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Plant Physiology 82:708-712 (1986)
© 1986 American Society of Plant Biologists

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Articles

Nitrate and Ammonium Induced Photosynthetic Suppression in N-Limited Selenastrum minutum1

II. Effects of NO3– and NH4+ Addition to CO2 Efflux in the Light

Douglas G. Birch, Ivor R. Elrifi and David H. Turpin

Department of Biology, Queen's University, Kingston, Ontario, Canada K7L 3N6

The effects of nitrate and ammonium addition on net and gross photosynthesis, CO2 efflux and the dissolved inorganic carbon compensation point of nitrogen-limited Selenastrum minutum Naeg. Collins (Chlorophyta) were studied. Cultures pulsed with nitrate or ammonium exhibited a marked decrease in both net and gross photosynthetic carbon fixation. During this period of suppression the specific activity of exogenous dissolved inorganic carbon decreased rapidly in comparison to control cells indicating an increase in the rate of CO2 efflux in the light. The nitrate and ammmonium induced rates of CO2 efflux were 31.0 and 33.8 micromoles CO2 per milligram chlorophyll per hour, respectively, and represented 49 and 48% of the rate of gross photosynthesis. Nitrate addition to cells at dissolved inorganic carbon compensation point caused an increase in compensation point while ammonium had no effect. In the presence of the tricarboxylic acid cycle inhibitor fluoroacetate, the nitrate-induced change in compensation point was greatly reduced suggesting the source of this CO2 was the tricarboxylic acid cycle. These results are consistent with the mechanism of N-induced photosynthetic suppression outlined by Elrifi and Turpin (1986 Plant Physiol 81: 273-279).

1 Supported by the School of Graduate Studies and Research of Queen's University and the Natural Sciences and Engineering Research Council of Canada.






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