, Warigal × (WI^*MMC) and Halberd × Warigal. The three genes, Bo1, Bo2 and Bo3, while transgressive segregation between two tolerant genotypes, G61450 and Halberd, suggested a fourth locus controlling tolerance to boron.     

Differential responses of bread wheat characters to high temperature

Summary Agronomic and yield data were collected from two trials each containing 16 bread wheat genotypes, planted two years under late sowing conditions of high temperature (above 30° C) and one year under a normal sowing time environment. The aim was to study the character response and yield correlations with yield components and other characters under high temperature conditions with full irrigation. The results show that yield, seeds per spike, biomass, and plant height are more thermo-sensitive than spike number per square meter, 1000 kernel weight, and test weight. The grain-filling rate was more temperature-sensitive than days to anthesis and duration of grain-filling. Simple phenotypic correlation analysis indicated that yield was highly and positively correlated with seeds per spike, biomass, and harvest index (HI), independent of seasons and genotypes under high temperatures. The seeds per spike accounted for variation of yield ranging from 35.2 to 78.1%. Effect of earliness on the yield under high temperature was highly dependent on the temperature regime during the heading stage. Grains per spike, biomass, HI, and test weight could be considered potential selection criteria for yield under high temperature. Analysis of yields under normal and late sowing conditions failed to reveal any association between the yield potential in normal sowing date and the performance of varieties under high temperature.     

Genetic control of frost tolerance in wheat (Triticum aestivum L.)

Summary The frost tolerance of winter wheat is one component of winter hardiness. If seedlings are frost resistant, it means that they can survive the frost effect without any considerable damage. To study the genetic control of frost tolerance, an artificial freezing test was used. Frost tolerance is controlled by an additive-dominance system. The results of diallel analyses indicate the importance of both additive and non-additive gene action in the inheritance of this character. The dominant genes act in the direction of lower frost tolerance and the recessive genes in the direction of a higher level of frost tolerance. The results of monosomic and substitution analyses show that at least 10 of the 21 pairs of chromosomes are involved in the control of frost tolerance and winter hardiness. Chromosomes 5A and 5D have been implicated most frequently. The geneFr1 (Frost 1) was located on the long arm of chromosome 5A. Crosses between cultivars, chromosome manipulation and the induction of somaclonal variation may be suitable methods for broadening the gene pool for frost tolerance.     

Response of four winter wheat cultivars to triallate

Summary Genetic variation among four winter wheat cultivars for response to treatment with triallate was examined. The cultivars showed differential tolerance based on stand count, however, differential tolerance was not seen for any of the other traits examined including visual score, and the mature plant traits, tiller count and yield. Significant effects due to increasing the rate of chemical were observed. An increase in yield was seen at the 0.5 and 1.0 kg/ha rates of triallate. This yield increase was paralleled by trends toward increasing seed number/head and tiller number/unit area. The results indicate that the effect of triallate on wheat has two components. The first component, toxicity, is seen on all cultivars, but the extent of damage varies significantly among cultivars. The second component is positive and leads to an increase in yield without interacting significantly with genotype.     

Genetic control of Russian wheat aphid (Diuraphis noxia) resistance in wheat accession PI 47545

The Russian wheat aphid, Diuraphis noxia (Mordvilko), is a major pest of cereal crops in many areas of the world, causing serious reduction in grain yield in wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.). Incorporating genetic resistance to D. noxia into wheat cultivars is paramount to effectively reduce damage inflicted by this pest. Genetic resistance to D. noxia has been identified in wheat, barley and rye germplasm, and several resistance genes are available for use for cultivar improvement. In the United States of America, only a few Russian wheat aphid (RWA) resistant winter wheat cultivars are currently available, and these cultivars contain only one of the six known RWA resistance genes. The objective of this study was to determine the inheritance of RWA resistance in wheat accession PI 47545, using a screening method based on differences in the leaf morphology of resistant and susceptible types following insect challenge. PI 47545 was selected for study, since it displayed high levels of resistance in a white-grained wheat background, the predominant wheat class produced in the Pacific Northwest of the USA. Segregation analysis was conducted on an F₂ population developed by cross-hybridizing the susceptible soft white winter wheat cultivar ‘Daws’ to the resistant accession PI 47545. Russian wheat aphid screening data from this population indicated that the resistance in PI 47545 is controlled by a single, dominant gene (χ² = 1.72; p ≤ 0.189). This revised version was published online in August 2006 with corrections to the Cover Date.     

