Fe-52 Translocation in Barley as Monitored by a Positron-Emitting Tracer Imaging System (PETIS): Evidence for the Direct Translocation of Fe from Roots to Young Leaves via Phloem

The real-time translocation of iron (Fe) in barley (Hordeum vulgare L. cv. Ehimehadaka no. 1) was visualized using the positron-emitting tracer Fe-52 and a positron-emitting tracer imaging system (PETIS). PETIS allowed us to monitor Fe translocation in barley non-destructively under various conditions. In all cases, Fe-52 first accumulated at the basal part of the shoot, suggesting that this region may play an important role in Fe distribution in graminaceous plants. Fe-deficient barley showed greater translocation of Fe-52 from roots to shoots than did Fe-sufficient barley, demonstrating that Fe deficiency causes enhanced Fe-52 uptake and translocation to shoots. In the dark, translocation of Fe-52 to the youngest leaf was equivalent to or higher than that under the light condition, while the translocation of Fe-52 to the older leaves was decreased, in both Fe-deficient and Fe-sufficient barley. This suggests the possibility that the mechanism and/or pathway of Fe translocation to the youngest leaf may be different from that to the older leaves. When phloem transport in the leaf was blocked by steam treatment, Fe-52 translocation from the roots to older leaves was not affected, while Fe-52 translocation to the youngest leaf was reduced, indicating that Fe is translocated to the youngest leaf via phloem in addition to xylem. We propose a novel model in which root-absorbed Fe is translocated from the basal part of the shoots and/or roots to the youngest leaf via phloem in graminaceous plants.