Novel heat tolerance and heat stress recovery traits have been discovered in a coordinated and multidisciplinary analysis of a diverse canola population

Australia’s climate has warmed by an average of 1.47°C since 1910 and increased extreme heat events are reducing crop yields. To put that into context, canola yields begin to decline when temperatures exceed 29.5°C during flowering. Three days of moderate heat stress (with daily maximum temperature of 32°C for four hours) during flowering reduce canola yield by 30 per cent at maturity.

To address the challenges this poses to canola production, GRDC invested in the establishment of a multidisciplinary research team that could take a coordinated approach to improving canola heat tolerance.

This research, taking place from 2019 to 2024, aimed to establish a protocol for canola heat tolerance screening under controlled conditions and in the field to identify genetic sources of heat tolerance.

This was achieved through a heat screening facility at the University of Western Australia (UWA) and validated in the field in portable heat chambers at NSW Department of Primary Industries and Regional Development (NSW DPIRD) and in multi-environment trials (METs) with multiple time-of-sowing treatments and irrigation at five locations across Australia.

The project is now nearing completion and has achieved several important outcomes.

1 Identification of genetic diversity for heat stress

Using high-density SNP marker genotyping, we confirmed broad genetic diversity in the Brassica napus population derived from the previous GRDC-invested UM00045 project. This germplasm – sourced from India, Europe, Asia and Australia – has been valuable in this study in heat tolerance and in parallel GRDC investments in disease resistance.

2 Heat screening

In the heat screening facility at UWA, 324 B. napus genotypes were screened over three years across multiple sowing dates. Genotypes were identified with either good heat stress tolerance or excellent recovery after heat stress.

Of these materials, 42 heat-tolerant lines ranked in the top 50 in both early and late sowing times, and 21 lines showed strong yield-recovery capability from a heat wave. These materials were promoted to canola breeders as valuable for canola heat tolerance breeding in Australia.

It is worth noting that to identify these heat-tolerant genotypes, water and nutrition were supplied before, during and after the heat wave in the facility to ensure these were not limiting.

3 Genomic mapping

Genome-wide association studies were used to map correlations between the observed heat tolerance traits and genetic variation at specific sites (loci) across the canola genome. Such correlations are called quantitative trait loci (QTLs).

In all, eight yield-related traits were mapped that have the power to protect yield from various types of heat stress. Included were the ‘heat stress tolerance index’ and ‘percentage change under heat’ traits. These traits detect good levels of heat tolerance and excellent recovery after heat stress.

This analysis resulted in the identification of 32 yield-related heat tolerance QTLs.

It is important to note that the analysis also provides insights about the utility of these QTLs in terms of likely impacts on breeding programs. That is because the analysis decodes the level of heritability associated with each QTL. The higher the heritability estimate, the greater the proportion of total phenotypic variability that is due to genetic, rather than environmental, causes.

For the canola heat stress traits, each QTL explained up to five per cent of the phenotypic variance for at least one trait. This means many genes with relatively minor impacts are at work with these traits. As such, it is possible to provide canola with a genetic basis for heat tolerance, but it requires combining the right mix of QTLs.

Given the level of understanding of the canola genome, the QTL analysis also led to the identification of 128 candidate genes that may account for the observed heat tolerance.

The function of these candidate genes can be confirmed using high-throughput ‘multi-omics’ platforms (genomics, phenomics, transcriptomic and metabolomics).

Once the data is available, genomic selection technology is then able to predict the best combination of genes needed to optimise heat tolerance within a canola breeding program.

However, given that heat events differ in different parts of the country, additional information was needed by breeders in the form of multi-environment field trials (METs). This allows for prediction about the optimum gene combinations given the different needs of different growing regions.

The project led by Dr Sheng Chen has successfully screened canola for heat tolerance in a heat screening facility, with the findings validated in the field in portable heat chambers and in multi-environment trials (METs). Photo: Evan Collis

4 Multi-environment validation

Field trials in portable heat chambers at NSW DPIRD from 2020 to 2023 provided a bridge between controlled-environment experiments at UWA and multi-environment trials at five locations across Australia.

Heat stress was simulated in canola plots in the field in portable heat chambers. This method was used to assess the heat tolerance of breeding lines at the whole plot level. This is a reliable indicator of heat tolerance based on a field-grown plant population.

These trials included three to five sowing times at up to five locations across Australia over four years. All were conducted with ample irrigation to avoid drought stress that would otherwise confuse or mask the heat stress measurements.

The METs validated the heat tolerance of presumed heat-tolerant germplasm and also provided useful evidence on the adaptation of these genotypes across different environments in Australia.

In the METs, the heat tolerance of up to 30 canola genotypes was validated in 2020 and 2021 and 48 canola genotypes in 2022 and 2023, with several common genotypes. Heat waves with long stress duration and strong stress intensity occurred in all five national field trials in 2023.

The top five heat-tolerant canola genotypes showed high levels of heat stress tolerance across multiple environments in Australia. For these five genotypes, there was no seed yield loss under moderate or high heat stress in some of the METs (see Figure 1). Averaged across all trials, the top five lines experienced 16 per cent yield loss compared with an average 35.2 per cent yield loss in 24 heat-sensitive lines (including several Australian control cultivars).

The Australian Climate Council reported 2023 as the hottest year on record in Australia and the area planted to canola in the same year was the second highest on record. If the new heat-tolerant canola lines were used in a hot year like 2023, an extra of 1.24 to 2.56 million tonnes of canola seeds could be produced. This equates to an extra $840 million to $1.7 billion in income, based on a canola price of $677 per tonne.

As such, this GRDC investment has identified heat-tolerant germplasm that is valuable for the future breeding and release of heat-tolerant canola cultivars for the benefit of Australian canola growers.

Figure 1: The grain yield performance of 11 representative lines selected from 48 canola lines tested in Leeton, NSW, in 2023.

Source: Dr Suman Rakshit, AAGI

Capitalising on the new traits

GRDC has now reinvested in this research for a further four years. The aim moving forward is to ensure that new varieties with enhanced heat tolerance are developed and deployed.

One of the key goals moving forward will be to bring together multiple heat tolerance genes from different sources to facilitate their collective transfer into breeding programs. This will entail both heat stress tolerance QTLs and genes associated with recovery after heat stress.

As they may involve two distinct type of traits, further investigations into the underlying mechanisms of heat stress tolerance in canola are needed.

Additionally, the now-completed project undertook heat screening with the provision of ample irrigation to avoid drought stress. This, of course, is not normal in the field, where crops are simultaneously exposed to an array of environmental stressors including drought, which can substantially impact yield.

Heat and drought stress are increasing in the growing season in Australia due to climate change. Therefore, a need has been identified for investigations into the response of canola to these combined stresses.

This will improve understanding of the interactions between heat and drought stress tolerance genetics in the field and enable the identification of unique and common QTLs and genes for heat and drought stress tolerance.

More information: Sheng Chen, sheng.chen@uwa.edu.au

GroundCover™ Supplement story: Canola being set to beat the heat