Respiration and nitrogenase activity in nodules ofCasuarina cunninghamiana and cultures ofFrankia sp. HFP020203: Effects of temperature and partial pressure of O2

The effects of time after exposure to acetylene and of nodule excision were examined using a flow-through system. After a transient depression in the rate of acetylene reduction that began about 1.5 min after exposure to acetylene, the rate recovered to 98% of the initial maximum value after 40 min. After nodule excision the rate stabilized to 90% of the initial maximum value observed in the intact plant.Excised nodules, measured at 6-min intervals in a closed system, with frequent changes of the gas mixture, were used for the remaining experiments. Acetylene reduction by the nodules increased rapidly as temperature was increased between 6 and 26°C. Between 26 and 36°C there was relatively little effect of temperature on acetylene reduction.Nodules and cultures ofFrankia were compared with respect to the effect of temperature and pO₂ (partial pressure of oxygen) on oxygen uptake. Cultures ofFrankia were grown on a nitrogen-free medium at either 0.3 kPa O₂ (vesicles absent) or 20 kPa O₂ (vesicles present). Oxygen uptake by nodules (vesicles absent) and by vesicle-containing cultures was strongly dependent on pO₂ at values below 20 kPa. This suggests the presence of a barrier to oxygen diffusion. Oxygen uptake was dependent on temperature as well as on pO₂, but the Q₁₀ was much larger for the cultures than for the nodules. This suggests that vesicles or related structures are not the source of the diffusion barrier in Casuarina nodules. Respiration by cultures ofFrankia lacking vesicles became O₂-saturated at low pO₂ values. Thus these cultures did not have a significant diffusion barrier. From these results it is concluded that nodules ofCasuarina cunninghamiana have a barrier to oxygen diffusion supplied by the host tissue and not byFrankia.