Measuring growth indicators in a simple and labor-saving method is necessary for diagnosing rice in the production sites. This study focused on the ratio of transmitted near infrared radiation (700-1000 nm, NIR) to photosynthetically active radiation (400-700 nm, PAR), that is, NIRt/PARt (t represents transmitted). The objective of this study was to investigate correlation between NIRt/PARt and rice growth indicators, which were leaf nitrogen content (LNC), dry matter weight (PDW), and nitrogen uptake (PNU) of rice plant, to examine the possibility of estimating the rice growth indicators using NIRt/PARt. The values of NIRt/PARt were measured at active tillering, panicle initiation, and flowering stages in test plots where paddy rice was cultivated under six different treatments of nitrogen application. Then, correlation was analyzed between NIRt/PARt and LNC, PDW, and PNU. Correlation with LNC was significant for active tillering stage (r=0.507, p<0.05) and data pooled for all growth stages (r=0.762, p<0.01). Correlation with PDW was significant for panicle initiation stage (r=0.564, p<0.01) and data pooled for all growth stages (r=0.599, p<0.01). Correlation with PNU was significant for active tillering stage (r=0.484, p<0.05), panicle initiation stage (r=0.413, p<0.05), and data pooled for all growth stages (r=0.727, p<0.01). Correlation between NIRt/PARt and the growth indicators was low at the flowering stage probably because characteristics of transmission and reflectance against PAR and NIR changed in rice canopy after heading. Further, the linear relationship between NIRt/PARt and PDW markedly changed after heading, which lowered correlation coefficients calculated using data pooled for all growth stages. Correlation coefficients using data only from active tillering and panicle initiation stages improved: r=0.856 (p<0.01) for LNC, r=0.934 (p<0.01) for PDW, and r=0.874 (p<0.01) for PNU. These results suggested that NIRt/PARt can be used to estimate rice growth indicators before heading.

We demonstrated a close-range sensing technique to seasonally track trends in canopy leaf greenness in rice paddies. A weatherproof narrow-band triplet digital camera system was positioned 3 m above a paddy rice field during the 2017 rice season in Kagoshima. The system automatically logged reflectance images at 10-min intervals from 8:00 to 16:00 in visible green （560 ± 10 nm）, red （650 ± 10 nm）, and near-infrared （855 ± 10 nm） bands. Two rice cultivars were grown at two fertilizer concentrations and seedling numbers in three replicates in concrete-framed plots. The camera was mounted on a platform at the top of an iron pole installed on a handcart built with scaffolding pipes. The east-looking camera captured oblique images of four plots （2 × 2 plots） with the camera’s axis tilted 65^o from vertical with the handcart positioned 3.2 m away from the west side of the plots. To capture images of a different group of plots each day, the handcart was manually moved to a randomly chosen set position among the five predetermined positions north and south along the field on the concrete-paved open side area every morning. Radiometric corrections for images utilized solar irradiance sensors and prior calibrations to calculate instantaneous reflectance in each band. The leaf greenness value （SPAD） and the leaf area index （LAI） were weekly collected for rice canopies, and daily values were obtained by interpolating the weekly data. From reflectance images, SPAD and LAI data were analyzed for 35-day period including the panicle formation stage. The canopy greenness index, defined as the product of （SPAD – 30） and LAI （which potentially represents canopy nitrogen condition）, highly correlated （r = 0.62, n = 4276） with the canopy surface averages of the bi-band ratio （NIR/Green）, where NIR and Green indicate the reflectance values in the near-infrared and green band, respectively. Further investigations including daily-averaged or hourly-sampled reflectance and/or bi-band ratios revealed that the correlation coefficient for the daily-averaged reflectance was 0.71(n = 128) and that for the hourly-sampled reflectance was 0.73 (n = 528) for the 15:00–16:00 period and 0.62 (n = 690) for the 11:00–14:00 period. We found that NIR/Green values obtained during the late afternoon time period best tracked the canopy greenness and differences in the cultivar and fertilization levels.