Diurnal regulation of NO3- uptake in soybean plants III. Implication of the Dijkshoorn-Ben Zioni model in relation with the diurnal changes in NO3- assimilation |
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Authors: | Delhon, Patricia Gojon, Alain Tillard, Pascal Passama, Lucien |
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Abstract: | According to the Dijkshoorn-Ben Zioni model, NO3 uptakein the roots is stimulated by NO3 assimilation in theshoots, through downward phloem transport of malate synthesizedin response to reduction of NO2 to NH3. In this paper,one hypothesis resulting from this model was tested, i.e. thatthe diurnal changes in NO3 uptake are due to the lightdependence of NO3 reduction in the leaves. This dependencewas studied in detached leaves transferred to deionized wateror supplied via the transpiration stream with similar amountsof 15NO3 in light or darkness. In the dark, the reductionof previously stored NO3 or xylem-borne 15NO3was generally about 4050% of that measured in the light.Glucose supply to the detached leaves stimulated NO3reduction in the dark, but not enough to increase it up to thesame rate as in the light. Nitrite reduction in detached leaveswas much less affected by darkness, and could be maintainedat a high level by exogenous supply of substrate. Advantagewas taken from this last observation to sustain NO2reductionin 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 theassociated synthesis of malate, it led to a marked inhibitionof NO3 uptake. In addition, the direct supply of malateto the shoots or to the roots failed to prevent the decreaseof NO3 uptake in darkness. Thus, our conclusion is thatthe mechanisms evoked in the Dijkshoorn-Ben Zioni model do notplay an important role in the diurnal variations of NO3uptake in soybean plants. Key words: Glycine max, light/dark cycle, malate synthesis, NO3 reduction, NO3 uptake |
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