Combining ability analysis for harvest index in winter wheat

Summary Cereal breeders have used harvest index (HI) as a selection criterion in segregating generations to identify physiologically superior lines with improved partitioning of total assimilate into grain. Information on combining ability for HI of the hard red winter wheat (Triticum aestivum L.) cultivars of the Southern Great Plains is not available. A study was undertaken to examine HI of seven genetically diverse winter wheat parents, evaluate their general combining ability (GCA) and specific combining ability (SCA) effects, and study correlations of HI with several agronomic traits. The seven parents were crossed in a half-diallel mating design to produce 21 crosses. The F₁'s, their F₂ progenies, and the parents were evaluated in replicated field tests at Stillwater and at Lahoma, OK. The combining ability analysis was performed using Griffing's Method 4, Model 1. The results showed significant variation among parents for HI. The GCA and the SCA effects were mostly inconsistent between generations and between environments. However, parents with consistently high HI and positive GCA estimates were identified. The progeny with high HI mostly resulted from parents with high GCA estimates. The correlations between HI and agronomic traits indicated that improvement in HI should also result in high grain yield, early maturity, and short plant height.     

Genotypic differences in wheat Al tolerance

Summary Aluminium tolerance of 83 genotypes from Croatian and Yugoslav Triticum aestivum germplasm was evaluated in nutrient solutions having Al³⁺ activities of 0, 12.5 and 25 M. Relative root length (25 M Al³⁺/0 Al) of various genotypes ranged from 2 to 97% (from very sensitive to tolerant to Al). No genotype with Al tolerance close to that of very tolerant cultivar Atlas-66 was found. Soil, climatic, fertilization, and liming effects that wheat plants giving seeds for the nutrient solution Al-tolerance screening had been subjected to during their growth cycle did not influence the Al-tolerance ranking. Significant correlation was found between screening wheat for Al tolerance in nutrient solutions and in acid Pseudogley soil amended with five rates of limestone in a greenhouse experiment. Seed protein concentration was significantly related to the Al-tolerance ranking (r² = 0.962). Such a significant correlation was not obtained in a case of rheological and other quality characteristics of seeds. Al-tolerant wheat genotypes identified in this study will be used in breeding for improved Al tolerance.Abbreviations HSD Tukey's Honestly Significant Difference - RRL-2 relative root length, in % (12.5 M Al³⁺/0 Al) - RRL-4 relative root length, in % (25 M Al³/0 Al)     

Genetic variation for nitrogen use efficiency in winter wheat (Triticum aestivum L.)

Summary The new European Common Agricultural Policy and environmental considerations are certainly to change agricultural practices toward low input cultivation systems. Nitrogen is one of the main inputs of winter wheat in northern France and it contributes highly to phreatic water pollution. A research programme has then been set up in order to study whether it is possible to breed for winter wheat cultivars using more efficiently N fertilisers. Less nitrogen would be applied, decreasing pollution risks and operational costs. It has been shown that a large variation exists for N related traits and for the resistance against N deficiency. On the one hand the cv Arche is very resistant to N deficiency, its yield on low N conditions (with no N fertiliser) is on average 89% of its yield on high N conditions (with a high N application). On the other hand, cv Récital is very susceptible to N deficiency as this same percentage is only 61%. A study on 10 hybrids showed that heterosis for grain yield was higher at low N level than at high N level. This was due to a higher number of grains per m².     

