Proton efflux and transfer cell formation as responses to Fe deficiency of soybean in nutrient solution culture

Abstract

Hydroponically grown Hawkeye soybeans with N supplied as NO ₃ ⁻ did not show any measurable pH decrease of the nutrient solution during the first week of Fe deficiency as has been observed for other Fe-efficient dicotyledonous species. Only after prolonged Fe stress with no renewal of the nutrient solution could an unspecific pH reduction be measured as a consequence of a decrease in the NO ₃ ⁻ content of the solution. On the other hand, Fe stress induced H⁺ efflux could be localized at the root tip region by day foru of-Fe treatment when intact plants were transferred from the nutrient solution to agar medium containing the pH indicator dye bromocresol purple. However, the activity of this H⁺ pump obviously was too weak to neutralize HCO₃-ions simultaneously excreted from older root parts and to acidify the bulk nutrient solution. Thus no remobilization of iron precipitated on older parts of the roots occurred and the plants remained chlorotic. Electron microscopy of the H⁺ extruding zone revealed hypodermal transfer cells with wall protuberances surrounded by cytoplasm especially rich in mitochondria. No transfer cells occurred in the rhizodermis as seen in other Fe-efficient dicots. Some cortical cells also showed transfer cell features with wall protuberances in the intercellular spaces. Often wall ingrowths were surrounded by a periplasmic space which reduced the potential surface amplification of the plasma membrane. It is concluded that the weak capacity of Hawkeye soybeans for Fe stress-induced H⁺ extrusion correlates with their less intense wall labyrinth formation as compared with other dicotyledonous species with higher Fe efficiency.