Abstract: | The influence of (NH4)2SO4 on 14C assimilation and cyclosisin internodal cells of Chara corallina was investigated. Severeinhibition of 14C assimilation was found at pH values above7·0, this inhibition being correlated with the exogenouslevel of NH3 rather than NH+4. Cyclosis was also affected athigher concentrations of (NH4)2SO4. This effect was similarlycorrelated with exogenous levels of NH3. 14C assimilation was inhibited non-competitively by (NH4)2SO4,the apparent Km being increased from 0·55 to 1·5mM. The results suggest that the site(s) of inhibition is locatedat the plasmalemma, rather than at the chloroplasts. (Evidencein support of in vivo uncoupling of photophosphorylation, bylow concentrations of (NH4)2SO4, was not obtained). Significant perturbation of the OH efflux pattern wasobserved as the level of (NH4)2SO4 was increased. Induced migrationof efflux sites indicates that NH3 may interfere with the cellularmechanism that controls OH transport. Using a cell-segmentisolating chamber it was shown that (NH4)2SO4 inhibited OHefflux rather than HCO3 transport. This inhibitory effectwas readily reversible. These data are discussed in terms of a possible relationshipbetween the observe NH4)2SO4 stimulation of 36Cl influxand the effect of this compound on 14C assimilation. |