XYLEM FLUXES OF FIXED N THROUGH NODULES OF THE LEGUME ACACIA-LITTOREAAND HAUSTORIA OF AN ASSOCIATED N-DEPENDENT ROOT HEMIPARASITE OLAX-PHYLLANTHI

Nodulated 1-1.5-year-old plants of Acacia littorea grown in minus nitr ogen culture were each partnered with a single seedling of the root he miparasite Olax phyllanthi. Partitioning of fixed N between plant orga ns of the host and parasite was studied for the period 4-8 months afte r introducing the parasite. N fluxes through nodules of Acacia and xyl em-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 mu g N g FW nodules(-1) d(-1)) corresp onded to a xylem loading flux of 0.20 mu g N mm(-2) d(-1) across the s ecretory membranes of the pericycle parenchyma of the nodule vascular strands. A much higher flux of N (4891 mu g mm(2) d(-1)) exited throug h xylem at the junction of nodule and root. The corresponding flux of N from host xylem across absorptive membranes of the endophyte parench yma of Olax haustorium was 1.15 mu g N mm(-2) d(-1), six times the loa ding flux in nodules. The exit flux from haustorium to parasite rootle t was 20.0 mu g N mm(-2) d(-1), 200-fold less than that passing throug h xylem elements of the nodule. Fluxes of individual amino compounds i n 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.