Figure 1. Leaf movement of Arabidopsis thaliana seedlings alters over the course of a day and a night. Plants were entrained to LD 12:12 then placed in constant light. The molecular clock is superimposed, showing peak expression times under the same conditions of genes linked to the circadian clock. The roles of these genes are described in the text. White circle, subjective day; grey circle, subjective night.

Figure 2. Anatomy of a circadian rhythm. (a) Diagram of a rhythm entrained by LD cycles then released into constant light. Note that all the parameters of the rhythm are retained in free-running conditions of constant light (LL). White bars, lights on; black bars, lights off; grey bars, subjective night. (b) An example of the effect of environment on the amplitude of a circadian rhythm. The rhythm is bioluminescence produced by A. thaliana (Landsberg erecta accession) carrying a CAB2::LUC transgene when grown on media containing increasing concentrations of sucrose. Seedlings were grown in LD 12:12 and released into darkness (DD) at dusk (12 h after dawn).

Figure 3. Transcription and translation feedback loops are at the heart of the plant circadian clock. (a) A simple transcription/translation feedback loop. A protein (A) activates transcription of a second gene (B). Following translation, the second protein (B) blocks transcription of the first gene (A). Levels of A thus drop until the point is reached when insufficient remains to allow production of B. Levels of B fall, in turn, to the point when the block on transcription of A is lifted, protein A is produced once more and the cycle repeats. A is thus the positive element and B the negative element of the feedback loop. The graph shows the expression profile of A (blue) is in antiphase to that of B (red). Levels of A fall as B rises, as B represses the transcription, and hence production, of A. (b) The extended three-loop model for the Arabidopsis clock. Red, CCA1/LHY-TOC1-X loop; orange, PRR7/PRR9-CCA1 ‘morning’ loop; green: TOC1-Y ‘evening’ loop. X and Y are hypothetical proteins, but Y is likely to be GI. Yellow arrows indicate light input via photoreceptor pathways, blue arrows indicate temperature input. Ueda (2006).

Figure 4. Phase response curves (PRC) for A. thaliana. This plots how much a light pulse (given at different times) causes the clock to speed up or slow down. Phase advances are plotted as positive changes, phase delays as negative changes. Red line, red light PRC; blue line, blue light PRC.

Figure 5. Photoperiod is a reliable cue for seasonal change but temperature and weather are unpredictable. These pictures were taken on 25th October (left) and 6th November (right) in northern Sweden (63°N) in 2006. At high latitudes, deciduous trees must stop growth early to be prepared to withstand harsh winter conditions. Winter may strike quickly and lack of preparation may lead to death.