Improved lodging resistance in sorghum is considered a pre-requisite if breeders are to continue to drive up the crop’s yield potential. A new GRDC investment is identifying the genetics needed to bolster the physical strength of sorghum stems
Like most grain crops, the need to increase sorghum yield is at the forefront of breeding efforts. This task, however, is made more complex in sorghum because plants with high yield potential have a greater propensity to lodge if they experience water stress during grain filling.
Crop physiologist at the University of Queensland Dr Geetika Geetika explains that water stress causes sorghum leaves to stop photosynthesis, which deprives the plant of the carbohydrates needed for grain filling.
To rescue the situation, the plant mobilises carbohydrates from the stem. The grain gets filled, but the stem is weakened and becomes prone to breaking.
“The greater the yield potential of the sorghum crop, the more likely the stem is to break if the crop experienced water stress,” says Dr Geetika.
Once lodging occurs, harvesting can be difficult and, in some cases, the entire crop can be lost. Even bumper crops that successfully filled grain can be lost in this way.
A GRDC investment is seeking to reduce this productivity-limiting tension between yield, water stress and lodging. Led by Professor David Jordan of the Queensland Alliance for Agriculture and Food Innovation (QAAFI), the team has successfully developed a method to measure stem strength directly.
Professor Jordan says this investment forms an important milestone for sorghum’s tenure as a grain crop in Australia because lodging has been a problem ever since the advent of high-yielding hybrid varieties in the 1960s.
“The challenge for sorghum breeders is that if they select the highest-yielding hybrids, they often select material that is more likely to lodge,” he says. “Similarly, if they select hybrids that do not lodge when water stress occurs during grain filling, then they often select hybrids with lower yield potential in good conditions.”
The breeder’s job is made even more difficult because conditions that favour lodging occur intermittently, making screening difficult.
Previously, sorghum breeders focused on helping plants avoid the remobilisation stress that causes lodging in the first place. This has included delivery of the stay-green trait, which was developed for use in breeding programs with previous GRDC investments. However, to push yields higher (without increasing lodging risk), inherent stem strength needs to be measured regardless
of remobilisation.
“That’s where our new method comes in as it allows plant breeders to measure stem strength under controlled conditions,” Professor Jordan says.
“It then becomes possible to screen diverse sorghum populations and identify new genetics that can bolster lodging resistance even if the plant needs to draw on stem carbohydrates to deal with water stress.“If we can crack this phenotyping challenge, then the way opens to increase sorghum yields faster.”
Phenotyping for stem strength
In collaboration with the Queensland Department of Agriculture and Fisheries and Advanta Seeds, Dr Geetika heads efforts to develop a way to measure and rank (or phenotype) sorghum for stem strength.
Figure 1: Cross-section of a sorghum stem.
Source: QAAFI
To do this, she first rated sorghum varieties relative to their genetic propensity to remobilise carbohydrates from the stem as a response to water stress.
This involves growing sorghum hybrids under favourable conditions up until flowering. At that stage, the leaves are removed to mimic a drought-triggered cessation of photosynthesis. To fill grain, the plant must now rely on stem carbohydrates.
Harvested stems then undergo a strength trial. This involves the use of a device called a ‘Vernier structures and materials tester’. It was modified for the sorghum lodging project by collaborating engineer Sean Reynolds. Modifications included parts made with a 3D-printer.
The stem breaker standardises measures of the force needed to break the sorghum stem.
The team is also drilling down into the chemical and morphological composition of the analysed stems, which adds additional data to the phenotyping.
“We know that factors that contribute to stem strength include structural features, such as rind thickness and overall stem shape and diameter, as well as compositional characteristics like differences in lignin, cellulose or hemicellulose content,” Dr Geetika says.
The phenotyping data creates a basis for increasing lodging resistance by selecting for the best stem strength given adequate levels of remobilisation to deal with water stress.
“So far we have conducted two years of field trials, and we are finding that there is diversity in the sorghum gene pool for stem strength,” Dr Geetika says. “We have three years of trials to go and each year we increase the number of sorghum material we phenotype.”
Ultimately, data will become available for hundreds of sorghum genotypes, with the 2025-26 season to include germplasm from collaborating breeding company Advanta Seeds (but with the information to eventually be available to the broader breeding community).
Capitalising on new phenotype data
Professor Jordan says that phenotyping breakthroughs feed into two pathways for improving lodging resistance. Both involve mapping diversity in stem strength back to differences at the genome levels in the form of quantitative trait loci (QTLs) or genes.
The first strategy for exploiting QTLs has less chance of success but will result in faster progress if successful, says Professor Jordan. It involves a search for small numbers of QTLs that alone can have a large impact on stem strength. The associated genes could then be exploited rapidly in a breeding program using marker-assisted selection.
However, complex traits that interact with many other biological and environmental factors rarely map back to single genes or QTLs.
The second strategy, therefore, makes use of conventional plant breeding enhanced by genomic selection technology.
Professor Jordan says the second approach will entail generating a “training population”, using data to train prediction algorithms.
These algorithms can then assign stem-strength breeding values to QTLs that are dispersed across the entire sorghum genome. The algorithms can also determine how to stack the best versions of these QTL into elite varieties.
“These breeding values make it possible to select for the stem strength component of lodging resistance and avoid the components that are negatively associated with yield,” Professor Jordan says.
That’s the power of genomic selection technology – it will allow us to concurrently account for two yield-related traits that drive lodging resistance in opposite directions.
The project is also maintaining a stored and curated collection of barcoded stems numbering in the thousands. This collection becomes the cornerstone for further gains in exploiting stem strength for lodging resistance going forward.
“The intertwined relationship between yields and lodging is one of the most difficult challenges that sorghum breeders face,” Professor Jordan says.
“To try to solve this problem, we brought together a large multidisciplinary team comprising geneticists, breeders’ molecular biologists, crop physiologists and engineers from the public and private sectors.
“With genomic selection technology, we are then able to integrate all those perspectives and derive a meaningful way forward despite the complexity.”
More information: David Jordan, david.jordan@uq.edu.au; Geetika Geetika, g.geetika@uq.edu.au
Read more in the GroundCoverTM story Lodging in sorghum remastered