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Shock absorbers

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Dr Andrea Leigh, Senior Lecturer at the University of Technology Sydney, explains how plants withstand extreme high temperatures, important work given climate change predictions that heatwaves will be more frequent and more intense. Desert plants live in extreme environments and one of the ways they cope with that is to produce leaves that are structurally stronger so they do not get eaten by herbivores or be subject to wind damage. The poor soils that they grow in mean that they do not have the energy reserves to drop their leaves and renew them annually, like deciduous plants. Desert plants hang on to their leaves for several years allowing them to conserve soil nutrients. When they do drop their leaves, like many plants, desert plants first draw unused nutrients from the leaves back into their vascular system for recycling.

Hibiscus growing on dune: Desert leaves are often covered in white hairs to reflect radiation and avoid overheating (Andrea Leigh)    
Desert plants live on the extreme edges of biological tolerance when it comes to temperature.
The morphology of plant leaves, such as their shape and thickness, assists plants to cope with really high temperatures, as does their reflectiveness. Many plants in the arid and semi-arid areas, some two thirds of our continent, are covered in white hairs or waxy coatings, both of which help to reflect radiation.

Infrared image shows a desert plant keeping cool among the hot rocks on the ground (Ellen Curtis)    
 For those plants that do not reflect radiation it is physiological means that must protect them. In plants the most important process to protect is photosynthesis, the process that gives the plants (and everything that eats plants) energy. Physiological methods can now be used to locate thresholds for damage to the photosynthetic machinery of plants, thresholds which mean damage can be irreparable or that the plant can repair damage and still survive.

These thermal damage thresholds are different for different plant species. A species of saltbush may have a thermal damage threshold of 47ºC whereas a species of spinifex may have one of 50ºC. These thermal damage thresholds also change over time. Just as people can cope with really hot days in summer but not if there is one in spring, plants follow a similar acclimatisation response. Work done by PhD student Ellen Curtis at the Australian Arid Lands Botanic Gardens in Port Augusta, South Australia, has shown thermal damage at moderate temperatures in winter and spring, when those plants species can survive unscathed at the same temperature in summer. This thermal damage threshold shift from winter to summer is unique for each plant species. When any organism experiences really high temperatures, heat shock proteins come into play. These heat shock proteins lock onto other proteins to stop them unravelling or refold them if they do unravel.

Leaf in fluorometer: measuring the thermal threshold for a species using a chlorophyll fluorescence technique (Andrea Leigh)    
For plants, more exposure to greater temperatures means that more heat shock proteins are produced. This is a gradual up-regulation, from season to season, so a sudden spring heatwave causes damage. There must be an energy cost for plants to make these heat shock proteins so understanding those trade-offs will help us understand which plant species are more vulnerable and which are more resilient. It may also be that widespread species such as mulga have better abilities to find nutrients in the poor soils potentially helping them to cope better with heat stress.

Plants transpire by losing water vapour through the stomata (tiny pores) on the undersides of their leaves, a cooling process similar to sweating in humans. Plants will always transpire if their roots can access water so they can keep their stomata open. When water is lacking plants close the stomata and stop transpiring, something they will do in drought. In deserts some plants will close their stomata for very long periods, an adaption to the dry conditions. Other plants, like those in riverbeds or floodplains, are adapted to higher levels of water and are more likely to keep their stomata open. Such plants also show lower thermal damage thresholds, meaning they cope less well with heat stress. The thermal damage thresholds also depend on whether the plant has naturally adapted to more (or less) water.

Recovery lights in leaf torture chambers in the desert laboratory, each one is a different temperature (Ellen Curtis)    
This research has implications for the parameters of climate change models that look at temperature stress on vegetation, as other factors need to be accounted for. Such models need to account for the differences between plant species, differences in the times of year and the differences in the microhabitat that the plant has naturally adapted to.
One size does not fit all, which is why models are refined all the time.

There is much work still to be done in the future. PhD student, Kirsty Milner, will be looking at how plants low in nutrients cope with heat stress. PhD student, Alicia Cook, will be looking at how heat stress affects very young plants like seedlings. Another area of research is looking at the recovery time needed for plant photosynthetic equipment to repair from the shock of consecutive heat stress events experienced over a short period of time. It seems that the inbuilt plant shock absorbers have a huge role to play in their survival.

Dr Andrea Leigh was interviewed by Ruby Vincent for A Question of Balance. Images from Dr Leigh. Summary text by Victor Barry, March 2015

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