Heat Tolerant Wheat Varieties Research Articles

Nuclear integration of MYB36 and APX-1 genes impart heat tolerance in wheat.

Elevated temperatures during grain filling stage, exceeding the optimal range by 3-4°C, not only results in a substantial yield reduction in wheat by 10-50% but activates disease and insect infestation. In this research, we introduced heat-tolerant MYB36 and APX-1 gene cassettes into wheat, employing an efficient Agrobacterium mediated transformation protocol, demonstrating higher transformation efficiency. The study encompassed the assembly of MYB36 and APX-1 gene cassettes, and confirmation of gene products in Agrobacterium, followed by the transformation of the MYB36 and APX-1 genes into wheat explants. We were able to select transgenic plant with various combinations. The transgenic plants with APX-1 gene alone produced medium sized grain and spike whereas with both APX-1 and MYB36 genes expressed individually under SPS and rd29a promoter respectively showed good tolerance to heat at 32oC at grain filling/milking stage and produced relatively bold grains. While non-transgenic plants grains were wrinkled with thin spike showing susceptibility to heat. This research contributes to the broader scientific understanding of plant stress responses and the combined effectiveness of MYB36 and APX-1 genes in crop improvement without disturbing normal nutritional values. The gene integration can serve as a valuable tool in breeding programs aimed at developing heat-tolerant wheat varieties. These findings also advance our comprehension of the functions of heat-induced genes and lay the foundation for selecting optimal candidates for in-depth functional studies of heat-responsive MYB36 and APX-1 genes in wheat.

Assessing Heat Stress Tolerance of Wheat Genotypes through Integrated Molecular and Physio-Biochemical Analyses

Heat as an abiotic stress significantly impairs the sustainable productivity of wheat (Triticum aestivum L.). To determine the tolerance of genotypes to heat stress, a comprehensive approach should be used that integrates simultaneous phenotyping and genotyping analyses. The aim of this study is to identify local heat-tolerant genotypes using simple sequence repeat (SSR) markers and evaluate the selected genotypes under field conditions for their tolerance to heat stress. Of the 12 SSR markers that showed polymorphism, eight were associated with six important traits. The use of hierarchical cluster analysis (HC) based on SSR markers led to the identification of 13 genotypes that showed varying results and were grouped into three distinct heat tolerance classes: tolerant (T), moderately tolerant (MT), and sensitive (S). The results showed that heat stress had a significant effect on 19 traits under this study, with significant variation in tolerance to heat stress between genotypes. The tolerant genotypes exhibited a range of average thousand-kernel weight (TKW) values between 40.56 and 44.85, while the sensitive genotype (Yecora Rojo) had an average TKW of 35.45. Furthermore, the tolerant genotypes showed two to three times higher levels of antioxidants compared to the sensitive genotypes when exposed to heat stress. Among the traits analyzed, six showed a favorable combination of high heritability (>60%) and genetic gain (>20%). Through the integration of principal component analysis and stepwise multiple linear regression, it was determined that six traits (grain yield, 1000-kernel weight, plant height, intercellular carbon dioxide, flag leaf area, and grain filling duration) revealed differences between the 13 genotypes. HC analysis of the six traits resulted in the same division of genotypes into three main categories as observed in an HC analysis based on SSR markers. It is worth noting that Saudi wheat, including KSU106, KSU105, and KSU115 as local genotypes, in addition to the 16HTWYT-22 genotype, showed higher heat tolerance compared to the other genotypes tested, indicating its potential suitability for agriculture in Saudi Arabia. These results contribute to breeding programs focused on developing heat-tolerant wheat varieties and accelerate progress in wheat productivity improvement programs.

