Genetic loci linking improved heat tolerance in wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) to lower leaf and spike temperatures under controlled conditions

Heat stress adversely affects wheat production in many regions of the world and is particularly detrimental during reproductive development. The objective of this study was to identify novel quantitative trait loci (QTL) associated with improved heat tolerance in wheat (Triticum aestivum L.) and to confirm previous QTL results. To accomplish this, a recombinant inbred line (RIL) population was subjected to a three-day 38°C daytime heat stress treatment during early grain-filling. At maturity, a heat susceptibility index (HSI) was calculated from the reduction of three main spike yield components; kernel number, total kernel weight, and single kernel weight. The HSI, as well as temperature depression (TD) of the main spike and main flag leaf during heat stress were used as phenotypic measures of heat tolerance. QTL analysis identified 14 QTL for HSI, with individual QTL explaining from 4.5 to 19.3% of the phenotypic variance. Seven of these QTL co-localized for both TD and HSI. At all seven loci, the allele for a cooler flag leaf or spike temperature (up to 0.81°C) was associated with greater heat tolerance, indicated by a lower HSI. In a comparison to previous QTL results in a RIL population utilizing the same source of heat tolerance, seven genome regions for heat tolerance were consistently detected across populations. The genetic effect of combining three of these QTL, located on chromosomes 1B, 5A, and 6D, demonstrate the potential benefit of selecting for multiple heat tolerance alleles simultaneously. The genome regions identified in this study serve as potential target regions for fine-mapping and development of molecular markers for more rapid development of heat tolerant germplasm.     

QTL for yield, yield components and canopy temperature depression in wheat under late sown field conditions

A wheat (Triticum aestivum L.) recombinant inbred line (RIL) population was used to identify quantitative trait loci (QTL) associated with yield, yield components, and canopy temperature depression (CTD) under field conditions. The RIL population, consisting of 118 lines derived from a cross between the stress tolerant cultivar ‘Halberd’ and heat stress sensitive cultivar ‘Karl92’, was grown under optimal and late sown conditions to impose heat stress. Yield and yield components including biomass, spikes m^?2, thousand kernel weight, kernel weight and kernel number per spike, as well as single kernel characteristics were determined. In addition, CTD was measured during both moderate (32–33 °C) and extreme heat stress (36–37 °C) during grain-filling. Yield traits showed moderate to high heritability across environments with a large percentage of the variance explained by genetic effects. Composite interval mapping detected 25 stable QTL for the 15 traits measured, with the amount of phenotypic variation explained by individual QTL ranging from 3.5 to 27.1 %. Two QTL for both yield and CTD were co-localized on chromosomes 3BL and 5DL and were independent of phenological QTL. At both loci, the allele from Halberd was associated with both higher yield and a cooler crop canopy. The QTL on 3BL was also pleiotropic for biomass, spikes m^?2, and heat susceptibility index. This region as well as other QTL identified in this study may serve as potential targets for fine mapping and marker assisted selection for improving yield potential and stress adaptation of wheat.     

Evaluation of heat stress tolerance in irrigated environment of T. aestivum and related species. I. Stability in yield and yield components

Wheat production is often limited by continual or terminal heat stress. The current study was aimed at the characterization of wild relatives and cultivated Triticum species for their heat tolerance in yield and its analysis in relation to yield components which confer yield stability at the three ploidy levels. Thirty-two non-cultivated and cultivated genotypes belonging to diploid, tetraploid and hexaploid wheat species were evaluated for heat stress tolerance in the field under full irrigation. Wheat species were sown in the field(New Delhi, India; 77°12′E, 28°40′N, 228.6 m m.s.l) at two dates of sowing, November (normal) and January (late Sowing) during winter seasons of 1994–95 and 1995–96. The late sown crop experienced 3°C warmer temperatures than that of the normal sown crop. Wide variability was observed for grain yield stability under heat stress, as the heat susceptibility index (S) ranged from 0.13 to 2.08. Hexaploidy conferred the productive and adaptive advantages as it combined high yield and stability when compared to the tetraploid and diploid groups. However within each ploidy group wide variation was observed for heat tolerance. T. aestivum cv C306 & HI1136, T. dicoccoides, T. monococcum acc. BSP1 and Ae. speltoides ssp. liqustica were highly heat tolerant in their grain yield. Stability in grain no. m- 2 conferred yield stability in all three ploidy levels, although grain weight stability also contributed to yield stability in moderately stable T. turgidum and T. sphaerococcum under heat stress. Higher biomass and grain no. m-2 are the two important traits which could be considered potential selection criteria for yield under heat stress. Of the two components of grain no. m-2, stability in spike no. m-2could be considered more important trait than grain no. spike-1. Since wide variation for heat tolerance of all the yield components are available among the wheat species, these species can be used for improving specific yield components of cultivated wheat. This revised version was published online in July 2006 with corrections to the Cover Date.     

