NITROGENASE ACTIVITY AND ASSOCIATED CARBON BUDGETS IN SEEDLINGS OF ACACIA-MANGIUM MEASURED WITH A FLOW-THROUGH SYSTEM OF THE ACETYLENE-REDUCTION ASSAY

Following introduction of acetylene into the flow-through gas system, nitrogenase activity of young seedlings of Acacia mangium Willd. increased during the initial gas mixing period but declined thereafter. Nitrogenase-linked respiration also declined rapidly for 3-4 min and slowly thereafter following introduction of acetylene. Seedlings of 12-42 weeks showed a decline in nitrogenase-linked respiration of 15-40%. Nitrogenase activity and nodulated root respiration declined further when the oxygen concentration in the gas flowing past the nodules was reduced. These concomitant declines in nodulated root respiration and nitrogenase activity were used to explore the relation between nodulated root respiration and nitrogenase activity, and to provide an estimate of the carbon cost of nitrogenase activity, and the growth and maintenance respiration of nodulated roots. The carbon cost of nitrogenase activity was 2.8-mu-mol CO2 mu-mol-1 C2H2 reduced for young seedlings (12-20 weeks) and was lower (1.3) for older seedlings (30-42 weeks). Nitrogenase activity was 1.2-mu-mol C2H4 g-1 nod. dw min-1 for plants at 12 weeks, but was 0.3-mu-mol C2H4 g-1 nod. dw min-1 for plants at 42 weeks after transplanting. The proportion of nitrogenase-linked respiration in nodulated root respiration also declined rapidly with plant age, being 70% at 12 weeks and only 6% at 42 weeks after transplanting.