Seedling survivability as a selection criterion for drought tolerance in wheat

Ninety genotypes of wheat (Triticum aestivum L.) were screened at the seedling stage in wooden boxes in greenhouse conditions (range of temperature 25‐35°C) for moisture stress. Boxes were filled with a mixture of soil : sand : FYM in a 50 : 45 : 5 ratio. Boxes were given equal quantities of water 12 h before sowing to ensure good germination. Seeds were sown in rows at a uniform depth of 3 cm. No irrigation was provided after sowing. When most of the genotypes started wilting, the boxes were irrigated to study the recovery response (seedling survivability) of the genotypes. Based on the days taken for recovery, wheat genotypes, JWS 98, HD 2329, HW 3081, Halna and MP 1136 withered early and were grouped as susceptible, while the genotypes HI 1494, HW 2044, Kundan, NIAW 588, PBW 514 and NI 5439 resumed growth, showed a better response and were classified as drought‐tolerant. The study on mode of inheritance revealed that seedling survivability is controlled by a single dominant gene.     

Method for screening bread wheat for tolerance to boron

A new method for screening wheat for boron tolerance has been developed to overcome the problems of methods used in the past. Seedlings were grown for 10 days while suspended over B-rich solutions. Their root lengths were then measured as an index of tolerance. Five varieties of wheat were screened at seven concentrations of B. Results were obtained more quickly and easily than from the alternative methodologies and compared favourably. Screening of 128 doubled haploid lines from a cross between Halberd and (Wl*MMC) suggested that transgressive segregation had occurred. This revised version was published online in August 2006 with corrections to the Cover Date.     

Evaluation of chlorophyll fluorescence as a tool for the identification of drought tolerance in upland cotton

Chlorophyll fluorescence (CF) is one tool used by researchers to quantify plant water status during periods of limited water availability. The research reported herein was designed to evaluate a CF-based protocol as a tool for use in cotton, Gossypium spp. breeding programs for the identification of drought tolerant genotypes. Twenty genotypes were selected to represent diverse and distinct US germplasm pools. Replicated tests were performed in Lubbock and College Station, TX in 2006 and 2007. Dryland and irrigated treatments, as main plots, were applied in a randomized complete block design, split to genotypes. CF measurements were taken at mid-bloom and late bloom growth stages. Source leaf tissue was harvested at predawn and subjected to high temperature incubation with CF measurements subsequently taken hourly for 5 h. Drought stressed plants had not mobilized their carbohydrate reserves from their source leaves overnight and thus maintained cell viability and therefore higher CF values throughout the incubation and measurement period with the opposite being true for non-stressed plants. Fiber lint yield and fiber properties were measured for comparison with the CF data. Genotype × treatment effects complicated the classification of genotypic response to drought. Few and inconsistent correlations were found among CF values and lint yield or fiber properties. Data suggested that this procedure provides little potential in selecting plants for drought tolerance when plants are grown under field culture.     

Use of chlorophyll fluorescence to evaluate the cold acclimation and freezing tolerance of winter and spring oats

The efficiency of the excitation capture by open Photosystem II (PSII) reaction centres was measured by the F_v/F_m ratios in a collection of winter and spring oats in order to assess the effects of hardening and freezing on the functionality of PSII and also the suitability of a chlorophyll fluorescence‐based method to screen oat cultivars for frost tolerance. A significant reversible decrease in F_v/F_m was found in all genotypes during acclimation to low, non‐freezing temperatures. F_v/F_m analysis appears to be an attractive test for the evaluation of frost tolerance in oats, being rapid, non‐invasive and capable of monitoring a trait related to a crucial stage in the acquisition of frost tolerance. It is more sensitive and precise than other standard methods and highly correlated with field‐evaluated frost damage. The measurements made during recovery 1 or 2 days after stress when the visual symptoms are not yet expressed, were especially advantageous because of the large variability in genotype response. The r‐values (close to 0.8) were reduced due to the non‐standard behaviour of the winter cultivar ‘Aintree’. The cold acclimation response of this genotype has been analysed in detail and the limits of artificial freezing tests are discussed.     

