Effect of heavy metals and environmental variables on the assimilation of CO2 and stomatal conductance of Ligustrum lucidum, an urban tree from Mexico City

Urban trees reduce CO₂ and pollutants that represent a risk for human health in cities. In this work, we assessed the potential effect of heavy metals and environmental variables on the CO₂ assimilation (A) and the stomatal conductace (g_S) of Ligustrum lucidum, a common urban tree in Mexico City. We compared two sites with contrasting pollution levels: 1) city centre (PPI-C, high pollution level); and 2) south of the city (CU-SW; low pollution level). At each site, we measured 1) phsysiological traits (A and g_S); 2) environmental variables (photosynthetically active radiation, PAR; air temperature, T_A; vapor pressure deficit, VPD; concentration of atmospheric CO₂); and 3) morphological leaf characteristics (stomatal size and density). Concentration of the heavy metals Pb, Cd, Cr, Cu, Mn, Fe, and Zn was determined in washed (internal metals) and unwashed (external plus internal metal) leaves at both sites. CO₂ assimilation at CU-SW was higher than at PPI-C. PAR had the greatest effect on A; whereas T_A and VPD had the greatest effect on g_S. Regarding heavy metals, although we found no significant differences in internal concentrations between sites, we found a lower stomatal density at PPI-C, which may indicate a response of the species to the local pollution conditions. This characteristic might be benefitial for the species, allowing it to maintain optimal physiological conditions by reducing the assimilation of pollutants. Our results suggest that L. lucidum is a well adapted species for the urban environment.