Monitoring nitrogen variability in two Mediterranean ornamental shrubs through proximal fluorescence-based sensors at leaf and canopy level

•Fluorescence-based sensors can help to reduce N losses in ornamental shrubs•An optimal N rate of 450 mg L−1 was identified for P. lauroceraus and L. japonicum•Optical indices allow the estimation of chlorophyll and N at leaf and canopy level•Sufficiency thresholds of Nitrogen Balance Index (NBI) were provided•NBI mapping can identify plant N variability under homogeneous fertilization Over - fertilization is one of the most relevant issues affecting container-grown nursery plants. The massive application of chemicals results in severe pollution due to nitrogen (N) leaching, and leads to the production of unhealthy plants sensitive to fungal diseases and insect attacks that increase the use of pesticides. The ability of two fluorescence-based sensors (Dualex and Multiplex) for assessing the N-sufficiency of container grown Prunus laurocerasus L. and Ligustrum japonicum L. as test plants was investigated in a two-year long experiment under experimental and commercial growing conditions. An optimal N rate of 450 mg L−1 was identified for both species growing under different N fertilization levels (0 - 1350 mg N L−1). Fluorescence-based indices allowed for reliable estimation of both chlorophyll and nitrogen content at the leaf and canopy level. Absolute and relative sufficiency threshold values were provided and spatially compared under standard nursery conditions. The optimal shoot N concentration of 2.04% (P. laurocerasus) and 1.35% (L. japonicum), and the corresponding non-destructive nitrogen balance index (NBI) values were assessed and used for decision-making on N application and monitoring of the species considered.