Studies on somaclonal variants for resistance to scab in bread wheat (Triticum aestivum L.) through in vitro selection for tolerance to deoxynivalenol

This study was conducted to develop an efficient in vitro selection system for scab resistance by using in vitro screening for tolerance to deoxynivalenol (DON). Immature embryos of two wheat varieties, a scab-resistant variety Sumai 3 and a susceptible variety Mianyang 11, and their reciprocal F₁ hybrids were cultured on MS medium supplemented with 2,4-D 2 mg/l and 0.6 × 10^-4 M DON for callus induction. The responses of callus induction and plant regeneration to 0.6 × 10^-4 M DON differed significantly between resistant and susceptible varieties, according to observed scab resistance levels at the plant level in the field. The percentage of callus formation of resistant variety Sumai 3 on induction medium containing DON was higher than that of susceptible variety Mianyang 11. Regeneration of DON-tolerant calli on DON-containing differentiation medium differed significantly between Sumai 3 and Mianyang 11. Averaged across the DON-tolerant calli of two varieties and their reciprocals, regeneration of DON-tolerant calli was decreased 3-fold on DON-containing medium. By an inoculation test with conidiospores of Fusarium graminearum Schw, we obtained several resistant lines from progenies of regenerated plants from DON-tolerant calli. These somaclonal lines had lower disease scoring (reaction index, infected spikelets and disease incidence), shorter plants and better yield components than Sumai 3, a famous Chinese resistant variety. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evidence of a major genetic factor conditioning freezing sensitivity in winter wheat

The inheritance of freezing tolerance in F₂‐derived F₄ populations from all possible crosses of winter wheat cultivars ‘Kestrel’, ‘Eltan’, ‘Tiber’, ‘Froid’ and germplasm line Oregon Feed Wheat #5 (ORFW) was investigated. When frozen to a temperature equal to the LT₅₀ of the least freezing tolerant parent (ORFW), survival frequency distributions were skewed to greater survival in six of the 10 crosses, however, very few of the progeny from the four crosses to ORFW survived. The inheritance of this freezing sensitivity was investigated with freezing of F_2:4 populations from the crosses of ORFW to ‘Eltan’ or ‘Tiber’ to the LT₅₀ of the hardier parent. Very few of the F_2:4 populations survived as well as ‘Eltan’ or ‘Tiber’, indicating a small number of strongly dominant genetic factors in ORFW that conditioned freezing sensitivity. Molecular analysis indicated these factors were not spring‐type vernalization alleles.     

Divergent selection for sprouting resistance in spring wheat

The development of sprouting-resistant spring-wheat ( Triticum aestivum L.) cultivars is a major breeding objective in many wheat-producing regions. Sprouting resistance is thought to be associated with delayed maturity. The primary objective of this study was to measure the reciprocal effects of selection for sprouting resistance and maturity. Two experiments were conducted over a 3–4-year period in Saskatoon, Canada. In the first experiment, two populations of hard red spring wheat were subjected to divergent selection (k = 10%) for maturity. In the second experiment, six populations derived from crosses between two sprouting-resistant, late-maturing, white-grained cultivars ('AUS1293' and 'AUS1408') and three early maturing, red–grained cultivars ('Park', 'PT516' and 'Roblin'), were subjected to divergent selection (k = 10%) for sprouting resistance. Selection for earliness reduced sprouting resistance in one population but had no effect in the second. For both populations, earlier maturity was associated with higher test weight but lower grain yield. In the second experiment, selection for increased sprouting resistance was effective, with realized heritabilities averaging 0.74. Increased sprouting resistance was associated with a slight delay (1–2.5 days) in time to spike emergence in four out of six populations, but had little effect on time to maturity in most populations. There was a trend towards redder grain in the sprouting-resistant selections. The recovery of sprouting-resistant, early maturing segregants was relatively low, averaging less than 10% over the six populations. In conclusion, selection for increased sprouting resistance can result in delayed maturity, but the magnitude of that delay will vary among populations.     

