The relationships between net nitrate uptake and root morphology and topology were investigated in wheat (Triticum durum Desf.) and citrus (Citrus volkameriana Ten. & Pasq.) seedlings in which root systems were modified by supplying different nutrient concentrations. Root morphological and topological changes were assessed by the number of root tips, the total root length, and the topological index. The net nitrate uptake process was evaluated during the induction and inhibition "feedback" regulation phases. In wheat seedlings, an increase in the external nitrate concentration, at either high or no phosphate, brought about an increase in number of root tips but did not affect total root length. A similar pattern was observed in citrus seedlings. Citrus seedlings also exhibited a shift in root branching pattern from herringbone to dichotomous with an increase in the external nitrate concentration. Conversely, wheat root topology was unaffected by nutrient treatments. A high level of nitrate supplied to wheat and citrus root systems reduced the full induction of the nitrate uptake rate and cumulative nitrate uptake but increased the half-time of reaching the full induction in net nitrate uptake (t1/2kind). Finally, a negative correlation between number of root tips and t1/2kindwas found in both species, while only in citrus seedlings was a positive correlation between t1/2kindand the topological index observed. The data suggest that root morphology has a greater effect on nitrate uptake than root topology.Key words: root morphology, root topology, net nitrate uptake, Citrus volkameriana, Triticum durum.