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Xylem fluxes of fixed N through nodules of the legume Acacia littorea and haustoria of an associated N-dependent root hemiparasite Olax phyllanthi

Nodulated 1-1.5-year-old plants of Acacia littorea grown in minus nitrogen culture were each partnered with a single seedling of the root hemiparasite Olax phyllanthi. Partitioning of fixed N between plant organs of the host and parasite was studied for the period 4-8 months after introducing the parasite. N fluxes through nodules of Acacia and xylem-tapping haustoria of Olax were compared using measured xylem flows of fixed N and anatomical information for the two organs. N 2 fixation during the study interval (635 μg N g FW nodules -1 d -1 ) corresponded to a xylem loading flux of 0.20 μg N mm -2 d -1 across the secretory membranes of the pericycle parenchyma of the nodule vascular strands. A much higher flux of N (4891 μg mm 2 d -1 ) exited through xylem at the junction of nodule and root. The corresponding flux of N from host xylem across absorptive membranes of the endophyte parenchyma of Olax haustorium was 1.15 μg N mm -2 d -1 , six times the loading flux in nodules. The exit flux from haustorium to parasite rootlet was 20.0 μg N mm -2 d -1 , 200-fold less than that passing through xylem elements of the nodule. Fluxes of individual amino compounds in xylem of nodule and haustorium were assessed on a molar and N basis. N flux values are related to data for transpiration and partitioning of C and N of the association recorded in a companion paper.