Physiological and Biochemical Responses of Evergreen and Deciduous Trees to Combined Drought and Lead Stress: Implications for Urban Green Spaces

Abstract

The impact of drought and Pb toxicity were evaluated relative to physiological and biochemical traits of four commonly used urban tree species. The experiment included six treatments: control, 300 ppm Pb, 600 ppm Pb, drought (50% field capacity), drought + 300 ppm Pb, and drought + 600 ppm Pb, applied to Platanus orientalis, Fraxinus excelsior, Cupressus sempervirens, and Pinus nigra. Results showed that Pb toxicity – particularly 600 ppm and drought – led to changes in most traits of the plants (P ≤ 0.05). The co-applied drought and 600 ppm Pb decreased total chlorophyll (Chl, 21%), relative water content (RWC, 23%), while increased malondialdehyde (MDA, 52%), proline accumulation (22%), total soluble sugar (TSS, 54%), superoxide dismutase (SOD, 110%), catalase activity (CAT, 137%), Pb accumulation in roots (1723%), and in shoots (611%) compared to the control. Compared to C. sempervirens and P. nigra, P. orientalis and F. excelsior accumulated more Pb and exhibited greater sensitivity to abiotic stress. The heat map results exhibited that traits under drought and Pb stress showed the most significant fluctuation, with TSS explaining the most variation among measured traits.