Heat Waves and Its Impact on Crop Production and Mitigation Techniques: A Review

Globally, heat waves are getting worse and more common. India experienced its warmest March and April on record, with maximum and minimum temperatures unusually rising in most of the nation. In 10 out of the 36 meteorological subdivisions during this time, the extreme temperatures were found to be higher by +8 to +10.8°C and the rainfall lower by -60 to -99 percent, respectively, compared to normal. Additionally, 2022 will be remembered as a prime example of the combined effects of high temperatures and low rainfall on agricultural production systems, particularly in northern and central India. Climate variability and change have an impact on India's agricultural production. In the states of Punjab, Haryana, and Rajasthan, abnormally high maximum and minimum temperatures in 2022 had an impact on livestock, crops, and fruits and vegetables. Himachal Pradesh, Uttar Pradesh, Madhya Pradesh, Bihar, and Maharashtra, as well as Jammu and Kashmir. The heat wave coincided with the wheat crop's grain filling and development stage, causing the grain to yellow and shrivel, forcing it to mature, and lowering yields by 15 to 25 percent. A variety of technologies are available to reduce the yield loss in wheat caused by a heat wave, and some of them have been implemented on farmer fields. As compared to the local variety HD-3086, several heat-tolerant wheat varieties, including PBW 803, DBW 187, and DBW 222, can withstand high temperatures while still producing normal yields.

Association mapping identifies loci and candidate genes for grain-related traits in spring wheat in response to heat stress

Heat stress is a limiting factor in wheat production along with global warming. Development of heat-tolerant wheat varieties and generation of suitable pre-breeding materials are the major goals in current wheat breeding programs. Our understanding on the genetic basis of thermotolerance remains sparse. In this study, we genotyped a collection of 211 core spring wheat accessions and conducted field trials to evaluate the grain-related traits under heat stress and non-stress conditions in two different locations for three consecutive years. Based on SNP datasets and grain-related traits, we performed genome-wide association study (GWAS) to detect stable loci related to thermotolerance. Thirty-three quantitative trait loci (QTL) were identified, nine of them are the same loci as previous studies, and 24 are potentially novel loci. Functional candidate genes at these QTL are predicted and proved to be relevant to heat stress and grain-related traits such as TaELF3-A1 (1A) for earliness per se (Eps), TaHSFA1-B1 (5B) influencing heat tolerance and TaVIN2-A1 (6A) for grain size. Functional markers of TaELF3-A1 were detected and converted to KASP markers, with their function and genetic diversity being analyzed in the natural populations. In addition, our results unveiled favor alleles controlling agronomic traits and/or heat stress tolerance. In summary, we provide insights into heritable correlation between yield and heat stress tolerance, which will accelerate the development of new cultivars with high and stable yield of wheat in the future.

Evaluation of wheat Genotypes for heat stress and disease resistance in multiple environments of Bangladesh

Twenty-seven advanced lines including three exiting varieties (BARI Gom 21, BARI Gom 32 and BARI Gom 33) were evaluated in alpha lattice design with two replications at Bangladesh Wheat and Maize Research Institute, Dinajpur and regional stations at Joydebpur, Jashore and Rajshahi under irrigated timely sown (ITS) and irrigated late sawn (ILS) conditions. The objective of the study was to find out the heat-tolerant wheat lines for future breeding programme to develop heat-tolerant wheat varieties. The genotypes were evaluated for phenological variation such as heading and maturity, yield and yield components, disease reaction, sterility, visual grain quality, etc. The results showed that the highest grain yield (5951 kg ha-1) was recorded with genotype BAW 1415 at Dinajpur under ITS condition and the lowest grain yield (1923 kg ha-1) was observed in BAW 1416 at Rajshahi under ILS condition. Considering the overall performances, genotypes BAW 1402, BAW 1403, BAW 1406, BAW 1407, BAW 1408, BAW 1411, BAW 1422 and BAW 1425 performed better. Considering disease infestation, genotypes BAW 1374, BAW 1393, BAW 1394, BAW 1397, BAW 1399 and BAW 1401 showed a good level of resistance to both leaf rust (LR) and wheat blast (WB). These genotypes could be selected for future breeding programmes to develop heat-tolerant and diseases resistance varieties. Bangladesh J. Agri. 2022, 47(2): 1-16

Impact of Climate Change on Phenology of Two Heat-Resistant Wheat Varieties and Future Adaptations.

