Effects of photosynthetic photon flux density, frequency, duty ratio, and their interactions on net photosynthetic rate of cos lettuce leaves under pulsed light: explanation based on photosynthetic-intermediate pool dynamics

Square-wave pulsed light is characterized by three parameters, namely average photosynthetic photon flux density (PPFD), pulsed-light frequency, and duty ratio (the ratio of light-period duration to that of the light-dark cycle). In addition, the light-period PPFD is determined by the averaged PPFD and duty ratio. We investigated the effects of these parameters and their interactions on net photosynthetic rate (P (n)) of cos lettuce leaves for every combination of parameters. Averaged PPFD values were 0-500 A mu mol m(-2) s(-1). Frequency values were 0.1-1000 Hz. White LED arrays were used as the light source. Every parameter affected P (n) and interactions between parameters were observed for all combinations. The P (n) under pulsed light was lower than that measured under continuous light of the same averaged PPFD, and this difference was enhanced with decreasing frequency and increasing light-period PPFD. A mechanistic model was constructed to estimate the amount of stored photosynthetic intermediates over time under pulsed light. The results indicated that all effects of parameters and their interactions on P (n) were explainable by consideration of the dynamics of accumulation and consumption of photosynthetic intermediates.