Availability and uptake of iron: Effect of ph changes during uptake of macronutrient ions

Abstract

Long-term greenhouse experiments with corn (Zea mays) established that uptake of Fe from nutrient solutions may be significantly enhanced by increasing the supply of potassium. A mechanism was proposed to describe how uptake of macronutrient ions may in turn influence availability and uptake of Fe and other elements from their chelated and sparingly soluble compounds. It was hypothesized that since differential accumulation of cations is accompanied by a net release of H ions, and excess anion accumulation accompanied by a net release of OH ions, uptake of Fe, for example, will be enhanced by increasing supply of salts favoring differential accumulation of cations: The released H ions will acidify the root free space, root surfaces, and the immediate root environment; minerals on root surfaces will be dissolved and chelated molecules will be dissociated; both reactions will liberate ions most of which will diffuse.through a relatively high H ion environment to absorption sites in the roots. The hypothesis was tested in short-term absorption experiments in which corn seedlings were allowed to absorb Fe from FeEDDHA or Fehydroxide sols in the presence of increasing concentrations of salts of K, Na, Li, Mg, or Ca. Uptake of Fe was highest in the presence of salts from which excess cation accumulation is known to occur and in which the largest pH drop was observed. Increasing the concentration of K, as KC1 or K2 SO4 , from 0.0 to 5.0 or 10.0 mM K in unbuffered solutions or exchange resin suspensions at pH >_ 6.9 resulted in considerable pH decline and increased Fe uptake. The pH decline correlated with the enhancement of Fe uptake. However, acidification to below pH of about 4.7 tended to reduce further Fe uptake. No detectable pH changes occurred in CaC0 3 plus Fe-hydroxide suspensions containing increasing supply of K, yet Fe uptake was enhanced during accumulation of K. Since essentially all Fe supplied as Fe-hydroxide occurred as colloidal particles or precipitates, enhancement of Fe uptake was attributed to a three-ion-contact effect: Plants accumulating excess cations released H ions which decomposed or dissolved the Fe sols or precipitates on root surfaces, thus freeing Fe ions for absorption. Theoretical calculations indicated that increasing H-ion concentration in the immediate root environment will significantly enhance liberation of Fe+ ^ from the stable chelate FeEDDHA. Old hypotheses claiming that potassium ions in plants are directly involved in reactions favoring translocation and utilization of Fe were replaced with one relating K uptake to subsequent reactions in the substrate.