Climate change (CC) is a global threat to the agricultural system. Changing climatic conditions are causing variations in temperature range, rainfall timing, humidity percentage, soil structure, and composition of gases in environment. All these factors have a great influence on the phenological events in plants’ life cycle. Alternation in phenological events, especially in crops, leads to either lower yield or crop failure. In light of respective statement, the present study is designed to evaluate the climatic impacts on two heat-resistant wheat varieties (Sialkot–2008 and Punjab–2018). During the study, impacts of CC on wheat phenology and annual yield were predicted considering six climatic factors: maximum temp, minimum temperature, precipitation, humidity, soil moisture content, and solar radiation using two quantitative approaches. First, a two-year field experimental plot was set up at five different sites of study—each plot a bisect of two sites. Phenological changes of both varieties were monitored with respect to climatic factors and changes were recorded in a scientific manner. Secondly, experimental results were compared with Global climate models (GMC) models with a baseline range of the past 40 years (1970–2010) and future fifty years (2019–2068) under Representative Concentration Pathway (RCP) 8.5 model analysis. Field experiment showed a (0.02) difference in maximum temperature, (0.04) in minimum temperature, (0.17) in humidity, and about (0.03) significant difference in soil moisture content during 2019–2021. Under these changing climatic parameters, a 0.21% difference was accounted in annual yield. Furthermore, the results were supported by GMC model analysis, which was analyzed by Decision Support System for Agrotechnology Transfer (DSSAT) model. Results depicted that non-heat-resistant wheat varieties could cause up to a 6~13% reduction in yield during future 50 years (2019–2068)) compared with the last 40 years (1970–2010). A larger decline in wheat grain number relative to grain weight is a key reducer of wheat yield, under future climate change circumstances. Using heat-tolerant wheat varieties will not only assist to overcome this plethora but also provide a potential increase of up to 7% to 10% in indigenous environment. On the other hand, it was concluded that cultivating these heat-resistant varieties that are also ripening late culminates into enhanced thermal time chucks during the grain-filling period; hence, wheat yield will increase by 8% to 12%. In changing climatic conditions and varieties, ‘Punjab–2018′ will be the better choice for peasants and farm-land owners to obtain a better yield of wheat to cope with the necessities of food on the domestic and national level.

Physiological and molecular insights on wheat responses to heat stress.

Increasing temperature is a key component of global climate change, affecting crop growth and productivity worldwide. Wheat is a major cereal crop grown in various parts of the globe, which is affected severely by heat stress. The morphological parameters affected include germination, seedling establishment, source-sink activity, leaf area, shoot and root growth. The physiological parameters such as photosynthesis, respiration, leaf senescence, water and nutrient relation are also affected by heat. At the cellular level, heat stress leads to the generation of reactive oxygen species that disrupt the membrane system of thylakoid, chloroplast and plasma membrane. The deactivation of the photosystem, reduction in photosynthesis and inactivation of rubisco affect the production of photoassimilates and their allocation. This ultimately affects anthesis, grain filling, size, number and maturity of wheat grains, which hamper crop productivity. The interplay of various systems comprising antioxidants and hormones plays a crucial role in imparting heat stress tolerance in wheat. Thus, implementation of various omics technologies could foster in-depth insights on heat stress effects, eventually devising heat stress mitigation strategies by conventional and modern breeding to develop heat-tolerant wheat varieties. This review provides an integrative view of heat stress responses in wheat and also discusses approaches to develop heat-tolerant wheat varieties.

Discovery of miRNAs and Development of Heat-Responsive miRNA-SSR Markers for Characterization of Wheat Germplasm for Terminal Heat Tolerance Breeding.

