Many plant species exhibit seasonal variation of flowering time in response to daylength. Arabidopsis (Arabidopsis thaliana) flowers earlier under long days (LDs) than under short days (SDs). This quantitative response to photoperiod is characterized by two parameters, the critical photoperiod (Pc), below which there is a delay in flowering, and the ceiling photoperiod (Pce), below which there is no further delay. Thus Pc and Pce define the thresholds beyond which maximum LD and SD responses are observed, respectively. We studied the quantitative response to photoperiod in 49 mutants selected for early flowering in SDs. Nine of these mutants exhibited normal Pce and Pc, showing that their precocious phenotype was not linked to abnormal measurement of daylength. However, we observed broad diversification in the patterns of quantitative responses in the other mutants. To identify factors involved in abnormal measurement of daylength, we analyzed the association of these various patterns with morphogenetic and rhythmic defects. A high proportion of mutants with altered Pce exhibited abnormal hypocotyl elongation in the dark and altered circadian periods of leaf movements. This suggested that the circadian clock and negative regulators of photomorphogenesis may contribute to the specification of SD responses. In contrast, altered Pc correlated with abnormal hypocotyl elongation in the light and reduced photosynthetic light-input requirements for bolting. This indicated that LD responses may be specified by positive elements of light signal transduction pathways and by regulators of resource allocation. Furthermore, the frequency of circadian defects in mutants with normal photoperiodic responses suggested that the circadian clock may regulate the number of leaves independently of its effect on daylength perception.
