M. van Ginkel

COMPARISON OF LEAF, SPIKE, PEDUNCLE AND CANOPY TEMPERATURE DEPRESSION IN WHEAT UNDER HEAT STRESS

Abstract This study examines genotype×organ–temperature depression (TD) interactions and whether differences in plant morphology influence organ-TD and its correlation with canopy temperature depression (CTD) and grain yield. Field experiments were conducted with 13 spring wheat genotypes planted on three dates in the 2000–2001 winter cropping cycle in NW Mexico. Surface temperatures of flag leaves, peduncles, spikes and canopy were measured with a hand-held infrared thermometer. Morphological and yield components were also measured. Results indicated that there is genetic variability for organ-TD. CTD showed strong positive correlations with organ-TD and grain yield. Organ-TD and CTD were positively correlated with leaf area index, and CTD was probably little affected by leaf rolling. Spike temperature was generally higher than leaf temperature, but lower than ambient air temperature. The interactions between grain yield and spike-TD and CTD were not significant. Results of this study indicate that CTD does not mask confounding interactions between organ temperatures and thus can be used reliably to measure TD during grain filling under heat stress conditions.

Bringing wild relatives back into the family: recovering genetic diversity in CIMMYT improved wheat germplasm

SummaryThe dangers of a narrow genetic base of the worlds major domesticated food crops have become a great global concern in recent decades. The efforts of the International Maize and Wheat Improvement Center (CIMMYT) to breed common wheat cultivars for resource poor farmers in the developing world (known as the Green Revolution wheats) has met with notable success in terms of improved yield, yield stability, increased disease resistance and utilization efficiency of agricultural inputs. However, much of the success was bought at the cost of an overall reduction in genetic diversity in the species; average Modified Rogers distances (MRD) within groups of germplasm fell from 0.64 in the landraces to a low of 0.58 in the improved lines in the 1980s. Recent efforts by CIMMYT breeders to expand the genetic base of common wheat has included the use of landraces, materials from other breeding programs, and synthetic wheats derived from wild species in the pedigrees of new advanced materials. The result, measured using SSR molecular markers, is a highly significant increase in the latent genetic diversity of recently developed CIMMYT breeding lines and cultivars compared to the original Green Revolution wheats (average MRD of the latest materials (0.63) is not significantly different from that of the landraces, as tested using confidence intervals). At the same time, yield and resistance to biotic and abiotic stresses, and end-use quality continue to increase, indicating that the Green Revolution continues to this day.

Evaluation of selection strategies for wheat adaptation across water regimes

Drought frequently constrains production of wheat (Triticum aestivum L.), but development of tolerant cultivars is hampered by low heritability for drought tolerance and a lack of effective selection strategies. Our objective was to identify an optimum selection regime for wheat in drought-prone environments. Six-hundred entries derived from 10 crosses were developed by selection under continuous high moisture, alternating high with low moisture, alternating low with high moisture, and continuous low moisture conditions for five generations. The selections were evaluated in two low-yield, a medium-yield, and a high-yield environment in the Yaqui Valley, Sonora, Mexico. The mean performance of entries derived from a particular selection regime was dependant on the stress level of the evaluation environment. Lines developed and selected under continuous high moisture and continuous low-moisture regimes produced the highest mean yields in the low moisture evaluation environment. There was no relationship between continuous selection under either high yielding conditions or low yielding conditions and the mean performance of the resultant lines in their respective high and low yielding evaluation environments. The mean yield of lines selected using the alternating high/low moisture regime as well as the five highest yielding lines were superior in the HY environment, and had similar performance with other regimes under the low yielding evaluation environment. Our results indicate that alternating selection between high and low yielding environments is the most effective way to develop wheat germplasm adapted to environments where intermittent drought occurs.

Plant traits related to yield of wheat in early, late, or continuous drought conditions

Bread wheats (Triticum aestivum L.) were evaluated for plant characteristics contributing to grain yield and plant adaptation under various drought patterns. The usefulness of these traits as explicit selection criteria in developing drought tolerant wheat varieties was investigated in three experiments. Cultivars from four germ-plasm groups, representing the four relevant major and distinct global wheat growing environments, were grown under the respective simulated early, late, continuous and no drought conditions by manipulating irrigation in north western Mexico. Additionally, 560 advanced lines from the CIMMYT breeding program were grown under late drought conditions, and 16 randomly selected advanced genotypes were studied in more detail under late and no drought conditions. In these three studies, the association between yield in drought-stressed environments and yield in non drought-stressed environments was interpreted to reflect genotypic high yield potential, mainly by way of high biomass development. However, yield potential only partly explained the superior performance under drought. For each pattern of drought stress, particular and often different plant traits were identified that further contributed specific adaptation to the distinct drought stress conditions. Knowledge of these traits will be useful for developing CIMMYT germplasm for specific drought-stressed areas. Ultimately, these studies demonstrate that both yield potential and specific adaptation traits are useful criteria in breeding for drought environments, and should be combined to achieve optimum performance and adaptation to drought stress.

