How good are vegetation indices to assess water status and biochemical parameters in olive tree?

As the water is an increasingly scarce resource, the adoption of deficit irrigation (DI) strategies has become essential to optimize its use in agriculture. To implement DI, it is important to monitor the leaf water status during crop development by indicators such as Relative Water Content (RWC) and Leaf Water Potential (LWP) to optimize the crop production. However, these methods are destructive and time-consuming. Many relationships between spectral data from remote sensing observations and various biophysical and physiological crops parameters have been proposed, in which, Vegetation Indices (VIs) are widely used. This study aims to evaluate the relationship between VIs with leaf water status (RWC, LWP) and biochemical parameters, in a drip irrigated olive orchard (cv. Cobrançosa), located in the Northeast of Portugal (Alfândega da Fé). Five irrigation strategies were studied: full irrigated (FI)_, that received a volume of water equivalent to satisfy crop water needs, FI₁₂₀ irrigated with 20% more than FI, two sustained deficit irrigation (SDI₆₀ and SDI₃₀) and farmer-managed irrigation (FMI). A list of 20 VIs was calculated and correlated with RWC, LWP and biochemical parameters. Although no good correlations were found between VIs and total polyphenols, a good agreement were found between: Photochemical Reflectance Index (PRI) and ortho-diphenols (R²=0.64); Green Normalized Difference Vegetation Index (GNDVI) and proline (R²=0.74); Normalized Difference Greenness Vegetation Index (NDGI) and glucose (R²=0.95); Transformed Chlorophyll Absorption Reflectance Index (TCARI) and LWP (R²=0.71); TCARI divided by Optimized Soil Adjusted Index (OSAVI) and RWC (R²=0.77). Thus, VIs appears as a useful tool to evaluate leaf water status and biochemical parameters from olive trees.