THE CAUSATION OF THE DIURNAL CHANGES IN NITROGEN FIXATION IN THE NODULES OF ALNUS GLUTINOSA

Abstract

SummaryThe translocation of 14C‐labelled photosynthate to the nodules of first‐year alder plants growing under natural illumination, but at constant temperature, is shown to be a rapid process which results in a maximum influx of new photosynthate at the time of the mid‐day peak in fixation. Analysis of fluctuations in the levels of the main free sugars present in the nodules at different times of day, and a study of the effects of interrupting supplies of photosynthate to the root system by stem ringing, suggests that a substantial part of the nodule carbohydrate is unavailable for fixation and that maximal rates of fixation are only attained when new photosynthates are entering the nodules in quantity. No evidence was obtained for an autonomous element in the mid‐day maximum in fixation when plants were kept in continuous darkness; a fall in acetylene reduction to an insignificant level 24 hours after darkening was accompanied by a fall in nodule sugars—notably in the amount of sucrose present.The rate of acetylene reduction by nodulated roots reached a maximum at about 25° C, and, at this temperature, was some six times faster than at 15° C.