Nickel translocation via the phloem in the hyperaccumulator Noccaea caerulescens (Brassicaceae)

Aims This study evaluated the effect of phloem translocation on Ni accumulation in the hyperaccumulator Noccaea caerulescens. Methods The first experiment assessed the metal and organic compound concentrations in phloem sap. Leaves were cut from plants grown in nutrient solutions added with 100 mu M Ni or Zn, and then used for phloem sap extraction by means of the EDTA-stimulated exudation method. In the second experiment, Ni-61(2+) was applied to old leaves as a foliar spray to assess bidirectional movement of Ni in phloem. Results In the first experiment, enrichment of Ni or Zn was found in phloem exudates, indicating high Ni and Zn phloem loading and translocation capacity in N. caerulescens. Amino acids, e.g. nicotianamine and histidine, were present at low concentrations in exudates, which are insufficient for Ni and Zn chelation. On the contrary, concentrations of organic acids, especially malate, were high in all treatments, which may be involved in Ni and Zn chelation in phloem. The second experiment showed that 89 % of Ni-61 exported from old leaves was translocated upward to young leaves, whereas only 11% moved downward to roots. Conclusions Phloem sap of N. caerulescens is enriched in Ni and malate, the majority of which moves upward to young tissues. Phloem translocation may play an important role for Ni accumulation in young leaves of N. caerulescens.