Response of respiration and nitrogenase activity in Datisca glomerata (Presl.) Baill. to changes in pO2

Respiration and nitrogenase activity by root systems of Datisca glomerata (Presl.) Baill. decline rapidly after exposure to C2H2. We measured C2H2 reduction and CO2 evolution to determine whether this decline is due to a variable barrier to O2 diffusion and to examine the nature of O2 regulation in these nodules. Changes in pO2 between 16 and 30 kPa during the decline had only small effects on respiration and nitrogenase activity. Thus the decline is not due to an increase in resistance to O2 diffusion. In contrast, in peas (Pisum sativum L.) nitrogenase activity was strongly reduced when the pO2 was lowered from 20 to 16 kPa during the decline. In Datisca there was little difference in nitrogenase activity when measurements in He-O2 mixtures were compared with Ar-O2 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 compartments that vary in the degree to which nitrogenase is protected from O2. As pO2 increases or decreases, various compartments turn on and off keeping the overall rate of nitrogenase activity and respiration nearly constant.Key words: actinorhizal plants, Datisca glomerata, Frankia, nitrogenase, oxygen protection, Pisum sativum.