Birdsfoot trefoil (Lotus corniculatus) root nodules: morphogenesis and the effect of forage harvest on structure and function

Abstract

Nodule structure, nodule enzymes of ammonia assimilation, nitrogenase-dependent acetylene reduction, and nodule soluble protein were studied during vegetative regrowth of detopped birdsfoot trefoil (Lotus corniculatus L.) seedlings grown in the glasshouse. Nodules senesced rapidly for a period of 14 days following shoot removal, but then pink nodule populations increased as shoot regrowth occurred. The structural sequence of senescence was similar in nodules whether the result of either aging or shoot removal. Membranes surrounding bacteroids showed degenerative changes as bacteroids senesced. Bacteroids aggregated within the nodule cells and ultimately disappeared in senescent cells. Infection threads and bacteria inside infection threads did not disintegrate.Nodule senescence as a result of shoot removal was accompanied by a marked decline in acetylene reduction, nodule soluble protein, nodule host plant glutamine synthetase (GS), and glutamate synthase (GOGAT). Nodule enzyme activity, soluble protein, and acetylene reduction activity recovered to initial values as shoot regrowth occurred and pink nodule populations increased. Nodule host plant glutamate dehydrogenase (GDH) did not change after shoot removal.This study shows that birdsfoot trefoil nodules respond to shoot removal by an increased senescence. Recovery of nodule function is associated with the formation of a new nodule population. The data also indicate that host plant GS and GOGAT function to assimilate fixed N. Spherical nodules with determinant growth may be less efficient than elongate nodules with indeterminant growth in nodule maintenance and function.