Mapping quantitative trait loci associated with grain filling duration and grain number under terminal heat stress in bread wheat (Triticum aestivum L.)

Terminal heat stress has the potential negative impact on wheat production across the world, especially in South Asia. Under the threat of terminal heat stress, wheat genotypes with stay green trait would suffer from high temperature stress during their long grain filling duration (GFD). The genotypes with short GFD would be advantageous. To identify quantitative trait loci (QTL) for heat tolerance, a RIL population of K 7903 (heat tolerant) and RAJ 4014 (heat sensitive) wheat genotypes was investigated under timely and late‐sown conditions. Heat susceptibility index of GFD, yield components and traits under late‐sown condition were used as phenotypic data for QTL identification. Stable QTLs associated with these traits were identified on chromosomes 1B, 2B, 3B, 5A and 6B. The LOD value ranged from 2.9 to 5.0 and the corresponding phenotyping variation explained ranged from 12.0–22%. QTL for heat susceptibility index for the grain filling duration were colocalized with QTL for productive tillers under late sown and GFD under late‐sown condition on chromosomes 1B and 5A, respectively. These genomic regions could be exploited for molecular wheat breeding programmes targeting heat tolerance.     

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.     

Identification of QTLs associated with heat tolerance at the heading and flowering stage in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

The ongoing rise in temperatures caused by global climate change is a critical climatic risk factor for rice production, and enhancing rice heat tolerance is an area of particular research interest. A recombinant inbred line (RIL) mapping population was developed from heat sensitive, rice cultivar IAPAR-9 crossed with heat tolerant, Liaoyan241. RIL and parental lines were exposed to high temperature at the heating and flowering stage in experiments in 2014 and 2015. As indicators of heat tolerance, the seed setting rate under natural (NS) and heat stress (HTS) conditions were measured, and the reduction rate of seed set (RRS) was calculated. Quantitative trait loci (QTL) analysis revealed eleven heat tolerance QTLs located on chromosomes 1, 3, 4, 5, and 6. Single QTL contribution rates were 4.75–13.81% and effect values were ? 5.98 to 5.00. Four major QTLs (qNS1, qNS4, qNS6, and qRRS1) were stable detected in different environments in both years. Thirteen QTLs with epistatic interactions and nine QTLs with environmental interactions were also detected. Major QTLs were all involved in epistatic and environmental interactions. Three QTLs from the SSR marker interval RM471 to RM177 region of chromosome 4 (qNS4, qHTS4, and qRRS4) were all involved in epistatic and environmental interactions and contributed to phenotypic variation, indicating that this region constituted a major QTL hotspot. The major QTL for heat tolerance identified in this study will aid in breeding tolerant cultivars and facilitating investigation of the molecular underpinnings of heat tolerance in rice.     

Mapping QTL for seed yield and canning quality following processing of black bean (<Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> L.)

Quantitative trait loci (QTL) analysis was conducted to identify QTL for seed yield and color retention following processing of a recombinant inbred line (RIL) black bean population. A population of 96 RILs were derived from the cross of black bean cultivars ‘Jaguar’ and 115M and evaluated in replicated trials at one location over 4 years (2004–2007) in Michigan. A 119-point genetic map constructed using simple sequence repeat (SSR), sequence related amplified polymorphism (SRAP), target region amplified polymorphism (TRAP) and phenotypic markers spanned fifteen linkage groups (LG) or 460 cM of the bean genome. Fourteen QTL for yield and color retention in four environments were identified by composite interval mapping on six linkage groups. A major QTL SY10.2^J115 for seed yield was identified on LG B10 with additional QTL on B3, B5, and B11. Color retention following processing was associated with loci on B1, B3, B5, B8, and B11. 115M possessed positive alleles for yield, but negative alleles for color retention. Some QTL for yield and color retention co-localized with regions identified in previous studies while others, particularly for color retention, were unique. Additional QTL for agronomic and canning quality traits were detected and individual contributions to future black bean breeding are discussed.     

