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Genotypic variation in ear morphology is linked to differences in photosynthetic potential to influence grain yield in winter cereals. Awns contribute to photosynthesis, particularly under water-limited conditions when canopy assimilation is restricted. We assessed performance of up to 45 backcross-derived, awned–awnletted NILs representing four diverse genetic backgrounds in 25 irrigated or rainfed, and droughted environments in Australia and Mexico. Mean environment grain yields were wide-ranging (1.38–7.93 t ha−1) with vegetative and maturity biomass, plant height, anthesis date, spike number, and harvest index all similar (P >0.05) for awned and awnletted NILs. Overall, grain yields of awned–awnletted sister-NILs were equivalent, irrespective of yield potential and genetic background. Awnletted wheats produced significantly more grains per unit area (+4%) and per spike (+5%) reflecting more fertile spikelets and grains in tertiary florets. Increases in grain number were compensated for by significant reductions in grain size (−5%) and increased frequency (+0.8%) of small, shrivelled grains (‘screenings’) to reduce seed-lot quality of awnletted NILs. Post-anthesis canopies of awnletted NILs were marginally warmer over all environments (+0.27 °C) but were not different and were sometimes cooler than awned NILs at cooler air temperatures. Awns develop early and represented up to 40% of total spikelet biomass prior to ear emergence. We hypothesize that the allocation of assimilate to large and rapidly developing awns decreases spikelet number and floret fertility to reduce grain number, particularly in distal florets. Individual grain size is increased to reduce screenings and to increase test weight and milling quality, particularly in droughted environments. Despite the average reduction in grain size, awnless lines could be identified that combined higher grain yield with larger grain size, increased grain protein concentration, and reduced screenings.

We aim to incorporate deep root traits into future wheat varieties to increase access to stored soil water during grain development, which is twice as valuable for yield as water captured at younger stages. Most root phenotyping efforts have been indirect studies in the laboratory, at young plant stages, or using indirect shoot measures. Here, soil coring to 2 m depth was used across three field environments to directly phenotype deep root traits on grain development (depth, descent rate, density, length, and distribution). Shoot phenotypes at coring included canopy temperature depression, chlorophyll reflectance, and green leaf scoring, with developmental stage, biomass, and yield. Current varieties, and genotypes with breeding histories and plant architectures expected to promote deep roots, were used to maximize identification of variation due to genetics. Variation was observed for deep root traits (e.g. 111.4–178.5cm (60%) for depth; 0.09–0.22cm/°C day (144%) for descent rate) using soil coring in the field environments. There was significant variation for root traits between sites, and variation in the relative performance of genotypes between sites. However, genotypes were identified that performed consistently well or poorly at both sites. Furthermore, high-performing genotypes were statistically superior in root traits than low-performing genotypes or commercial varieties. There was a weak but significant negative correlation between green leaf score (–0.5), CTD (0.45), and rooting depth and a positive correlation for chlorophyll reflectance (0.32). Shoot phenotypes did not predict other root traits. This study suggests that field coring can directly identify variation in deep root traits to speed up selection of genotypes for breeding programmes. Variation in deep root traits among wheat genotypes were identified in the field using high-throughput soil coring. Some weakly significant relationships were found between deep root traits and above-ground surrogates.

There is a pressing need to improve the water-use efficiency of rain-fed and irrigated crop production. Breeding crop varieties with higher water-use efficiency is seen as providing part of the solution. Three key processes can be exploited in breeding for high water-use efficiency: (i) moving more of the available water through the crop rather than it being wasted as evaporation from the soil surface or drainage beyond the root zone or being left behind in the root zone at harvest; (ii) acquiring more carbon (biomass) in exchange for the water transpired by the crop, i.e. improving crop transpiration efficiency; (iii) partitioning more of the achieved biomass into the harvested product. The relative importance of any one of these processes will vary depending on how water availability varies during the crop cycle. However, these three processes are not independent. Targeting specific traits to improve one process may have detrimental effects on the other two, but there may also be positive interactions. Progress in breeding for improved water-use efficiency of rain-fed wheat is reviewed to illustrate the nature of some of these interactions and to highlight opportunities that may be exploited in other crops as well as potential pitfalls. For C3 species, measuring carbon isotope discrimination provides a powerful means of improving water-use efficiency of leaf gas exchange, but experience has shown that improvements in leaf-level water-use efficiency may not always translate into higher crop water-use efficiency or yield. In fact, the reverse has frequently been observed. Reasons for this are explored in some detail. Crop simulation modelling can be used to assess the likely impact on water-use efficiency and yield of changing the expression of traits of interest. Results of such simulations indicate that greater progress may be achieved by pyramiding traits so that potential negative effects of individual traits are neutralized. DNA-based selection techniques may assist in such a strategy.

