How Summer Heat Kills Fragile Flowers

New research by Madeline Carins-Murphy and colleagues has revealed how the combination of heatwaves and drought can cause rapid and irreversible damage to flowers, limiting reproduction in plants. Scientists found that extreme heat triggers the formation of bubble-like air pockets called embolisms that block the plant’s water transport system, leading to hydraulic failure when soil moisture is low. It’s critically important as the embolisms disable the pathways that supply flowers with water, leading to rapid desiccation and flower death. These findings explain why heat and drought together have an especially severe impact on crop yields and plant reproduction worldwide.

The researchers studied pyrethrum daisy flowers, which are cultivated for insecticide production. They found several surprising factors that make flowers more vulnerable to hydraulic failure during heatwaves than leaves.

Pyrethrum flowers have much higher rates of residual transpiration than leaves when stomata, the plant’s breathing holes, are closed. Transpiration is the loss of water vapour from plant tissues. Residual transpiration refers to water movement through the petals after stomata close in response to drought. This water loss is typically an adaptation to cool flowers. But during heatwaves, it accelerates dehydration.

Modelling showed the soil only needs to be mildly dry for heat to trigger complete hydraulic failure in flower stems. Leaves remained unaffected even with substantial soil drying.

The scientists found that the onset of hydraulic failure in flowers matched the exceedance of a theoretical threshold for runaway cavitation. Cavitation refers to the formation of embolisms. Runaway cavitation is an uncontrolled feedback loop where rising embolisms create more embolisms as water tension increases. This runaway process in flower stems cuts the water supply, rapidly desiccating flowers.

Carins-Murphy and colleagues discovered how critical water was to flower survival by examining how well the flowers coped with heat. They found that direct heat exposure did not damage well-watered flowers, proving that flower mortality was tied to dehydration rather than thermal damage.

While we may think of flowers as pretty and attracting pollinators, they’re a crucial organ for plant reproduction. It’s fertilised flowers that go on to create the seeds and fruits that form many of our crops. Understanding why flowers may be more sensitive to drought conditions could help improve crop yields in drier summers. Carins-Murphy and colleagues found that this sensitivity was due to runaway cavitation in flowers, and this could have serious consequences for crops. They write:

Theoretical predictions and empirical data indicate that runaway cavitation in the flowering stem of mildly water-stressed pyrethrum plants during transient heat induced a rapid decline in water potential resulting in the lethal desiccation of flowers. Validating the role of this process during heat-induced damage to flowers, and by extension, yield losses, and reproductive failure, highlights the importance of incorporating runaway cavitation into process-based modeling to understand the impact of hot and dry conditions on cultivated and natural plant systems. With rising global temperatures and changing rainfall patterns (IPCC 2014), obtaining a greater understanding of the impacts of combined heat and drought stress on plant reproduction is of the utmost urgency. The response of pyrethrum flower mortality to these stresses is likely to reflect a more general response of perennial plants to promote long-term survival of vegetative tissues; however, this remains to be tested. If commonalities exist, then the increasingly frequent cooccurrence of hot and dry weather with flowering (Hedhly et al. 2009) will have substantial negative impact on crop production, species’ persistence, and ecosystem function.

Carins-Murphy et al. 2023.

READ THE ARTICLE
Carins-Murphy, M.R., Cochard, H., Deans, R.M., Gracie, A.J. and Brodribb, T.J. (2023) “Combined heat and water stress leads to local xylem failure and tissue damage in pyrethrum flowers,” Plant Physiology, p. kiad349. Available at: https://doi.org/10.1093/plphys/kiad349.

Cover Image: Pyrethrum by Canva.