From green to red: Urban heat stress drives leaf color evolution

Urban environments, occupying approximately 1% of total land area, often
impose novel biotic and abiotic selective pressures on organisms and
provide valuable opportunities to understand the eco-evolutionary dynamics
between nature and human societies. Prevalence of impervious surface and
resulting higher temperatures in urban areas, known as urban heat islands,
comprises prominent characteristics in global cities. However, it is not
known whether and how urban plants adapt to such heat stress. This study
focused on Oxalis corniculata, which has intraspecific polymorphism in
leaf color (green, red), and examined whether the leaf color variation is
associated with urban heat stress. Field observations revealed consistent
associations between leaf color and habitat types (green vs. urban) at
local (< 500m), landscape (< 50km), and global scales.
Green-leaved plants were dominant in green habitats, and red-leaved
individuals had increased in number in urban habitats. Growth and
photosynthesis experiments indicated the adaptive benefit and cost of
red/green leaves associated with heat stresses. Red-leaved individuals had
higher growth rates and photosynthetic efficiency under heat stress, while
green-leaved individuals displayed higher growth rates and photosynthetic
efficiency under non-stressful conditions. Genome-wide SNP analysis
suggests that the red leaf trait may have evolved multiple times from the
ancestral green leaf, rather than spreading from a single origin of red
leaf evolution. Overall, the results suggested that the dominance of red
leaves of O. corniculata seen in cities worldwide would be evidence of
plant adaptative evolution due to urban heat islands.