Identification of QTLs controlling rice drought tolerance at seedling stage in hydroponic culture

‘Drought avoidance’ and ‘drought tolerance’ are two mechanisms by which plants adapt under water stress. These mechanisms are difficult to evaluate separately in field experiments. Using hydroponic culture, we studied the genetic control of drought tolerance in rice (Oryza sativa L.) without the effect of drought avoidance. A backcross inbred population of ‘Akihikari’ (lowland cultivar) × ‘IRAT109’ (upland cultivar) with 106 lines was cultured with (stressed condition) and without (non-stressed condition) polyethylene glycol (PEG) at seedling stage. The relative growth rate (RGR), specific water use (SWU), and water use efficiency (WUE) showed significant genotype × environment interactions with or without PEG, indicating that each line responded differently to water stress. A quantitative trait locus (QTL) analysis revealed that these interactions were QTL specific. A total of three QTLs on chromosomes 2, 4, and 7 were detected for RGR. The QTL on chromosome 7 had a constant effect across environments, while the QTL on chromosome 4 had an effect only under non-stressed condition and that on chromosome 2 only under stressed condition. The stress-specific QTL on chromosome 2 was not co-located with any QTLs for root system depth previously reported from the same mapping population. However, this QTL was co-located with a stress-specific QTL for SWU, suggesting that the control of transpiration was relevant to dry matter production under drought. We concluded that PEG-treated hydroponic culture is very effective for use in genetic analyses of drought tolerance at seedling stage.     

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.     

A strategy of ideotype development for heat-tolerant wheat

Heat stress significantly limits yield in many wheat-growing areas globally including north-western NSW. While various traits linked to high-temperature tolerance have been identified, the combination of traits that optimize the heat tolerance of wheat has not been established in most environments. A total of 554 genotypes were evaluated in the field at different times of sowing in north-western NSW for three consecutive years to develop a heat-tolerant wheat ideotype for this environment. The later sown experiments were exposed to higher temperatures at the critical reproductive and grain-filling stages of development. The impact of high temperature was greatest at anthesis, and eventual grain yield was reduced by between 4% and 7% with every 1°C rise in average maximum temperature above the optimum of 25°C. High temperature reduced yield, plant height, grain weight and days to anthesis and maturity, and increased the percentage of screenings and grain protein content. Genotypes that produced higher yield under heat stress had shorter days to flowering and maturity, higher NDVI during grain filling, greater chlorophyll content at the milk stage of grain fill, taller plants, greater grain weight and number, and lower screenings compared with the benchmark cultivar Suntop. The genotype closest to the predicted heat-tolerant wheat ideotype identified from trait ranges had 79.6% similarity.     

Exposure to heat stress during flowering period reduces flower yield and pyrethrins in Pyrethrum (Tanacetum cinerariifolium)

Heat stress is one of the major limitations to crop productivity worldwide. Global warming effects are expected to increase the number of hot days and increase the probability and intensity of heat stress events. Short periods (3–5 days) of heat stress with maximum temperatures exceeding 35°C often occur during late spring and early summer in some pyrethrum growing regions of Australia. These heat stress events usually coincide with pyrethrum flowering period. Pyrethrum is a perennial herbaceous plant which is commercially grown for extraction of pyrethrins which accumulate in the achenes of the flowers and are used as a natural insecticide. This experiment was conducted to understand the effects of timing of short periods of heat stress on flower development and pyrethrum yield. Plants were subjected to short periods of high temperature treatments (12 hr at 35–40°C) for three consecutive days at three flower maturity stages (early, mid, late). Control plants were grown at ambient temperature (10–25°C) throughout the flowering period. Exposure of pyrethrum plants to short periods of high temperature during the flowering period caused a significant reduction in the flower and pyrethrin yield. This was associated with the reduction in flower size and accelerated flower senescence. Exposure of pyrethrum plants to heat stress significantly increased the rate of flower development resulting in a shorter flowering period. Overall, plants grown under control treatment showed slower rate of flower development and longer duration flowering period. This resulted in longer duration of pyrethrin accumulation and higher yield of pyrethrins per flower. Timing and duration of heat stress significantly influenced pyrethrin yield per flower. Heat stress caused more severe yield reductions at early flowering than later in the flowering period. Research focusing on agronomic strategies, phenology and breeding for tolerance to heat stress is therefore important to cope with future climate changes and to obtain maximum pyrethrin yield.     

