Influence of xylem vascular architecture on the translocation of phosphorus from nodal roots in a genotype of Trifolium repens during undisturbed growth

Our objective was to establish whether the xylem vascular connections (architecture) of a genotype of Trifolium repens L. had implications for the intraplant allocation of recently assimilated phosphorus (P). One nodal root of each plant was isolated and fed P-32-labelled nutrient solution for 24 h. The fed root was either on the parent axis (11-12 nodes proximal to the apex) or on a branch at the third or fourth node along the branch when counting from the junction with the parent stolen. Allocation patterns were obtained by dissecting plants and assaying each dissected component for P-32. Under conditions of undisturbed growth in a controlled environment xylem architecture was found to have an important influence on the intraplant distribution of P-32 from nodal roots. Allocation patterns of P-32 were consistent with those predicted from knowledge of the xylem vascular architecture of the genotype and a predominantly acropetal direction of the transpiration stream. For instance, very little P-32 (< 1% of exported P-32 from the fed root) was found in branches on the opposite side of the stolen to the fed root, and the strong acropetal direction of the transpiration stream resulted in little allocation of P-32 to leaves that had traces that arose from axial bundles at positions proximal to that of the fed root (the leaf subtending the fed root and the next distal leaf had low P-32 content), to all plant organs proximal to the fed root (< 6%) and to all other roots (< 4%). The high allocation of P-32 to the branch arising at the same node as the fed root (64%) reflects the numerous links that the fed root has to vascular bundles in the branch (3-6, of which two are axial bundles) against the single connection it has to the adjacent lower axial bundle in the parent stolen. Allocation of P-32 to the leaf and axillary bud at each node along the branch strongly favoured nodes on the adaxial side, a result consistent with the fact that the root connections are to the upper and lower axial bundles on the adaxial side of the branch. Allocation to a particular node along the branch was also influenced by its nodal position relative to the fed root and the sink strength of its tissues for the transpiration stream.