Biomass allocation and leaf trait responses of rice genotypes to iron (Fe2⁺) toxicity

Iron (Fe) toxicity, particularly in the ferrous form (Fe²⁺), poses a serious threat to rice productivity in lowland regions of Bangladesh due to waterlogging and the use of Fe-rich irrigation. This study evaluated six rice genotypes-four previously identified as tolerant (BU Line-2, BU Line-5, BU Line-8 and BU Line-12), one moderately susceptible (BU Line-7), and one susceptible (BRRI Dhan-102)—under three Fe²⁺ levels i.e. Fe₀=Control, Fe₆₀₀=600 mg L⁻¹, and Fe₁₂₀₀=1200 mg L⁻¹ in semi-controlled pot conditions. Key physiological traits, including biomass allocation, root and leaf characteristics, leaf bronzing score (LBS), and tissue damage, were measured at 75 days after transplanting. Genotypes responded differentially to Fe stress. At 1200 mg L⁻¹ Fe, BU Line-12 showed the lowest total dry weight reduction (28.2%), compared to 55.7% in BU Line-5 and 50.5% in BRRI Dhan-102. BU Line-12 also recorded the lowest LBS (2.7 at Fe₆₀₀), while BU Line-2 exhibited strong root resilience (only 9.7% root DW reduction at Fe₆₀₀) but suffered higher shoot damage (LBS 7.84 at Fe₁₂₀₀). Reproductive biomass was especially sensitive, with up to 96.2% reduction in BU Line-5 under severe Fe stress. BU Line-12 consistently maintained shoot-root balance and showed fewer fully damaged leaves, making it the most promising candidate for Fe-toxic environments. The findings highlight the complex nature of iron (Fe) tolerance in rice and emphasize the effectiveness of trait-based screening in identifying tolerant genotypes. These results are valuable for guiding rice breeding efforts and selecting genotypes suitable for Fe-rich lowland ecosystems.