Cation‐anion uptake balance and proton efflux in plant efficiency for iron

Abstract

Iron efficiency in plants appears to be related to several phenomena which differentially function in different species and cultivars of plants although 2 or more of the mechanisms may be coupled and present in the same individual. Proton expulsion from roots is important in iron efficiency in dicots and it may or may not be associated with excess cation uptake where a cation is exchanged for H⁺. When protons are expelled, the residual OH^‐ in roots must be oxidized but this oxidation releases electrons for redox reactions which may.be coupled with enzymatic Fe⁺⁺⁺ reduction. If this set of reactions is related to catylase or peroxidase, the presence of these enzymes in plants would be more rational. Some plants, particularly monocots, do not expel protons unless much of the nitrogen is in the NH4⁺ form. Such plants then cannot acidify or reduce the rhizosphere. There is evidence that these monocot plants produce phytosiderophores which function in iron mobilization and reduction. Monocots take up more anions than cations and hence tend to increase the pH of the rhizosphere. This increase in pH may improve the stability of the phytosiderophores. The differential role of H⁺ efflux in monocots and dicots may be related to low Ca uptake by the latter. It is of interest that the PI54315–9–1 soybean, which is Fe inefficient, has less Ca uptake than efficient Hawkeye soybean.

Key words:

• Lime induced chlorosis
• Organic acid
• Proton Release
• Reductants
• Siderophores