Linking the diurnal and seasonal dynamic of photochemical reflectance index and photosynthesis in a paddy rice field

• We developed a novel multispectral rotating camera for photochemical reflectance index (PRI) observation • We observed the PRI and carbon flux data in a paddy rice field at diurnal cycle over the whole growing season • The diurnal change of PRI (PRId) well captures the light use efficiency changes even the canopy structure and pigment pools change significantly • A novel method (PRId) for the assessment of VPD induced physiological stress in paddy rice crops. The photochemical reflectance index (PRI) has been proven to be a robust proxy for photosynthetic light use efficiency (LUE). Prior studies have shown that PRI alone exhibits a weak correlation with LUE in croplands, because the observed PRI is significantly influenced by canopy structure and pigment pools, whereas LUE is not driven by these factors. To address this issue, studies have separated the PRI into two components: the early morning PRI (PRI0) to capture the seasonal change of canopy structure and pigment pools, and the diurnal change of PRI (PRId) to track the xanthophyll cycle, allowing them to monitor the seasonal and diurnal change of LUE in forest stands. However, few studies have applied this method to track LUE in croplands, particularly in paddy rice fields. Moreover, this approach requires continuous, high-frequency (≤30-minute) observations of PRI using hyperspectral cameras mounted on towers or drones, which is challenging to achieve in natural environments. To address these challenges, we developed an ultra-spatial resolution multispectral camera to obtain high-frequency PRI observations economically in a paddy rice field, and explored the diurnal relationship between LUE and PRId. Results showed that PRId effectively tracked the diurnal cycle of LUE and captured the stress of atmospheric dryness on plant photosynthesis. The coefficient of determination (R²) between PRId and LUE was 0.65, significantly higher than that between PRI and LUE (0.46) during the greening-up period with significant changes of canopy structure and pigment pools. This indicated that separating PRI into PRI0 and PRId is an effective approach for linking PRI observations to LUE in paddy rice fields.