Tree Physiological Variables as a Proxy of Heavy Metal and Platinum Group Elements Pollution in Urban Areas

SIMPLE SUMMARY: Air pollution in urban areas represents a major environmental risk to human and ecosystem health. Our aim was to test the adequacy of the Ligustrum lucidum physiological variables as proxies for heavy metal and platinum group element pollution. Chlorophyll, nitrogen and F(v)/F(m) generally showed high values typical of healthy plants, and they did not seem to be consistently affected by the different pollutants. Regarding flavonoid content, it was negatively correlated with heavy metals, which did not confirm its role as a protectant against metal stress. The relatively low levels of pollution usually found in the city of Logroño, together with the influence of other environmental factors and the relative tolerance of Ligustrum lucidum to modest atmospheric pollution, probably determined the only slight response of physiological variables to heavy metals and platinum group elements. ABSTRACT: Physiological variables (the content of chlorophyll, flavonoids and nitrogen, together with F(v)/F(m)) and the content of ten heavy metals (As, Cd, Cu, Hg, Mn, Ni, Pb, Sb, V and Zn) and two platinum group elements (PGEs: Pd and Rh) were measured in the leaves of 50 individuals of Ligustrum lucidum trees regularly distributed in the city of Logroño (Northern Spain). Three of these variables increased with increasing physiological vitality (chlorophyll, nitrogen and F(v)/F(m)), whereas flavonoids increased in response to different abiotic stresses, including pollution. Our aim was to test their adequacy as proxies for the pollution due to heavy metals and PGEs. The three vitality indicators generally showed high values typical of healthy plants, and they did not seem to be consistently affected by the different pollutants. In fact, the three vitality variables were positively correlated with the first factor of a PCA that was dominated by heavy metals (mainly Pb, but also Sb, V and Ni). In addition, F(v)/F(m) was negatively correlated with the second factor of the PCA, which was dominated by PGEs, but the trees showing F(v)/F(m) values below the damage threshold did not coincide with those showing high PGE content. Regarding flavonoid content, it was negatively correlated with PCA factors dominated by heavy metals, which did not confirm its role as a protectant against metal stress. The relatively low levels of pollution usually found in the city of Logroño, together with the influence of other environmental factors and the relative tolerance of Ligustrum lucidum to modest atmospheric pollution, probably determined the only slight response of the physiological variables to heavy metals and PGEs.