A Periodic 4-h Extension of the Dark Period Did Not Cause Long-Term Changes in the Circadian Regulation of Photosynthesis and Sugar Levels in Lettuces.

The photoperiod in controlled environment agriculture can be adjusted to minimize electricity consumption, even if it differs from the plant's circadian rhythm. Daily modifications of the photoperiod disrupt the plant's circadian resonance state, resulting in altered growth and yield. However, the effects of periodic, rather than daily, photoperiod adjustments remain less understood. This study aims to investigate the effects of a 4-h extension of the dark period every 3 days on the circadian regulation of photosynthetic activity and sugar content, as well as on lettuce yield. Control lettuces were grown under a 16/8 photoperiod, while EPD lettuces ("Exceptionally long Period of Darkness") were grown under a repeated 16/12-16/8-16/8 photoperiod pattern from the beginning to the end of cultivation. The experiment was repeated twice, and the 4-h extension induced a loss of photosynthetic activity of 7% and 11% during the following lighting period in the first and second experiments, respectively. The yields were not affected. The stomatal conductance followed the circadian rhythm of lettuce rather than directly responding to photoperiod modifications. Furthermore, no long-term changes in starch and sucrose content were observed. Taken together, these results show that extending the dark period by 4 h every 3 days did not cause long-term disruption of the circadian regulation of photosynthesis and sugar levels in lettuce. These results provide new insights for optimizing light management in controlled environment agriculture, suggesting that the management of dark periods is crucial for maintaining yields and reducing energy consumption.