Appropriate time interval of PPFD measurement to estimate daily photosynthetic gain.

Photosynthetic models sometimes incorporate meteorological elements typically recorded at a time interval of 10 min or 1 h. Because these data are calculated by averaging instantaneous values over time, short-term environmental fluctuations are concealed, which may affect outputs of the model. To assess an appropriate time interval of photosynthetic photon flux density (PPFD) measurement for accurate estimation of photosynthetic gain under open field conditions, we simulated the daily integral net photosynthetic gain using photosynthetic models with or without considering induction kinetics in response to changes in PPFD. Compared with the daily gain calculated from 60-min-interval PPFD data using a steady-state model that ignored the induction kinetics (i.e. a baseline gain), the gains simulated using higher-resolution PPFD data (10-s, 1-min, and 10-min intervals) and using a dynamic model that considered slow induction kinetics were both smaller by ~2%. The gain estimated by the slow dynamic model with 10-s-interval PPFD data was smaller than the baseline gain by more than 5% with a probability of 66%. Thus, the use of low-resolution PPFD data causes overestimation of daily photosynthetic gain in open fields. An appropriate time interval for PPFD measurement is 1 min or shorter to ensure accuracy of the estimates.