Effect of Grafting on Nodulation of Trifolium ambiguum

THE clover Trifolium ambiguum is a perennial species which has a somewhat restricted natural range in the Caucasus region and the neighbouring steppes, extending westwards to about the Crimea. As an introduction in the United States, it has attracted much attention as a prospective pasture plant for more favoured northern sections because of its deep-rooting, habit, capacity to spread by rhizomes and considerable degrees of both drought and winter hardiness. Furthermore, as it produces nectar freely and the corolla tube is shallow, it is of particular interest with respect to honey production. However, as a legume it has an outstanding weakness in that effective nodulation of this species has never been found. Parker 1 tested on it some thirty-five strains of Rhizobium trifolii and, although most of them produced a certain degree of nodulation, the nodules almost without exception were very small, quite withered, brown, unhealthy and obviously ineffective. In general, these nodules occurred at the point of origin of a small root, as though rhizobial infection of this species occurred as the result of the rupture of the root tissue by an emerging branch root, as reported by Allen and Allen 2 for Arachis hypogea , and not through root hairs. Normally, of course, the relationship between Rhizobium and plant is more positive, direct and symbiotic, the bacteria reaching the cortical tissue through an infected root hair and there causing a proliferation of cells and an appreciable development of healthy nodular tissue. In the case of nodulation effective in symbiotic nitrogen fixation, the nodules are comparatively large, plump and pinkish in colour owing to the presence within them of the product leghæmoglobin.