Exploring the potential of red-leafed trees to counter excess light stress and drought in urban plazas: A case study with Acer platanoides

We tested the hypothesis that trees with red foliage are better suited than co-specific green-leafed individuals to cope with excess light, a stress condition typically experienced by plants growing in urban plazas. As a corollary, we surmised that red-leafed trees better counteract photooxidative stress than the green-leafed plants when drought stress reduces the usage of PAR for photosynthesis. For this purpose, we analysed the responses of the red-leafed (‘Crimson King’, CK) and the green-leafed (‘Summershade’, SS) cultivars of Acer platanoides to (1) chronic light stress, imposed by increasing solar light reflectance through the use of light-coloured concrete slabs; (2) to one-month imposition of drought stress of increasing severity. In CK, the maximum daily light harvesting capacity was higher than in SS, because of a greater photosynthetic light saturation point. Net photosynthesis was higher in CK than in SS under chronic light stress. Similarly, during drought, CK maintained higher photosynthetic rate during the central hours of the day than SS, although neither of them underwent photoinhibition and their recovery was similar after relief from drought. We suggest that the ability of anthocyanins to limit the flux of solar photons to the chloroplasts, makes red-leafed CK well suited to grown in urban sites suffering from chronic or drought-induced light stress. On the contrary, attenuation of solar irradiation coupled with high cost of anthocyanin biosynthesis largely limits carbon gain and consequential whole-plant growth when red-leafed cultivars are grown at natural irradiance.