IRON TRANSPORT TO DEVELOPING OVULES OF PISUM-SATIVUM .1. SEED IMPORT CHARACTERISTICS AND PHLOEM IRON-LOADING CAPACITY OF SOURCE REGIONS

To understand the processes that control Fe transport to developing se eds, we have characterized seed growth and Fe accretion and have devel oped a radiotracer technique for quantifying phloem Fe loading in vege tative source regions of Pisum sativum. In hydroponically grown plants of cv Sparkle, developing ovules exhibited a seed-growth period of 22 d, with Fe import occurring throughout the 22-d period. Average Fe co ntent of mature seeds was 19 mu g. Source tissues of intact plants wer e abraded and pulse labeled for 4 h with 100 mu M Fe-59(III)-citrate. Fe was successfully phloem loaded and transported to seeds from leafle ts, stipules, and pod walls. Total export of Fe-59 from labeled source regions was used to calculate tissue-loading rates of 36, 40, and 51 pmol of Fe cm(-2) h(-1) for the leaflet, stipule, and pod wall surface s, respectively. By comparison, surface area measurements, along with seed-growth results, allowed us to calculate average theoretical influ x values of 42 or 68 pmol of Fe cm(-2) h(-1) for vegetative tissues at nodes with one or two pods, respectively. Additional studies with the regulatory pea mutant, E107(a single-gene mutant of cv Sparkle that c an overaccumulate Fe), enabled us to increase Fe delivery endogenously to the vegetative tissues. A 36-fold increase in Fe content of E107 l eaves, relative to Sparkle, resulted in no increase in Fe content of E 107 seeds. Based on these findings, we hypothesized that Fe is phloem loaded in a chelated form, and the expression/synthesis of the endogen ous chelator is an important factor in the control of Fe transport to the seeds.