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

Summary

The assimilation of H¹⁴CO₃ ^-, ¹⁵NO₃ ^-, ¹⁵NO₂ ^- and ¹⁵NH₄ ⁺ 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 ¹⁴CO₂ fixation and ¹⁵NO₃ ^- and ¹⁵NO₂ ^- assimilation in the light. ¹⁵NH₄ ⁺ assimilation in the light was only inhibited 70% by 10^-4M DCMU. ¹⁵NH₄ ⁺ assimilation was stimulated by 10^-5M CCCP but was inhibited by concentrations of CCCP above 5×10^-5M. In double-label experiments ¹⁵NO₃ ^- assimilation was less sensitive than ¹⁴CO₂ 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.