Nitrate, nitrite and ammonia assimilation by leaves: Effects of inhibitors

The assimilation of H(14)CO3 (-), (15)NO3 (-), (15)NO2 (-) and (15)NH4 (+) by barley (Hordeum vulgare L.) leaf segments in the presence of a number of metabolic inhibitors was studied in experiments where the substrates and inhibitors were vacuum infiltrated into the tissue. 3-(3',4'-Dichlorophenyl)-1,1-dimethylurea (DCMU), carbonyl-cyanide-m-chlorophenylhydrazone (CCCP), and iodoacetate (IOA) inhibited (14)CO2 fixation and (15)NO3 (-) and (15)NO2 (-) assimilation in the light. (15)NH4 (+) assimilation in the light was only inhibited 70% by 10(-4)M DCMU. (15)NH4 (+) assimilation was stimulated by 10(-5)M CCCP but was inhibited by concentrations of CCCP above 5×10(-5)M. In double-label experiments (15)NO3 (-) assimilation was less sensitive than (14)CO2 fixation to both DCMU and CCCP. CCCP but not DCMU stimulated NO2 (-) accumulation in dark and IOA was inhibitory. The rate of NO2 (-) accumulation in the light in the presence of DCMU or atrazine was similar to that in the dark and in all cases NO2 (-) accumulation was inhibited about 90% by oxygenation of the medium. The results indicate that the assimilation of all nitrogen species is closely linked to photosynthetic electron transport, that rate of assimilation of nitrogen species is independent of rate of assimilation of CO2, and that the dark in-vivo nitrate reduction is a useful analogue of the mechanism operating in the light only if electron flow to oxygen is impaired.