Plasmalemma Transport of HCO−3and OH−inChara corallina: Inhibitory Effect of Ammonium Sulphate

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Abstract
The influence of (NH4)2SO4 on 14C assimilation and cyclosis in internodal cells of Chara corallina was investigated. Severe inhibition of 14C assimilation was found at pH values above 7·0, this inhibition being correlated with the exogenous level of NH3 rather than NH+4. Cyclosis was also affected at higher concentrations of (NH4)2SO4. This effect was similarly correlated with exogenous levels of NH3. 14C assimilation was inhibited non-competitively by (NH4)2SO4, the apparent Km being increased from 0·55 to 1·5 mM. The results suggest that the site(s) of inhibition is located at the plasmalemma, rather than at the chloroplasts. (Evidence in support of in vivo uncoupling of photophosphorylation, by low concentrations of (NH4)2SO4, was not obtained). Significant perturbation of the OH− efflux pattern was observed as the level of (NH4)2SO4 was increased. Induced migration of efflux sites indicates that NH3 may interfere with the cellular mechanism that controls OH− transport. Using a cell-segment isolating chamber it was shown that (NH4)2SO4 inhibited OH− efflux rather than HCO−3 transport. This inhibitory effect was readily reversible. These data are discussed in terms of a possible relationship between the observe NH4)2SO4 stimulation of 36Cl− influx and the effect of this compound on 14C assimilation.