Complexity of iron nutrition: Lessons for plant‐soil interaction research

Abstract

Iron deficiency chlorosis remains an economically important plant nutrition problem after decades of research. However, basic research on Fe nutrition has provided much information important to a general understanding of plant nutrition, including: 1) chemical equilibria in nutrient solutions and xylem and phloem fluids; 2) regulation of the root's Fe uptake potential to meet the availability of Fe to the root; 3) localization of ferric reduction, Fe‐uptake, and proton excretion in young parts of roots (<3cm from tip) of main and lateral roots; 4) excretion of ligands (phytosiderophores) by graminae to facilitate cation diffusion and uptake; 5) excessive phosphate use as pH buffer in nutrient solutions confounds plant research compared to other buffers; and 6) uptake of Fe by plants under sterile conditions shows uptake is a fundamental plant capability; however, plants may also use Fe from some microbial siderophores.

One of the most important iron nutrition research findings is regulatory control of Fe uptake and translocation. Severe Fe‐stress causes derepression of the potential of dicots to absorb‐translocate Fe from Fe‐chelates by up to 500‐fold faster than fully Fe‐sufficient plants. The increased reduction, uptake, translocation, and proton excretion are highly localized within a few cm of main and lateral root tips. For many species, Fe deficiency causes root tip growth to be decreased, and many lateral roots are initiated. Where studied, these Fe‐deficient young epidermal cells (including root hairs on many species) are derepressed for all the Fe‐stress‐response activities. Roots show regulatory control on Fe uptake potential even in the absence of shoots. Roots supplied adequate total Fe, but in the presence of excess chelator which lowers Fe availability, display strong Fe‐stress‐responses, but remain green with normal foliar Fe levels. Although feedback control on shoot Fe is very effective, keeping Fe in young laminae at about 80 mg/kg dry weight, the mechanism of this regulation is not yet understood.

Key words:

• Fe‐stress‐response
• acidification
• reduction
• uptake
• chlorosis
• phosphate
• bicarbonate
• regulatory control
• chelates