Performance of reflectance indices and of a handheld device for estimating in‐field the nitrogen status of grapevine leaves

BACKGROUND AND AIMS: Optical leaf sensing of plant nitrogen (N) status enables fertilisation based on plant demand. The aim of this study was to assess the suitability of leaf reflectance indices and of a handheld device for the estimation of N and chlorophyll (Chl) in Vitis vinifera L. cv. Riesling. METHODS AND RESULTS: Riesling leaves from a long‐term fertilisation trial were analysed for nitrogen and pigments and their spectral reflectance in the Vis/near‐infrared range was recorded for two consecutive seasons. In the second season the leaves were also measured with a Chl fluorescence based portable device (Dualex). Published reflectance indices and fluorescence indices were regressed against the concentration of pigment and N. Temporal changes in the Chl/N ratio complicated the indirect assessment of N via Chl. Red edge inflection points and several three‐ to four‐waveband indices allowed for a good prediction of leaf N. Fluorescence indices allowed for reliable estimation of N and Chl. CONCLUSIONS: While leaf pigments can be analysed by simple two‐band indices, accurate N estimation requires more complex indices such as three‐band indices or indices calculated from derivative spectra. SIGNIFICANCE OF THE STUDY: Data from this study will assist growers to monitor non‐destructively the spatial and temporal variability of grapevine N status and thereby, improve fertilisation.