Physiological and Antioxidant Responses of Rough Lemon (Citrus jambhiri L.) Rootstocks to Foliar-Applied Green-Synthesized ZnO Nanoparticles Under Drought Stress

Drought stress affects citrus growth and physiological responses by impairing photosynthesis and antioxidant homeostasis. In this study, we produced green-synthesized zinc oxide nanoparticles (ZnO NPs) using leaves from Calotropis gigantea L. We assessed their effects on drought-stressed rough lemon (Citrus jambhiri L.) seedlings. UV–Visible spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and dynamic light scattering (DLS) confirmed the formation of hexagonal wurtzite ZnO nanoparticles with predominantly spherical morphology and nanoscale particle size. The synthetic material had a particle size of about 10 nm. We conducted a pot experiment under well-watered and drought conditions, withholding irrigation for 15 days and applying foliar ZnO-NP at 0, 50, 100, 150, and 200 ppm. Drought caused decreased plant growth, biomass, gas exchange, PSII performance, and chlorophyll relative content. ZnO application reduced these effects, and 150 ppm produced the strongest responses in plant height, number of leaves, biomass, SPAD, and PSII-related traits. Additionally, under drought conditions, 100–150 ppm ZnO also positively affected photosynthetic performance and antioxidant responses such as SOD, CAT, POD, TPC, and DPPH activity. Overall, green-synthesized ZnO NPs, especially at 100–150 ppm, enhanced the physiological attributes and antioxidant defense of rough lemon rootstocks under water-deficit conditions. These results indicate that they could be a nano-enabled method for citrus drought management.