Figure 1.

Nitrogen Cycling between Root and Shoots.

Arrows represent the fluxes of nitrogen uptake, transport, and utilization in whole plants of Ricinus communis. Part of the taken up nitrate is reduced in the root as amino acids and is transported to the leaves and shoot apex by the xylem (black arrow). The remaining nitrate is reduced in the leaves, and exported to the root and the shoot apex by the phloem (gray arrow). (A) Xylem to phoem and (B) phloem to xylem transfers occur during the transport in the root and the stem, leading to amino acid cycling. Adapted from Jeschke and Hartung (Jeschke and Hartung 2000).