The circadian clock is the endogenous timing system based on self-sustained oscillations (period is ca. 24-h = circadian) generated by gene regulatory networks. Some experiments suggested that the circadian clock whose period is 24-h is the most adaptive on the earth, 24-h cycling environment. However, plants show a diversity in their circadian periods, suggesting a driving force for the period diversity. We focused on the local adaptation of photoperiodic responses based on the day-length measurement by the circadian clock.
Japanese short-day duckweed, Lemna aequinoctialis is an annual paddy weed showing a latitudinal cline in critical day-lengths. This duckweed provides a good platform for linkage analysis between circadian periods and critical day-lengths because of rapid flower induction and well-established bioluminescent reporter system using a gene gun. We collected 26 strains from all over Japan and determined their critical day-lengths and circadian periods and found the negative correlation between them. This suggests that the local adaptation of critical day-lengths is likely to involve the modulation of circadian periods.
The intra-species natural variation of circadian periods is broadly observed in various organisms. These period variations should be considered in the context of the local adaptation of phase-specific physiological regulations.