The green, the red, and the sunlight

Sunlight irradiance is extremely heterogeneous in the urban environment. In urban canyons, trees do not receive direct sunlight over extended periods and may experience chronic low-light stress, particularly if light-demanding species are planted. In contrast, in urban plazas, where reflected radiation sums up to direct sunlight, plants may suffer from a chronic excess of solar irradiance. In the latter case, incoming total irradiance may largely exceed the amount required for photosynthesis, and leaves are consequently exposed to photo-oxidative stress and generation of Reactive Oxygen Species (ROS). Excess light stress may become severe when coupled with other environmental stressors, such as drought, that severely constrain photosynthesis and, consequently, the usage of sunlight energy for carbon fixation. ROS disrupt cell membranes, lead to leaf necrosis, and hinder recovery of plant performance when stress is relieved. This reduces benefit provisioning and increases management costs of urban trees. Harsh, highly irradiated urban plazas are already quite an inhospitable place for trees, and will become even more unfavorable in the near future, because of rapid climate change. In a previous study conducted on a herbaceous species it was observed that the red cultivar was more tolerant to a sudden drought spell, and the associated photo-oxidative damage than the green cultivar. This was attributed to the photoprotective role provided by anthocyanins located in the epidermal layer of the red cultivar. Here we hypothesized that cyanic cultivars of woody species might be particularly suitable for planting in urban plazas, because of epidermal anthocyanins may attenuate the detrimental effects of excess light stress. To test this hypothesis, we conducted experiments in which plants were exposed to either sudden (Exp. 1) or chronic (Exp. 2) excess light stress using constitutive cyanic (‘Crimson King’), transiently cyanic (‘Deborah’), and acyanic (‘Summershade’) cultivars of Acer platanoides. In Exp. 1, sudden excess light stress was imposed by 1-month drought cycle. In Exp. 2, chronic excess light stress was imposed by increasing pavement albedo from 5% to 30%. Results illustrate evidence of the photoprotective role of epidermal anthocyanins, but also highlight some physiological consequences of “being red” which may affect the landscape performance of red cultivars.