Résumé
According to the Dijkshoorn-Ben Zioni model, NO3− uptake in the roots is stimulated by NO3− assimilation in the shoots, through downward phloem transport of malate synthesized in response to reduction of NO2− to NH3. In this paper, one hypothesis resulting from this model was tested, i.e. that the diurnal changes in NO3− uptake are due to the light dependence of NO3− reduction in the leaves. This dependence was studied in detached leaves transferred to deionized water or supplied via the transpiration stream with similar amounts of 15NO3− in light or darkness. In the dark, the reduction of previously stored NO3− or xylem-borne 15NO3− was generally about 40–50% of that measured in the light. Glucose supply to the detached leaves stimulated NO3− reduction in the dark, but not enough to increase it up to the same rate as in the light. Nitrite reduction in detached leaves was much less affected by darkness, and could be maintained at a high level by exogenous supply of substrate. Advantage was taken from this last observation to sustain NO2−reduction in attached darkened shoots at the same rate as in the light, by ensuring an appropriate delivery of NO2− from the xylem. Although this was assumed to restore the light level of the associated synthesis of malate, it led to a marked inhibition of NO3− uptake. In addition, the direct supply of malate to the shoots or to the roots failed to prevent the decrease of NO3− uptake in darkness. Thus, our conclusion is that the mechanisms evoked in the Dijkshoorn-Ben Zioni model do not play an important role in the diurnal variations of NO3− uptake in soybean plants.