Evaluation of the Capability of Spectral Water Indices for Assessing Water stress in Olive Tree

Spectrometric measurements have the potential for fast and non-destructive measurements of plant water stress. The aim of this work was to investigate the ability of several spectral water indices, including water index (WI), normalized spectral water indices 1-5 (NWI 1-5), and normalized water index based on wavelengths in 960 and 940 nm (NWI 960-940) for detection of water stress in olive trees. The experimental treatments involved two olive cultivars (Koroneiki and T2) and four water regimes (100%, 85%, 70%, and 55% of crop water requirement). Results showed that the olive trees in different water supplies 85%, 70%, and 55% of ETc were subjected to soil moisture deficit equal to 11, 15, and 20%, respectively, as compared to soil moisture of control treatment. Because of the high resistance of olive trees to water stress, water reduction at levels of 15, 30, and 45 percent did not have significant effects on spectral indices. However, spectral indices were closely and significantly linear associated with relative water content of the crop leaf (). Among all tested water spectral indices, NWI-2 showed the least consistent associations with relative water content of the leaf (ranging from 1–23% less than the ones in other tested indices). Based on the average amount of spectral indices and relative water content during the study period, NWI4, NWI5, NWI1, WI, NWI960-940, NWI3, and NWI2 showed a stronger relationship with the relative water content of olive leaves, respectively. In conclusion, spectral reflectance indices, WI, NWI 1-5, and NWI 960-940, could be useful for fast and non-destructive estimating of plant water stress.