Assessment of Photochemical Reflectance Index Measured at Different Spatial Scales Utilizing Leaf Reflectometer

Vegetation indices on the basis of optical characteristics of vegetation can represent various conditions such as canopy biomass and physiological activity. Those have been mostly developed with the large-scaled applications of multi-band optical sensors on-board satellites. However, the sensitivity of vegetation indices for detecting vegetation features will be different depending on the spatial scales. Therefore, in this study, the investigation of photochemical reflectance index (PRI), known as one of useful vegetation indices for detecting photosynthetic ability and vegetation stress, under the three spatial scales was conducted using multi-spectral camera installed in unmanned aerial vehicle (UAV), field spectrometer, and leaf reflectometer. In the leaf scale, diurnal PRI had minimum values at different local-time according to the compass direction of leaf face. It meant that each leaf in some moment had the different degree of light use efficiency (LUE). In early growth stage of crop, PRIleaf was higher than PRIstands and PRIcanopy because the leaf scale is completely not governed by the vegetation cover fraction. In the stands and canopy scales, PRI showed a large spatial variability unlike normalized difference vegetation index (NDVI). However, the bias for the relationship between PRIstands and PRIcanopy is lower than that in NDVIstands and NDVIcanopy. Our results will help to understand and utilize PRIs observed at different spatial scales.