Xylem fluxes of fixed N through nodules of the legumeAcacia littoreaand haustoria of an associated N-dependent root hemiparasiteOlax phyllanthi

Abstract

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. N2 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 pencycle 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−1 d−1, six times the loading flux in nodules. The exit flux from haustorium to parasite rootlet was 20.0 pg N mm−1 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.