Genetic changes in resistance to environmental stresses by U.S. Great Plains wheat cultivars

Increased grain yield potential of newcultivars of wheat (Triticum aestivumL.) is attributed to morphological traits,but actual yield is determined mostly byenvironmental conditions. Our objectivewas to ascertain the contribution ofresistance to freezing, high temperature,drought, and defoliation to advances inyield of landmark cultivars of winter wheatin the U.S. Great Plains. Eight cultivarsthat represented significant improvementbetween 1874 and 1994 were compared bystandard electroconductivity measurementsof stability of seedling cell membranes tofreezing, high temperature, and desiccationand by grain yield and its components inplants subjected to freezing during theseedling stage and to high temperature,drought, and defoliation during maturation. Genetic changes relative to `Turkey'(introduced 1874) in stability of cellmembranes to freezing, high temperature,and desiccation were small andinconsistent. Advances in grain yieldunder control conditions were similar togains in field studies. Most cultivars hadlittle genetic change in yield afterfreezing, drought, or defoliation, probablybecause high levels of resistance areincompatible with high yield potential andthe stresses are episodic. Genetic advancein grain yield under high temperature wasphenotypically correlated with change inyield under control conditions, suggestingthat the trait is essential forproductivity because of the ubiquitousoccurrence of the stress in the region. Weconcluded that changes in resistances tofreezing, drought, and defoliationcontributed little to advances inproductivity of winter wheat in the GreatPlains, but that resistance to hightemperature was important for new cultivars.     

Heat tolerance for yield and its components in different wheat cultivars

Summary Twenty one diverse, standard and experimental cultivars of common spring wheat (Triticum aestivum L.) were tested for the effect of heat stress on phenology, yield and its components by growing the materials for 2 years under full irrigation during the hot summer (offseason), and the cool winter (normal) conditions. Heat tolerance was estimated for each variable by the heat susceptibility index (S) which scales the reduction in cultivar performance from cool to hot conditions relative to the respective mean reduction over all cultivars.Genotypes differed significantly in S for yield and its components. The ranking of cultivars in S over the 2 years was consistent for yield, kernels per spike and kernel weight, but not for spike number. Of the three yield components, the greatest genotypic variation in S was expressed for kernels per spike. However, S for yield could not be simply attributed to S in a unique component across all cultivars. On the other hand, a general linear model regression of summer yield on its components revealed that the most important yield component affecting yield variation among cultivars under heat stress was kernel number per spike. Kernel number per spike was positively associated across cultivars with longer duration and greater stabilty of thermal time requirement from emergence to double ridge. It is therefore concluded that kernel number per spike under heat stress is a reasonable estimate of heat tolerance in yield of wheat and that this tolerance is operative already during the first 2 to 3 weeks of growth.     

Quantitative and molecular characterization of heat tolerance in hexaploid wheat

Understanding the genetic basis of tolerance to high temperature is important for improving the productivity of wheat (Triticum aestivum L.) in regions where the stress occurs. The objective of this study was to estimate inheritance of heat tolerance and the minimum number of genes for the trait in bread wheat by combining quantitative genetic estimates and molecular marker analyses. Two cultivars, Ventnor (heat-tolerant) and Karl92 (heat-susceptible), were crossed to produce F₁, F₂, and F₃populations, and their grain-filling duration (GFD) at 30/25 ^°C 16/8 h day/night was determined as a measure of heat tolerance. Distribution of GFD in the F₁ and F₂ populations followed the normal model (χ², p > 0.10). A minimum of 1.4 genes with both additive and dominance effects, broad-sense heritability of 80%, and realized heritability of 96%for GFD were determined from F₂ and F₃ populations. Products from 59primer pairs among 232 simple sequence repeat (SSR) pairs were polymorphic between the parents. Two markers, Xgwm11 andXgwm293, were linked to GFD by quantitative trait loci (QTL) analysis of the F₂ population. The Xgwm11-linked QTL had only additive gene action and contributed 11% to the total phenotypic variation in GFD in the F₂population, whereas the Xgwm293-linked QTL had both additive and dominance action and contributed 12% to the total variation in GFD. The results demonstrated that heat tolerance of common wheat is controlled by multiple genes and suggested that marker-assisted selection with microsatellite primers might be useful for developing improved cultivars. This revised version was published online in August 2006 with corrections to the Cover Date.     

