Dawn signal as a rhythmical timer for the seasonal adaptive variation of CAM: a model

Abstract Photoperiodism, known to control the level of CAM in different types of CAM plants (e.g. Kalanchoe blossfeldiana, Bryophyllum daigremontianum), would act as a reliable timer for seasonal coherent adaptation. Two different endogenous rhythms (malic enzyme activity, PEP carboxylase capacity) appear to be separately coupled to dawn and dusk, respectively, thus achieving time-compartmentation of CAM; this feature suggests involvement of an ‘internal coincidence’ type of clock mechanism. Persistence in continuous darkness of the rhythm of malic enzyme activity (measured by label incorporation into pyruvate or by CO₂ output) establishes its endogenous character and shows that light is not necessary for malate decarboxylation. The role of the dawn signal would be to entrain the CAM system, i.e. to phase the endogenous rhythm of malic enzyme activity correctly to local time. The possibility of an effect of the phase of the PEP carboxylase rhythm on the phase of the decarboxylation rhythm is ruled out by the presence of the intermediary malate storage step. Phase responses to red and to far-red show that phytochrome is involved in rephasing the rhythm of malic enzyme activity. The relative position of dusk affords a ‘measurement’ of the season by the CAM system entrained by dawn. According to the dusk-dawn interval, the level of PEP carboxylase capacity (amount of active enzyme) is modified, resulting in changes of CAM level (high activity under short days).