Background A study on photosynthetic and enzyme activity changes and mineral content in lettuce under cadmium stress has been conducted in a greenhouse, utilizing the modulated effect of zinc (Zn) application in the nutrient solution on lettuce. Zn is a micronutrient that plays an essential role in various critical plant processes. Accordingly, three concentrations of Zn (0.022, 5, and 10 mg L−1 ) were applied to hydroponically grown lettuce (Lactuca sativa L. cv. Ferdos) under three concentrations of Cd toxicity (0, 2.5, and 5 mg L−1 ). Results The results showed that along with increasing concentrations of zinc in the nutrient solution, growth traits such as plant performance, chlorophyll index (SPAD), minimum fluorescence (F0), leaf zinc content (Zn), leaf and root iron (Fe) content, manganese (Mn), copper (Cu), and cadmium increased as well. The maximum amounts of chlorophyll a (33.9 mg g−1 FW), chlorophyll b (17.3 mg g−1 FW), carotenoids (10.7 mg g−1 FW), maximum fluorescence (Fm) (7.1), and variable fluorescence (Fv) (3.47) were observed in the treatment with Zn without Cd. Along with an increase in Cd concentration in the nutrient solution, the maximum amounts of leaf proline (5.93 mmol g−1 FW), malondialdehyde (MDA) (0.96 μm g−1 FW), hydrogen peroxide (H2O2) (22.1 μm g−1 FW), and superoxide dismutase (SOD) (90.3 Unit mg−1 protein) were recorded in lettuce treated with 5 mg L−1 of Cd without Zn. Additionally, the maximum activity of leaf guaiacol peroxidase (6.46 Unit mg−1 protein) was obtained with the application of Cd at a 5 mg L−1 concentration. Conclusions In general, an increase in Zn concentration in the nutrient solution decreased the absorption and toxicity of Cd in lettuce leaves, as demonstrated in most of the measured traits. These findings suggest that supplementing hydroponic nutrient solutions with zinc can mitigate the detrimental effects of cadmium toxicity on lettuce growth and physiological processes, offering a promising strategy to enhanc
