Ontogenetic Changes in Root Nodule Subpopulations of Common Bean (Phaseolus vulgaris L.): III. NODULE FORMATION, GROWTH AND DEGRADATION

• Published in: Journal of experimental botany 1993-03, Vol.44 (260), p.579-586
• Main Authors: VIKMAN, PER-ÅKE, VESSEY, J. KEVIN
• Format: Article
• Language: English
• Subjects: Agronomy. Soil science and plant productions; Biological and medical sciences; Biomass; Correlations; Fundamental and applied biological sciences. Psychology; Image analysis; Nitrogen fixation; Nodulation; Nodules; Parasitism and symbiosis; Plant physiology and development; Plants; Rhizosphere; Root nodules; Root systems; Symbiosis
• ISSN: 0022-0957; 1460-2431

In root nodule subpopulations from different parts of the root system, nitrogenase activity was previously found to vary. At the time for rapid pod-filling nitrogenase activity declined in nodules located at the crown of the root system, while nodule biomass and specific activity increased considerably in distal parts. This could be explained either by formation of new nodules or by growth of existing nodules. The objective of the present study was to characterize nodule development and to quantify the amount of active nodules in the crown and distal root zones. Here, we address the mechanisms that determine ontogenetic changes in nitrogen fixation. A new method was developed for nodule classification by size and surface colour, using an image analysis system. This method conveniently gives objective and highly reproducible values even on large nodule samples. All root nodules from crown and distal parts of the root system were collected throughout the life-span of common beans (Phaseolus vulgaris L. cv. GN1140). After freeze-drying, the nodules were classified into five different types, and the numbers and biomass of each type were determined. The correlations between relative proportions of certain nodule types and nitrogenase activity in mixed nodule populations was studied. Bright nodules with a projected area larger than 3.3 mm2 appeared to be the major site for active nitrogenase. Hence, the capacity for nitrogen fixation would be dependent on the formation and degradation of this nodule type. The different nitrogenase activities observed in crown and distal nodule populations, was explained by the proportions of these large and bright nodules. Before pod-filling, fewer distal nodules were formed, and instead of growing into the active type, these nodules remained small and senesced. Apparently, in addition to autoregulation of nodule initiation, complete nodule development can be restricted in parts of the root system. A bi-directional scheme is suggested for the development of root nodules in this symbiosis.