Translocation of Iron from Soybe Cotyledons 1

Soybean seeds, Glycine max L. Merrill, were produced by plants treated from anthesis to seed maturity with 'Fe supplied as ferric ethylenediaminedi(o-hydroxyphenylacetate). Seed coats accounted for 7.4% of dry seed weight and had Fe concentrations 5 times greater than the embryos. After germinating 2 days, cotyledons contained 69.6% and radicles 5.0% of original seed Fe. Fractions of seed Fe unavailable to seedlings were: 19.8% in seed coats, 1.7% in germination paper, 0.1% in the water under germinating seeds, and 3.8% unaccounted for. Every 3 days seedlings received nutrient solution without Fe or with 10 gM ferric ethylenediaminedi(ohydroxyphenylacetate) and developed as -deficient Fe or normal Fe plants. The deficient Fe cotyledons on day 18 retained 13% of the labeled Fe originally present. Cotyledons of normal Fe plants retained 50 to 70% of their original Fe. Moreover, cotyledons of the normal Fe plants accumulated externally supplied Fe and finally contained twice the quantity of Fe originally present. Stem exudate collected above cotyledons of deficient Fe plants contained 5.3 /M 'Fe. Eleetrophoresis of exudate showed that most of the 'Fe migrated anodically as a single band and was in the position of ferric citrate. Knowledge of Fe translocation in plants generally focuses on moveinents of this metal from roots to leaves. Much less has been done to study conditions that induce or suppress translocation of Fe stored internally, e.g. in cotyledons. Or.e might consider this fraction unimportant, for Fe reserves in seeds of flowering plants, even the large seeded legumes, provide only a small part of that needed for mature growth. It is reasonable, however, to consider this fraction physiologically important in delaying the onset for Fe deficiency in seedlings temporarily exposed to Fe-deficient media. 'This work was partially supported by United States Atomic Energy Commission funds available to Lee 0. Tiffin. I Contribution from the Agricultural Environmental Quality Institute and Plant Physiology Institute, Agricultural Research Center, ARS, United States Department of Agriculture, Beltsville, Md. 20705. Obviously, the seedling that depends on cotyledonary Fe for its establishment must have an efficient transport mechanism, for it must export the stored Fe before its cotyledons are shed, and the available time for this may be only a few days. This study had three objectives: to determine Fe distribution from soybean cotyledons to other plant parts, to measure effects of externally applied Fe on that distribution, and to identify the form of Fe in vascular exudate of seedlings whose main source of Fe was in the cotyledons. A brief preliminary report of this work has been given (9). MATERIAILS AND METHODS Plant Culture before Anthesis. Hawkeye soybean, Glycine max L. Merrill, was the experimental plant. General cultural methods have been described (7). In the prelabeling period, 32 plants were grown in complete nutrient solutions (4 plants/8 liters) containing 2 ,M FeEDDHA'. Radioiron Treatment at Anthesis. At anthesis (approximately 30 days after germination) plants were given nutrient solutions containing 5 /uM 'FeEDDHA (2 juc 'Fe/ tmole Fe). These solutions received isotope periodically and contained 3 to 5 /LM Fe throughout the labeling period (from 30 to 100 days after germination). Each 8 liters of nutrient solution received 120 pmoles of Fe in the labeling period. Mature seeds dried on the vine and were harvested over the period of 70 to 100 days from germination. Analysis of Seeds. The distribution of 'Fe between seed coats and embryos and the specific activity of 'Fe in whole seeds were determined on oven-dry (70 C) seeds. All assays of seeds and plant parts were made with a y-scintillation spectrometer. Germination and Treatments. A total of 145 seeds of about equal size were assayed for 'Fe and germinated (24 ± 2 C) between moist Whatman No. 1 papers. The paper extended down the sides of a stainless steel support into 2 cm of water in a covered glass dish. After germination for 2 days, the seed coats, embryos, germination papers, and water were analyzed for 'Fe. On day 2 all seedlings with radicles 1 to 2 cm long were again assayed for radioiron and, except for five seedlings, were returned to the growth chamber under 8 hr dark, 16 hr light 'Abbreviations: FeEDDHA: ferric ethylenediaminedi(o-hydroxyphenylacetate); -Fe: deficient Fe; +Fe: normal Fe.