Distribution of phosphorus and calcium from nodal roots of Trifolium repens: The relative importance of transport via xylem or phloem

The intra-plant distribution of radioactive phosphorus (P-32) and calcium (Ca-45) exported from a single nodal root of Trifolium repens (white clover) was measured to ascertain if initial allocation of radioisotopes via the xylem was significantly altered over time by redistribution via the phloem. The radioisotopes were simultaneously supplied in a 2 h pulse which was followed by supply of non-radioactive nutrient solution until harvest 3, 6, 9, 21 or 72 h after the start of labelling. By the end of the 3 d experimental period net export from the source root was 94% of total absorbed Ca-45 but only 56% of P-32. After movement through the stolen of the parent axis, main sinks for both nutrients were the leaf and the branch at the second node distal to the source root (64 and 30% of total absorbed Ca-45 and P-32 respectively after 72 h). Very little of either radioisotope moved into branches located along the stolen of the parent axis, on the side opposite to the source root (1 and 2% of total absorbed Ca-45 and P-32 respectively after 72 h). This evidence, along with knowledge of the vascular architecture of the genotype, indicated that the allocation of mineral nutrients was determined chiefly by the vascular links between the source root and sink organs rather than sink size or proximity to the source root. Data indicated that some distribution of 3?P occurred via the phloem but that it was quantitatively small in relation to allocation via the xylem both in the short (3 h) and medium (72 h) terms.