Evaluation of a semi‐controlled test as a selection tool for frost tolerance in durum wheat (Triticum durum)

Most durum wheat (Triticum durum) varieties possess only low winter hardiness due to their frost susceptibility. In North America and Central Europe, durum wheat is therefore typically sown in spring to circumvent the local winter conditions. However, the yield potential of durum in these regions could be much better exploited if durum varieties with increased frost tolerance were available, which could be sown in autumn. A factor limiting breeding for increased frost tolerance is the variation in the occurrence of frost stress across years. The ‘Weihenstephaner Auswinterungsanlage’ is a semi‐controlled test that exposes the plants to all weather conditions. Snow coverage of the plants, serving as frost protection, is prevented by the movable glass lid of the semi‐controlled test. In this study, different scorings for frost tolerance based on this semi‐controlled test were evaluated and compared with frost tolerance data in the field. Our results illustrate the potential of the ‘Weihenstephaner Auswinterungsanlage’ as an indirect selection tool for frost tolerance in durum breeding programmes, especially when regular frost tolerance data from the field are not available.     

Reciprocal substitutions analysis of freezing resistance in wheat (Triticum aestivum L.)

To investigate the effects of individual chromosomes on freezing resistance, as well as their interactions with the genetic background, reciprocal sets of chromosome substitution lines between two hard red winter wheat cultivars, ‘Cheyenne’ and ‘Wichita’, were used in this study. Duplicate lines for each chromosome were included to check background homogeneity. Two experiments were carried out in complete block designs with two replications for each duplicate. Crown and leaf water content and leaf wet weight were measured in the field experiments. Crown survival, electrolyte leakage and 50% lethality temperature (LT50) were measured in the laboratory. The results showed that ‘Cheyenne’ was more resistant than ‘Wichita’. Crown survival was significantly correlated with crown water content, crown wet weight and electrolyte leakage. Chromosomes 6A, 3B and 5D substituted from ‘Wichita’ into ‘Cheyenne’ (‘CNN‐WI’), decreased the crown survival, and increased membrane stability, crown water content and crown wet weight of ‘Cheyenne’. Thus, these chromosomes from ‘Wichita’ decreased freezing resistance in ‘Cheyenne’. Reciprocally, chromosomes 5A, 5D, 3B and 4D from ‘Cheyenne’ into ‘Wichita’ increased crown survival and decreased crown water content and crown wet weight of ‘Wichita’. It was concluded that these chromosomes from ‘Cheyenne’ cause freezing resistance in ‘Wichita’ and carry freezing‐resistance genes.     

Quantitative trait loci mapping of freezing tolerance and photosynthetic acclimation to cold in winter two‐ and six‐rowed barley

Photoacclimation (PA) and freezing tolerance (FT) have been identified as closely related traits, due to common mechanisms of environmental control. In this study, diversity array technology (DArT) was used for identification of the quantitative trait loci (QTL) of FT and PA in winter barley. Simple sequence repeat (SSR) and single nucleotide polymorphism (SNP) markers were subsequently used to saturate QTL regions. Two F₂ mapping populations were created, for two‐rowed (P44) and six‐rowed barley (CaP). Different regions of the genome were responsible for differences in traits between parents in these two populations. Eleven QTLs were identified in the P44 population, including five typical for FT and PA, on chromosomes 2H, 3H and 7H. In the CaP population, only one QTL connected with PA and 10 connected with FT were found on all chromosomes except 2H. Our results demonstrate that different sets of markers should be applied in marker‐assisted selection for FT in two‐ and six‐rowed barley, as several loci determine FT at the level of biparental crosses.     

QTL mapping and marker-assisted selection for Fusarium head blight resistance in wheat: a review

During the past decade, numerous studies have been published on molecular mapping of Fusarium head blight (FHB) resistance in wheat. We summarize the relevant findings from 52 quantitative trait loci (QTL) mapping studies, nine research articles on marker-assisted selection and seven on marker-assisted germplasm evaluation. QTL for FHB resistance were found on all wheat chromosomes except chromosome 7D. Some QTL were found in several independent mapping studies indicating that such QTL are stable and therefore useful in breeding programmes. We summarize and update current knowledge on the genetics of FHB resistance in wheat resulting from QTL mapping investigations and review and suggest FHB breeding strategies based on the available information and DNA markers.     

