WHOLE-ROOT IRON(III)-REDUCTASE ACTIVITY THROUGHOUT THE LIFE-CYCLE OF IRON-GROWN PISUM-SATIVUM L (FABACEAE) - RELEVANCE TO THE IRON NUTRITION OF DEVELOPING SEEDS

To understand the whole-plant processes which influence the Fe nutriti on of developing seeds, we have characterized root Fe(III)-reductase a ctivity and quantified whole-plant Fe balance throughout the complete 10-week (10-wk) life cycle of pea (Pisum sativum L., cv. Sparkle). Pla nts were grown hydroponically in complete nutrient solution with a con tinuous supply of chelated Fe; all side shoots were removed at first a ppearance to yield plants with one main shoot. Root Fe(III)-reductase activity was assayed with Fe(III)-EDTA. Flowering of the experimental plants began on wk 4 and continued until wk 6; seed growth and active seed import occurred during wks 5-10. Vegetative growth terminated at wk 6. Iron(III) reduction in whole-root systems was found to be dynami cally modulated throughout the plant's life cycle, even though the pla nts were maintained on an Fe source. Iron(III)-reductase activity rang ed from 1-3 mu mol Fe reduced . g(-1) DW . h(-1) at early and late sta ges of the life cycle to 9.5 mu mol Fe reduced . g(-1) DW . h(-1) at w k 6. Visual assays demonstrated that Fe(III)-reductase activity was lo calized to extensive regions of secondary and tertiary lateral roots d uring this peak activity. At midstages of growth (wks 6-7), root Fe(II I)-reductase activity could be altered by changes in internal shoot Fe demand or external root Fe supply: removal of all pods or interruptio n of phloem transport from the reproductive portion of the shoot (to t he roots) resulted in lowered root Fe(III)-reductase activity, while r emoval of Fe from the nutrient solution resulted in a stimulation of t his activity. Total shoot Fe content increased throughout the 10-wk gr owth period, with Fe content in the non-seed tissues of the shoot decl ining by 50% of their maximal level and accounting for 35% of final se ed Fe content. At maturity, total seed Fe represented 74% of total sho ot Fe; total Fe in the roots (apoplasmic and symplasmic Fe combined) w as minimal. These studies demonstrate that the root Fe(III)-reductase system responds to Fe status and/or Fe requirements of the shoot, appa rently through shoot-to-root communication involving a phloem-mobile s ignal. During active seed-fill, enhanced root Fe(III)-reductase activi ty is necessary to generate sufficient Fe2+ for continued root Fe acqu isition. This continuing Fe supply to the shoot is essential for the d eveloping seeds to attain their Fe-content potential. Increased rates of root Fe(III) reduction would be necessary for seed Fe content to be enhanced in Pisum sativum.