Comparative evaluation of factors involved in Fe stress response in tomato and soybean

Abstract

Iron‐efficient and Fe‐inefficient tomato (Lycopersicon esculentum Mill; T3238FER and T3238fer, respectively) and soybean (Glycine max (L.) Merr.; A7 and T203, respectively) were grown in modified Hoagland solutions with varying levels of Fe (0, 0.025, 0.05, and 0.75 mg Fe L^‐1). The release of H⁺ ions, reductants and reduction of Fe³⁺ to Fe²⁺ by the root, and concentration of nutrients in plant leaves and roots were measured to determine if these factors interact during the Festress response mechanism. The FER tomato responded to Fe stress by increasing H ion and reductant release concurrently with increased reduction of Fe³⁺ by the roots; all three reached maximum levels at the same time (day 7). Iron‐stressed A7 soybean responded first with increased H⁺ ion release and reduction by the roots, while reductant release lagged about 1 day behind. In both cases, increased concentration of leaf Fe corresponded to the time of maximum Fe‐stress response. The Fe‐inefficient T3238fer tomato and T203 soybean did not show any of these responses. When soybean plants (A7) were under Fe stress and then received either 0 or 0.20 mg Fe L^‐1, the Fe‐stressed plants responded to added Fe by reduced levels of H⁺ ion and reductant release and reduction of Fe by the roots. We propose that the three steps of the Fe stress response mechanism‐‐release of H⁺ ions, release of reductants, and reduction of Fe³⁺ by the roots‐‐work together to favor maximum reduction of Fe³⁺. The H⁺ ions provide maximum solubility of Fe, reduction by the roots provides reduced Fe, and reductants in the roots provide the environment to maintain the reduced Fe²⁺ in the root.