Morphophysiological and biochemical leaf responses of Moringa oleifera Lam. to Zn stress during vegetative growth

This study demonstrates that Moringa oleifera Lam. exhibits morphophysiological adjustments to excess zinc, maintaining photosynthesis and cell membrane integrity through antioxidant responses and resilience to oxidative stress. The ecophysiology of tree growth under excess zinc (Zn) provides insight into tolerance and resistance mechanisms essential for survival in contaminated soils and for the restoration of areas degraded by potentially toxic elements (PTEs). We hypothesized that increasing soil Zn concentrations induce dose-dependent oxidative stress, triggering physiological and antioxidant acclimation mechanisms in Moringa oleifera Lam. This study evaluated the morphophysiological responses of M. oleifera during six months of cultivation under increasing soil Zn concentrations supplied as zinc chloride (0, 100, 200, and 300 mg Zn dm⁻³) under greenhouse conditions. Photosynthetic performance, gas exchange, stomatal traits, antioxidant metabolism, and membrane integrity were assessed. Zinc exposure increased malondialdehyde (MDA) content by 69% at 300 mg Zn dm⁻³ and raised electrolyte leakage by 122% and 141% at 200 and 300 mg Zn dm⁻³, respectively, indicating oxidative damage. The number of leaflets declined by up to 58% at the highest Zn concentration, whereas stomatal structure and pheophytin indices remained preserved. Total chlorophyll and carotenoid contents decreased by 33% and 28%, respectively, at 300 mg Zn dm⁻³, without significant impairment of chlorophyll fluorescence or photosynthetic capacity. Functional stability was associated with compensatory responses, including a 65% increase in water-use efficiency at 200 mg Zn dm⁻³ and modulation of antioxidant metabolism. At moderate Zn concentrations (100 mg Zn dm⁻³), carotenoids helped mitigate oxidative damage, whereas, at higher concentrations, antioxidant defense relied predominantly on enzymatic mechanisms, with significant increases in SOD, catalase (CAT), and peroxidase (POX) activities at 300 mg Zn dm⁻³ (965%, 883%, and 445%, respectively, compared to the control). Overall, M. oleifera demonstrates physiological acclimation and tolerance to soil Zn concentrations up to 300 mg dm⁻³; however, this tolerance is accompanied by oxidative damage and structural changes, indicating measurable physiological costs under high Zn stress.