Correlation between the ratio of transmitted near infrared radiation to photosynthetically active radiation and rice growth indicators

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.