Investigating the Physiological Mechanisms of Spontaneous Plants on Urban Walls: Adaptation to Multiple Environmental Stresses

Giorgia Guardigli ¹, Stefano Mancuso ², Nadia Bazihizina ¹

Department of Biology, University of Florence, via Micheli 1, 50121 Florence ¹, Department of Agriculture, Food, Environment and Forestry, University of Florence, via Micheli 1, 50121 Florence; Viale delle Idee, 30, 50019 Sesto Fiorentino, Florence ²

Urban environments pose significant challenges for vegetation due to limited space resulting from extensive urbanization and high levels of surface sealing. The predominance of impervious surfaces, such as asphalt and concrete, not only restricts available soil but also exacerbates urban heat island effects and reduces water infiltration, creating a harsh environment for plant growth. Despite these constraints, cities are characterized by numerous vertical surfaces, such as walls and facades, which are often regarded as inhospitable areas for plants due to their lack of soil, nutrients, and water. Nevertheless, these unutilized vertical spaces are frequently colonized by spontaneous plant species, which are naturally adapted to resource-scarce and extreme conditions. These species exhibit remarkable ecological resilience, enduring high temperatures, water scarcity, and exposure to pollutants, particularly during the summer months. Beyond their survival capabilities, spontaneous urban vegetation contributes to ecosystem services such as enhancing biodiversity, regulating microclimates, and reducing air pollution. Studying these species offers valuable insights into the mechanisms of plant adaptation and resilience, potentially informing strategies for urban greening and the development of sustainable green infrastructure. This study investigates the diurnal eco-physiological responses of four spontaneous species—Capparis spinosa, Cymbalaria muralis, Dittrichia viscosa, and Parietaria judaica—growing on a south-facing wall adjacent to a busy street in central Florence during the summer months. Field measurements of gas exchange, photosynthetic efficiency, and temperature were conducted hourly from 8:00 to 18:00 using a porometer to assess the physiological response of the plants to variations in temperature, light, and humidity throughout the day. During the day, temperature was also monitored to determine how different gas exchange patterns affected plant thermal regulation and its impact on the temperature of the wall beneath the plants and in non-covered parts of the wall surface. Subsequently leaf tissues were sampled for an ionomic analysis. Finally, dehydration-rehydration cycles and their impact on the PSII performance were used to assess to what extent foliar water uptake might help these four species to cope with drought stress. The results highlight inherent differences in physiological parameters among the four species. Despite initial disparities, all species exhibited a common trend in photosystem II efficiency (PSII), which decreased during the central hours of the day and recovered toward the evening. This trend was inversely correlated with stomatal conductance. Thermal regulation effects also varied between species, with Capparis spinosa demonstrating the most pronounced wall-cooling effect, likely due to higher transpiration rates and larger plant size. During dehydration, C. muralis and C. spinosa displayed smaller declines in PSII and electron transport rate (ETR) compared to the other two species, while during rehydration, D. viscosa showed a notable recovery in ETR. The results from the combined field and laboratory measurements will provide insights into the mechanisms underlying plant resilience to urban environmental stresses, highlighting both the differences and commonalities among species, with implications for sustainable urban greening strategies.

Main author career stage: PhD student

Contribution type: Talk

First choice session: 4. Structure, physiology, and development

Second choice session: 2. Ecology