Salicylic Acid Mitigates Lead-Induced Stress in the Endangered Shrub Cistus heterophyllus subsp. carthaginensis

Lead (Pb) contamination severely threatens plant health and biodiversity, particularly in mining-affected ecosystems. The phytohormone, salicylic acid (SA), plays a crucial role in regulating plant stress responses. Here, the effect of SA supplementation on the in vitro response of Cistus heterophyllus subsp. carthaginensis, a critically endangered Mediterranean shrub, to Pb stress (50 μM Pb(NO3)2) was evaluated. SA dose pretreatment (100 μM) was selected based on phenolic accumulation in leaf tissues. Physiological and biochemical parameters—including mineral content, photosynthetic performance, total phenolics, and antioxidant activity—were quantitatively analyzed. SA pretreatment markedly reduced Pb accumulation (25%) while promoting Fe (73%), K (29%), and Mn (15%) uptake. It also alleviated Pb-induced photosynthetic impairment, preserved chloroplast integrity, increased chlorophyll content, and reduced the accumulation of lipid peroxidation products. Furthermore, SA promoted the accumulation of phenolic compounds—such as flavonoids, (+)-catechin, gallic acid, and hydroxycinnamic acid derivatives—in Pb-treated shoots, resulting in increased antioxidant capacity, as reflected by DPPH and FRAP assays, and protection against lipid autooxidation. However, no differential effect of SA pretreatment on DNA protection against oxidative damage was observed. Overall, SA acted as an effective priming agent, maintaining mineral homeostasis, photosynthetic stability, and antioxidant defense under Pb stress. These findings highlight its potential for enhancing plant resilience to Pb toxicity and for supporting the conservation and reintroduction of C. heterophyllus in contaminated habitats.