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It is well established that nitrate is a potent inhibitor of nodulation and nitrogen fixation in legumes. The objective of this study was to demonstrate the relative insensitivity of these processes to nitrate with Calopogonium mucunoides, a tropical South American perennial legume, native to the cerrado (savannah) region. It was found that nodule number was reduced by about half in the presence of high levels of nitrate (15 mM) but nodule growth (total nodule mass per plant) and nitrogen fixation (acetylene reduction activity and xylem sap ureide levels) were not affected. Other sources of N (ammonium and urea) were also without effect at these concentrations. At even higher concentrations (30 mM), nitrate did promote significant inhibition (ca. 50%) of acetylene reduction activity, but no significant reduction in xylem sap ureides was found. The extraordinary insensitivity of nodulation and N 2 fixation of C. mucunoides to nitrate suggests that this species should be useful in studies aimed at elucidating the mechanisms of nitrate inhibition of these processes.

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Calopogonium mucunoides Desv. is able to form nodules, root organs in which biological nitrogen fixation takes place, after a symbiotic interaction with soil bacteria known as rhizobia. Such distinct advantage of some legume species faces different environmental abiotic factors such as acid and alkaline pH conditions of soil. Nodulation and symbiotic nitrogen fixation response in different pH ranges were determined under greenhouse conditions. Plants were cultivated in vermiculite and treated with nutritive solution adjusted to pH 4.0, 5.5, and 7.0, and after three months, the following variables were measured: nodule number, nitrogen fixation, tissue protein, amino acids, total ureides, allantoin, and allantoate. The number of nodules and nitrogen fixation were enhanced under acidic conditions, but nitrogenase activity was drastically decreased at pH 7.0. Acidic conditions decreased the amount of protein, amino acids, total ureides, allantoate, and allantoin in leaves and nodules, but at pH 5.5, only protein content was decreased. Symbiosis with C. mucunoides and biological nitrogen fixation were kept under acidic conditions, but it was negatively affected under conditions that are near to neutral. It is reasonable to conclude that the association between C. mucunoides and nitrogen-fixing bacteria from ''Cerrado'' evolved in this context, which is supported by the difficulties faced to keep interaction under pH near neutrality.

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The distribution of amino acids in distinct tissues of Canavalia ensiformes was determined during the life cycle of the plant. Glycine was shown to be the main amino acid in mature seeds, while the nonprotein amino acid canavanine exhibited a high concentration in 7-d-old seedlings. Canavanine was lower in the seeds when compared to other tissues analyzed. This does not support the nitrogen-storage function of canavanine, however, it suggests that it is involved in the translocation of amines during the early stages of the development.

ResumoEm Calopogonium mucunoides, o metabolismo de nitrogênio foi signiicativamente alterado entre os diferentes estágios de desenvolvimento das plantas. Ao longo do ciclo de vida, mesmo na presença de nitrato, a planta manteve-se nodulada e altas concentrações de ureídeos foram identiicadas nos tecidos e seiva de xilema, evidência de que a espécie é tolerante ao nitrato e que este não afeta a ixação de nitrogênio ao longo do desenvolvimento. A redução bioquímica de nitrato apresenta alterações ao longo do ciclo de vida, com mudança no sítio de redução junto com mudanças na concentração de aminoácidos solúveis totais. No entanto, a atividade de nitrato redutase constitutiva em nódulos desaparece com a exposição ao nitrato. Palavras-chave: Leguminosa; Ureídeos; Nodulação AbstractNitrate inluences the metabolism of nitrogenous compounds in calopo (Calopogonium mucunoides) throughout the life cycle. In Calopogonium mucunoides, nitrogen metabolism was signiicantly altered between the various plant development stages. Throughout the life cycle, even in the presence of nitrate, the plant kept itself nodulated and high ureide concentrations were identiied in xylem tissues and sap, evidencing that the species is tolerant to nitrate and that this does not affect nitrogen ixation over development. Nitrate biochemical reduction presents alterations throughout the life cycle, with change in the reduction site along with changes in total soluble amino acid concentrations. However, constitutive reductase nitrate activity in nodules disappears with the exposition to nitrate.