On defining a winter wheat

Summary An international survey of wheat breeders and scientists was conducted to determine whether there was a common understanding of the term winter wheat. Response to photoperiod was mentioned by only seven of the 30 survey respondents. It was consequently suggested that it be excluded from any definition. Response to vernalization was mentioned by 28 of the 30 respondents. However, there was considerable variation in the method of quantifying this response and the degree of response necessary for wheat to be called winter. It was concluded that the only objective and internationally consistent definition of winter wheat was a genetic definition based on the complement of Vrn genes a wheat possesses. Varieties not possessing any of the dominant Vrn alleles appear to take a distinctively longer and more variable time to head when grown under non-vernalizing, long day conditions. These are genetically winter wheats.     

Comparison of phenotypic and molecular marker-based classifications of hard red winter wheat cultivars

Genetic diversity is the basis for successful crop improvement and can be estimated by different methods. The objectives of this study were to estimate the genetic diversity of 30 ancestral to modern hard red winter wheat (Triticum aestivum L.) cultivars adapted to the Northern Great Plains using pedigree information, morphological traits (agronomic measurements from six environments), end-use quality traits (micro-quality assays on 50 g grain or milled flour samples for the six environments), and molecular markers (seed storage proteins separated using SDS-PAGE, 51 SSRs, and 23 SRAP DNA markers), and to determine the relationships of genetic distance estimates obtained from these methods. Relationships among diversity estimates were determined using simple (Pearson) and rank (Spearman) correlation coefficients between distance estimates and by clustering cultivars using genetic-distances for different traits. All methods found a wide range in genetic diversity. The genetic distance estimates based on pedigree had the highest values due to possible over-estimation arising from model assumptions. The genetic diversity estimates based on seed storage protein were lowest because they were the major determinants of end-use quality, which is a highly selected trait. In general, the diversity estimates from each of the methods were positively correlated at a low level with the exceptions of SRAP diversity estimates being independent of morphologic traits (simple correlation), SDS-PAGE, and SSR diversity estimates (rank correlation). However, SSR markers, thought to be among the most efficient markers for estimating genetic diversity, were most highly correlated with seed storage proteins. The procedures used to accurately estimate genetic diversity will depend largely upon the tools available to the researcher and their application to the breeding scheme.     

Genetic analysis of in vitro wheat somatic embryogenesis

Summary A spring wheat genotype which produces somatic embryos in vitro, after short and long-term culture, was tested for its ability to sexually transmit this embryogenic trait. Reciprocal crosses were performed between a embryogenic line and a nonembryogenic variety.Immature embryos were cultured on Murashige and Skoog medium plus 2 mg/l 2,4-dichlorophenoxyacetic acid, gelled with 5.5 g/l agarose. Somatic embryogenesis was not expressed in the F₁'s. In contrast, from several hundred immature embryos of the F₂ generation of one cross, 10.7% and 1.6% expressed somatic embryogenesis in short and long-term cultures respectively. These percentages of embryogenic: non-embryogenic fits a model of a few complementary genes. The embryogenic capacity of the F₂ genotypes depends on the presence of recessive alleles at these gene loci. The long-term wheat somatic embryogenesis capacity requires a more complex mechanism than the short-term one.Abbreviations CS Chinese Spring - Aq Aquila - E Embryogenic - NE Nonembryogenic - SC Subculture     

Genetic improvement trends in agronomic performances and end-use quality characteristics among hard red winter wheat cultivars in Nebraska

