Iron (Fe) toxicity is a major stress to rice in many lowland environments worldwide. Due to excessive uptake of Fe2+ by the roots and its acropetal translocation into the leaves, toxic oxygen radicals may form and damage cell structural components, thus impairing physiological processes. The typical visual symptom is the "bronzing" of the rice leaves, leading to substantial yield losses, particularly when toxicity occurs during early vegetative growth stages. The problem is best addressed through genotype improvement, i.e., tolerant cultivars. However, the time of occurrence and the severity of symptoms and yield responses vary widely among soil types, years, seasons, and genotypes. Cultivars resistant in one system may fail when transferred to another. Better targeting of varietal improvement requires selection tools improving our understanding of the resistance mechanisms and strategies of rice in the presence of excess iron. A phytotron study was conducted to develop a screen for seedling resistance to Fe toxicity based on individual plants subjected to varying levels of Fe (0-3000 mg L-1 Fe supplied as Fe(II)SO4), stress duration (1-5 d of exposure), vapor-pressure deficit (VPD; 1.1 and 1.8 kPa), and seedling age (14 and 28 d). Genotypes were evaluated based on leaf-bronzing score and tissue Fe concentrations. A clear segregation of the genotypic tolerance spectrum was obtained when scoring 28 d old seedlings after 3 d of exposure to 2000 mg L-1 Fe in a high-VPD environment. In most cases, leaf-bronzing scores were highly correlated with tissue Fe concentration (visual differentiation in includer and excluder types). The combination of these two parameters also identified genotypes tolerating high levels of Fe in the tissue while showing only few leaf symptoms (tolerant includers). The screen allows selecting genotypes with low leaf-bronzing score as resistant to Fe toxicity, and additional analyses of the tissue Fe concentration of those can identify the general adaptation strategy to be utilized in breeding programs.
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