An assessment of the salt tolerance of wheat/Thinopyrum bessarabicum 5Eb addition and substitution lines

Lines of Triticum aestivum cv. Chinese Spring carrying an additional chromosome 5E^b from Thinopyrum bessarabicum or having chromosome 5A or 5D replaced by chromosome 5E^b were screened in hydroculture for tolerance to salt. The previously reported tolerance of the 5E^b addition line was confirmed and the two substitution lines were shown to have a higher level of survival in 175 mol/m³ NaCl than both the addition and the ‘Chinese Spring’ parent. Reasons for the better tolerance of the substitutions are discussed.     

Recurrent selection using male sterility and hydroponic tiller culture in pedigree breeding of wheat

A procedure for recurrent selection based on the male sterility gene, Ms3, was implemented. To facilitate the production of large numbers of hybrid progeny, a simple hydroponic system was developed in which male‐sterile tillers cut at the flowering stage can be pollinated and maintained for about 8 weeks‐long enough to produce a large quantity of viable hybrid seeds. The recurrent selection steps were integrated with a pedigree breeding programme employing different selection cycles for male and female plants. F₁ female plants are subjected to a single screening for seedling resistance. In addition to F₁ seedling screening, F₂‐F₄ male families are field‐selected for disease resistance, agrotype and quality in a pedigree programme before being used in crosses.     

Fluorescence in situ hybridization polymorphism using two repetitive DNA clones in different cultivars of wheat

Twenty‐two wheat cultivars and a wheat line were analysed with two‐colour fluorescence in situ hybridization (FISH) using the pSc119.2 and pAs1 repetitive DNA clones to detect if polymorphism could be observed in the hybridization patterns of different wheat cultivars. The FISH hybridization pattern of ‘Chinese Spring’ was compared with wheat cultivars of different origins. Differences were observed in the hybridization patterns of chromosomes 4A, 5A, 1B, 2B, 3B, 5B, 6B, 7B, 1D, 2D, 3D and 4D. Although a low level of polymorphism exists in the FISH pattern of different wheat cultivars, it is possible to identify 17 pairs of chromosomes according to their hybridization patterns with these two probes. This study will help to predict the expected variation in the FISH pattern when analysing wheat genetic stocks of different origin. It is presumed that variation in hybridization patterns are caused by chromosome structural rearrangements and by differences in the amount and location of repetitive sequences in the cultivars analysed.     

Computer simulation of a selection strategy to accommodate genotype environment interactions in a wheat recurrent selection programme

Multi-environment trials (METs) are used in plant breeding programmes to evaluate genotypes (lines/families) as a basis for selection on expected performance (yield and/or quality) in a target population of environments (TPE). When a large component of the genotype environment (G × E) interactions results from crossover interactions, samples of environments in METs that deviate from the TPE provide a suboptimal basis for selection of genotypes on performance expected in the TPE. To adjust for the negative effects of these deviations, a selection strategy that weights the data from the MET according to their expected frequency of occurrence in the TPE (i.e. a weighted selection strategy) was investigated. Computer simulation methodology was used to obtain preliminary information on the weighted selection strategy and compare it to the traditional unweighted selection strategy for a range of MET scenarios and G × E interaction models. The evaluation of the weighted selection strategy was conducted in context with the germplasm enhancement programme (GEP) of the Northern Wheat Improvement Programme in Australia. The results indicated that when the environments sampled in the MET matched those expected in the TPE, the unweighted and weighted selection strategies achieved a similar response to selection in the TPE. However, when the environments sampled in the MET did not match the expectations in the TPE and a large component of the G × E interactions resulted from crossover interactions, the weighted selection strategy achieved a greater response to selection in the TPE. The advantage of the weighted strategy increased as the amount of crossover G × E interaction increased or fewer environments were sampled in the METs.     

