How nitrogen form and concentration affect growth, nutrient accumulation and photosynthetic performance of Olea europaea L. (cv. 'Kalamon')

•Olive plants showed better growth when they received urea at high supply.•Plants showed better growth when they received NO3− at low to medium supply.•N form (NO3−, NH4+, urea, NH4+ + NO3−) influenced nutrient uptake by olive plants.•High N supply in the form of NH4+-N significantly inhibited net photosynthesis.•The most appropriate N form for the nutrition of olive cultivar 'Kalamon' was urea.

A greenhouse experiment was conducted, in order to study the effect of nitrogen (N) form (NO3−, NH4+, urea, NH4+ + NO3−) and concentration (1, 8 and 16 mM) on growth, mineral nutrition and photosynthetic performance of olive plants (Olea europaea L., cv. 'Kalamon'). Our data indicate that when plants at high supply (16 mM) received urea as N source better growth performance was recorded, compared to the cases olive plants received other N forms; however, at low to medium N supply (1 or 8 mM) better plant growth performance observed when they were fertilized with NO3−-N. Total N uptake at 16 mM was significantly lower when plants received NO3−-N, in comparison to the other N forms. NO3− form at 16 mM resulted in lower total uptake of P, Fe, Mn and Zn, compared to the other treatments. On the other hand, NH4+-N at high N supply (16 mM) resulted in lower total uptake of Ca and Mg. However, significantly higher K uptake was found when plants were treated with NH4+, at high N rate (16 mM). High N supply in the form of NH4+-N significantly inhibited net photosynthetic rate (A), although the chlorophyll fluorescence (as an indicator of the function of PSII) did not significantly differ among the N treatments. In conclusion, urea-N proved to be the most appropriate N form for the mineral nutrition of the olive cultivar 'Kalamon'; thus, it should be preferred as a source of N in olive groves receiving high N fertilizations.