Search

Both nitrogenase activity and the accumulation of 14C-labelled photosynthates in the nodules of pea plants in nitrogen-free culture reached maxima shortly before flowering and fruit development. During the period from flowering to fruiting, nitrogenase activity and accumulation of 14C-photosynthates in the nodules declined by 60%, whereas the photosynthesis of the plant doubled. Studies of the translocation of the photosynthates within the plant during this period suggested that this might be due to an increase from 17% to 50% of the photosynthates which accumulated at the shoot apex. Consistent increases in nitrogenase activity over the 24. hours following removal of the shoot apex were only achieved in fruiting plants, however, although vegetative, flowering and fruiting plants all showed increased accumulation of photo-synthates in the nodules. Continuous removal of flowers as they were formed over a 2-week period resulted in a large increase in accumulation of photosynthates in the nodules and in nitrogenase activity. Other factors which may regulate nitrogenase activity in these plants are discussed. [ABSTRACT FROM AUTHOR]

The translocation of [sup14]C-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. [ABSTRACT FROM AUTHOR]