SummaryCurrent theories of phosphorus uptake by plants imply that they can augment diffusion to their root axes by the development of abundant root hairs or mycorrhizas. Some phosphorus efficient plants have root morphology with multi-branched roots and localised regions of densely packed root hairs, which we suggest is better suited to the retention of substances exuded by the roots than uptake of substances moving to the root by diffusion. Evidence of substantial exudation by the proteoid roots of Lupinus albus is presented.
Large quantities of citrate ions have been shown to be secreted by the roots of Lupinus albus. It is postulated that these react in the soil to form ferric hydroxy phosphate polymers which diffuse to the root surface where they are degraded by the action of reducing agents in the presence of an Fe II uptake mechanism balanced by hydrogen ion secretion. Some known chemical behaviour of Fe III and citrate which supports this postulate is reviewed. Evidence is also presented which suggests that much of the Fe absorbed circulates within the root system and is subsequently precipitated.
The development of proteoid roots by Lupinus albus was studied under glasshouse conditions in acid-washed sand with varying iron and phosphorus supply and in three contrasting soils (acid, neutral and alkaline) at a range of phosphorus levels. Aqueous extracts of soil from around the roots were compared with regard to their pH and their reducing and chelating activity. In both experiments, proteoid formation was predominantly affected by the phosphorus status of the plant. Manganese uptake in the second experiment was related to proteoid root development and decreased as phosphorus supply increased. The ability ofL. albus to utilise soil and added phosphorus decreased as pH of the soil increased.
Proteoid roots of Lupinus albus can develop under aseptic conditions, but many more are produced when
microorganisms are present. The roots appear to be the source of the hydrogen ions. reductants and
chelating agents observed in their vicinity. The activity of these secretions is reduced in the presence of
microorganisms which presumably use them as sources of energy.
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