Translocation of iron citrate and phosphorus in xylem exudate of soybean.

Soybean plants, Glycine max (L.) Merrill, in standard solution received 2.5 mum ferric ethylenediamine di(o-hydroxyphenylacetate (FeEDDHA) and 0 to 128 mum phosphorus. Their stem exudates contained: 32 to 52 mum Fe, 120 to 5000 mum P, and 120 to 165 mum citrate. Electrophoresis of exudates with high P caused Fe trailing that precluded identification of any major form of Fe. Exudate with low P gave an anodic band of Fe citrate as the major Fe compound. Phosphate added to exudate in vitro depressed the Fe citrate peak and cause Fe trailing. EDDHA added to exudate in vitro pulled Fe from Fe citrate; citrate then migrated as a slower form and Fe migrated as FeEDDHA. A modified preculture system, involving 2-day renewals of 0.2 mum FeEDDHA with 3.2, 9.6, or 16 mum P and low levels of other ions, controlled pH depression and produced considerable change in citrate and P levels. The exudates contained: 45 to 57 mum Fe, 200 to 925 mum P, and 340 to 1025 mum citrate. The high citrate was from plants grown with low P. The major form of Fe in the exudates was Fe citrate. This is probably the form translocated in the plants.