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 calc ium (Ca-45) exported from a single nodal root of Trifolium repens (whi te clover) was measured to ascertain if initial allocation of radioiso topes via the xylem was significantly altered over time by redistribut ion via the phloem. The radioisotopes were simultaneously supplied in a 2 h pulse which was followed by supply of non-radioactive nutrient s olution 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 move ment through the stolen of the parent axis, main sinks for both nutrie nts were the leaf and the branch at the second node distal to the sour ce root (64 and 30% of total absorbed Ca-45 and P-32 respectively afte r 72 h). Very little of either radioisotope moved into branches locate d along the stolen of the parent axis, on the side opposite to the sou rce root (1 and 2% of total absorbed Ca-45 and P-32 respectively after 72 h). This evidence, along with knowledge of the vascular architectu re 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.