Biogenic zinc nanoparticles modulate physiological, biochemical, and molecular mechanisms to enhance drought resilience in Citrus reticulata

Water stress significantly impairs citrus productivity and threatens global food security. We evaluated the comparative efficacy of ionic zinc (iZn) and biogenic zinc nanoparticles (ZnNPs), synthesized from castor (Ricinus communis) leaf extract, in enhancing drought tolerance of Citrus reticulata seedlings. ZnNPs showed superior bioavailability due to their nanoscale size, uniform morphology, and crystalline structure. Under drought conditions, foliar application of 50 mg L⁻¹ ZnNPs enhanced relative water content (RWC), chlorophyll and carotenoid levels, and photosynthetic rate, resulting in greater biomass accumulation. Antioxidant enzyme activity (SOD, CAT, POD, and GST) was significantly enhanced. In contrast, the levels of oxidative stress indicators, including electrolyte leakage, H₂O₂, and MDA, were significantly reduced, indicating effective ROS scavenging and improved membrane stability. ZnNPs also modulated hormonal balance by elevating stress-responsive hormones (ABA, SA, and JA) while maintaining growth-related hormones (IAA and tZR). Transcriptomic analysis revealed 1545 differentially expressed genes (DEGs) in ZnNPs-treated plants compared with 639 in iZn-treated seedlings. Notably, upregulated genes included CrHSP70, CrWRKY40, CrSOD1, CrNCED2, CrLOX2–1, CrPE53, and CrAMT3–1. Additionally, ZnNPs uniquely activated key metabolic pathways, such as phenylpropanoid and flavonoid biosynthesis for oxidative stress mitigation and cutin/suberin biosynthesis to reinforce physical barriers against water loss. These findings provide the first integrated evidence that ZnNPs enhance citrus drought tolerance through coordinated molecular and physiological mechanisms, offering a sustainable nanotechnology-based strategy for climate-resilient horticulture.