Toxicity of Zinc Chloride Modulates Chlorophyll, Oxidative Stress and TCA Cycle Associated Organic Acids Exudation in Hydroponically Grown Maize Seedlings

Zinc (Zn) is an essential trace element; however, its excess can lead to toxicity, thereby disrupting plant physiology and rhizosphere interactions. This hydroponic research analyzed the consequence of increasing Zn concentrations (Zn1–Zn3) on maize ( Zea mays L.) seedlings, focusing on growth, photosynthesis, oxidative stress, and root exudation responses. Growth was boosted with a modest Zn supply (Zn1), with root and shoot lengths rising 19.8% and 38.3%, respectively, in evaluation to the control. In contrast, higher Zn levels suppressed shoot length by 25.4% and reduced both fresh and dry biomass. Photosynthetic pigments were adversely affected, with chlorophyll a , b , and total chlorophyll decreasing by 1.9%, 0.8%, and 1.6%, respectively, while carotenoids declined by 0.06–0.08% under Zn stress. Lipid peroxidation intensified, as indicated by a 1.9–2.8% increase in MDA content, and ROS accumulation rose, with O₂ ^•− levels in leaves by 2.4–5.5% at Zn2–Zn3. Zn stress also significantly altered root exudates composition, for example, lactic acid secretion surged under Zn1 (+ 8697%) but decreased sharply under Zn3 (− 3224%), while citric (+ 58%) and malic (+ 54%) acids increased at Zn1, and oxalic acid consistently declined (1109–158%). These changes, along with a 4–5% reduction in rhizosphere pH, suggest that Zn-induced organic acid exudation serves as a detoxification mechanism. Overall, seedlings exhibited a dual response, where moderate Zn levels enhanced growth and exudation-mediated tolerance, whereas excessive Zn induced oxidative stress, pigment loss, and metabolic disruption, underscoring root exudate modulation as a key indicator of Zn stress resilience in maize.