Ion Circulation via Phloem and Xylem Between Root and Shoot of Nodulated White Lupin

The exchange rates of mineral cations in the xylem and phloem between root and shoot of white lupin (Lupinus albus L., cv. Ultra) were measured using nodulated plants grown in a defined liquid culture medium low in Na and lacking nitrogen. Harvests were taken at 39 and 49 days after sowing and plant parts analysed for C, N, and the mineral cations K(+), Mg(++), Ca(++), and Na(+). Respiration losses of carbon by nodulated roots were assessed using Pettenkofer assemblies, and concentrations of C, N, and cations in xylem and phloem assayed by collecting root bleeding xylem sap and phloem sap of stem base during the day and night at several times throughout the study period. Flow rates of C and N between root and shoot were determined as in earlier modeling studies (Pate et al., 1979 b), using data on consumption of C by roots, increments of C and N in shoot and root dry matter and, C : N ratios in xylem and phloem sap. Using ratios of cation : C in xylem and phloem, net flow of the various ions between shoot and root were computed. The data showed substantial return of K(+) and Mg(++) from shoot to root with phloem translocate. This return flow provided roots with more K(+) or Mg(++) than was required for growth. It was estimated that 76 % or 87 % of the phloem-borne K(+) and Mg(++) respectively reentered the xylem and was thus circulated within the plant. Rates of return flow to roots and circulation within the plant were very small for Ca(++) and for Na(+) under the conditions of the experiment.