2.3. NDVI measurement
For measuring spectral reflectance of rice canopies, we used
a spectroradiometer (MS-720, EKO Instruments Co., Ltd., Tokyo).
MS-720 measures radiation from 350 nm to 1050 nm by 3.3 nm
intervals. We measured the sky radiation with a FOV 180◦ attachment
and the plant reflection with a FOV 45◦ attachment from 1 m
above the rice canopies, with 3 replications at each field when
we measured LAI. We calculated canopy reflectance by dividing
plant radiation by sky radiation. We calculated the normalized difference
vegetation index (NDVI) using canopy reflectance in RED
(620−670 nm) and NIR (841–875 nm):
NDVI = (reflectance in NIR − reflectance in RED)
(reflectance in NIR + reflectance in RED)