Diurnal regulation of NO3-uptake in soybean plants II. Relationship with accumulation of NO and asparagine in the roots

Experiments were performed with soybean plants to test the hypothesis that the inhibition of NO3− uptake in darkness is due to feedback control by NO3− 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 NO3− 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 NO3− and Asn accumulations in the roots. However, the light/dark rhythmicity of NO3− uptake was only marginally affected by these treatments, and thusappeared quite independent from transpiration and root NO3− or Asn levels. Typically, the maintainance of a constant transpiration during the day/night cycle did not suppress the inhibition of NO3− uptake in darkness, whereas it almost prevented the dark increase in root NO3− and Asn contents. These data strongly support the conclusion that the effect of light on NO3− uptake is not mediated by changes in translocation and accumulation of N compounds.