Temporal Patterns of Uptake, Flow and Utilization of Nitrate, Reduced Nitrogen and Carbon in a Leaf of Salt-Treated Castor Bean (Ricinus communis L.)

Changes in net photosynthesis, respiration, transpiration and contents of total C, NO3-N and reduced N were followed throughout the life of leaf 6 of nitrate-dependent plants of castor bean exposed to moderate salinity stress (71 mol m-3 NaCl). Salt treatment was applied for measuring mineral flows in a parallel study (Jeschke and Pate, 1991b). Concurrent measurements were made of solute composition and C:N molar ratios and concentrations of reduced N and collected NO3-N in phloem sap bleeding from shallow incisions in the top and at the base of petioles and in xylem exudates from flaps of proximal leaf midribs following pressurization of the root system. The resulting data were used to construct empirical models of the respective economies of C, total N, NO3 and reduced N for a sequence of defined phases of leaf life. Water use efficiency increased 3-fold from emergence to a maximum of 1.5 mmol CO2 mol-1 H2O before declining to 0.5 mmol CO2 mol-1 H2O at senescence. Xylem molar ratios of C:N varied from 1.2-2.8, with nitrate always a smaller component than reduced N. Phloem sap C:N increased from 10-40 with leaf expansion and was then maintained in the range of 40-50 until falling steeply to 20 at leaf senescence. Nitrate comprised less than 1% of total N in all phloem sap samples. The models of C uptake, flow, and utilization showed a major role of phloem import and then increasingly of laminar photosynthesis in providing C for leaf growth. The carbon budget was thereafter characterized by rates of phloem export closely matched to net rates of CO2 fixation by the lamina. Corresponding data for total N depicted an early major role of both xylem and phloem import, but the eventual dominance of xylem import as the N source for leaf growth. Cycling of N by xylem to phloem exchange commenced before the leaf had achieved maximum N content, and was the major contributor to phloem export until leaf senescence when mobilized N provided most exported N. The nitrate economy of the leaf was characterized by early establishment of tissue pools of the ion in the petiole and to a lesser extent in the lamina, continued high rates of nitrate reduction in the lamina but negligible assimilation in the petiole, and a release through xylem of previously accumulated NO3 from petiole to lamina. Related data for reduced N illustrated the much greater importance of this form of N than nitrate in transport, storage and cycling of N at all stages of leaf and petiole life. Xylem to phloem interchanges of reduced N in petiole were minimal in comparison with cycling through the lamina. The ratio of CO2 reduction to NO3 reduction in the lamina was at first low (57 mol mol-1) increasing to a peak value of 294 during mature leaf functioning before declining to 190 during the presenescence phase of leaf development. This pattern reflected age-related effects on water use efficiency, changes in NO3 levels in the xylem stream entering the lamina, and the relatively low photosynthetic performances of very young and senescent laminae.