Background and AimsRoot length and depth determine capture of water and nutrients by plants, and are targets for crop improvement. Here we assess a controlled-environment wheat seedling screen to determine speed, repeatability and relatedness to performance of young and adult plants in the field.MethodsRecombinant inbred lines (RILs) and diverse genotypes were grown in rolled, moist germination paper in growth cabinets, and primary root number and length were measured when leaf 1 or 2 were fully expanded. For comparison, plants were grown in the field and root systems were harvested at the two-leaf stage with either a shovel or a soil core. From about the four-leaf stage, roots were extracted with a steel coring tube only, placed directly over the plant and pushed to the required depth with a hydraulic ram attached to a tractor.Key ResultsIn growth cabinets, repeatability was greatest (r = 0·8, P < 0·01) when the paper was maintained moist and seed weight, pathogens and germination times were controlled. Scanned total root length (slow) was strongly correlated (r = 0·7, P < 0·01) with length of the two longest seminal axile roots measured with a ruler (fast), such that 100–200 genotypes were measured per day. Correlation to field-grown roots at two sites at two leaves was positive and significant within the RILs and cultivars (r = 0·6, P = 0·01), and at one of the two sites at the five-leaf stage within the RILs (r = 0·8, P = 0·05). Measurements made in the field with a shovel or extracted soil cores were fast (5 min per core) and had significant positive correlations to scanner measurements after root washing and cleaning (>2 h per core). Field measurements at two- and five-leaf stages did not correlate with root depth at flowering.ConclusionsThe seedling screen was fast, repeatable and reliable for selecting lines with greater total root length in the young vegetative phase in the field. Lack of significant correlation with reproductive stage root system depth at the field sites used in this study reflected factors not captured in the screen such as time, soil properties, climate variation and plant phenology.

Greater yield per unit rainfall is one of the most important challenges in dryland agriculture. Improving intrinsic water-use efficiency (W(T)), the ratio of CO(2) assimilation rate to transpiration rate at the stomata, may be one means of achieving this goal. Carbon isotope discrimination (Delta(13)C) is recognized as a reliable surrogate for W(T) and there have now been numerous studies which have examined the relationship between crop yield and W(T) (measured as Delta(13)C). These studies have shown the relationship between yield and W(T) to be highly variable. The impact on crop yield of genotypic variation in W(T) will depend on three factors: (i) the impact of variation in W(T) on crop growth rate, (ii) the impact of variation in W(T) on the rate of crop water use, and (iii) how growth and water use interact over the crop's duration to produce grain yield. The relative importance of these three factors will differ depending on the crop species being grown and the nature of the cropping environment. Here we consider these interactions using (i) the results of field trials with bread wheat (Triticum aestivum L.), durum wheat (T. turgidum L.), and barley (Hordeum vulgare L.) that have examined the association between yield and Delta(13)C and (ii) computer simulations with the SIMTAG wheat crop growth model. We present details of progress in breeding to improve W(T) and yield of wheat for Australian environments where crop growth is strongly dependent on subsoil moisture stored from out-of-season rains and assess other opportunities to improve crop yield using W(T).

PCR-based markers were developed to detect the point mutations responsible for the two major semi-dwarfing genes Rht-B1b ( Rht1) and Rht-D1b ( Rht2) in wheat. These markers were validated by testing 19 wheat varieties of known Rht genotype. They included Rht-B1b and Rht-D1b dwarfs, double-mutant varieties and tall wheats. These were correctly genotyped with the Rht-B1b and Rht-D1b-specific primers, as well as markers specific for the tall alleles Rht-B1a and Rht-D1a. Using a family of doubled-haploid lines segregating for Rht-B1b and Rht-D1b, the markers were mapped to the expected homoeologous regions of chromosomes 4B and 4D, respectively. Both markers were strongly correlated with a reduction in height, accounting for 23% ( Rht-B1b) and 44% ( Rht-D1b) of the phenotypic variance in the population. These markers will have utility in marker-assisted selection of the Rht-B1b and Rht-D1b genes in wheat breeding programs.

Leaf rust and stripe rust are important diseases of wheat world-wide and deployment of cultivars with genetic resistance is an effective and environmentally sound control method. The use of minor, additive genes conferring adult plant resistance (APR) has been shown to provide resistance that is durable. The wheat cultivar ‘Pastor’ originated from the CIMMYT breeding program that focuses on minor gene-based APR to both diseases by selecting and advancing generations alternately under leaf rust and stripe rust pressures. As a consequence, Pastor has good resistance to both rusts and was used as the resistant parent to develop a mapping population by crossing with the susceptible ‘Avocet’. All 148 F5 recombinant inbred lines were evaluated under artificially inoculated epidemic environments for leaf rust (3 environments) and stripe rust (4 environments, 2 of which represent two evaluation dates in final year due to the late build-up of a new race virulent to Yr31) in Mexico. Map construction and QTL analysis were completed with 223 polymorphic markers on 84 randomly selected lines in the population. Pastor contributed Yr31, a moderately effective race-specific gene for stripe rust resistance, which was overcome during this study, and this was clearly shown in the statistical analysis. Linked or pleiotropic chromosomal regions contributing to resistance against both pathogens included Lr46/Yr29 on 1BL, the Yr31 region on 2BS, and additional minor genes on 5A, 6B and 7BL. Other minor genes for leaf rust resistance were located on 1B, 2A and 2D and for stripe rust on 1AL, 1B, 3A, 3B, 4D, 6A, 7AS and 7AL. The 1AL, 1BS and 7AL QTLs are in regions that were not identified previously as having QTLs for stripe rust resistance. The development of uniform and severe epidemics facilitated excellent phenotyping, and when combined with multi-environment analysis, resulted in the relatively large number of QTLs identified in this study.

