Diurnal regulation of NO3- uptake in soybean plants I. Changes in NO3- influx, efflux, and N utilization in the plant during the day/night cycle

Abstract

The effect of light on NO 3 - utilization was investigated in non-nodulated soybean (Glycine 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 NO 3 - uptake rate was observed 2-6 h after the lights were turned off. This was specifically due to an inhibition of NO 3 - influx as measured by 15 N incorporation during 5 min. The absolute values of NO 3 - 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 15 NO 3 - in both roots and shoots, as well as xylem transport of 15 NO 3 - and reduced 15 N. Concurrently with this slowing down of transport and metabolic processes, accumulations of NO 3 - and Asn were significantly stimulated in roots during the dark period. These data are discussed in view of the hypothesis that darkness adversely affects NO 3 - uptake through specific feedback control, in response to alterations in the later steps of N utilization which are more directly dependent on light.