Transport of Foliar Applied Iron (⁵⁹Fe) in Vicia faba

Abstract

Radioactively labeled iron (⁵⁹Fe) was used to study iron retranslocation from mature leaves of Broad bean (Vicia faba L. var. Scirocco). Our experiments offered the possibility to detect and quantify the translocation of foliar applied iron by imaging technique in combination with tissue analysis. ⁵⁹Fe labeled solution was placed as a droplet onto the leafs upper surface of intact plants. Distribution of ⁵⁹Fe was analyzed after 0.5 h up to 2 days. Iron was translocated acropetally (towards the tip of the treated leaf) as well as basipetally. Movement in the apical direction was predominant, amounting to about 65% of ⁵⁹Fe translocated from the application site. About 35% of ⁵⁹Fe were transported basipetally, corresponding to absolute amounts of 2.8–53.6 pmol h⁻¹. After 30 min, it was detectable in the petiole, which included a translocation of 20 mm basipetal from the application site. A mean of 15% of the iron retranslocated from a leaflet was detected in non‐treated leaflets of the same leaf. This iron was supposed to have been exchanged from the phloem into the xylem pathway, probably within the petiole. When the loading rate into the phloem was estimated on basis of the sum of retranslocated ⁵⁹Fe per time and per area of the leaf treated, a range of 0.031–2.21 pmol h⁻¹ mm⁻² (mean: 0.62 pmol h⁻¹ mm⁻²) was obtained. This was not sufficient to meet an estimated demand for iron in the growing terminal bud, but could cover about 25% of it. In conclusion, average iron retranslocation from leaves of Fe‐sufficient plants was not large enough to meet the iron demand of the growing shoot. This was not due to a limitation in iron availability for transport, as an excess amount of iron was supplied which was not biologically bound, but a limitation due to transport facilities, probably in the phloem, seemed to be more likely in this case.

Keywords:

• Iron
• Retranslocation
• Phloem transport
• Vicia faba

Acknowledgments

This work was supported by the DFG Priority Research Program 717 “The Apoplast of Higher Plants.” We wish to thank Dr. Jürgen Augustin for helpful discussions and technical support, and Mrs. Renate Krüger (both Institute of Primary Production and Microbial Ecology, Centre for Agricultural Landscape and Land Use Research) for dedicated and skillful technical assistance.