大穗小麦多小穗基因的染色体定位

采用中国春单体系列对大穗普通小麦品系“88F2185”的多小穗性状进行了基因定位研究。结果表明，“88F2185”决定多小穗的基因位于其1B、3D和5A染色体上，其中3D染色体的效应最强。“88F2185”1B和3D染色体上的基因表现显性，而5A染色体上的基因表现隐性。此外，“88F2185”4D染色体上还存在减少小穗数目的隐性基因。据前人研究及本试验结果分析认为，“88F2185”5”的1B及4D染色体上具控制小穗数目的新基因。     

A YA-type cytoplasmic male-sterile source in common wheat

A new cytoplasmic male‐sterile (CMS) system for hybrid wheat breeding, YA‐type CMS line with the cytoplasmic mutant from the common wheat variety ‘CA8057’, was developed by the Institute of Genetics and Developmental Biology, Chinese Academy of Sciences. The pollen sterility of YA‐type CMS line was easily maintained but difficult to restore. Some sterile lines with desirable agronomic performance, such as msYA‐‘CA8057’ (BC₁₇), msYA‐‘Yuandong 6’ (BC₉), msYA‐‘Jin 411’ (BC₉), msYA‐‘WL1’ (BC₁₀), msYA‐‘Yanshi 9’ (BC₁₀), msYA‐‘BPm16’ (BC₉), msYA‐‘Jindong 8’ (BC₉) and msYA‐‘Jinmai 33’ (BC₉), were bred and a restorer line GR1 was screened with 26 new restorer lines being developed by transferring restorer genes from GR1. It was found that abnormal phenomena occurred at the uninucleate‐pollen stage and the abortive pollen was poor in starch content and other components. The variance analysis of agronomic traits in eight sterile lines indicated that there was no general negative effect of cytoplasm. The genetic analysis for fertility restoration showed that two pairs of independent major genes (designated YARf₁YARf₁YArf₂YArf₂) and some minor genes could be involved in the fertility restoration in restorer line GR1, and YARf₁ was epistatic over YARf₂ for the genetic effect of fertility restoration. As a new CMS system, the YA‐type CMS line was of potential value for hybrid wheat breeding and should be further studied.     

Mass selection of wheat for grain filling without transient photosynthesis

Summary Post-anthesis chemical desiccation of wheat (Triticum aestivum L.) plants in the field eliminates transtent photosynthesis by killing all green tissues, thus revealing the plant's capacity for grain filling from stored stem reserves, as the case is for post-anthesis stress such as drought or leaf diseases. This study was conducted to investigate whether mass selection for large kernels under chemical desiccation would lead to the improve ment of grain filling in the absence of transient photosynthesis.Six crosses of common spring wheat were subjected to three cycles of mass selection from F₂ through F₁ when selection was performed for large kernels by sieving grains from plants that were erther chemically desiccated after anthesis, or not (controls). The resulting 36 bulks (six crosses by three selection cycles by two selection environments) were compared with their respective F₂ base populations, when tested with and without chemical desiccation.Selection for large kernels under potential conditions (without chemical desiccation) did not improve kernel weight under potnetial conditions, evidently because these materials were lacking in genetic variation for kernel weight under potential conditions. In four of the crosses, 3rd cycle selection for large kernels under potential conditions decreased kernel weight under chemical desiccation. On the other hand, selection for large kernels under chemical desiccation was effective in improving kernel weight and test weight under chemical desiccation, depending on the cross and the selection cycle, with no genetic shift in mean days to heading or mean plant height. Selection for large kernels under chemical desiccation was also effective in some cases in increasing kernel weight under potential conditions. The results are interpreted to show that selection under potential conditions and under chemical desiccation operate on two different sources for grain filling, namely transient photosynthesis and stem reserve utilization, respectively. In order to expose genetic variability for stem reserve utilization to selection pressure, transient photosynthesis must be eliminated, as done by chemical desiccation in this study.