Effect of gypsum-derived calcium on cadmium stress in citrus rootstocks

Cadmium (Cd) contamination severely limits plant growth and physiological performance in citrus by disrupting redox homeostasis, membrane integrity, and plant water relations. The objective of this study was to elucidate whether calcium supplied via gypsum can mitigate Cd-induced physiological and oxidative damage in citrus rootstocks and to identify genotype-dependent tolerance responses. Three citrus rootstocks (Citrus reshni, Citrus aurantium L., and Citrus sinensis × Poncirus trifoliata) were exposed to increasing CdCl₂ concentrations (0–50 mg L⁻¹), either alone or combined with exogenous gypsum (CaSO₄·2H₂O) applications (5.3 and 10.6 g kg⁻¹ soil). Cd stress markedly reduced plant biomass, relative water content, and chlorophyll index, while increasing electrolyte leakage, oxidative damage markers, and osmolyte accumulation. At 50 mg L⁻¹ Cd, RWC declined by up to 65%, whereas EL and MDA increased by >200% compared to control plants (p ≤ 0.05). These alterations were closely associated with excessive reactive oxygen species production and disruption of membrane integrity. Cd exposure also triggered strong activation of enzymatic and non-enzymatic antioxidant defenses, including superoxide dismutase, catalase, ascorbate peroxidase, and phenolic compounds. Gypsum application significantly alleviated Cd-induced physiological dysfunction, with the highest dose (10.6 g kg⁻¹) increasing RWC by up to 170% and reducing oxidative stress markers by 20–30% relative to Cd-stressed plants (p ≤ 0.05). Among the tested genotypes, C. sinensis × Poncirus trifoliata exhibited the highest tolerance to Cd stress, followed by C. aurantium L., whereas C. reshni was the most sensitive. Overall, these findings provide mechanistic insight into calcium-regulated coordination between redox homeostasis and water relations as a key driver of Cd tolerance in citrus rootstocks, highlighting gypsum as an effective strategy to enhance stress resilience in woody perennial species.