DIURNAL REGULATION OF NO3- UPTAKE IN SOYBEAN PLANTS .1. CHANGES IN NO3- INFLUX, EFFLUX, AND N UTILIZATION IN THE PLANT DURING THE DAY-NIGHTCYCLE

The effect of light on NO3- utilization was investigated in non-nodula ted soybean (Glycine max L. Merr., cv, Kingsoy) plants during a 14/10 h light/dark period at a constant temperature of 26 degrees 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- i nflux as measured by N-15 incorporation during 5 min. The absolute val ues of NO3- efflux depended on whether the labelling protocol involved manipulation of the plants or not, but were not affected by illuminat ion of the shoots. Darkness had an even more marked effect in lowering the reduction of (NO3-)-N-15 in both roots and shoots, as well as xyl em transport of (NO3-)-N-15 and reduced N-15. Concurrently with this s lowing down of transport and metabolic processes, accumulations of NO3 - and Asn were significantly stimulated in roots during the dark perio d. these data are discussed in view of the hypothesis that darkness ad versely affects NO3- uptake through specific feedback control, in resp onse to alterations in the later steps of N utilization which are more directly dependent on light.