Urban environments are built from impermeable surfaces such as asphalt, concrete, brick and stone — materials highly efficient at absorbing and re-emitting heat. This study begins with a simple but striking observation. Occupying diverse habitats, O. corniculata exhibits a high degree of intraspecific variation in leaf colour, ranging from green to red. While rarely found in rural environments, researchers noticed that red-leaved individuals were much more likely to grow in urban spaces. The role of red pigments, also known as anthocyanins, in alleviating heat stress is well known — they function by intercepting excess sunlight or neutralising reactive oxygen species. This therefore led the researchers to question whether the high frequency of red-leaved individuals in these environments was a rapid, adaptive response to urban heat stress.
To test this hypothesis, green and red-leaved individuals were grown under heat stress in controlled environments. While green-leaved individuals thrived under normal conditions, when grown under heat stress, red-leaved individuals fared better with higher growth and photosynthetic rates. The poorer performance of red-leaved individuals under normal conditions suggests that adoption of this phenotype comes at a cost. A genome-wide population genetic analysis was then undertaken, revealing that the red-leaf variant of O. corniculata had evolved multiple times from the ancestral green-leaf variant. Altogether, the results of this study indicate that the red leaf phenotype of O. corniculata has emerged in cities across the globe as an adaptive response to urban heat islands.
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