DOES IRON-DEFICIENCY IN PISUM-SATIVUM ENHANCE THE ACTIVITY OF THE ROOT PLASMALEMMA IRON TRANSPORT PROTEIN

Roots of Fe-sufficient and Fe-deficient pea (Pisum sativum L.) were studied to determine the effect of Fe-deficiency on the activity of the root-cell plasmalemma Fe2+transport protein. Rates of Fe(III) reduction and short-term Fe2+ influx were sequentially determined in excised primary lateral roots using Fe(III)-ethylene- diaminetetraacetic acid (Fe[III]-EDTA). Since the extracellular Fe2+ for membrane transport was generated by root Fe(III) reduction, rates of Fe2+ influx for each root system were normalized on the basis of Fe(III) reducing activity. Ratios of Fe2+ influx to Fe(III) reduction (micromole Fe2+ absorbed/micromole Fe[III] reduced) revealed no enhanced Fe2+ transport capacity in roots of Fe- deficient peas (from the parental genotype, Sparkle) or the functional Fe-deficiency pea mutant, E107 (derived from Sparkle), relative to roots of Fe-sufficient Sparkle plants. Data from studies using 30 to 100 micromolar Fe(III)-EDTA indicated a linear relationship between Fe2+ influx and Fe(III) reduction (Fe2+ generation), while Fe2+ influx saturated at higher concentrations of Fe(III)-EDTA. Estimations based on current data suggest the Fe2+ transport protein may saturate in the range of 10-4.8 to 10-4 molar Fe2+. These results imply that for peas, the physiological rate limitation to Fe acquisition in most well-aerated soils would be the root system's ability to reduce soluble Fe(III)-compounds.