Spatial–temporal variation of the regulation and the kinetics of net nitrate (NO₃⁻) uptake rate (NNUR) along the tap root of Citrus aurantium L. were analysed. Suberin incrustation in the peripheral cell layers and plasma membrane (PM) H⁺-ATPase localisation, anatomical and physiological factors involved in NO₃⁻ uptake were also investigated. The results clearly indicated a spatially uniform distribution of the regulation process, accompanied by a temporal heterogeneous pattern of the kinetics of NO₃⁻ uptake along citrus tap root. In particular, kinetic analysis had a biphasic pattern, saturating (high affinity transport system) and linear (low affinity transport system), in response to increasing external NO₃⁻ concentrations in each root region, where 200 µM NO₃⁻ represented the threshold separating these two systems. Kinetic parameters, Km and Vmax, clearly indicated that apical segments reached the maximum value of induction before basal segments. Hence, the apical root zones, early exhibiting the maximum of potential capacity to absorb the NO₃⁻, could be considered more efficient than basal root segments for acquiring NO₃⁻ from external solution. Suberin incrustations in the hypodermal cell layer, characterised by uniform fluorescence intensity among the root segments, could be responsible for the unchanged NNUR, while the PM H⁺-ATPase could explain the temporal pattern of NNUR.
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