DIURNAL REGULATION OF NO3- UPTAKE IN SOYBEAN PLANTS .2. RELATIONSHIP WITH ACCUMULATION OF NO3- 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 cont rol by NO3- and/or Asn accumulating in the roots. Xylem export of N co mpounds was shown to depend on water flux in both excised root systems and N-15-labelled intact plants, suggesting that the shortage of tran spiration in darkness may be responsible for the retention of NO3- and Asn in the roots. This was verified in experiments where the light/da rk pattern of transpiration was modulated in intact plants by changing the relative humidity of the atmosphere. Any decrease of transpiratio n at night was associated with a concurrent stimulation of NO3- and As n accumulations in the roots. However, the light/dark rhythmicity of N O3- uptake was only marginally affected by these treatments, and thus appeared quite independent from transpiration and root NO3- or Asn lev els. Typically, the maintainance of a constant transpiration during th e day/night cycle did not suppress the inhibition of NO3- uptake in da rkness, whereas it almost prevented the dark increase in root NO3- and Asn contents. These data strongly support the conclusion that the eff ect of light on NO3- uptake is not mediated by changes in translocatio n and accumulation of N compounds.