A large proportion of the Asian population fulfills their energy requirements from wheat (Triticum aestivum L.). Wheat quality and yield are critically affected by the terminal heat stress across the globe. It affects approximately 40% of the wheat-cultivating regions of the world. Therefore, there is a critical need to develop improved terminal heat-tolerant wheat varieties. Marker-assisted breeding with genic simple sequence repeats (SSR) markers have been used for developing terminal heat-tolerant wheat varieties; however, only few studies involved the use of microRNA (miRNA)-based SSR markers (miRNA-SSRs) in wheat, which were found as key players in various abiotic stresses. In the present study, we identified 104 heat-stress-responsive miRNAs reported in various crops. Out of these, 70 miRNA-SSR markers have been validated on a set of 20 terminal heat-tolerant and heat-susceptible wheat genotypes. Among these, only 19 miRNA-SSR markers were found to be polymorphic, which were further used to study the genetic diversity and population structure. The polymorphic miRNA-SSRs amplified 61 SSR loci with an average of 2.9 alleles per locus. The polymorphic information content (PIC) value of polymorphic miRNA-SSRs ranged from 0.10 to 0.87 with a mean value of 0.48. The dendrogram constructed using unweighted neighbor-joining method and population structure analysis clustered these 20 wheat genotypes into 3 clusters. The target genes of these miRNAs are involved either directly or indirectly in providing tolerance to heat stress. Furthermore, two polymorphic markers miR159c and miR165b were declared as very promising diagnostic markers, since these markers showed specific alleles and discriminated terminal heat-tolerant genotypes from the susceptible genotypes. Thus, these identified miRNA-SSR markers will prove useful in the characterization of wheat germplasm through the study of genetic diversity and population structural analysis and in wheat molecular breeding programs aimed at terminal heat tolerance of wheat varieties.

Development and use of miRNA-derived SSR markers for the study of genetic diversity, population structure, and characterization of genotypes for breeding heat tolerant wheat varieties.

Heat stress is an important abiotic factor that limits wheat production globally, including south-east Asia. The importance of micro (mi) RNAs in gene expression under various biotic and abiotic stresses is well documented. Molecular markers, specifically simple sequence repeats (SSRs), play an important role in the wheat improvement breeding programs. Given the role of miRNAs in heat stress-induced transcriptional regulation and acclimatization, the development of miRNA-derived SSRs would prove useful in studying the allelic diversity at the heat-responsive miRNA-genes in wheat. In the present study, efforts have been made to identify SSRs from 96 wheat heat-responsive miRNA-genes and their characterization using a panel of wheat genotypes with contrasting reactions (tolerance/susceptible) to heat stress. A set of 13 miRNA-derived SSR markers were successfully developed as an outcome. These miRNA-SSRs are located on 11 different common wheat chromosomes (2A, 3A, 3B, 3D, 4D, 5A, 5B, 5D, 6A, 6D, and 7A). Among 13 miRNA-SSRs, seven were polymorphic on a set of 37 selected wheat genotypes. Within these polymorphic SSRs, three makers, namely HT-169j, HT-160a, and HT-160b, were found promising as they could discriminate heat-tolerant and heat-susceptible genotypes. This is the first report of miRNA-SSR development in wheat and their deployment in genetic diversity and population structure studies and characterization of trait-specific germplasm. The study suggests that this new class of molecular makers has great potential in the marker-assisted breeding (MAB) programs targeted at improving heat tolerance and other adaptability or developmental traits in wheat and other crops.

Tolerance mechanisms for breeding wheat against heat stress: A review

Heat stress is one of the major abiotic stresses affecting the growth and yield-related characters of wheat. Using various genetic approaches we can develop heat tolerant wheat varieties in order to mitigate the effect of heat stress on wheat production. Currently different strategies have been adopted to develop thermotolerance in wheat. Development of thermotolerant wheat varieties is one of the major steps toward the improvement of wheat yield against heat stress. For this purpose, it is important to have a complete and clear concept of the morpho-physiological, biochemical and molecular mechanisms of heat tolerance in wheat. This review may provide better knowledge about heat tolerance through discussing the morphological, physiological, biochemical and molecular mechanisms of heat tolerance in wheat based on different parameters such as grain filling duration, grain yield, leaf senescence, canopy temperature depression, photosynthesis, chlorophyll content, membrane thermostability, translocation of photo-assimilates, starch synthesis, antioxidant response, protein synthesis and omics approaches.

Heat Sensing and Lipid Reprograming as a Signaling Switch for Heat Stress Responses in Wheat.

