Abstract
Spectral indices can be used as fast and non-destructive indicators of plant water status or stress. It is the objective of the present study to evaluate the feasibility of using several spectral indices including water index (WI) and normalized spectral water indices 1-5 (NWI 1-5) to estimate water status in olive trees in arid regions in Iran. The experimental treatments involved two olive cultivars (Koroneiki and T2) and four irrigation regimes (irrigated with 100%, 85%, 70%, and 55% estimated crop evapotranspiration [ETc]). The results obtained showed that olive trees subjected to the different irrigation regimes of 85%, 70%, and 55% ETc experienced soil water content (SWC) deficits by 4.5%, 12%, and 20.5% that of the control, respectively. Significant differences were observed among the treatments with respect to measured relative water content (RWC), SWC, and the spectral indices of WI and NWI 1-5. The normalized spectral indices combining NIR and NIR wavelengths were found more effective in tracking changes in RWC and SWC than those that combine NIR and VIS or VIS and VIS wavelengths, respectively. Spectral indices were closely and significantly associated with RWC (.63** < R2 < .77**) and SWC (.51** < R2 < .67**). Among all the spectral indices investigated, NWI-2 showed the least consistent associations with RWC (ranging from 4-15% lower than the other indices examined) and SWC (ranging from 1-23% lower than the others). Based on the pooled data on spectral indices, RWC, and SWC collected during the study period, WI, NWI-1, NWI-4, and NWI-5 showed stronger correlations with RWC and SWC than did NWI-3 and NWI-2. In conclusion, the spectral indices of WI and NWI 1-5 measured at the leaf level are found useful as fast and non-destructive estimators of plant water stress in arid regions.