Evaluation of wheat cultivars from different eras allows breeders to determine changes in agronomic and end-use quality characteristics associated with grain yield and end-use quality improvement over time. The objective of this research was to examine the trends in agronomic and end-use quality characteristics of hard red winter wheat cultivars grown in Nebraska. Thirty historically important and popular hard red winter wheat cultivars introduced or released between 1874 and 2000 were evaluated at Lincoln, Mead and North Platte, Nebraska in 2002 and 2003. An alpha lattice design with 15 incomplete blocks of two plots and three replications was used at all locations. Agronomic (days to flowering, plant height, spike length, culm length, grain yield and yield components, and grain volume weight) and end-use quality (flour yield, SDS-sedimentation value, flour protein content, and mixograph time and tolerance) traits were measured in each environment. Highly significant differences were observed among environments, genotypes and their interactions for most agronomic and end-use quality characteristics. Unlike modern cultivars, older cultivars were low yielding, and less responsive to favorable environments for grain yield and yield components. Semidwarf cultivars were more stable for plant height than traditional medium to tall cultivars. All cultivars had high grain volume weight since it is part of the grading system and highly selected for in cultivar release. Modern cultivars were less stable than older cultivars for SDS-sedimentation and mixing tolerance. However, the stability of older cultivars was attributed to their having weak mixing tolerance and reduced SDS-sedimentation values. The reduced protein content of modern cultivars was offset by increased functionality, as measured by mixograph and SDS sedimentation. In conclusion, breeders have tailored agronomic and end-use quality traits essential for hard red winter wheat production and marketing in Nebraska.     

Diallel analysis of resistance to head blight caused by Fusarium culmorum in winter wheat

Summary Ten homozygous winter wheat genotypes representing different levels of resistance to Fusarium head blight were crossed in all possible combinations excluding reciprocals. Parents, F₁ and F₂ were inoculated with one pathogenic strain of Fusarium culmorum. Data for head blight, observed 21 days after first inoculation (OBS-2), and for the area under the disease progress curve, based on observations 14, 21 and 28 days after first inoculation (AUDPC), were analyzed. The contrast between parents and F₁ crosses indicated dommance effects of the resistance genes. Diallel analysis according to Griffing's Method 4, Model 1 showed significant general combining ability (GCA) effects for both F₁ and F₂; specific combining ability effects were not significant. With the exception of one genotype for which general performance for Fusarium resistance was not in agreement with its GCA, the resistance to F. culmorum was uniformly transmitted to all offspring, and the parents can be described in terms of GCA. It is suggested that in the progenies with one of the awned lines as parent, one resistance gene was linked with the gene coding for presence of awns, located on chromosome 4B. A single observation date, taken at the right time, was as effective in assessing resistance as the AUDPC.     

An evaluation of seed and seedling drought tolerance screening tests in wheat

Summary A series of experiments was performed in order to evaluate the significance of seed germination and seedling growth in osmotic media as screening methods for drought tolerance.Ten spring wheat (Triticum aestivum L. em Thell.) and one durum wheat (Triticum durum Desf.) were tested under controlled environments, using polyethylene glycol-6000 (PEG) solutions as the moisture stress inducing media.Tolerance in the rate of endosperm utilization, under stress, prior to the onset of germination varied among cultivars.Germination rate or injury to germination at various concentrations of PEG differed significantly among cultivars. Cultivar rating with respect to injury to germination changed with stress levels. Injury to germination did not correlate with endosperm utilization rate in PEG or in water.Germinating seedlings were tolerant to extreme desiccation up to the stage of emergence of the first leaf from the coleoptile.Growth of photosynthesizing seedlings was monitored as they were carried through an increasing concentration gradient of PEG solutions, ranging from –5.9 to –11.3 bars of water potential. Cultivars significantly differed in seedling growth tolerance to increasing levels of water stress. Seedling growth tolerance across cultivars was not correlated with their germination responses under srress.It is concluded that tolerance to water stress in growing seedlings can be screened for by using PEG-containing nutrient solutions. It can not be predicted from germination tests in osmotica.Work was done under a US-Israel Binational Science Foundation (BSF) Grant no. 1654/78.Contribution from Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel, No. 192-E, 1979 series.     

