Xylem and Phloem Transport and the Functional Economy of Carbon and Nitrogen of a Legume Leaf

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Abstract
Exchanges of CO2 and changes in content of C and N were studied over the life of a leaf of Lupinus albus L. These data were combined with measurements of C:N weight ratios of xylem (upper stem tracheal) and phloem (petiole) sap to determine net fluxes of C and N between leaf and plant. Phase 1 of leaf development (first 11 days, leaf to 1/3 area) showed increasing net import of C and N, with phloem contributing 61% of the imported C and 18% of the N. 14C feeding studies suggested the potential for simultaneous import and export through phloem over the period 9-12 days. Phase 2 (11-20 days, leaf attaining maximum area and net photosynthesis rate) exhibited net import through xylem and increasing export through phloem. Eighty-two percent of xylem-delivered N was consumed in leaf growth, the remainder exported in phloem. Phase 3 (20-38 days) showed high but declining rates of photosynthesis, translocation and net export of N. Phase 4 (38-66 days) exhibited substantial losses of N and declining photosynthesis and translocation of C. C:N ratio of xylem sap remained constant (2.3-2.6) during leaf life; petiole phloem sap C:N ratio varied from 25 to 135 over leaf development. The relationships between net photosynthesis and N import in xylem were: phase 1, 4.8 mg C/mg N; phase 2, 24.7 mg C/mg N; phase 3, 91.9 mg C/mg N; and phase 4, 47.7 mg C/mg N.