The spatial variability of the nitrate (NO₃^‐) uptake along the maize primary root axis was investigated at physiological and molecular levels. Net NO₃^‐ uptake rate (NNUR) and its kinetic parameters, together with the gene expression of a high‐affinity NO₃^‐ transporter (NRT2.1), were evaluated. The activity and the expression of plasma membrane H⁺‐ATPase (PM H⁺‐ATPase), key enzyme in plant nutrition, were also analysed. The NNUR showed a heterogeneous spatial pattern along the root, where the regions closer to the root tip early exhibited higher capacity to absorb NO₃^‐ than the basal regions, because of a higher maximum NNUR and faster induction of the inducible high‐affinity transport system (iHATS), the presence of the high‐affinity transport system (HATS) also at external NO₃^‐ concentrations >100 µm and an improved NO₃^‐ transport because of lower K_m values. ZmNRT2.1 transcript abundances were not spatially correlated with NNUR, suggesting that post‐translational effects or NAR2 protein co‐expression could be involved. By contrast, PM H⁺‐ATPase displayed a similar spatial–temporal pattern as that of nitrate uptake, resulting in higher activity in the root tip than in the basal regions. Increased activities of the enzyme after nitrate supply resulted in enhanced expression of MAH3 and MAH4, PM H⁺‐ATPase subfamily II genes, while MAH1 was not expressed.

Seven-day-old maize seedlings grown in a nitrogen-free hydroponic culture were exposed for 48 h to 0, 100 and 300 μM trans-cinnamic, p-coumaric, ferulic, caffeic acids, umbelliferone and 200 μM KNO₃. Net nitrate uptake was affected by trans-cinnamic, ferulic and p-coumaric acids in a concentration-dependent manner, and trans-cinnamic acid appeared to be the strongest inhibitor. Conversely, at low concentrations, caffeic acid stimulated net nitrate uptake while umbelliferone did not influence it. After 24 h of treatment, plasma membrane H⁺-ATPase activity significantly decreased in a concentration-dependent manner in response to trans-cinnamic, ferulic and p-coumaric acids, while umbelliferone and caffeic acid had no effect on H⁺-ATPase activity.