Comparison of Gunnera tinctoria (Molina) Mirbel plants grown in the presence and absence of nitrogen showed that N2 fixed by the cyanobacterium (Nostoc punctiforme L.) can supply all of the requirements for vegetative growth. Estimates of the proportion of tissue occupied by Nostoc indicates that an actively-fixing cyanobacterial component (the cyanobiont) of somewhat less than 1 % of the total (host + cyanobiont) biomass was sufficient to meet the demands of the host. On the basis of low nitrogen contents and high C: N ratios the cyanobiont was nitrogen-limited.
In situ nitrate assimilation was limited by nitrate uptake and/or transport. Whole plants did not accumulate nitrate and had little capacity to assimilate nitrate, although nitrate reductase activity could be induced in intact leaves or leaf segments supplied directly with aqueous nitrate solutions. The possible significance of these results is discussed. Growth of G. tinctoria on ammonium was depressed at high concentrations and this species may only be able to use low ammonium concentrations. These results indicate that G. tinctoria is almost entirely dependent on N2 fixed by the cyanobacterium to meet its nitrogen demands and has little capacity to use alternative forms of nitrogen.