LAUSR

Abstract Optical sensors are useful tools for rapid and periodic assessment of plant nutritional status. However, the utility and potential of these sensors may be reduced if their readings change throughout the day. Therefore, this study aimed to assess the effects of time of day on measurements of the SPAD-502 chlorophyll meter and the GreenSeeker throughout the bean crop cycle. The treatments consisted of time of day for sensor measurements (8:00, 12:00, and 16:00 h) throughout six dates over the crop cycle. The results showed that measurements from the SPAD-502 and the GreenSeeker significantly changed according to time of day. The SPAD index at 16:00 h showed the lowest coefficient of variation (CV, 2.5%) and was on average, 3.05 ± 0.43, and 1.12 ± 0.25 SPAD units higher when compared to readings obtained at 8:00 and 12:00 h. Differently, the NDVI from the GreenSeeker showed the lowest CV (4.34%) at 8:00 h and was, on average, 0.06 ± 0.028 and 0.03 ± 0.01 units higher than measurements taken at 16:00 h and 12:00 h, respectively. Furthermore, the different times of sensor measurements present variations in air temperature and solar radiation, which directly influence the leaf water content and paraheliotropic movements. Thus, the indices from both sensors tend to show a high variability during different times of the day. Therefore, it is essential to create a consistent sampling protocol to reduce the variability of sensor measurements during the crop cycle.