Hand-held spectral sensors for in-field grapevine nutrient monitoring

Monitoring the nutrient status of grapevines is important for optimizing fertilization strategies, ensuring grape quality, and minimizing environmental impacts in precision viticulture. The traditional laboratory leaf analysis for nutrient monitoring is time-consuming and costly, so rapid, non-destructive, on-site measuring techniques are needed. The aim of present study was to investigate the seasonal variability of grapevine nutrient status using hand-held spectral sensors, specifically focusing on key phenological stages. The study was carried out in a vineyard located in the Balaton Uplands region (Hungary), which serves as a long-term monitoring site for various environmental and viticultural research projects.

Measurements were conducted at four distinct phenological stages (flowering and fruit set, bunch closure, veraison, harvest), with 40 leaf samples collected per occasion. The physical and physiological characteristics of the leaves were recorded, including fresh and dry weight, leaf area, leaf mass area (LMA), chlorophyll content (CCI), and Normalized Difference Vegetation Index (NDVI). In addition, particular emphasis was placed on the Normalized Difference Greenness Index (NDGI), to examine the correlation with the nitrogen content (N) of the leaves. CCI was measured using an MC-100 (Apogee Instruments), while NDVI and NDGI were collected using PlantPen NDVI 310 and N-Pen N 110 handheld instruments (Photon Systems Instruments). Leaf parameters were determined in a laboratory. This multi-temporal approach allowed us to evaluate the sensitivity of these spectral indices in tracking nutrient fluctuations throughout the growing season.

In general, the highest average NDVI (0.771) was measured at the flowering and fruit set stage, while the highest values of the other indicators were usually recorded during the veraison and harvest periods. The highest LMA-adjusted NDGI values were recorded at veraison, suggesting a peak in leaf N concentration. Ongoing laboratory analyses are expected to confirm these results. Significant differences were found in LMA-adjusted NDGI values between veraison and other phenological stages. A strong positive correlation was between NDGI and CCI (r=0.60, p<0.05) for the whole season. This connection became even stronger in the later stages, reaching r=0.68 at bunch closure and r=0.89 at veraison (p<0.05). Principal Component Analysis (PCA) showed no clear separation between the phenological stages based on the investigated parameters.

Acknowledgments: Tibor Zsigmond is grateful for internal funding by HUN-REN ATK (Project number 0405B1481P). The research was funded by the Sustainable Development and Technologies National Programme of the Hungarian Academy of Sciences (FFT NP FTA) and the 2023-1.2.4-TÉT-2023-0009 project.