SUMMARY: Light caused up to a 20-fold increase in the rate of nitrate and nitrite assimilation by the marine chlorophyte While higher rates of oxygen evolution were observed during both nitrate and nitrite assimilation, the extra oxygen released was not related to the amounts of nitrate or nitrite assimilated. Carbon dioxide was required for light to increase nitrate or nitrite assimilation; the addition of glucose did not overcome this requirement. The light intensities at which nitrate and nitrite assimilation reached their maximum rates were 300 and 1000 ft. candles respectively, while oxygen evolution and carbon fixation required 1800 ft. candles. In the presence of 3-(4-chlorophenyl)-1,1-dimethylurea (10 ), nitrate and nitrite assimilation were still increased by 30% in light, although oxygen evolution decreased to 2% of the control rate. Assimilation of nitrate and nitrite in light was decreased to 50% by sodium azide or potassium cyanide at 1-5 x 10 ; oxygen evolution was less sensitive, requiring more than 3 x 10 to decrease the evolution to 50%. Azide and cyanide both blocked dark nitrate and nitrite assimilation but at slightly higher concentrations than those required in the light. Iodoacetate (10 ) decreased dark nitrate and nitrite assimilation to 2% and 38% of control values, respectively; but, in the light, nitrate assimilation was not affected and nitrite assimilation was decreased by only 35% at this concentration. It is concluded that the action of light in stimulating nitrate and nitrite assimilation cannot be explained on the basis of a single site of action as has been proposed for spinach chloroplasts and that two relatively independent sites exist in the living cell.


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