LAUSR

ABSTRACT It is essential to manage nitrogen (N) fertilization to obtain the appropriate protein contents in wheat grains. In this study, N uptake from the booting stage to flag leaf appearance, and the tiller number at the booting and panicle formation stages was estimated using a handheld active optical sensor of the normalized difference vegetation index (NDVI) under different measurement conditions in Hokkaido, Japan. A stable NDVI was obtained if measured horizontally at 60–120 cm height from the canopy regardless of ambient radiation at different local times . However, the relationship between N uptake and NDVI differed significantly if the NDVI was measured at a 20 cm height (P < 0.01) or at 45° angle to the soil surface (P < 0.001). Regression analysis revealed that N uptake could be fitted by a quadratic curve of NDVI (N uptake = 0.39 + 17.4 × NDVI2; P < 0.001) across the different regions, years, and growth stages if the NDVI was less than 0.75. However, the accuracy was poor if the NDVI was higher than 0.75 probably because the emitted and reflected light was scattered in the canopy owing to increased leaf area and causing saturation of the NDVI. Significant positive correlations were observed between tiller number and NDVI at booting (P < 0.001) and panicle formation (P < 0.001). However, the relationship at the booting stage differed among regions and years. The significant positive correlation between the residuals of the relationship and NDVI of bare soil (P < 0.001) suggested that the NDVI at booting was influenced by soil conditions owing to low green coverage. In conclusion, it is expected the rapid monitoring of N uptake and tiller number of winter wheat using an active optical NDVI sensor will be helpful for simple and rapid decision-making for N fertilization management.