Résumé
Experiments were performed with soybean plants to test the hypothesis that the inhibition of $\mathrm{N}{\mathrm{O}}_{3}^{-}$ uptake in darkness is due to feedback control by $\mathrm{N}{\mathrm{O}}_{3}^{-}$ and/or Asn accumulating in the roots. Xylem export of N compounds was shown to depend on water flux in both excised root systems and 15N-labelled intact plants, suggesting that the shortage of transpiration in darkness may be responsible for the retention of $\mathrm{N}{\mathrm{O}}_{3}^{-}$ and Asn in the roots. This was verified in experiments where the light/dark pattern of transpiration was modulated in intact plants by changing the relative humidity of the atmosphere. Any decrease of transpiration at night was associated with a concurrent stimulation of $\mathrm{N}{\mathrm{O}}_{3}^{-}$ and Asn accumulations in the roots. However, the light/dark rhythmicity of $\mathrm{N}{\mathrm{O}}_{3}^{-}$ uptake was only marginally affected by these treatments, and thus appeared quite independent from transpiration and root $\mathrm{N}{\mathrm{O}}_{3}^{-}$ or Asn levels. Typically, the maintainance of a constant transpiration during the day/night cycle did not suppress the inhibition of $\mathrm{N}{\mathrm{O}}_{3}^{-}$ uptake in darkness, whereas it almost prevented the dark increase in root $\mathrm{N}{\mathrm{O}}_{3}^{-}$ and Asn contents. These data strongly support the conclusion that the effect of light on $\mathrm{N}{\mathrm{O}}_{3}^{-}$ uptake is not mediated by changes in translocation and accumulation of N compounds.