Photosynthetic Nitrogen Metabolism in High and Low CO2-adaptedScenedesmus

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Abstract
Larsson, C.-M., Larsson, M. and Guerrero, M. G. 1985. Photosynthetic nitrogen metabolism in high and low CO2-adapted Scenedesmus. II. Effect of ammonium and methionine sulphoximine on nitrate utilization.—J. exp. Bot. 36: 1387–1395 In 3% CO2-grown Scenedesmus obtusiusculus Chod. utilizing NO3− J as the N source, NH4+ addition caused a prompt inhibition of NO3− utilization. Nitrate reductase (NR) activity declined rapidly in response to the presence of NO4+, but the cessation of NO3− utilization was too rapid to be accounted for by the loss in NR activity. The first site of NO4+ inhibition in these cells seems to be the entrance of NO3− into the cells. Upon exhaustion of NO4+ from the medium, NO3− utilization was rapidly restored and NR activity increased. Air-grown cells were much less sensitive to the effect of NO4+, more than 30 min being required for added NO4+ to cause complete inhibition of NO3− utilization. In these cells, NO3− uptake and NR activity decreased in parallel in response to NO4+ addition. In 3% CO2-grown cells simultaneously subjected to NO4+ and air-level of CO2, NO4+ initially inhibited NO3− utilization completely, but a slight recovery took place after approximately 20 min The glutamine synthetase (GS) inhibitor L-methionine D, L-sulphoximine (MSO) behaved as a potent inhibitor of NO3− uptake in 3% CO2-grown cells, but had considerably less effect in air-grown cells, although the time-course of the MSO-induced inhibition of GS was the same in both cases