Effects of ferrous iron toxicity on the growth and mineral composition of an interspecific rice

Ferrous iron (Fe2+) toxicity is one of the major constraints to lowland rice production. It disrupts the rice plant physiology in several respects. The critical iron toxicity concentration in plant tissue depends partly on the overall nutritional status of the plant. African cultivars are generally less sensitive to this stress than Asian ones, but their yield potential is significantly lower. Interspecific hybridization between African and Asian rices is considered a possible solution to iron toxicity in lowland rice production. The objective of this work was to study the influence of high applied ferrous iron concentrations on the growth and mineral composition of the tissue of an interspecific rice, Oryza sativa L. x Oryza glaberrima Steud. Experiments were performed in hydroponics by applying different ferrous iron concentrations (0, 125, 250, and 500 mg L-1 Fe2+) to an interspecific rice line at two different plant ages. Iron toxicity conditions and symptom development were achieved in a hydroculture system provided with a frequent adjustment of pH, oxygen content, associated iron redox state, and iron availability in the nutrient solution. Symptoms (bronzing, as well as reduction in plant growth and survival rate) appeared at and above 250 mg L-1 applied Fe2+ after 4 weeks of iron-toxicity stress. The hybrid line did not show iron toxicity symptoms at 125 mg L-1 Fe2+, despite an iron concentration in its leaves (3356 mg kg(-1)) well above the usual critical toxicity concentration in rice (700 mg kg(-1)). The mineral elements were classified into three groups according to their concentration in the tissue under the different applied ferrous iron concentrations. In Group I [phosphorus (P), calcium (Ca), magnesium (Mg), copper (Cu), manganese (Mn), molybdenum (Mo), and zinc (Zn)], the concentrations were always the lowest at 125, intermediate at 250, and highest at 0 or 500 mg L-1 applied Fe2+. In Group II [iron (Fe) and sodium (Na)], the concentrations increased with ferrous iron applications. In Group III [nitrogen (N) and potassium (K)], the concentrations did not vary significantly with applied ferrous iron. However, the concentrations of all mineral elements, except iron, were maintained between their critical deficiency and toxicity limits. This property may have contributed to the tolerance - at 125 mg L-1 applied Fe2+ - of the present line to leaf iron concentrations generally considered toxic to rice. Varietal screenings to evaluate of the resistance to ferrous iron toxicity will be performed in accordance with the results of the present study: after 4 weeks of iron-stress, at 250 mg L-1 applied Fe2+ and at the youngest tested age of stress application.