Response of respiration and nitrogenase activity in Datisca glomerata (Presl.) Baill. to changes in pO(2)

Respiration and nitrogenase activity by root systems of Datisca glomerata ( Presl.) Baill. decline rapidly after exposure to C2H2. We measured C2H2 red uction and CO2 evolution to determine whether this decline is due to a vari able barrier to O-2 diffusion and to examine the nature of O-2 regulation i n these nodules. Changes in pO(2) between 16 and 30 kPa during the decline had only small effects on respiration and nitrogenase activity. Thus the de cline is not due to an increase in resistance to O-2 diffusion. In contrast , in peas (Pisum sativum L.) nitrogenase activity was strongly reduced when the pO(2) was lowered from 20 to 16 kPa during the decline. In Datisca the re was little difference in nitrogenase activity when measurements in He-O- 2 mixtures were compared with Ar-O-2 mixtures, showing that there is no gas phase limitation on diffusion. Therefore the major diffusion resistance in Datisca nodules is in the infected cells. We propose that this resistance is in the vesicles of Frankia and that the vesicles represent a series of c ompartments that vary in the degree to which nitrogenase is protected from O-2. As pO(2) increases or decreases, various compartments turn on and off keeping the overall rate of nitrogenase activity and respiration nearly con stant.