Plant regeneration from immature embryos of 48 elite CIMMYT bread wheats

Forty-eight bread wheat (Triticum aestivum L.) released cultivars and elite advanced lines were evaluated for their ability to produce embryogenic callus using three different media. Basal N6 medium supplemented with dicamba (E1), MS medium containing 2,4-D (E3) or MS medium containing 2,4-D plus different amino acids (E5) were used for callus initiation and maintenance. Plant regeneration was achieved on basal MS medium with indole-3-acetic acid (IAA) and 6-benzylamino purine (BAP) and rooting on MS with 1-naphthaleneacetic acid (NAA). Percentage regeneration varied widely with both genotype and initiation medium, with values ranging from 2% to 94%. The number of plantlets produced per embryo ranged from 6 to 42. Thirteen genotypes showed at least 50% regeneration after culture on E5 medium; 3 genotypes after culture on E3 initiation medium and 1 after initiation on E1. After four subcultures, over a 16-week period, 41 genotypes (85%) lost their ability to regenerate plants while the remaining 7 lines (15%) retained plant regeneration potential but at reduced levels. E3 medium was found to be the best for maintaining regeneration potential after four subcultures.

Importance of P uptake efficiency versus P utilization for wheat yield in acid and calcareous soils in Mexico

Abstract There are large agricultural areas in the world where wheat yields are limited by low phosphorus (P) availability. Breeding for P uptake and P utilization efficiency may reduce this problem. This study was conducted to determine the contribution of P uptake and utilization efficiency to grain yield of selected spring wheat genotypes in different environments. Thirty-eight semidwarf spring bread wheat (Triticum aestivum) genotypes were grown in two experiments in Mexico, each on an acid Andisol under rainfed conditions and on a calcareous Aridisol with irrigation, without (−P) and with 35 kg P per ha fertilized (+P). Without P fertilization, grain yield ranged from 0.8 to 4.6 t ha−1 in the acid soil and from 2.4 to 5.2 t ha−1 in the calcareous soil. With P fertilization, this range was even larger. Under conditions of P deficiency, i.e. in the acid soil at −P and +P (high P adsorption) and calcareous soil at −P (P-depleted soil), P uptake explained 71–100% of the variation in grain yield, and was highly correlated with grain yield (r=0.79–0.95). In contrast, at +P in the calcareous soil, P utilization efficiency explained 60–63% of the variation in grain yield. Here, low grain P concentration was related to high grain yield (r=−0.40 to −0.59). In the calcareous soil, the harvest index was correlated with grain yield, irrespective of the P level. In the acid soil, post-anthesis P accumulation was important. It was positively correlated with grain yield, whereas in the calcareous soil, no post-anthesis-P accumulation occurred. Here, grain P accumulation at maturity was completely determined by translocation of pre-anthesis shoot P. We conclude that the combination of improved P uptake and P utilization efficiency in the same genotypes requires selection under both high and low-P conditions.

Expression and inheritance of tolerance to waterlogging stress in wheat

Approximately 10 million hectares of wheat (Triticum aestivum L.) globally experience medium toserious waterlogging. The inheritance of waterloggingtolerance was determined in reciprocal crosses ofthree tolerant (Prl/Sara, Ducula and Vee/Myna), andtwo sensitive (Seri-82 and Kite/Glen) spring breadwheat lines. Parents, F1, F2, F3, and backcrossgenerations were studied under field conditions in Cd.Obregon, State of Sonora, Mexico. Flooding was appliedwhen plants were at the three-leaf and first-internodestages. Basins were drained after 40 days of flooding.Leaf chlorosis was used as a measure of waterloggingtolerance. The sensitive by sensitive cross, Seri-82 × Kite/Glen, showed the highest mean values forpercentage leaf chlorosis and area under chlorosisprogress curve (AUCPC), and the lowest mean values forplant height, biomass, grain yield, and kernel weight.The F3 of the cross between the two tolerant parentsDucula and Vee/Myna had the lowest mean values forpercentage leaf chlorosis and AUCPC, and the highestmean values for plant height, biomass, and grainyield. The expression of waterlogging tolerance wasnot influenced by a maternal effect. The F1 hybridswere intermediate for leaf chlorosis, indicating thattolerance was additive. Quantitative analysis alsoindicated that additive gene effects mainly controlledwaterlogging tolerance in these crosses. Segregationratios of F3 lines indicated that up to four genescontrolled waterlogging tolerance in these crosses,with two genes adequate to provide significanttolerance. Early-generation selection for tolerancewould be effective in these populations.

