NODULATION OF WHITE CLOVER (TRIFOLIUM-REPENS) IN THE ABSENCE OF RHIZOBIUM

Spontaneous nodules developed on the roots of white clover (Trifolium repens cv. Ladino) in the absence of Rhizobium, A small subpopulation of uninoculated clover plants (0.2%) exhibited white, single-to-multil obed elongated structures on their root systems when grown without fix ed nitrogen. Clonal propagation using aseptic stolons confirmed the ge netic stability of the observation. Few if any viable bacteria of unkn own origin were recovered from surface-sterilized structures. Nodule c ontents were incapable of eliciting nodulation. Histological observati ons showed that these structures possessed all the characteristic feat ures of indeterminate nodules, such as active meristem, cortex, endode rmal layer, vascular strands, and a central zone with parenchyma cells . Infection threads, intercellular or intracellular bacteria were abse nt. Instead, numerous starch grains were observed in the central zone, a feature absent in normal nitrogen-fixing nodules. Our observation b roadens the concept of spontaneous nodulation, believed to be restrict ed to alfalfa (Medicago sativa), to other legumes, and suggests a degr ee of generality among indeterminately nodulated legumes displaying na tural heterozygosity.