Diurnal changes in nitrogen fixation in the root nodules of Casuarina

Measurements were made of the rate of nitrogen fixation and of the carbohydrate content of nodules detached at different times of day from plants of Casuarina cunninghamiana growing in water culture under semi-natural conditions in a greenhouse lit by daylight and subject to considerable diurnal variation in temperature also. The $^{15}$N method was used for the measurement of fixation. A distinction was made between 9fixation potential9 - the rate shown by nodules incubated under $^{15}$N at a standard favourable temperature, and 9actual fixation9, namely the rate shown by nodules incubated at the temperature prevailing in the greenhouse at the time. It was assumed that the values obtained for actual fixation, though known to be considerably below those prevailing in the nodules at the times of detachment, would be proportional to the latter. Actual fixation was low in the early morning, but soon rose and remained at a relatively high level for several hours centred about mid-afternoon, and then fell. A separate study showed that the rate of fixation increased steadily over the temperature range 10-36 degrees C, and indicated that temperature fluctuations in the greenhouse were partly responsible for the diurnal changes in the rate of fixation. An additional factor, presumably light intensity acting through its effect on the supply of carbohydrates to the nodules, also appeared to be involved, but although evidence was obtained that during periods of low light or darkness the fixation potential was related to the level of nodular carbohydrates, over the part of the day when actual fixation was high the nodules showed a low carbohydrate content. The possible explanation of this situation is considered.