Trait discovery work in wheat has identified new indicators for heat tolerance and efficient selection technology for use by breeders for rapid progression into new varieties and on to Australian paddocks
After completing just one year of experimental trial work, a new project has identified novel ways to select for heat tolerance in wheat. These methods are fast, efficient and cheap enough to be adopted by breeders.
Recovery after heat stress varied among the wheat population screened for heat tolerance. Better-performing lines included H-137, which was able to recover following exposure to heat stress. Photo: Dr Jingjuan Zhang, Murdoch University
Additionally, 16 heat-tolerant and 12 heat-sensitive wheat lines were identified from a population of 319 genotypes. These have been passed on to project partner Australian Grain Technologies (AGT) for field analysis at Narrabri, New South Wales.
This project is headed by Professor Zhong-Hua Chen (Australian Research Council Future Fellow) and his research team at Western Sydney University.
Professor Chen says the project targeted differences in ‘source–sink’ biology within a diverse wheat population. This target involves the biological steps between sunlight hitting the leaf (the ‘source’) and carbohydrates being used for grain filling (the ‘sink’).
This encompasses leaf structural characteristics, stomata parameters, chloroplast traits where photosynthesis occurs, flowering and pollen synthesis as well as the leaf veins that connect the source and sink and root characteristics.
To probe such a rich series of complex and dynamic biological processes, Professor Chen assembled an international team of collaborators to help undertake the breakthrough analysis.
Included in Australia are Jay Bose and Oula Ghannoum at Western Sydney University, Rajeev Varshney and Chengdao Li at Murdoch University, and Meixue Zhou at University of Tasmania. This network is also training four PhD students.
The team also collaborates with the University of Haifa in Israel, the International Maize and Wheat Improvement Center (CIMMYT) in Mexico, and AGT.
New basis for wheat heat tolerance
This project involved running heat-screening trials both in the field and in glasshouses across three sites in NSW, Western Australia and Tasmania.
The focus was on 319 wheat genotypes especially assembled for the project. Included were approximately 150 Australian varieties, both new and old cultivars, approximately 160 varieties collected in China during a previous GRDC investment conducted by the University of Tasmania, breeding lines imported from CIMMYT and wild wheat relatives imported from Israel.
Field, glasshouse and laboratory experiments were conducted on a large range of biochemical traits and processes:
- flag leaf vapour pressure deficit (VPD);
- leaf temperature (Tleaf);
- stomatal conductance (gs);
- linear electron flow (LEF);
- vein density;
- stomatal density;
- stomatal index;
- soil plant analysis development (SPAD) value;
- proline content;
- sugar content;
- leaf length;
- leaf width;
- plant height;
- heading time;
- flowering time;
- pollen viability; and
- canopy normalised difference vegetation index (NDVI).
Also measured were crop yield and biomass for both the correlation analyses and weighted ranking of heat tolerance of the 319 wheat genotypes.
“It took us seven months to get the complete dataset that we used for this analysis,” Professor Chen says. “We had to consolidate many different experimental and analytical systems to be able to probe wheat in this novel way.”
To help translate the new understanding for use by breeders, the genetic basis for the observed physiological, phenotypical, morphological and yield traits were mapped. That means quantitative trait loci (QTLs) are being identified for the new heat tolerance traits.
New insights
The resulting data unearthed several measurements that are reliable indicators of heat tolerance.
Figure 1: Heat stress is a common cause of loss in grain number and grain weight in wheat.
Source: Dr Jingjuan Zhang, Murdoch University
These are: VPD, Tleaf, gs, LEF, NDVI, stomatal index, stomatal density, proline content, sugar content and relative pollen viability.
“For fast screening of heat tolerance in the field, we suggest using low-cost and rapid laboratory equipment and handheld instruments,” Professor Chen says. “That means two of the identified indicators are particularly good targets for breeding.”
The first is VPD as it can be measured using a handheld device (such as the Li-600 porometer) in the field and glasshouse.
The other is pollen viability as it can be tested using a PCR (polymerase chain reaction) thermocycler. This test allows for high-throughput testing of 96 lines for pollen viability in one hit.
Overall, 16 heat-tolerant wheat genotypes were identified. The genotypes were selected based on the weighted average score by ranking many phenotypic traits. Included were stay green, pollen viability, chloroplast heat tolerance, leaf structural and biochemical traits as well as yield and agronomic traits of wheat.
The data has also clarified how heat stress disrupts source–sink interactions in wheat by impacting the production or uptake, transport, storage and remobilisation of critical metabolites.
Professor Chen explains that optimising source–sink relations through the transport of more photoassimilates to sink tissues (and the enhancement of the use of more photoassimilates by sink tissues) is essential to reduce yield losses under heat stress.
“The development of new, improved wheat genotypes is greatly facilitated by understanding the plasticity of the response to heat stress that exists between wheat genotypes, especially in source–sink relationships at the reproductive and grain-filling stages,” he says.
Rapid development phase
The project was designed to ensure rapid progression of useful traits to breeders and onto Australian paddocks. That means the new resources are available to all breeding programs.
AGT is already accessing these heat-tolerant genotypes and information, but findings have also been communicated to wheat breeders at LongReach Plant Breeders, Intergrain, Australian Crop Breeders (ACB), RAGT Australia, Seednet and the NSW Department of Primary Industries and Regional Development.
To facilitate uptake, AGT is additionally undertaking breeder-relevant trials with the 16 newly identified lines at Narrabri (where heat stress is common). It will make its findings publicly available. AGT also provided field support to the research work at Murdoch University for the WA field trials during the phenotyping phase.
Looking ahead, field and greenhouse trials of selected genotypes are being repeated in NSW, WA and Tasmania to validate the findings. The measurements are being made using a doubled haploid (DH) population.
“This means the various heat tolerance traits will be tested in the same genetic background, allowing for a direct comparison and selection of promising heat tolerance DH lines,” Professor Chen says.
The resulting data is critical in terms of supporting the selection and adoption of materials by breeders interested in deploying the new heat tolerance traits in future varieties.
More information: Zhong-Hua Chen, z.chen@westernsydney.edu.au
Read also: Re-imagining heat tolerance traits in wheat – part 2.