Developmental patterns related to nitrogen fixation in theNostoc-Gunnera magellanica Lam. symbiosis.

Developmental patterns related to nitrogen fixation in the heterocystous cyanobacteriumNostoc harboured in distinct colonies along the stem ofGunnera magellanica Lam. plantlets were examined using successive plant sections. Pronounced morphological, physiological and biochemical alterations in the cyanobacterium were demonstrated. Close to the growing apex the cyanobacterial biomass, contained in smallGunnera cells, was low and consisted mostly of vegetative cells showing a high density of different storage structures except for cyanophycin granules. In contrast, both the total and specific nitrogenase activity and the relative nitrogenase protein level were at maximum within this part; while the frequency of heterocysts increased from zero to 30% within the same area. The nitrogenase protein was localized only in the heterocysts throughout the plant. Further down theGunnera stem there was a progressive increase in both the cyanobacterial biomass and the heterocyst frequency, which finally constituted about 60% of the cyanobacterial cell population. Throughout this part of the stem, cyanophycin granules were frequent in the vegetativeNostoc cells. At the base of the stem, degeneratedNostoc cells dominated and the nitrogenase activity was close to zero, although the nitrogenase protein remained. Degeneration of theNostoc cells and leaf shedding coincided. Both intact plants (approx. 20 mm in height) and plant stem sections (2 mm in length) showed substantial nitrogenase activity, although sectioning caused a 30% reduction in total nitrogenase activity.