Aspects of symbiotic fixation of nitrogen (with special reference to non-leguminous root nodules)

The thesis relates to the nitrogen-fixing root nodules inhabited by actinomycete-like organisms which occur on a number of non-leguminous species of angiosperms. Some parts of the thesis describe comparative studies of the nodules of various examples (mostly foreign) of the above plants which were in culture in the glasshouse of Glasgow University Botany Department, The species available were as follows: -Myrica gale (Britain), M. faya (Canary Islands) M.cordifolia and pilulifera (Africa)., M. cerifera, M. pensylvanica and X. carolinensis (all -.A.), N. .javanica (Indonesia) alnus felutinosa (Britain), A, viridis and A, incana (urope), One such study concerned the structure of the nodules and the appearance of the endophyte, under the light and electron microscope, in species which previously had received little or no such investigation. Close similarities to the nodules of previously-examined related species were found. A second comparative study involved the determination of the number of chromosomes in the meristematic cells of the nodules of six of the species as compared with the number in the dividing cells of the root tips. In each species the numbers were found to be identical, contrasting with the tetraploidy in the nodule cells reported for many leguminous species. In a third such study, confined this time to the genera Nyrica and Alnus, the extent to which an endophyte associated with a particular host specie's is able to symbiose with another host species in the same genus was studied by means of cross-inoculation trials. Among Myrica species little evidence of specialisation was obtained, except that the failed to set up satisfactory symbiosis with any other species. It did induce the formation of numerous nodules, but they remained of minute size and failed to fix nitrogen. It is concluded that in any classification of these nodule endophytes, the M. gale endophyte should be separate from the rest. The microscopic examination of the ineffective nodules induced by M.gale showed that they lacked vesicles. In Alnus the endophyte of A.glutinosa was found to symbiose satisfactorily with A.viridis plants. The first of two more specialised studies related to the time of appearance of nitrogenise activity detected by the acetylene-reduction technique in young alder plants which were just beginning to nodulate, and to its relation to endophyte differentiation. Activity was detected two weeks before evidence of fixation was provided by the greening of the leaves, and was found to be closely related to vesicle development by the endophyte, studied by means of the microscopic examination of squash preparations. In a further study the effect of the level of carbon dioxide in the rooting medium on growth and nitrogen fixation in Alnus glutinosa and Myrica gale plants was examined by growing plants in water culture with either air or air with 2% carbon dioxide bubbled constantly through the culture solution. Rather variable results were obtained, but the final conclusion was that the extra carbon dioxide has a harmful effect on nitrogen fixation and plant growth. Acetylene-reduction tests on detached nodules of the two species also indicated an inhibiting effect of excess carbon dioxide on nitrogenise activity, possibly due primarily to a reduction in respiration. The latter effect appears to prevent any enhancement of fixation which might otherwise follow the greater activity of carboxylase enzymes in the 'dark' fixation of carbon dioxide in the presence of an increased level of that gas