Genetic control of esterase-6 isozymes in hexaploid wheat and rye

Summary The EST-6 leaf esterase phenotypes from euploid, nullisomic-tetrasomic and rye chromosome addition and substitution lines of common wheat were determined using polyacrylamide gel electrophoresis. Evidence is presented to demonstrate that Est-6 is a new set of genes, that are expressed in the leaf. The Est-6 gene set were clearly distinguished from the Est-5 genes which are expressed in the grain. The three homoeoallelic loci, Est-A6, Est-B6 and Est-D6, were located on chromosomes 3A, 3B and 3D. An Est-R6 gene was located on chromosome 6R is involved in rye. Some considerations concerning homoeology between homoeologous group 3 of wheat and the rye chromosome 6R are made.     

Genetic analysis and selection for wheat yield in drought-stressed and irrigated environments

Summary Wheat (Triticum aestivum L.) cultivars grown in the southern Great Plains of the U.S.A. are exposed to a wide range of moisture conditions due to large fluctuations in the amount and frequency of rainfall. Yield stability under those conditions is therefore a desirable trait for wheat breeders. Our primary objective was to quantify various genetic parameters for grain production in drought-stressed and irrigated environments. We also attempted to predict and measure yield responses when selection is practiced in either drought-stressed or irrigated environments, or both. Seventy F₂-derived lines from the cross, TAM W-101/Sturdy, were evaluated at Goodwell, OK, under irrigated and naturally drought-stressed conditions in 1987 and 1988. Genetic variance and heritability estimates were higher in the irrigated environment than in the drought-stressed environment. The genetic correlation coefficient for yields in the two environments was 0.20±0.16, indicating that selection of widely adapted genotypes requires testing in both environments. Based on the genetic variance/covariance structure of this particular population, the linear index which maximized the combined expected gain in both environments was 0.66Y₁ + 0.34Y₂, in which Y₁ and Y₂ are yields in the irrigated and drought-stressed environments. This index is not expected to apply across all populations; rather, it further supports the hypothesis that testing in either environment alone (drought stressed or irrigated) may not be most effective for increasing either mean productivity or yield under drought stress.     

Genetic control of photoperiodic sensivity and maturity in spring wheat within narrow limits of adaptation

Summary Photoperiodic respose, as assessed by a regression technique, exhibited complete dominance averaged over the crosses of an eight parent diallel in the vernalized condition. Photoperiodic response as final leaf number for the vernalized 8-hour photoperiod diallel was closely related to photoeriodic response of the regression method. However, the diallel analyses of both sets of data showed little agreement in terms of respectieve array positions.The inheritance of photoperiodic response in diallels using regression values showed little agreement between the vernalized and unvernalized conditions. This difference was postulated to be due to interaction of vernalization and photoperiodic response in the unvernalized situation. In the unvernalized condition photoperiodic response exhibited non-allelic interaction, attributable mainly to the cultivar Pinnacle in general behaviour in its crosses. Its removal gave a situation of high average dominance for photoperiodic response with a clear indication that high photoperiodic sensitivity was dominant to comparative insensitivity.Days to ear emergence (vernalized and 18-hour photoperiod) exhibited non-allelic interaction in its expression, due mainly to the general behaviour of the cultivar Pinnacle in its crosses. Removal of its array gave a situation of a moderately strong degree of overdominance in the expression of days to ear emergence. Maturity differences amongst parents and F₁'s, vernalized and under 18-hour photoperiod, are postulated to be due to a factor other than vernalization or photoperiodic response beheved to be growth temperature in differentially in fluencing growth and/or developmental rates between genotypes.