Genotypic variation in physiological and molecular responses underpins differential germanium tolerance in rice (Oryza sativa L.)

• Screening identified Ge-tolerant (TXY2115/CLY669) and sensitive (ZXY1205/ZZY8) rice. • Tolerant genotypes maintain superior growth, biomass, and photosynthesis under stress. • Tolerance involves preemptive antioxidant defense with higher SOD/CAT activity. • Elevated root glutathione and upregulated OsSOD1 / OsCAT1 reduce oxidative damage. • Findings enable Ge assessment and provide targets for breeding resilient rice. Germanium (Ge) exhibits dual effects in plants, ranging from beneficial to toxic, depending on its concentration and the plant genotype. A germination-based screening of 18 diverse rice varieties identified TXY2115 and CLY669 as tolerant, and ZXY1205 and ZZY8 as sensitive to Ge stress. This study elucidates the physiological and molecular mechanisms underpinning this differential tolerance under escalating Ge stress (0–40 mg l -1 ). The tolerant genotypes effectively mitigated Ge toxicity, sustaining superior growth, higher biomass, and preserving photosynthetic function, evidenced by 18–25 % higher stomatal conductance. This superior photosynthetic function, despite similar percent reductions in total chlorophyll content (56 % in tolerant vs. 54 % in sensitive varieties at 40 mg l -1 Ge), is explained by two key traits: (1) tolerant varieties retained a higher chlorophyll a/b ratio (1.8 ± 0.1 vs. 1.4 ± 0.1 in sensitive genotypes), preserving photosystem II (PSII) stability, and (2) stronger coupling between stomatal conductance (Gs) and net photosynthetic rate (Pn) ( r = 0.83, p < 0.01), as observed in arsenic-tolerant rice. The cornerstone of their tolerance was a preemptive and highly coordinated antioxidant defense system. This was characterized by a significantly enhanced capacity to activate antioxidant enzymes (11–40 % higher Superoxide Dismutase,(SOD) and Catalase (CAT) activity), boost key metabolites like glutathione (33 % higher root GSH), and strongly upregulate the expression of core antioxidant genes ( OsSOD1 and OsCAT1 ). This robust multi-level response effectively curtailed oxidative damage, resulting in 37–53 % lower ROS and 45–58 % lower Malondialdehyde (MDA) levels than the sensitive varieties at high Ge concentrations. This research provides a critical framework for assessing Ge’s role in agriculture, its mitigation, and identifies specific targets for breeding crops resilient to Ge stress or suitable for Ge biofortification.