Australia broke its “hottest day” record this week, and heat waves are becoming more common in Australia. Heat waves are projected to increase in duration and intensity with global warming and climate change.
As recently as 2012, heat waves have caused corn and cotton crop failures in the United States and Europe.
More frequent heat waves with global warming may result in crop failures. Fortunately, there is a national and international effort by plant breeders and physiologists to develop new varieties of crops with heat tolerance.
In many cereal crops such as wheat, chickpea and rice, the reproductive stage is most sensitive to high temperatures, and it is important to ensure that physiological traits (such as pollen viability) for heat tolerance are targeted.
Cotton is a major summer crop in Australia. At Narrabri, in NSW, Dr Nicola Cottee at CSIRO Plant Industry is collaborating with the University of Sydney to screen cotton cultivars at a gene, cell, leaf, plant and crop level. She is trying to identify contributors to high temperature tolerance.
I was surprised to see Dr Cottee had well-watered cotton plants growing in a heat chamber at 45˚C for over two weeks in good condition. In fact, cotton mainly uses water to cool its leaves to an optimal leaf temperature of 28˚C. This works almost like an evaporative cooler in an air-conditioner.
So if air temperatures are close to 28˚C, cotton can grow with little water, because it doesn’t need as much transpirational water to cool itself. In the USA, one cotton variety - VH260 (35˚C )- has a higher photosynthetic optimum than another - ST4554 (28˚C). This means there is an opportunity to develop cotton which can photosynthesise at higher temperatures.
Chickpea is a major winter grain legume grown in rotation with cereal crops in the Australian grain belt. Heat waves are a serious threat to chickpea yield and grain quality.
Chickpea is a major source of protein for people living in the Indian Sub-Continent; it is like a meat substitute, as many Indians are vegetarians. One of my PhD students, Viola Devasirvatham, explored chickpea varieties with superior tolerance to high temperatures (above 35°C). She has used various approaches in the field and controlled environments in Narrabri, NSW and at the International Crops Research Institute for Semi-Arid Tropics (ICRISAT) in India.
Heat stress during the reproductive stage was the main cause of yield loss in chickpea. High temperatures reduced pod set by reducing pollen viability and pollen production per flower. Pollen in the tolerant variety (ICCV92944) was viable at 35°C and at 40°C resulting in pod set. Pollen in the sensitive variety (ICC5912) was completely sterile at 35°C resulting in no pod set. Heat tolerant chickpea varieties have been identified, and these could maintain chickpea yields under heat wave conditions.
Worldwide, more than 3.5 billion people depend on rice for more than 20% of their daily calories. One-fifth of the world’s population - more than a billion people mostly living in Asia - depend on rice cultivation for their livelihoods.
Researchers at Macquarie University have found that an Australian wild rice species, Oryza meridionalis, native throughout northern Australia, had a higher growth rate and leaf photosynthesis than domestic rice, Oryza sativa, at 45°C.
Wheat is Australia’s largest winter cereal crop by area and production. Effects of high temperature stress on yield and grain quality are often most important during the pollen development and early grain filling stages.
At the University of Sydney, Professors Richard Trethowan, Jeff Amthor and I have begun looking at genetic variation for heat tolerance in wheat germplasm. This work is relevant to the northern grains region of Australia. We will use a range of germplasm already showing promise for high temperature tolerance. These lines will be tested in the field and in high temperature chambers and glasshouses.