Research that will be published soon has quantified the toll that increased temperatures and heat stress are having on wheat yields.

Queensland Alliance for Agriculture and Food Innovation (QAAFI) researcher, Dr Karine Chenu, has found that heat effects – when studied in isolation from other factors such as new varieties and practices – caused grain yields to decrease by an average of 240 kilograms per hectare per decade since 1985.

She says the trend is striking.

“Heat events during reproductive and grain-filling phases can heavily impact on yield. We are quantifying heat effects at those critical stages in carefully designed trials in glasshouse trials at Gatton.”

Working at QAAFI's Centre for Plant Science, Dr Chenu's collaborative research using the mid-maturing wheat cultivar Janz (sown on 15 May) aims to investigate wheat's response to high temperatures. The aim is to help develop management practices and breeding strategies to improve wheat’s adaptation to changing climatic conditions.

Dr Chenu used historical weather data from the Scientific Information for Land Owners Patched Point Data set for 60 sites across the Australian wheatbelt.

Nine sowing dates (from 1 April to 31 July at two-week intervals), three cultivars of contrasting maturity types [Axe (PBR), Bolac (PBR) and Janz and an improved version of the Agricultural Production Systems sIMulator (APSIM) were also called on to investigate wheat responses to increased temperatures over the 33-year period.

It showed that the frequency of hot days (where temperatures were higher than 26 degrees Celsius) around flowering (anthesis) and during grain-filling have, on average, increased by 0.6 and 1.2 days per decade, respectively, for the cultivar Janz sown on 15 May.

Dr Chenu says heat has two types of effect: stressful and non-stressful, both of which affect yield.

"Firstly, there are the effects due to a greater average temperature, which are not necessarily classed as directly stressful in regards to heat.

"These include accelerated phenological development, shortened growth stages and increased evaporative demand, which results in decreased transpiration efficiency and increased drought stress."

Secondly, there are the impacts of heat shock or heat 'stress' events. These include damage to the photosynthetic apparatus of plants, pollen and developing embryo mortality, and scorched leaves. An important effect is that, since 1985, accelerating phenological development has shortened key growth stages.

“The increasing temperatures have led the crop cycle to shorten across the whole wheatbelt for all the sowing dates and genotypes tested,” she says.

As an example, at the national scale, Janz sown on 15 May has experienced shorter pre and post flowering phases and whole crop cycles of 0.09 days per year, 0.07 days per year and 0.16 days per year, respectively.

Dr Chenu says this shortening of the cycle is projected to substantially affect potential yield in the near to mid-term if varieties and sowing dates remain unchanged.

To keep the same example, at the national scale, Janz sown on 15 May is projected to have its growth scale shortened by 13 days by 2050 (averaged across 33 general circulation models).

Then there are the impacts of ‘heat stress’, not related to shortening phenological periods.

“Assuming the same increasing rates in heat impact across the wheatbelt – a very conservative assumption in terms of temperature increase – the effect of heat stress on Janz grain number and weight (sown on 15 May) would increase by 3.3 per cent and 15.4 per cent by 2050, respectively, without considering the interactions with drought and carbon dioxide concentration.

“While it is expected that breeders will continue to deliver varieties with increased yields, heat is expected to decrease yields by more than 15 per cent by 2050, again without considering the interactions with drought and carbon dioxide concentration.”

Frost impacts

Dr Chenu is plant ecophysiologist and modeller.

Her research also encompasses frost and drought and the impacts they have on winter cereals. Across these areas, she works with Dr Behnam Ababaei, Dr Bangyou Zheng, Professor Scott Chapman, Dr James Watson, Dr Troy Frederick and Dr Jack Christopher.

She says that since 1957, when reliable temperature records became available, researchers have noted a number of trends in relation to frost.

"There are more frosts in the south and west, with frost days occurring later in the year, and potentially increasing yield losses," she says.

"What's important is when the last frost occurs.

"Since 1957, the last frost days have been getting later in most parts of the wheatbelt, thus affecting yield more often, or with a bigger impact.

"Taking expert knowledge from frost physiologists into account, we have found that frost accounts for 10 per cent of yield losses in Australian wheat, with another 10 per cent due to the indirect costs related to later sowing constraints. In the northern region, these indirect costs can be as high as 20 per cent."

Together, the researchers' work feeds into breeding and adaptive management programs and points to the need for earlier sowing and longer-season crops, particularly in the eastern part of the wheatbelt.

Dr Chenu says the idea is simple.

“The first step is to get the combination of sowing by genotype maturity right. It is the best way to reduce the effects of abiotic stress on yield and is a strategy of escape or avoidance.

"The second step is to look at crop tolerance to different stresses, such as heat, drought and frost. This is what we are trying to do."

More information: Karine Chenu, 07 4529 4127, karine.chenu@uq.edu.au