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

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.

Reversible Dark-Induced Senescence of Soybean Root Nodules

“…The green color of senescing nodules has been reported previously (1,16,21,27). This color is probably due to the destruction of the Lb heme group.…”

“…Senescence of alfalfa nodules can be accelerated as a result of defoliation or prolonged darkness (4,26,27), and the capacity of a single nodule to fix nitrogen can be regained by virtue of reinfection of the meristematic tissue (27 Initial experiments in this laboratory suggested that the metabolic changes observed in soybean nodules as a result of foliar dark treatment, including loss of nitrogenase activity, could be reversed. This study was conducted to investigate the changes in nodular metabolism which occurred as a result of dark-induced senescence, and to document the reversal of this process in soybean nodules.…”

“…Acetylene reduction activity returned to normal, and both total soluble protein and leghemoglobin were resynthesized while protease activity against azocasein decreased to the level of control nodules. The nodule population that had turned green after 8 days in the dark exhibited a progressive increase in red color starting nearest the exterior of the nodule, and after 16 days of recovery nodules were indistinguishable from control nodules maintained under a normal photoperiod.and a senescent zone connected to the lateral root (9,16,19,27). …”

Reversible-Dark Induced Senescence of Soybean Root Nodules

“…Senescence of alfalfa nodules can be accelerated as a result of defoliation or prolonged darkness (4,26,27), and the capacity of a single nodule to fix nitrogen can be regained by virtue of reinfection of the meristematic tissue (27). The morphology of soybean nodules has been found to be quite different, however.…”

Dinitrogen (C2H2) fixation, total plant yield, and total nitrogen percentage in alfalfa (Medicago sativa L) as affected by plant density, cutting management, and cultivar

“…In a greenhouse study, alfalfa was harvested at flowering stage, then regrown for 28 days to 1% flower before being cut again at approximately 3.5-cm stubble height (Vance et al, 1980 …”