Ontogenetic changes in root nodule subpopulations of common bean (Phaseolus vulgaris L.). I. Nitrogenase activity and respiration during pod-filling

Nitrogenase activity is commonly measured on a whole plant basis, or only on parts of the root system. In the present paper, activity in different root nodule subpopulations was followed throughout reproductive growth, in order to characterize the pod-filling decline in nitrogenase activity. Inoculated common bean plants were grown to maturity under controlled environment conditions. Nitrogenase activity (H2 evolution in air) and nodule respiration (CO2 evolution) were measured in three separate zones of the root system with a non-destructive, open flow, gas-exchange system. Nitrogenase activity in the top zone drastically declined at the initiation of pod-filling, whereas nitrogenase activity in the mid zone was stable during the same period. Hence, the pod-filling decline was limited to a certain nodule subpopulation and not of a systemic type. Nodule respiration showed a similar, but less pronounced pattern. The sharp decline in nitrogenase activity was not paralleled in nitrogenase specific activity. Nitrogenase activity is not likely to be limited by the availability of oxygen or carbohydrates at the onset of pod-filling because specific nodule respiration did not change significantly at this time. In the top zone, nitrogenase specific activity declined gradually throughout the measurements, whereas in the mid-part of the root system specific activity peaked and gradually declined 2-4 weeks later. The dissimilarities between specific and total nitrogenase activity were explained by differences in nodule growth rates. The data suggest that the oldest nodule population loses activity at the onset of pod-filling. At the same time, nodules grow and nitrogenase activity increases in younger distal parts of the root system. Estimating total nitrogen fixation in this symbiosis by partial sampling of nodulated root systems is likely to be very misleading.