The influence of an additional supply of ammonium to a nitrate containing hydroponic solution on
the mineral nutrition of greenhouse rose plants (Rosa hybrida cv. ‘Lambada’) has been investigated.
The effect on NPK uptake, mineral contents in roots and leaves as well as nitrate reductase (NR) and
glutamine synthetase (GS) activities were examined. The addition of ammonium in a nutrient solution
containing nitrate produced a total nitrogen uptake increase during shoot elongation, while in the
absence of ammonium, nitrate uptake was lower during shoot elongation. Adding ammonium to the
hydroponics solution caused an increase of potassium uptake while ammonium absorption was
constant. Phosphate uptake was always higher in combined nitrate plus ammonium treatment,
likewise in this treatment the most relevant effect on plant mineral content was the increase of P
concentration in the roots. In both treatments with or without NH+4, free nitrate was notably higher
in the roots than in leaves, indicating that nitrate reduction in rose plants takes place mainly in the
leaves. NR activity in the leaves was higher when ammonium was present, whereas the root GS
activity was similar in both treatments. The influence of ammonium on phosphate uptake and the
subsequent effects on transport of other ions and enzymatic activities are discussed.