Dopamine promotes tolerance in rice under iron excess by improving root anatomy, ionic balance, photosyntheticperformance and biomass.

Abstract

Rice (Oryza sativa L.) is an essential food for more than half of the world's population, and is crucial for food security and the economy. Grown mainly in flooded soils, this crop faces challenges such as low pH and excess iron (Fe), resulting from redox conditions that favor the availability of this element. High concentrations of Fe are toxic to plants, impairing photosynthesis, gas exchange, anatomical structures, the antioxidant system, nutritional status, and biomass production. Therefore, management strategies and the use of biomolecules have been investigated to reduce these effects. Dopamine (DOP) has been studied as a neurotransmitter with a regulatory role in biotic and abiotic stresses. DOP attenuates the toxicity of heavy metals, in addition to mitigating the impacts of salinity, flooding and water stress, through the activation of antioxidant enzymes, improvement of photosynthetic efficiency, modulation of anatomical structures and increase in nutrient absorption and biomass. This study aimed to evaluate whether the exogenous application of DOP reduces oxidative damage in the photosynthetic system of rice plants exposed to excess Fe, in addition to analyzing changes in leaf structure, production of reactive oxygen species (ROS), activity of antioxidant enzymes, nutritional status and biomass. The research was conducted at the Federal Rural University of the Amazon, Paragominas Campus, in a greenhouse under controlled conditions. Ten-day-old rice seedlings were grown in a hydroponic system and subjected to four treatments: two concentrations of Fe (250 μM, control; 5000 μM, excess) and two of DOP (0 μM and 50 μM). DOP was applied to the nutrient solution from the 20th to the 40th day, while Fe was supplied from the 30th to the 40th day. On the 40th day, physiological and morphological parameters were measured, and samples were collected for anatomical, biochemical, and nutritional analyses. Data were submitted to one-way ANOVA and Scott-Knott test (p < 0.05). The results demonstrated that excess Fe causes significant damage to plants. However, DOP promotes nutrient accumulation in roots and leaves, providing Fe content in these structures, in addition to increasing leaf and root anatomy. DOP application minimizes oxidative damage, elevating photosynthetic pigments and increasing PSII quantum efficiency (ΦPSII) and electron transport rate (ETR). In gas exchange, increases in net photosynthesis (PN) and instantaneous carboxylation efficiency (PN/Cᵢ) were observed. DOP also strengthens the antioxidant defense, increasing the activities of superoxide dismutase (33%), catalase (29%), ascorbate peroxidase (75%) and peroxidase (17%), while reducing the levels of reactive oxygen species (O2⁻ and H2O2) and oxidative stress markers (MDA and EL). Although excess Fe reduced biomass, DOP increased shoot, root and total dry matter. Therefore, DOP alleviates the effects ofstress caused by excess Fe in rice plants.