Spatial and temporal patterns of net nitrate uptake regulation and kinetics along the tap root of Citrus aurantium

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 lM 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.