Nondestructive optical crop phenotyping to monitor nitrogen content in field-grown ornamentals

Nitrogen management for open-field production of ornamental plants is challenging due to the large number of species and cultivars. Sub-optimal nitrogen nutrition negatively influences the commercial quality of the plants while supra-optimal N has an environmental impact due to nitrate leaching. Therefore, (a) reliable indicator(s) of plant nitrogen phenotyping is/are needed. Sensors based on the optical properties of chlorophyll have already provenchoo their use for plant phenotyping in various agronomic cropping systems. They can be used as quick, nondestructive and relatively inexpensive tools to support decisions concerning in-season N fertilization. Research on these optical N sensors in outdoor ornamentals is scarce. For this study, 4 woody perennials were chosen, Acer pseudoplatanus, Ligustrum ovalifolium, Prunus laurocerasus ‘Rotundifolia’ and Tilia cordata, as well as 2 herbaceous species Chrysanthemum morifolium and Begonia x tuberhybrida. All plants received 3 different N treatments in 3 replicates. The experiment was first conducted in 2016 and repeated in 2017. The leaf and whole-plant nitrogen content and non-destructive measurements were monitored regularly during the growing season. At leaf level, the SPAD-502 chlorophyll meter was evaluated. At crop level, the species were monitored with a GreenSeeker RT100 (Trimble) which calculates the normalized difference vegetation index (NDVI). This canopy reflectance index can be correlated with N and biomass. Early results show significant correlations between foliar nitrogen and SPAD values for Begonia, Ligustrum, Acer and Tilia, while for Prunus and Chrysanthemum correlations were absent. At canopy level, only for Acer, Ligustrum and Chrysanthemum a correlation was found between NDVI and N in 2016. In 2017 this relation was less clear. Although NDVI was an estimator for biomass for Acer, Chrysanthemum, Prunus and Tilia, robustness is further investigated.