Résumé
The short-term dependence of $\mathrm{N}{\mathrm{O}}_{3}^{-}$ uptake upon photosynthesis and sugar supply to the roots of soybean plants was investigated in a series of experiments where CO2 availability, light intensity or conduction of phloem sap to the roots were severely limited. Removal of CO2 from the atmosphere or girdling of the stem equally prevented the stimulation of $\mathrm{N}{\mathrm{O}}_{3}^{-}$ uptake when plants were transferred from darkness to the light. The effect of these two treatments can be reversed by CO2 re-supply or by addition of 10 mM glucose in the nutrient solution, respectively. Glucose was also more effective in stimulating $\mathrm{N}{\mathrm{O}}_{3}^{-}$ uptake by intact plants in darkness than in light. Collectively, these observations are interpreted as evidence that the diurnal changes in $\mathrm{N}{\mathrm{O}}_{3}^{-}$ uptake are due to decreased phloem transport of photosynthates in darkness. Accordingly, the magnitude of these changes was much dependent on starch accumulation in the leaves at the end of the photoperiod. Shading the plants lowered this accumulation, and resulted in an amplification of the diurnal changes in $\mathrm{N}{\mathrm{O}}_{3}^{-}$ uptake. These results are discussed in connection with the hypothesis that the carbon-dependent plasticity of the night/day ratio of $\mathrm{N}{\mathrm{O}}_{3}^{-}$ uptake is an important feature of the co-ordination of the acquisition of N and C by the plant.