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Creeping woodsorrels changed their color to adapt to urbanization 

Urbanization has undeniably left a permanent mark on our planet. From sprawling skyscrapers to endless highways, man-made infrastructures have dramatically altered the landscape, imposing selective pressures on the organisms that inhabit these regions. 

While many studies have explored how urban heat stress impacts animal evolution, a recent study has investigated its effects on plants, specifically the creeping woodsorrel.

Urban heat islands

Urban regions are predominantly characterized by heat-retaining surfaces constructed of materials like brick, asphalt, and concrete. These surfaces give rise to what is termed as urban heat islands, regions that experience elevated surface temperatures. 

Urban heat islands have a profound impact on the behavior, physiology, and evolutionary trajectories of resident organisms. According to the researchers, it has remained unclear whether (and how) urban plants adapt to such heat stress

The creeping woodsorrel

Oxalis corniculata, commonly known as the creeping woodsorrel, serves as a prime example. This cosmopolitan plant boasts a vibrant spectrum of leaf colors, ranging from green to red. 

These color variations are not just aesthetic but are believed to be evolutionary adaptations that help the plant cope with environmental stress. 

Specifically, the red pigments (anthocyanins) in the leaves play a protective role against heat and light damage by intercepting light and acting as antioxidants.

Focus of the study 

Dr. Yuya Fukano and his colleagues from various Japanese institutions set out to investigate the relationship between urban heat islands and the leaf color distribution of the creeping woodsorrel. 

“We noticed that the red-leaved variants of the creeping woodsorrel commonly grew near impervious surfaces in urban areas but rarely grew in farmlands or green spaces in and around the city,” said Dr. Fukano. By contrast, the green-leaved variants flourished in rural areas and city green spaces.

This pattern was not restricted to Tokyo. An online database indicated similar distribution patterns worldwide, linking urbanization directly to leaf color variations in this plant species.

Red or green: Which is more adaptive?

Driven by these observations, the team explored the adaptive benefits of the leaf colors. Their findings revealed that red-leaved variants had the upper hand in high-temperature conditions, exhibiting better growth rates and photosynthetic efficiency. 

On the other hand, green-leaved variants outperformed the red in cooler conditions, thriving in dense green areas. This could explain their global distribution patterns, with red-leaved wood sorrels dominating urban heat islands and green-leaved variants preferring non-urban green spaces.

The study also hints that the red-leaf variant evolved multiple times from its green-leaf ancestor, further underlining the adaptability of plants in response to their environments.

Study implications 

While the creeping woodsorrel’s leaf color variations might not transform our world overnight, Dr. Fukano believes that the study offers a glimpse into ongoing evolution in urban settings. 

“Although these findings will not change much in the immediate future, this study showcases one of the most popular examples of ongoing evolution that can be observed in urban areas,” said Dr. Fukano.

“Urban heat islands are precursors to global warming. Understanding the rapid adaptive evolution of urban organisms to high temperatures will provide valuable insights on ecosystem dynamics and sustainable crop production.”

The research is published in the journal Science Advances.

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