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, activityin different root nodule subpopulations was followed throughoutreproductive growth, in order to characterize the pod-fillingdecline in nitrogenase activity. Inoculated common bean plantswere 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 theroot system with a non-destructive, open flow, gas-exchangesystem. Nitrogenase activity in the top zone drastically declinedat the initiation of pod-filling, whereas nitrogenase activityin the mid zone was stable during the same period. Hence, thepod-filling decline was limited to a certain nodule subpopulationand not of a systemic type. Nodule respiration showed a similar,but less pronounced pattern. The sharp decline in nitrogenaseactivity was not paralleled in nitrogenase specific activity.Nitrogenase activity is not likely to be limited by the availabilityof oxygen or carbohydrates at the onset of pod-filling becausespecific nodule respiration did not change significantly atthis time. In the top zone, nitrogenase specific activity declinedgradually throughout the measurements, whereas in the mid-partof the root system specific activity peaked and gradually declined2-4 weeks later. The dissimilarities between specific and totalnitrogenase activity were explained by differences in nodulegrowth rates. The data suggest that the oldest nodule populationloses activity at the onset of pod-filling. At the same time,nodules grow and nitrogenase activity increases in younger distalparts of the root system. Estimating total nitrogen fixationin this symbiosis by partial sampling of nodulated root systemsis likely to be very misleading. Key words: Nitrogen fixation, respiration, pod-filling decline, Phaseolus vulgaris, ontogeny