Researchers are calculating the risk and assessing the impact of heat stress at flowering to help growers choose the most appropriate varieties for their location
On 12 October 2004, the southern grainbelt was hit with an extreme heat event, leaving growers anxious and unsure about how much damage the heat had done to their wheat.
Dr Peter Hayman, principal scientist in climate applications with the South Australian Research and Development Institute (SARDI), says wheat yields were reduced, but the problem for growers was knowing how much of the damage was due to that one hot day and how much might have been due to the preceding dry month.
Pots of flowering wheat were heated in polycarbonate chambers at at the University of Adelaide’s Waite Campus, Urrbrae, October 2012.
It is generally understood anthesis, or flowering, is when the wheat plant is most sensitive to heat stress. But Dr Hayman says there is still a lot that scientists do not know about the impact of heat stress on grain in Australian conditions.
His current project, ‘Assessing and managing heat stress in cereals’, aims to provide some answers.
One of the ironies of heat stress is that crops grown in wetter regions, where growers plant later or sow longer-season varieties that flower later in the spring, can be at greater risk of heat stress compared with crops grown in hotter, lower-rainfall areas. “Some of these high-rainfall sites are pushing flowering into November,” he says.
The project, which is run by SARDI and the University of Adelaide, has studied two areas of South Australia: Minnipa on the Eyre Peninsula, and Roseworthy, north of Adelaide.
“If you just do the number of hot days in spring, Minnipa seems much hotter than Roseworthy. But when you look at it from the wheat crop’s perspective you actually get a different picture,” Dr Hayman says.
“Wheat crops in Minnipa usually flower in mid-September, before the hot spring days, while most Roseworthy crops flower in mid-October and are more likely to experience heat events.”
The project has two parts: calculating the likelihood of high temperatures in spring, defined as 35°C and higher; and working out the consequences. While climate change brings warmer mean temperatures, the main issue is an increase in extreme events.
“How that change in mean temperature is delivered in spring extremes is our concern,” Dr Hayman says.
For the first three years of the project, 2009 to 2011, the scientists set up chambers in wheat paddocks. They then subjected that wheat to 35°C for a single day.
“The first year there were some significant results, then in the next two years the variability between crops seemed to swamp any clear results,” Dr Hayman says.
Dr Peter Hayman and Bronya Alexander
PHOTOS: Asa Wahlquist
In 2012 they set up the experiment differently. Research officer Bronya Alexander explains that they used 80 wheat in pots, and also looked at water stress. In the week before the day of heat stress they stopped watering half the pots so they would be water stressed on the day of heating, and the other half were kept well watered.
They also worked with Scott Mills from ScienceMob, a science support company, and used many more temperature sensors – 150 in all – in 2012.
“From each pot we chose one plant, which we stuck a sensor to, and one head that also had a sensor right next to it, to measure the air temperature just next to the wheat.” The sensors logged the temperature every 10 seconds.
The researchers put 70 pots in six chambers, heating two chambers to 30°C, two to 35°C and two to 38°C, as well as leaving 10 outside for a control.
Dr Hayman says growers today face three interrelated climate risks: running out of moisture, frost and spring heat events. He hopes his project will help growers assess the risk of damaging heat events, just as frost risk can be assessed.
“We can say more about the risks of running into spring drought and probably more about frosts but less about heatwaves. High-consequence/low-frequency events are difficult to manage, difficult to predict and it is also difficult to really be clear about the exact impact.”
Ms Alexander hopes to find results for how much yield is affected by heat.
She points out growers do have options. “If the chance of getting a really hot temperature at the time the crop flowers is a big risk for you, you might consider sowing your crop earlier or sowing varieties that flower earlier before they reach the high-risk phase.”
The final results will be available by the coming sowing season.
Dr Peter Hayman,
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