A critical evaluation of iron mobilization mechanisms with special reference to the contact effect phenomenon

Abstract

Abstract The mechanism whereby plants growing on aerated and neutral to basic soils obtain adequate amounts of iron is, in spite of several experimentally observed iron stress responses, not yet clear. We have evaluated the currently held models of iron mobilization, and have discussed whether they could, as they presently stand, explain a very old observation, namely the contact phenomenon according to which mere physical contact between plant roots and iron‐containing solid particles enhances the uptake of iron by plant roots. It seems that in aerated well‐buffered soils, the iron mobilization mechanisms such as H+ efflux, reduction of iron, or chelation by organic ligands can be operational to an effective and significant extent only if these chemical processes take place in domains isolated from the chemical reactions taking place in the soil milieu. It is shown that roots of corn attach themselves firmly to sand particles and build extensive root hairs at the contact sites. In the two‐phase experiments in which the nutrient phase lacked iron, corn plants with roots in contact with quartz sand containing traces of iron impurities grew better and absorbed more Fe, Cu, and Mn than plants with roots suspended in nutrient solution. When iron was present in the nutrient phase, however, the roots’ contact with quartz sand had no effect on the plant's growth and the content of the above nutrients. It is postulated that protected micropockets, formed at the root and solid interfaces and sealed off from the rest of the soil milieu by mucigel plugs, shelter a chemical environment favorable to nutrient mobilization. The nutrients thus mobilized diffuse to the root cell membrane but cannot leak out and are not accessible to other roots.