Opportunities exist for replacing reduced height (Rht) genes Rht-B1b and Rht-D1b with alternative dwarfing genes for bread wheat improvement. In this study, the chromosomal locations of several height-reducing genes were determined by screening populations of recombinant inbred lines or doubled haploid lines varying for plant height with microsatellite markers. Linked markers were found for Rht5 (on chromosome 3BS), Rht12 (5AL) and Rht13 (7BS), which accounted for most of the phenotypic variance in height in the respective populations. Large height differences between genotypes (up to 43 cm) indicated linkage to major height-reducing genes. Rht4 was associated with molecular markers on chromosome 2BL, accounting for up to 30% of the variance in height. Confirming previous studies, Rht8 was linked to markers on chromosome 2DS, whereas a population varying for Rht9 revealed a region with a small but significant height effect on chromosome 5AL. The height-reducing effect of these dwarfing genes was repeatable across a range of environments. The molecular markers developed in this study will be useful for marker-assisted selection of alternative height-reducing genes, and to better understand the effects of different Rht genes on wheat growth and agronomic performance.

Key messageGreater embryo size in a large and carefully phenotyped mapping population was genetically associated with a greater number of longer seminal roots to increase grain yield in droughted field environments.Breeding modification of root architecture is challenging in field environments owing to genetic and phenotypic complexity, and poor repeatability with root sampling. Seeds from a large mapping population varying in embryo size were harvested from a common glasshouse and standardised to a common size before assessing in rolled germination paper at 12 and 20 °C for seedling growth. Differences in genotype means were large and heritabilities high (h2 = 0.55–0.93) indicating strong and repeatable genotypic differences for most root traits. Seminal roots 1 to 3 were produced on all seedlings, whereas growth of seminal roots 4, 5 and 6 was associated with differences in embryo size. Increases in seminal root number from 4 to 6 per plant were strongly, genetically correlated with increases in total seminal length (rg = 0.84, < 0.01). Multivariate analysis confirmed initiation and growth of seminal roots 1, 2 and 3, and of roots 4, 5 and 6 behaved as genetically independent (rPg = 0.15 ns) cohorts. Tails representing extremes in seedling root length and number were associated with significant differences in grain yield of up to 35% in droughted field environments but were not different in irrigated environments. Increases in grain yield were linked to greater lengths of seminal roots 4, 5 and 6 and were largely independent of plant height or development. This is the first report on the genetic relationship of seedling root architecture and embryo size, and potential in selection of seminal root size for accessing deep-soil moisture in droughted environments.

Wheat productivity is commonly limited by a lack of water essential for growth. Carbon isotope discrimination (Δ), through its negative relationship with transpiration efficiency, has been used in selection of higher wheat yields in breeding for rainfed environments. The potential also exists for selection of increased Δ for improved adaptation to irrigated and high rainfall environments. Selection efficiency of Δ would be enhanced with a better understanding of its genetic control. Three wheat mapping populations (Cranbrook/Halberd, Sunco/Tasman and CD87/Katepwa) containing between 161 and 190 F₁-derived, doubled-haploid progeny were phenotyped for Δ and agronomic traits in 3-5 well-watered environments. The range for Δ was large among progeny (c. 1.2-2.3[per thousand]), contributing to moderate-to-high single environment (h ² = 0.37-0.91) and line-mean (0.63-0.86) heritabilities. Transgressive segregation was large and genetic control complex with between 9 and 13 Δ quantitative trait loci (QTL) identified in each cross. The Δ QTL effects were commonly small, accounting for a modest 1-10% of the total additive genetic variance, while a number of chromosomal regions appeared in two or more populations (e.g. 1BL, 2BS, 3BS, 4AS, 4BS, 5AS, 7AS and 7BS). Some of the Δ genomic regions were associated with variation in heading date (e.g. 2DS, 4AS and 7AL) and/or plant height (e.g. 1BL, 4BS and 4DS) to confound genotypic associations between Δ and grain yield. As a group, high Δ progeny were significantly (P < 0.10-0.01) taller and flowered earlier but produced more biomass and grain yield in favorable environments. After removing the effect of height and heading date, strong genotypic correlations were observed for Δ and both yield and biomass across populations (r g = 0.29-0.57, P < 0.05) as might be expected for the favorable experimental conditions. Thus selection for Δ appears beneficial in increasing grain yield and biomass in favorable environments. However, care must be taken to avoid confounding genotypic differences in Δ with stature and development time when selecting for improved biomass and yield especially in environments experiencing terminal droughts. Polygenic control and small size of individual QTL for Δ may reduce the potential for QTL in marker-assisted selection for improved yield of wheat.