Temperature is an essential physical factor that affects the plant life cycle. Almost all plant species have evolved a robust signal transduction system that enables them to sense changes in the surrounding temperature, relay this message and accordingly adjust their metabolism and cellular functions to avoid heat stress-related damage. Wheat (Triticum aestivum), being a cool-season crop, is very sensitive to heat stress. Any increase in the ambient temperature, especially at the reproductive and grain-filling stages, can cause a drastic loss in wheat yield. Heat stress causes lipid peroxidation due to oxidative stress, resulting in the damage of thylakoid membranes and the disruption of their function, which ultimately decreases photosynthesis and crop yield. The cell membrane/plasma membrane plays prominent roles as an interface system that perceives and translates the changes in environmental signals into intracellular responses. Thus, membrane lipid composition is a critical factor in heat stress tolerance or susceptibility in wheat. In this review, we elucidate the possible involvement of calcium influx as an early heat stress-responsive mechanism in wheat plants. In addition, the physiological implications underlying the changes in lipid metabolism under high-temperature stress in wheat and other plant species will be discussed. In-depth knowledge about wheat lipid reprograming can help develop heat-tolerant wheat varieties and provide approaches to solve the impact of global climate change.

Over-Expression of the Heat-Responsive Wheat Gene TaHSP23.9 in Transgenic Arabidopsis Conferred Tolerance to Heat and Salt Stress.

The small heat shock proteins (sHSP) are stress-induced proteins with molecular weights ranging from 12 to 42 kDa that act as molecular chaperones to prevent the irreversible aggregation of denaturing proteins. In this study, we cloned the heat responsive gene TaHSP23.9 from wheat (Triticum aestivum) based on TMT-labeled quantitative proteomic analysis in our previous work and examined its function in the response of transgenic Arabidopsis to heat and salt stress. Amino acid alignment and phylogenetic tree analysis showed that TaHSP23.9 contained a typically conserved structure of the alpha-crystallin domain and is closely related to OsHSP23.2 in rice. Transient expression assays demonstrated that TaHSP23.9 is located on the endoplasmic reticulum. Quantitative real-time PCR demonstrated that TaHSP23.9 was expressed much more in filling grains under normal conditions and was significantly upregulated by heat and salt stress. Transgenic Arabidopsis plants that constitutively over-expressed TaHSP23.9 had no visible differences or adverse phenotypes compared with the wild type under normal conditions but exhibited enhanced tolerance to heat and salt stress under stress conditions. In addition, we found that the expression level of TaHSP23.9 was significantly higher in the heat-tolerant wheat varieties than in the heat-sensitive varieties. Our results suggest that TaHSP23.9 may function as a protein chaperone to positively regulate plant responses to heat and salt stress and could be developed as a molecular marker for screening heat-tolerant wheat varieties.

Cell membrane stability- an important criterion for selection of heat tolerant genotypes in wheat (Triticum aestivum L.)

Cell membrane stability, grain filling rate, grain filling duration, canopy temperature and grain yield were used to evaluate performance of 100 diverse bread wheat (Triticum aestivum L.) genotypes under timely sown and late sown heat stress conditions for two cropping season. The genotypes differed significantly for all the traits show-ing considerable variation for improvement of characters. The genotypes WH1165 had significant high grain yield (14.6* g and 11.4g) and (11.3* g and 11.4* g) followed by cell membrane stability under timely sown and heat stress conditions, respectively indicating potential tolerance against heat stress. Correlation coefficients revealed that cell membrane stability (0.451**) and (0.639**) in timely sown and in late sown conditions, respectively were the most important trait followed by grain filling rate (0.882** and 0.744**) under timely sown and late sown conditions respec-tively. Results revealed that bread wheat genotypes which had high value of cell membrane stability had high grain yield showed potential photorespiration and high grain filling rate under heat stress condition. Twenty two genotypes WH1021, WH1155, VL803, WH787, NW1014, Raj3765, HD1869, 2042, WH1124, HD2285, WH1133, HUW234, 4066, Sonak, UP2425, UP2473, PBW503, PBW373, PBW533, SGP13, HD2643 and WH789 were identified as heat tolerant genotypes based on their relative performance in yield components, grain yield and heat susceptibility indi-ces. These genotypes were found to be ideal candidates to be used in developing heat tolerant wheat varieties. Canopy temperature, membrane thermostability and grain filling rate have also shown strong correlation with grain yield. Because of this association, these traits constitute the best available ‘tool’ for genetic improvement of wheat suitable for cultivation under heat stressed environments. Thus, these could be used as indirect selection criteria for developing heat tolerant wheat genotypes that would provide sufficient yields to meet the ever increasing wheat demand.

