AbstractsBiology & Animal Science

The primary intent of this study was to clarify the role of the endophyte in the Ceanothus symbiosis. A technique utilizing a 20 percent solution of H₂O₂ was developed which allowed the isolation of a Streptomyces sp. from the root nodules of some Ceanothus species. Further characterization suggested that the isolates belonged to a single species of the genus Streptomyce s. It was shown that infection and nodulation of snowbrush (C. velutinus) occurs in at least four distinct phases. The first begins as a swelling at the tip of the root hair, which is followed by a marked swelling. At this point unidentifiable objects appear within the root hair and are presumed to correspond to the presence of the endophyte within the root hair. As the infection continues the root hair becomes swollen to its base. Shortly after this point has been reached the fourth phase of the process is recognized as the formation of visible nodules on the root surface. In the presence of the isolate and a sterile nodule extract, the infection process has been observed to the point where the entire root hair has become swollen. However, without the sterile extract the infection was not as complete and proceeds much slower. Where the isolate is omitted from the inoculum and the seedling inoculated with the sterile extract, a slight swelling of the root hair tip occurs. This corresponds to the first stage of infection. It appears that the infection of snowbrush root hairs is a unique character of the isolated organism. Free-living, soil isolated Streptomyces species either had no effect on root hair morphology or caused the death of the inoculated seedling. On the basis of the similarity of the isolates, infective ability of the isolates which appears to be a unique feature of the endophyte, the tentative name Streptomyces ceanothi (Atkinson) Wollum, 1965 was proposed. This species encompasses those organisms which are capable of inhabiting the root nodules of snowbrush and presumably other Ceanothus species. It was also suggested that some predisposing agent or agents may be required for the rapid initiation of the infection process in snowbrush seedlings. Also it was demonstrated that nodulated snowbrush seedlings were capable of fixing atmospheric nitrogen. The rate of fixation was not determined. Ceanothus species as a source of nitrogen in the forest ecosystem are becoming a distinct possibility. As additional information is learned about the symbiotic system we can look for introduced changes in the nodulation potential on natural sites. When endophyte selection is practiced in these programs, it will lead to the maximum fixation of nitrogen. It is possible to envision inoculated plantings of Ceanothus species with the more desirable conifers to assure the greatest utilization of the fixed nitrogen. The field of symbiotic nitrogen fixation in Ceanothus and other non-leguminous plants is beginning to broaden. It appears to have a limitless future.