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

Abstract

Experiments were performed with soybean plants to test the hypothesis that the inhibition of NO 3 - uptake in darkness is due to feedback control by NO 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 15 N-labelled intact plants, suggesting that the shortage of transpiration in darkness may be responsible for the retention of NO 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 NO 3 - and Asn accumulations in the roots. However, the light/dark rhythmicity of NO 3 - uptake was only marginally affected by these treatments, and thus appeared quite independent from transpiration and root NO 3 - or Asn levels. Typically, the maintainance of a constant transpiration during the day/night cycle did not suppress the inhibition of NO 3 - uptake in darkness, whereas it almost prevented the dark increase in root NO 3 - and Asn contents. These data strongly support the conclusion that the effect of light on NO 3 - uptake is not mediated by changes in translocation and accumulation of N compounds.