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

Abstract

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.