Wheat is a cool season crop with an optimal daytime growing temperature of 15 C during the critical reproductive stage. Here on the Prairies, we can see 30 C during this stage.
Eighty percent of wheat plants exposed to 30 C during a three-day period around anthesis had abnormal anthers, both structurally and functionally. For every degree above the optimum 15 C, wheat experiences a yield reduction of three to four percent. Pre-flowering and anthesis stages are impacted more by high temperature than post-flowering stages, according to the Crop Journal, February 2018.
As the Earth continues to warm, major wheat growing areas will experience, or may already be experiencing, temperatures of 40 C and higher during critical growth stages. Conservative estimates say the average global temperature is rising at a rate of 0.18 C every decade.
Richard Trethowan, director of the Watson Grains Research Centre in New South Wales, says a large component of the breeding work at the centre is on producing wheat, chickpeas and fababeans with high temperature tolerance and improved yield stability.
“We have been running heat tolerance trials for some years,” Trethowan said.
“What we realize is that for every degree rise in mean maximum temperature above the optimum for the season, there is a 250 to 400 kilogram per hectare (100 to 160 kg per acre) yield loss. It is really significant.”
None of this bodes well for an expanding human population that increasingly depends on wheat, much of which is grown close to the equator where temperatures are rising faster, as is the population.
However, there have been recent breakthroughs in identifying genetic and molecular factors affecting heat tolerance in wheat. Plant breeders are moving ahead to improve heat tolerance using existing germplasm found in modern cultivars and indigenous species.
Excessive heat can damage cellular structure and affect metabolic pathways, especially those relating to membrane thermostability, photosynthesis and starch synthesis. This occurs through a molecular process called denaturation, in which proteins lose their natural structure because of an external force such as excessive heat.
This degenerative force increases levels of unsaturated fatty acids and disrupts water, ion and organic solute movement across cellular membranes. The increased cell membrane permeability inhibits cellular function, impacting photosynthesis.
Starch accumulation in wheat kernels is reduced by more than 30 percent at temperatures between 30 C and 40 C. Thus, the ability to synthesize, store and remobilize starch at high temperature is crucial to determination of grain sink strength.
Thylakoids are flattened sacs inside a chloroplast contained by pigmented membranes, upon which photosynthesis takes place. These sacs store the chlorophyll necessary for photosythesis.
Thylakoid membranes are the most easily damaged of all cell components. Membrane damage caused by excessive heat leads to chlorophyll loss. Thylakoid membranes under high temperature show swelling, increased leakiness and physical separation of the chlorophyll light harvesting complex.
Heat stress affects agronomic traits at every developmental stage. Short periods of high temperature at the pre-flowering and flowering stages reduce overall yield and kernel numbers per spike. This is attributed to pollen’s lower ability to germinate and to the rate of pollen tube growth.
Yield losses during post-flowering are due to the abortion of grains and decreased grain weight. The early grain filling period is relatively more heat sensitive than later periods.
Australian researchers have found that variation in average growing season temperatures of 2 C can cut grain yield by as much as 50 percent, most of which is due to the leaves’ rapid aging or deterioration when temperatures are above 34 C. Heat stress during the grain filling stages also has a negative impact on grain quality.
According to the September 2017 edition of the Australian magazine Grain Central, plant breeders in that country are responding to looming climate change by increasing their focus on breeding wheat lines with better yield potential under heat stress conditions.
“Among the Australian cultivars there is generally a degree of heat tolerance that has come from empirical selection in our environment. Breeders are making progress. There are lines like Mace that are one of the most heat-tolerant of all the Australian cultivars available,” said Trethowan.
What can farmers do?
But it’s not all up to the plant breeders, said Trethowan. Farmers have to do their part too if they expect to achieve the best results with the new heat-tolerant lines. The new varieties need to be grown in conjunction with improved farming practices.
“If we are going to do something about the dips in yields in the difficult years, we have to sow crops on time,” he said.
“We want a conservation agriculture system where you are retaining stubble. That gives you cooler soils, cooler roots and plants more able to deal with temperature shock.
