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 f or the intraplant allocation of recently assimilated phosphorus (P). O ne nodal root of each plant was isolated and fed P-32-labelled nutrien t 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 nod e along the branch when counting from the junction with the parent sto len. Allocation patterns were obtained by dissecting plants and assayi ng each dissected component for P-32. Under conditions of undisturbed growth in a controlled environment xylem architecture was found to hav e an important influence on the intraplant distribution of P-32 from n odal roots. Allocation patterns of P-32 were consistent with those pre dicted from knowledge of the xylem vascular architecture of the genoty pe and a predominantly acropetal direction of the transpiration stream . For instance, very little P-32 (< 1% of exported P-32 from the fed r oot) was found in branches on the opposite side of the stolen to the f ed root, and the strong acropetal direction of the transpiration strea m 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 roo t (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 a rising at the same node as the fed root (64%) reflects the numerous li nks that the fed root has to vascular bundles in the branch (3-6, of w hich two are axial bundles) against the single connection it has to th e 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 f avoured nodes on the adaxial side, a result consistent with the fact t hat the root connections are to the upper and lower axial bundles on t he adaxial side of the branch. Allocation to a particular node along t he branch was also influenced by its nodal position relative to the fe d root and the sink strength of its tissues for the transpiration stre am.