Seaweed biostimulant enhances freezing tolerance in citrus via coordinated redox regulation and metabolic adjustment

Frequent freezing events pose a significant challenge to citrus production in subtropical regions, necessitating sustainable strategies to improve cold hardiness. This study investigated the effect of foliar (F), root-zone (R), and combined (FR) applications of seaweed extract (SWE) on Valencia orange plants under freezing stress (FZS). Freezing stress reduced total biomass (12.8%) and photosynthetic efficiency (23.3%), accompanied by increased oxidative damage, as indicated by elevated levels of hydrogen peroxide (H₂O₂), superoxide radicals (O₂ •− ), and lipid peroxidation (LPO) by 2x as compared to control plants. Seaweed extract, primarily through the combined foliar and root-zone (FR) treatment, enhanced biomass and photosynthetic performance, relative to unstressed treated plants. This improvement was associated with reduced oxidative stress markers and increased antioxidant enzyme activities, including catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD), which increased by 35.4, 15.5, and 16.5%, respectively. Moreover, soluble sugars, such as glucose, fructose, and sucrose, accumulated to higher levels (36.8%) as compared to the control (20.7%), indicating improved osmotic adjustment and cryoprotection. In summary, these findings demonstrate that the combined FR method using seaweed extract enhances freeze tolerance in citrus by strengthening antioxidant defenses, limiting oxidative injury, and promoting osmoprotectant accumulation, thereby offering a sustainable approach to mitigating freezing stress in the citrus production system.