“Conservation agriculture also reduces moisture stress. A plant under moisture stress is less able to deal with temperature shock. Weed control is very important. It’s also a water use issue, and adequate nutrition. Plants that don’t have adequate nutrition are less able to cope with temperature shocks.
Rebecca Thistlethwaite, a plant scientist at the University of Sydney, said Australian plant breeders are sourcing genetic material from lines that thrive in hot climates such as India, Mexico and Pakistan. In 2016 and 2017, the breeders looked at 4,200 heat tolerant lines from around the world.
“We have found there is significant genetic diversity both within the Australian and international germplasm,” said Thistlethwaite.
“There have been two particularly promising lines from Australian cultivars: EGA Gregory and Livingston were the two best,” she said.
“Those that did better than the commercial varieties were international germplasm, mainly from Mexico and India.”
Thistlethwaite said another part of the project at Narrabri was developing phenotyping protocols for the best heat tolerance germplasm. In modern wheat, breeders are usually selecting genes on the best yield. Because of that, the genetic base of wheat has been narrowed.
It’s important to broaden the genetic base, and wild wheat ancestors can contribute a lot. Emmer wheat, the first domesticated wheat, contains immense variation, contributing to tolerance of high temperature and drought.
Stay-green capacity is another factor. When a crop is under stress, genotypes with long stay-green ability continue with photosynthesis for very long periods, thus contributing to grain filling.
Heat helps nitrogen
There may be a slight benefit to high heat events. Australian scientists found that in some situations, extreme heat actually increased nitrogen content.
The researchers know they can’t help a wheat plant by turning down the oven or making it rain. But if they better understand how heat waves impact wheat, they will be better prepared for whatever the future brings. To assist in this endeavor, they are entering real time field observations into software models to perhaps help Australian grain growers deal with rising temperatures.
The scientists study the impact of heat on soil microbial activity, soil structure, soil moisture, air around the plants and of course the wheat plants themselves. Knowing how these factors affect crops might help farmers protect their plants against the negative impact of heat waves, said James Nuttall, a scientist with Australia’s federal agriculture department.
“Heat waves greatly reduce wheat yields. Computer modelling helps find strategies to limit the impact of extreme weather and climate change. This can specifically come in handy during the sensitive periods of crop flowering and the grain filling phase,” said Nuttall, who is working with the Soil Science Societies of America.
Nuttall and his team performed three sets of heat stress studies looking at timing, intensity and duration:
- They tested how plants responded to a multi-day heat wave and if it affected plants more during the flowering or grain-filling phase.
- They studied how water availability during the heat wave affected the wheat.
- They grew the plants in outdoor grow chambers, which they could heat up to reach their desired temperatures.
“We have data from three experiment sets on the response of wheat to a range of acute high-temperature treatments for 35 C, 37 C and 42 C. It includes one day, three days and five days of exposure,” Nuttall said.
“Results showed that high temperatures five days before the wheat began to flower reduced the number of wheat kernels on a plant. Also, a high-temperature event while the grain of wheat was growing reduced the grain size.
“We developed algorithms they called ‘heat sum,’ which encompassed temperature, duration in hours and plant stage at which the stress occurred. Some of their findings showed that high temperature five days prior to anthesis reduced kernel numbers and overall grain yield, but individual kernel weight and nitrogen concentration increased.
“For extreme high temperature after anthesis, grain yield and individual kernel weight decreased but nitrogen concentration increased. Water availability prior to anthesis helped wheat respond positively to high temperature.”
They put all the results together into a computer simulation model, which allowed them to predict how broad acre wheat crops in the future will be affected by heat waves of different intensity and duration.
Scientists studying climate change are now delving more deeply into plants’ response to carbon dioxide levels, temperature and rainfall. This crop model allows them to test combinations of these factors on growth and yield.
“These computer models help predict how wheat will react to extreme heat so we can try to prevent negative effects in advance,” Nuttall said.
“Such algorithms contribute to developing strategies for crop adaptation to climate change.”
With a population of only 24 million, Australia does not account for a big piece of the human pie and they don’t consume much of the wheat they grow. But Australia is in a semi-arid zone and it does grow about 32 million tonnes of wheat annually, so wheat plant survival in severe heat is a matter of great concern Down Under.