The Potential of Using Spectral Reflectance Indices to Estimate Yield in Wheat Grown Under Reduced Irrigation

SummaryThe objectives of this research were to study the association in bread wheat between spectral reflectance indices (SRIs) and grain yield, estimate their heritability, and correlated response to selection (CR) for grain yield estimated from SRIs under reduced irrigation conditions. Reflectance was measured at three different growth stages (booting, heading and grainfilling) and five SRIs were calculated, namely normalized difference vegetation index (NDVI), simple ratio (SR), water index (WI), normalized water index-1 (NWI-1), and normalized water index-2 (NWI-2). Three field experiments were conducted (each with 30 advanced lines) in three different years. Two reduced irrigation environments were created: (1) one-irrigation level (pre-planting), and (2) two-irrigation level (pre-planting and at booting stage), both representing levels of reduced moisture. Maximum yield levels in the experimental zone were generally obtained with 4–6 irrigations. Genotypic variations for all SRIs were significant. Three NIR (near infrared radiation) based indices (WI, NWI-1, and NWI-2) gave the highest level of association (both phenotypic and genotypic) with grain yield under both reduced irrigation environments. Use of the mean SRI values averaged over growth stages and the progressive integration of SRIs from booting to grainfilling increased the capacity to explain variation among genotypes for yield under these reduced irrigation conditions. A higher level of broad-sense heritability was found with the two-irrigation environment (0.80) than with the one-irrigation environment (0.63). Overall, 50% to 75% of the 12.5% highest yielding genotypes, and 50% to 87% of the 25% highest yielding genotypes were selected when the NWI-2 index was applied as an indirect selection tool. Strong genetic correlations, moderate to high heritability, a correlated response for grain yield close to direct selection for grain yield, and a very high efficiency of selecting superior genotypes indicate the potential of using these three SRIs in breeding programs for selecting increased genetic gains in grain yield under reduced irrigation conditions.

A diagnostic molecular marker allowing the study of Th. intermedium-derived resistance to BYDV in bread wheat segregating populations

Abstract Barley yellow dwarf (BYD) is the most important viral disease of small cereal grains. True resistance to the disease is not found in wheat (Triticum aestivum L.), but it has been introgressed from Thinopyrum intermedium (Ti) on chromosome 7DL of recombinant wheat lines designated TC. The objectives of our study were to identify a high through-put scoring tool for the presence of the translocated Th. intermedium fragment and to assess its suitability for evaluating resistance to BYDV in segregating populations. Segregation of the Ti fragment was followed in the F2 population of an Anza (bread wheat) by TC14/2*Spear (TC14) cross. Resistance to BYDV isolates PAV-Mex and MAV-Mex in F3, F4, and F5 populations was evaluated under field and/or greenhouse conditions by measuring the virus titers of infected plants using ELISA, and the number of infected plants per plot. The SSR marker gwm37 was polymorphic for the translocation. In F4 lines it was associated with the physical presence of an intact translocation on chromosome 7DL and with low virus titers of BYDV-PAV. Reductions in virus titer of 27% and 55% in the F3 and 18% and 45% in the F5 populations were observed when the fragment was present in the heterozygous and homozygous states, respectively, confirming a dosage effect of the resistance allele. A lower proportion of infected individuals in the field was associated with the presence of the fragment, indicating a mechanism that may interfere with aphid feeding or virus translocation within infected plants. Despite significant differences between groups with and without the fragment, the OD values of infected lines overlapped, and it was not possible to definitively detect the fragment based solely on ELISA. We conclude that gwm37 is a reliable marker for the Ti translocation that will allow efficient detection of the translocation in breeding populations and greatly assist in selecting BYDV-resistant wheats in the absence of the disease.

Quantifying novel sequence variation and selective advantage in synthetic hexaploid wheats and their backcross-derived lines using SSR markers

Synthetic hexaploid wheats (SHWs) and synthetic backcross-derived lines (SBLs) obtained from them are novel sources of useful traits for broadening the diversity in breeding germplasm of hexaploid bread wheat (Triticum aestivum). Fifty-one EST-derived and 39 genomic-derived microsatellite markers (SSRs) covering the A, B, and D genomes were used to assess the genetic diversity present in 11 SHWs, their backcross derived families, and their durum and bread wheat parents and to test for the selective advantage of SHW alleles in SBL families after several generations of selection. The 90 SSR markers amplified 91 loci with 474 alleles across all genotypes. In many of the SHWs, novel alleles were observed which were stably inherited in the SBL families. Gene diversity, the average number of alleles per locus, cluster analysis, and principal coordinate analysis revealed a high level of genetic diversity in the Aegilops tauschii and durum parents of the SHWs, and also in the SBLs. In the latter, alleles from the SHW parent had a selective advantage for six SSR markers. This indicates that SHWs and SBLs are a valuable resource for broadening the genetic base of elite wheat breeding germplasm. Fingerprinting of SBLs and their corresponding SHW and bread wheat parents, and testing for selective advantage of SHWs alleles promises to be a useful method for detecting chromosomal regions of interest for bread wheat improvement.