Several stably expressed QTL for spike density of common wheat (Triticum aestivum) in multiple environments

Spike density (SD), an important spike morphological trait associated with wheat yield, is the spikelet number per spike (SNS) divided by spike length (SL). In this study, phenotypic data from eight environments were collected and a recombinant inbred line population (RIL) constructed by the wheat line 20828 and the cultivar 'Chuannong16' and a Wheat55K SNP array-based constructed genetic linkage map were used to identify SD quantitative trait locus (QTL). Correlation between SD and other agronomic traits was calculated. Genes associated with plant growth and development for major loci were predicted. The results showed that 24 QTLs associated with SD were detected in eight environments. Among them, three major QTL, namely QSd.sicau-5B.2, QSd.sicau-2D.3 and QSd.sicau-4B.1, explained up to 35.62%, 14.21% and 11.23% of phenotypic variation, respectively. The positive alleles of them were all derived from 'Chuannong16'. The significant relationships between SD and other agronomic traits were detected and discussed. Taken together, the stably expressed SD QTL under different environments identified in this study provided theoretical guidance for further fine mapping and germplasm improvement.     

Using three selected overlapping RILs to fine-map the yield component QTL on Chro.D8 in Upland cotton

Cotton yield improvement is vital to fulfill rising global demands. The identification of major quantitative trait loci (QTL) for yield components was helpful in molecular marker-assisted selection (MAS) to improve cotton yield. We previously identified a densely populated QTL region for fiber qualities and yield components on chromosome D8 (Chro.D8) of Upland cotton from a (7235 × TM-1)RIL. In the present study, to fine-map yield component QTLs, we chose three overlapped recombinant inbred lines (RILs) with different intervals included the yield component QTLs, and backcrossed each line with TM-1 to develop three large sized mapping populations. Phenotypic data for yield components were collected in Nanjing (JES/NAU) and Xinjiang (BES/XJ) in 2006 and 2007. Three simple sequence repeat (SSR) genetic linkage maps on chro.D8 were constructed using 907 individuals in (7TR-133 × TM-1)F₂ (Pop A), 670 in (7TR-132 × TM-1)F₂ (Pop B), and 940 in (7TR-214 × TM-1)F₂ (Pop C). Three stable QTLs for boll size, two for lint percentage and one for boll number per plant,were detected on chro.D8 following analysis of three RIL backcrossed F₂/F_2:3 progeny at JES/NAU and BES/XJ although their cultivation practices differ greatly between these two cotton-growing regions. One QTL for boll number per plant exhibited a phenotypic variance (PV) of 5.6–10.1%, three QTLs for boll size exhibited 15.0–35.5% PV and two lint percentage QTLs exhibited 10.9–19.3% PV. Negative correlation between lint yield and fiber strength was confirmed.     

Genetic markers for doubled haploid response in barley

In order to analyse the genetic control of anther culture response in barley, a doubled-haploid (DH) population from the cross between a medium responsive cultivar ‘Dobla’ and the model cultivar ‘Igri’ was produced. A linkage map was constructed with 91 markers. A sub-population of 41 lines was characterised for different components of the anther culture response, and was used for quantitative trait loci (QTL) analysis. The vrs1 locus region on chromosome 2H, which determines inflorescence row type, was coincident with the largest putative QTL for number of embryos (nEMB) and albino plants. A region of chromosome 6H was associated with QTLs for nEMB and green plants. QTLs for number and percentage of green plants were located on the long arm of chromosome 5H. Therefore, new QTLs for main components of barley anther culture response were identified on chromosomes 2H, 5H and 6H, indicating that anther culture response in barley could be controlled by relative few genes of large effect. This work is a useful step towards the identification of new regions on the barley genome that could be associated with fundamental biological process implicated in the anther culture response.     

