How plants cope with heat stress

As Australia swelters through a scorching summer, the nation’s flora is facing not just extreme heat, but also prolonged heat exposure. This deadly combination can damage plant health and resilience, with potentially devastating consequences. 

Understanding how plants withstand heat stress is key to developing strategies to help both native plants and crops survive in an increasingly warming climate, supporting both ecosystem stability and food security.

Plant ecologist Associate Professor Andy Leigh from the University of Technology Sydney (UTS) has travelled to some of the world's most extreme environments, to study how temperature affects plant biological processes, and how they survive and adapt.

“Plants play a vital role not only in providing food and habitat, but also in cooling the environment and storing carbon from the atmosphere. Even the vast desert regions of Australia are vegetated, unlike some other deserts around the world,” said Associate Professor Leigh.

“Many plant species have evolved remarkable strategies to deal with heatwaves. Leaf shape, size and colour all affect heat absorption. In the desert and in the alps, leaves often have a very pale waxy or hairy surface, which reflects excess radiation from the sun. Small leaves also help reduce water loss and generally stay cooler than large ones.”

“However, when you combine drought and heatwaves, that spells disaster. Heat stress impairs photosynthesis, which is how plants convert sunlight into energy, and it disrupts growth and flower and seed production,” she said.

A study led by UTS PhD candidate Alicia Cook, under Associate Professor Leigh’s supervision, has challenged the traditional understanding of heat thresholds in plants. It shows that a plant’s heat tolerance is better measured by cumulative heat exposure rather than peak temperatures.

“In food processing, this concept has long been used to prevent food poisoning. The higher the temperature, the less time it takes to kill any nasty bacteria. Conversely, if the temperature is lower, it takes longer to kill the bacteria,” said Associate Professor Leigh.

“We tested this idea in plants by measuring the decline in the efficiency of leaf photosynthesis after exposure to high temperatures over different lengths of time.

“Like bacteria and other organisms, heat tolerance in plants follows a similar pattern which suggests a fundamental metabolic similarity across all life forms. It also more accurately mirrors the type of heat stress found in nature,” she said.

Humans are also impacted by cumulative heat exposure, with more hospital visits and deaths during prolonged heatwaves. And what doesn’t kill you makes you stronger – many plants and animals will acclimatise and become more heat tolerant if they experience some heat stress.

The study, Beyond a single temperature threshold: Applying a cumulative thermal stress framework to plant heat tolerance, was published in Ecology Letters.

Associate Professor Leigh has just returned from Kosciuszko National Park where she is working on a collaborative project, assisted by the NSW Government through its Environmental Trust, with the NSW Department of Climate Change and Energy, NSW National Parks and Wildlife Service (NPWS), the Australian National University and the Royal Botanic Gardens Sydney.

“This project is looking at threatened ecological plant communities found in alpine environments, and whether different genetic strains of some alpine species are more resilient than others. This is important because the plants used by NPWS to rehabilitate eroded areas need to be resilient.

“As the temperature warms, cold tolerant species don’t have very far to go. They occupy a tight environmental niche, and interestingly, as the temperature rises this can actually lead to more cold stress as there is less insulating snow cover to protect the plants.

“We use bespoke equipment to simulate heatwave conditions, applying this to different foundational alpine species in situ. This kind of experiment is virtually never done, as applying heatwave conditions to plants in a remote alpine region is incredibly challenging,” she said.

The project is being led by UTS PhD candidate Lisa Danzey, who recently published a research article on the topic: Environmental and geographical drivers behind alpine plant thermal tolerance and genetic variation in the journal Plants.

While the natural environment has finely tuned heatwave defences, home gardeners often struggle to protect their plants from harsh conditions. Heat stress can cause garden plants to wilt, scorch, and drop their leaves.

“If you know a heatwave is coming, the best thing to do is to water your plants at the base, not on the leaves, and water them really well, not just a sprinkle. Early morning or late evening is best, allowing the soil to absorb moisture before temperatures rise.”

“When it gets really hot, humans sweat because it cools us down. When water evaporates off our skin, it takes heat with it. Plants are the same, they've got little pores on their leaves called stomata. If they have enough water, they will open their stomata and water will evaporate.

“Mulching to retain soil moisture, and placing pots in the shade during extreme heat events, or using temporary covers, can make a huge difference. Even grouping plants together can create a microclimate that helps them survive.”

Studying how plants function under extreme temperatures is essential for conservation and agriculture. By understanding which species are most vulnerable, we can plan for a future where plants – and people – continue to thrive.