Résumé
The effect of light on NO3− utilization was investigated in non-nodulated soybean (Clycine max L. Merr., cv. Kingsoy) plants during a 14/10 h light/dark period at a constant temperature of 26°C. A 30–50% decrease of net NO3− uptake rate was observed 2–6 h after the lights were turned off. This was specifically due to an inhibition of NO3− influx as measured by 15N incorporation during 5 min. The absolute values of NO3− efflux depended on whether the labelling protocol involved manipulation of the plants or not, but were not affected by illumination of the shoots. Darkness had an even more marked effect in lowering the reduction of 15NO3− in both roots and shoots, as well as xylem transport of 15NO3− and reduced 15N. Concurrently with this slowing down of transport and metabolic processes, accumulations of NO3− and Asn were significantly stimulated in roots during the dark period. These data are discussed in view of the hypothesis that darkness adversely affects NO3− uptake through specific feedback control, in response to alterations in the later steps of N utilization which are more directly dependent on light.