ALTERATION OF N-NUTRITION IN MYRICA-GALE INDUCES CHANGES IN NODULE GROWTH, NODULE ACTIVITY AND AMINO-ACID-COMPOSITION

Changes in nodule growth and activity and in the concentrations of sol uble N compounds in nodules, leaves and xylem sap under conditions of altered N nutrition in the actinorhizal plant Myrica gale L. are repor ted. Altering the N nutrition of symbiotic plants may alter the intern al regulation of combined N which in turn may regulate nodule growth a nd activity. Flushing nodules daily with 100% O-2 caused a decline in amide concentration and an increase in nodule growth although plants h ad recovered some nitrogenase activity within 4 h of exposure to O-2. Samples of nodules, leaves and xylem sap were derivatized an amino aci ds identified and quantified using either reverse phase high performan ce liquid chromatography or gas chromatography-mass spectrometry in si ngle ion monitoring mode. The ratio of asparagine in the nodules to th at in the xylem was much higher in plants fed N (6.7 for NH4+-fed and 8.3 for NO3--fed plants) than for N-2-fixing plants (2.5). Significant amounts of N-15 added as (NH4+)-N-15 or (NO3-)-N-15 accumulated in no dules following accumulation in the shoot which is consistent with the translocation of N to the nodules via the phloem. The uptake of (NH4)-N-15 led to the synthesis and subsequent translocation of glutamine in the xylem sap. These results are discussed in terms of the feedback mechanisms that may regulate nitrogen fixation in Myrica root nodules .