UC Davis

Grape quality and yield are goals in viticultural management that are affected by numerous abiotic and biotic factors. Field heterogeneity complicates vineyard management by producing grapes with variable stress responses and ripening patterns. Manipulation of vine water stress via irrigation is a powerful management tool influencing canopy growth and berry development. To optimize water use in a changing climate, novel techniques in precision viticulture are being developed. Increasingly, growers use satellite remote sensing tools to evaluate the heterogeneity of vineyards and to help achieve viticultural targets. Hyperspectral Vegetation Indices (HVIs) facilitate comparisons of plant stress responses at the leaf, canopy, or vineyard level. We leveraged leaf-level data collected through the Grape Remote sensing Atmospheric Profile and Evapotranspiration eXperiment (GRAPEX) to evaluate linkages between grapevine water stress and spectral indices. This study takes place in an experimental vineyard near Madera, in the Central Valley of California, an area of large seasonal inputs of irrigation. The dataset includes a three-week period during which water stress was imposed. HVIs measured pre-, during, and post-water stress were collected in parallel with conventional plant stress measurements, including leaf water potential. The purpose of this experiment is to evaluate the practical use of leaf spectroscopy to monitor water status. 88 HVIs were evaluated and among these, we found that HVIs exhibited two patterns in sensitivity to water-stressed plant responses: (1) 12 HVIs detected stress earlier at 930 or 1130, while (2) 29 detected stress at 1600. This suggests that timing of satellite or other aerial overpasses, which typically occur around solar noon, and HVI(s) chosen to evaluate plant water status may be consequential in detection of plant stress.