Flatpea (Lathyrus sylvestris L. cv. 'Lathco') plants, grown hydroponically, were inoculated with rhizobia (Rhizobium leguminosarum strain 92F2), supplied with nitrate nitrogen (2 meq 1-1), or provided with both rhizobia and NO3-. Nodulated plants supplied with NO3- had the highest biomass and the highest level of free amino acids in leaves and roots. Nitrogen fixation was depressed 80% in inoculated plants supplemented with NO3-. Compared to plants whose nitrogen requirement was satisfied either partially or fully by NO3-, plants that were totally dependent upon biological nitrogen fixation had fewer lateral shoots and lower concentrations of nitrogen, soluble protein, and total free amino acids in the shoots, especially in the leaves. The symbiotic association between plants and rhizobia for nitrogen fixation was not essential to the production and accumulation of 2,4-diaminobutyric acid (A2bu) and other non-protein amino acids in flatpea. Levels of A2bu, and most of the other free amino acids quantified, were significantly lower in the nodulated plants grown in the absence of NO3- than in the inoculated or noninoculated plants supplied with NO3-. Arginine and glutamic acid were exceptions in that levels of these two amino acids were higher in roots bearing effective nodules. Plant responses to the different treatments are attributed to the amount of nitrogen available to the plant, rather than the method or form of nitrogen supplied. © 1990 Oxford University Press.
