Copper in xylem and phloem saps from rice (Oryza sativa): the effect of moderate copper concentrations in the growth medium on the accumulation of five essential metals and a speciation analysis of copper-containing compounds

Copper (Cu) is an essential element for cereals, playing important roles as a cofactor of several enzymes. Copper and four other metals (Fe, Mn, Zn and Mo) taken up by roots are efficiently delivered to the shoots via xylem and phloem. Here we investigated the concentrations of Cu, Fe, Mn, Zn and Mo in the xylem and phloem saps as well as in tissues of rice (Oryza sativa L.) seedlings when they were grown under different Cu levels in culture solution. Although the Cu concentrations in the roots and the Mn concentrations in the mature shoot tissues were increased with the increase of the Cu level in the culture solution, the concentrations of Cu and the other four metals in the xylem and phloem saps and the Cu contents in the shoot tissues were only slightly affected by moderate increases in the Cu medium level. The results of our analyses using membrane filtration, size-exclusion chromatography and electrospray ionisation time-of-flight mass spectrometry indicate that Cu in the xylem sap is dominantly complexed by 2′-deoxymugineic acid, whereas Cu in the phloem sap is bound to several compounds, i.e. nicotianamine, histidine and other >3-kDa compounds.