Excess light and drought stress in the urban environment: response in cyanic and acyanic Norway Maple cultivars

The trees in the city, important from an economic, social and environmental point of view, are frequently found to vegetate under particularly severe micro- and macro-environmental conditions, due to the modification of the chemical-physical characteristics of the soil and the air atmospheric pollution, the reduced availability of space for root systems and foliage as well as the peculiar water balance of urban systems. There are also a multitude of abiotic stresses, especially during the summer season, in which there is a combination of high temperatures, high solar radiation and low availability of water in the soil, which further complicate the correct development of the trees. In this context there are some tree species that have at their disposal a series of morpho-anatomical, biochemical and physiological characteristics that contribute to increasing their tolerance to ‘drought’ and to light stress. In particular, ‘red’ plants are able to synthesize a wide range of metabolites including anthocyanins. These absorb in vivo the green and yellow light bands of light, commonly between 500 and 600 nm. They protect leaves from the stress of photo-inhibiting flows by absorbing excess photons that would otherwise be intercepted by chlorophyll. Furthermore, they serve as a useful optical filter, diverting the excess of high energy amount away from the already saturated photosynthetic electron transport chain. Anthocyanins reduce the oxidative load in a leaf by simply filtering the yellow-green light, since most of the reactive oxygen in plant cells comes from the excitation of chlorophyll. These compounds are also excellent scavengers of free radicals for which the induction of leaf anthocyanins has been implicated in the acquisition of tolerance to many different types of environmental stressors. In this context it becomes fundamental to know the physiological responses of the arboreal species, in order to enhance them and use them for a better urban green design. The objective of this thesis, therefore, is essentially to evaluate the ecophysiological strategies adopted by three cultivars of the species Acer platanoides ('Summershade', 'Crimson King' and 'Deborah') to two different types of light stress: the first a ‘sudden’ stress, the second a chronic one. What differentiates these cultivars is the colour of the leaves: the first cultivar is constitutively green, the second is constitutively red and the third is temporarily (in the juvenile phase) red. To this end, in 2016 and 2017 two experiments were conducted at the Minoprio Foundation in Vertemate con Minoprio (CO). During the first (duration 3 months) the sudden light stress was induced by a water stress, while during the second (duration 16 months) the chronic light stress was realized with the use of white concrete slabs that reflected an excess of light. Physiological measurements (gas exchanges, water relations and photochemistry of the photosystem) and optics were carried out. Overall, the results obtained from this doctoral project were as follows: 1. Leaf optical characterization of the three cultivars in question. 2. Cyanic cultivars are suitable for urban sites with very high irradiance through the year and are better equipped to cope with sudden, short drought spell. 3. Under optimal conditions the cultivars with green leaves provided higher benefits, in terms of CO2 storage and transpirational cooling than cyanic cultivars because anthocyanins are “costly” to plants.