Physiology and biology of non-legume root-nodule plants

(1) Investigations were carried out into several aspects of diurnal variation in nitrogen fixation, the 15N method being mainly employed. Most of the experiments involved second year plants of Casuarina cunninghamiana, but in several cases first year plants of Hippophae rhamnoides and Myrica cerifera were used as well. Fluctuations in nitrogen fixation and the carbohydrate content of the nodules during the day were studied under greenhouse conditions. The effects of temperature and plant darkening on the fixation process were also investigated. (2) A well marked diurnal rhythm in fixation, with maximal rates during the early afternoon and minimal rates in the early morning and late evening, was found in second year Casuarina plants held under greenhouse conditions. Temperature was found to have a marked effect on the fixation rate, and it was concluded that this was mainly responsible for the fluctuations in fixation during the day. The effect of light intensity was less immediate, since darkening of the plants had no effect on fixation until after three days. It was concluded that changes in light intensity during a particular day would have little effect on the rate of fixation on that day. (3) Fluctuations in the level of nodular carbohydrates did not parallel fluctuations in the nitrogen fixation rate during the day. (4) The effect of darkening on the fixation of first year plants was more immediate and it was considered that in these light intensity on a particular day might well have some control over fixation on that day