Histological and ultrastructural observations of Medicago sativa root nodule senescence after foliage removal

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Abstract
Root nodules from control and defoliated alfalfa (Medicago sativa L.) were compared for a period of 18 days following defoliation. Bacteroid containing cells of the nodules decreased markedly for the first 10 days following defoliation, but then increased during the next 8 days. Nodule length increased continuously during the 18 days in both control and defoliated plants. Cellular integrity of vascular bundles, meristems, and infection threads was retained in nodules from treated and control plants throughout the experiment. The sequence of senescence in bacteroid containing cells was similar in nodules from both treatments. Senescence was initially observed in the cytoplasm of bacteroid containing cells. Membranes surrounding bacteroids showed degenerative changes as bacteroids senesced. Bacteroids aggregated within nodule cells. Infection threads and bacteria inside infection threads did not disintegrate. Bacteroids ultimately disappeared in senescent cells.Nodules of alfalfa appear to respond to foliar harvesting by a partial but temporary senescence from which they recover by regeneration of bacteroid containing cells.
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