Grain development and endoreduplication in maize and the impact of heat stress

The post-anthesis development of growing maize kernels is strongly affected by heat stress. The maize cultivar “Spezi” was used to quantify this effect in kernels from 14 days after flowering until maturity. Day/night temperature of control plants was 25/20°C. Stress of 40/25°C was given for seven days or continuously up to maturity. Kernels were analysed weekly for dry matter and extractable DNA. In addition the ploidy levels and the DNA content in intact cell nuclei were determined by flow cytometry. The dry matter reduction started immediately after heat treatment and reached, at maturity, 40% for temporary heat stress and 60% for permanent heat stress. The reduction of extractable DNA started later and was not as extensive. Endopolyploidy was found in all kernel tissues, namely embryo, endosperm and pericarp. In endosperm, 3C nuclei reached their maximum number at approximately 14–17 days, and cells with higher ploidy levels between 21 and 26 days after flowering. Later on 6C nuclei were dominant. The DNA content in intact cells of the endosperm reached a maximum 21 days after flowering. This maximum was lower for heat stress variants and decreased more rapidly. Heat stress can vary from year to year under field conditions. Since heat stress changes the ratio between embryo and endosperm DNA in the direction of a higher portion of embryo DNA at maturity, this has an influence on the measured content of GM DNA from GM pollen transfer into conventional maize fields.     

Quantitative trait loci for seed dormancy in rice

Seed dormancy (SD) is controlled by its own complicated genetic factors and environmental factors. SD is an important trait affecting grain yield and quality in cereal crops. A population comprising 240 recombinant inbred lines (RIL) was used for detecting quantitative trait locus (QTL) for SD in rice. To minimize the effect of environment, data for lines for which the optimum temperature during the late ripening stage is either below 20°C or above 30°C were excluded from the analysis, which left 185 lines. In a dynamic germination test of the parents of the population, Minghui 63 showed clear SD and Zhenshan 97 showed none. The seeds of each RIL, harvested 32 days after heading, were divided into two lots: seeds in one lot were sown immediately, without any treatment to break their dormancy, whereas seeds in the other lot were sown after they had been exposed to dry heat (50°C) for 72 h. Composite interval mapping showed the presence of qDGE1, qDGR5a, qDGR5b, and qDGR7 in the first lot whereas only qDGR7 was detected in the second lot––which had been treated to break SD––indicating the strong influence of qDGR7 in controlling SD. A recently cloned Sdr4 is also located in the qDGR7 region. Coincidently, three rice homologues of Arabidopsis SD gene DOG1 were found in qDGE1 and qDGR5b regions where no SD QTL had been reported so far. These results indicate that the QTL found in this study are reliable, and that it would be worthwhile to clone qDGE1 and qDGR5b.     

QTLs for chlorophyll and chlorophyll fluorescence parameters in barley under post-flowering drought

Drought is one of the major factors limiting barley yields in many developing countries worldwide. The identification of molecular markers linked to genes controlling drought tolerance in barley is one way to improve breeding efficiency. In this study, we analyzed the quantitative trait loci (QTL) controlling chlorophyll content and chlorophyll fluorescence in 194 recombinant inbred lines (RILs) developed from the cross between the cultivar ‘Arta’ and Hordeum spontaneum 41-1. Five traits, chlorophyll content, and four chlorophyll fluorescence parameters, namely initial fluorescence (F_o), maximum fluorescence (F_m), variable fluorescence (F_v), and maximum quantum efficiency of PSII (F_v/F_m) which are related to the activity of the photosynthetic apparatus, were measured under well-watered and drought stress conditions at post-flowering stage. QTL analysis identified a total of nine and five genomic regions, under well-watered and drought stress conditions, respectively, that were significantly associated with the expression of the five target traits at post-flowering stage. No common QTL was detected except one for chlorophyll content, which was identified in both growth conditions, demonstrating that the genetic control of the expression of the traits related to photosynthesis differed under different water conditions. A QTL for F_v/F_m, which is related to the drought tolerance of photosynthesis was identified on chromosome 2H at 116 cM in the linkage map under drought stress. This QTL alone explained more than 15% of phenotypic variance of maximum quantum yield of PSII, and was also associated with the expression of four other traits. In addition, another QTL for F_v/F_m was also located on the same chromosome (2H) but at 135.7 cM explaining around 9% of the phenotypic variance under drought conditions. The result presented here suggest that two major loci, located on chromosome 2H, are involved in the development of functional chloroplast at post-flowering stage for drought tolerance of photosynthesis in barley under drought stress. If validated in other populations, chlorophyll fluorescence parameters could be used as selection criteria for drought tolerance.     

