Diurnal rhythm in nitrate absorption by roots of soybeans (Glycine max)

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Abstract
The diurnal rhythm in NO3 - absorption by the intact soybean roots was investigated by using a continuous sampling system of the culture solution. During the day and night period, the soybean absorbed NO3 - almost linearly irrespective of the time or NO3 - concentration in the medium. The NO3 - absorption rate was found to be different between the day and night period, and the level in the night was about 60-75% of that in the daytime. The temporary interruption of NO3 - absorption was observed twice a day at dawn and dusk. It was suggested that this rhythmic pattern of NO3 - absorption was not directly controlled by the shoots, based on the fact that the rhythm appeared under on extended dark period or by the cutting of the shoots. The diurnal rhythm was also observed in the N concentration of the xylem exudate collected from decapitated roots. The NO3 - absorption was completely inhibited by the addition of a metabolic inhibitor such as CCCP or KCN, but the rate was not affected by DNP. Concomitant K+ efflux from the roots was observed in the CCCP treatment. When the 1 mM NH4 + was added to the medium, the NO3 - absorption rate decreased to about 33% of the initial rate after a time-lag of 1 or 2 h. On the other hand, by the addition of NO2 - (1 mM), the NO3 - uptake was almost completely interrupted for several hours. The efflux of NO3 - from the roots to the medium was confirmed when the plants were transferred from the medium containing 15NO3 - to the non-labeled NO3 - medium. The results suggest the presence of three steps in the efflux: first a rapid efflux (5 mg N/h plant)) was observed for the initial few min, and the second efflux (0.9 mg N/(h plant)) occurred subsequently after about 10 min, then the third low but constant efflux (0.4 mg N/(hplant)) continued for a long time.