Urban lighting alters chlorophyll metabolism and promotes CO2 assimilation during the night in Tilia plathyphyllos Scop. and Platanus x acerifolia (Aiton) Willd

Light pollution is one of the most impacting factors on plant photodynamics in the urban environment. An experiment was conducted in Tilia plathyphyllos (TP) and Platanus x acerifolia (PA) to test whether streetlamp illumination (similar to 700 mu mol m(-2) s(-1) PAR during the night) alters chlorophyll daily rhythmicity and modifies the leaf CO2 assimilation rate (P-n). Lighted plants (L) showed positive values in P-n during the night indicating the presence of active CO2 assimilation processes compared to controls (C) (-1.68 vs 1.67 mu mol CO2 m(2) S(-1)s(-1) in TP and -2.16 vs 1.90 in PA, L vs C plants, respectively). In parallel, intercellular CO2 concentration values were lower and stomatal conductance higher in L than C plants. Chlorophyll (CM) content was higher in L plants (16.49 vs 24.27 mu g cm(-2) in TP and 16.79 vs 18.46 in PA, L vs C plants, respectively). We suggest that the streetlamp illumination and similar red-to-blue ratio to that of midday sunlight reduced the natural down-regulation of Chl biosynthesis during the night, thereby altering Chl content. However, no alteration in the daily rhythmicity of Chl content was observed. We offer new evidence that streetlamp emissions should be carefully considered to reduce their impact on urban trees.