QTL analysis of kernel shape and weight using recombinant inbred lines in wheat

Quantitative trait locus (QTL) analysis of kernel shape and weight in common wheat was conducted using a set of 131 recombinant inbred lines (RIL) derived from ‘Chuan 35050’ × ‘Shannong 483’. The RIL and their two parental genotypes were evaluated for kernel length (KL), kernel width (KW), thousand-kernel weight (TKW), and test weight (TW) in four different environments. Twenty QTL were located on 12 chromosomes, 1A, 1B, 1D, 2A, 2B, 3B, 4A, 4B, 5D, 6A, 6B, and 7B, with single QTL in different environments explaining 5.9–26.4% of the phenotypic variation. Six, three, four, and seven QTL were detected for KL, KW, TKW, and TW, respectively. The additive effects for 17 QTL were positive with Chuan 35050 increasing the QTL effects, whereas the remaining three QTL were negative with Shannong 483 increasing the effects. Eight QTL (40%) were detected in two or more environments. Two QTL clusters relating to KW, TKW, and TW were located on chromosomes 2A and 5D, and the co-located QTL on chromosome 6A involved a QTL for KW found in two environments and a QTL for TKW detected in four environments.     

Molecular mapping of quantitative trait loci (QTLs) controlling aluminium tolerance in bread wheat

Aluminium (Al) toxicity is a major constraint to crop productivity in acidic soils. A quantitative trait locus (QTL) analysis was performed to identify the genetic basis of Al tolerance in the wheat cultivar ‘Chinese Spring’. A nutrient solution culture approach was undertaken with the root tolerance index (RTI) and hematoxylin staining method as parameters to assess the Al tolerance. Using a set of D genome introgression lines, a major Al tolerance QTL was located on chromosome arm 4DL, explaining 31% of the phenotypic variance present in the population. A doubled haploid population was used to map a second major Al tolerance QTL to chromosome arm 3BL. This major QTL (Qalt _CS .ipk-3B) in ‘Chinese Spring’ accounted for 49% of the phenotypic variation. Linkage of this latter QTL to SSR markers opens the possibility to apply marker-assisted selection (MAS) and pyramiding of this new QTL to improve the Al tolerance of wheat cultivars in breeding programmes.     

Australian rice varieties vary in grain yield response to heat stress during reproductive and grain filling stages

Climate change may lead to an increase in both day and night time temperatures in rice (Oryza sativa L) growing regions, but the impact of such temperature increases on yields of Australian rice varieties is not known. We evaluated the biomass and grain yield response of eleven Australian rice varieties including long, medium and short grain types, and the Californian cultivar M205, to heat stress during the reproductive phase and grain filling stages. Heat stress (day/night = 35/25°C) was applied at one of three stages: from panicle exertion to anthesis (PE), from anthesis to 10 days after anthesis (EGF) and from 10–20 days after anthesis (LGF) periods after which the effect on biomass and grain yield was compared to control plants. When heat stress was applied at PE and early grain filling stages, mean grain yield losses across rice varieties were 83% and 53%, respectively, though significant genotype × heat stress treatment interactions were observed. Notably, three varieties—YRM 67, Koshihikari and Opus—appeared to possess greater tolerance to heat stress at these growth stages. A significant genotype × heat stress treatment interaction was also observed in the LGF treatment, where significant yield reductions were only observed in Opus (21% loss) and YRM 67 (25% loss). A lack of effect of heat stress on total grain yield in most varieties at late grain filling appeared to be due to late tiller grain yields which were either unaffected by the heat stress or increased significantly compared to control plants. While genetic variation for tolerance to heat stress across the three growth stages was observed, there was no rice genotype that was consistently tolerant (in terms of yield under stress) across all three heat stress treatments. In the absence of a genotype that showed broad heat stress tolerance during reproductive growth, we suggest screening of a wider pool of more diverse rice germplasm is warranted.