Spatial and temporal patterns of net nitrate uptake regulation and kinetics along the tap root of <Emphasis Type="Italic">Citrus aurantium</Emphasis>

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, K _m and V _max, 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.