Using satellite data to identify the causes of and potential solutions for yield gaps in India’s Wheat Belt

Food security will be increasingly challenged by climate change, natural resource degradation, and population growth. Wheat yields, in particular, have already stagnated in many regions and will be further affected by warming temperatures. Despite these challenges, wheat yields can be increased by improving management practices in regions with existing yield gaps. To identify the magnitude and causes of current yield gaps in India, one of the largest wheat producers globally, we produced 30 meter resolution yield maps from 2001 to 2015 across the Indo-Gangetic Plains (IGP), the nation’s main wheat belt. Yield maps were derived using a new method that translates satellite vegetation indices to yield estimates using crop model simulations, bypassing the need for ground calibration data. This is one of the first attempts to apply this method to a smallholder agriculture system, where ground calibration data are rarely available. We find that yields can be increased by 11% on average and up to 32% in the eastern IGP by improving management to current best practices within a given district. Additionally, if current best practices from the highest-yielding state of Punjab are implemented in the eastern IGP, yields could increase by almost 110%. Considering the factors that most influence yields, later sow dates and warmer temperatures are most associated with low yields across the IGP. This suggests that strategies to reduce the negative effects of heat stress, like earlier sowing and planting heat-tolerant wheat varieties, are critical to increasing wheat yields in this globally-important agricultural region.

Wheat leaf lipids during heat stress: II. Lipids experiencing coordinated metabolism are detected by analysis of lipid co-occurrence.

Identifying lipids that experience coordinated metabolism during heat stress would provide information regarding lipid dynamics under stress conditions and assist in developing heat-tolerant wheat varieties. We hypothesized that co-occurring lipids, which are up-regulated or down-regulated together through time during heat stress, represent groups that can be explained by coordinated metabolism. Wheat plants (Triticum aestivum L.) were subjected to 12 days of high day and/or night temperature stress, followed by a 4-day recovery period. Leaves were sampled at four time points, and 165 lipids were measured by electrospray ionization-tandem mass spectrometry. Correlation analysis of lipid levels in 160 leaf samples from each of two wheat genotypes revealed 13 groups of lipids. Lipids within each group co-occurred through the high day and night temperature stress treatments. The lipid groups can be broadly classified as groups containing extraplastidic phospholipids, plastidic glycerolipids, oxidized glycerolipids, triacylglycerols, acylated sterol glycosides and sterol glycosides. Current knowledge of lipid metabolism suggests that the lipids in each group co-occur because they are regulated by the same enzyme(s). The results suggest that increases in activities of desaturating, oxidizing, glycosylating and acylating enzymes lead to simultaneous changes in levels of multiple lipid species during high day and night temperature stress in wheat.

Screening for heat tolerance in spring wheat (Triticum aestivum l.)

The present investigation was undertaken to identify the promising heat tolerant lines and to evaluate their heat stress responses. Twenty five spring wheat genotypes were studied in non-stressed (optimum sowing) and stressed (late sowing) environments. The experiments were conducted at Regional Wheat Research Centre, Bangladesh Agricultural Research Institute, Gazipur, during the cropping season of 2009-10. Randomized complete block design was used with three replications. Yield and yield contributing phenological and physiological characters varied among the genotypes under both optimum and late sowing conditions. Ground cover, grain filling duration, canopy temperature at grain filling and biomass production were severely affected by the heat stress leading to low grain yield under late sowing condition. G-12, G-13, G-14, G-18, and G-19 were identified as heat tolerant genotypes based on their relative performance in yield components, grain yield and heat susceptibility indices. Present genotypes are found to be ideal candidates to be used in developing heat tolerant wheat varieties. Tropical Agricultural Research and Extension 17(1): 2014: page 26-37

Evaluation of Some Wheat Hybrids under Normal and Heat Stress Conditions

Heat shocks at anthesis are one of the major limitations in wheat productivity in many countries worldwide including Pakistan. Present investigations were designed to battle out this calamity which may further be exploited in breeding heat tolerant cultivars. Seven parents including tolerant, moderately tolerant and susceptible but high yielders were utilized for this purpose and later hybridized in a 7 ×7 diallel fashion. Analysis of variance revealed significant variability for all the parameters studied. Highest value of negative and desirable significant heterosis and heterobeltiosis for plant height was shown by the cross combinations Weebli-1 × Uqab-2000, Inqilab-91 × Shalimar-88 under normal conditions and under stress by the hybrids Uqab-2000 × Weebli-1, Shalimar-88 × Maya/Pavon. Maximum positive heterosis for tillers per plant was recorded by the cross combination Shalimar-88 × Chenab-2000 (12.04%) and maximum positive and highly significant heterobeltiosis was recorded by the cross combination Shalimar-88 × Chenab-2000 (11.46%). Under stress for tillers per plant none of the crosses showed positive estimates. Maximum positive heterosis for 100- grain weight was shown by the cross combination Chenab-2000 × Inqilab-91 (28.22%). Maximum positive heterosis under stress for 100-grain weight was shown by the cross Inqilab-91 × Weebli-1 (23.35%) followed by Chenab-2000 × Inqilab-91 (22.22%). Maximum positive heterosis for grain yield under normal conditions was manifested by the cross combinations Uqab-2000 × Punjab-85 (28.70%) and under stress maximum positive heterosis was recorded by cross combination Shalimar-88 × Uqab-2000 (27.02%). The results of heterosis and heterobeltiosis estimation revealed that the hybrid vigor is available in the current experiment for all the traits and selection of desirable hybrids is the best way to improve the grain yield of bread wheat. Many cross combinations in the current studies may be utilized following pedigree or bulk method to develop heat tolerant wheat varieties because of their ability to perform well under normal and heat stress conditions.

Production of early maturing heat tolerant wheat varieties for southern warm zone by haploid breeding

This research was conducted to compare haploid production methods in wheat using chromosome elimination by wheat and maize crosses. The three methods used were: a) classic b) detached tiller c) intermediary. Parental materials were 11 F1 wheat genotypes (female parents) and maize genotype BC572 (male parent). The traits studied were: seed set, embryo formation, and haploid plant production. The comparison among these methods revealed that the C method was superior with respect to seed set, embryo formation, and haploid production. Overall performance of the three methods was about 76.84%, 25.22% and 51.89% for seed setting, embryo formation and haploid production respectively. Among wheat genotypes DH-133 with 87.28% seed set DH-132 with 32.27% embryo formation and 65.08% haploid production were the best genotypes studied. Considering human resources and glass house conditions all other lines were produced by B method detachedtiller.As far as evaluation is concerned, overall, 1155 wheat lines were evaluated for earliness and heat tolerance. This included 788 lines from international nurseries and local collection and 367 DH lines. The evaluations were conducted preliminary at Karaj and finally at Ahwaz and other tropical sites from 2005 to 2010. Totally, 160 from international nurseries and local collection were identified as early maturing and heat tolerant. These lines will be used as parental materials for DH production in the second phase of the project. From the evaluated DH lines, 21 lined could reach replicated yield trials and finally 6 lines were found to be superior to the check varieties (Chamran and Vee.Nac) both under normal and late sowing conditions at 3 sites namely Ahwaz, Zabol and Darab stations. Three lines of these have already been assessed under farmer conditions at 2 locations and 1 line overyield the best check variety (Vee.Nac) besides being resistant to all kinds of rusts yellow, brown andblackunderlatesowingconditions. * Corresponding Author: Tayebeh Bakhshi  adelehbakhshi@yahoo.com