PHOTOPERIOD CONTROL OF DIAPAUSE IN DAPHNIA. IV. LIGHT AND CO₂-SENSITIVE PHASES WITHIN THE CYCLE OF ACTIVATION

¹ Department of Biological Sciences, State University of New York at Albany, Albany, New York 12203

The diapaused embryo of the cladoceran, Daphnia pulex, may require light for terminating the diapause, and a single photocycle may be adequate. In D. pulex the light-refractory phase of diapause may be broken or completed in a variety of environments. In the Supply House strain employed in these studies, the usual low temperature treatment is unnecessary when the embryos are placed in constant darkness in sealed containers. Alternatively, the refractory state in constant darkness may be broken with low 0₂ and high CO₂ tensions. Both modifications were shown to be necessary and in that sequence.

A single long-day light signal may terminate the diapause when the embryo passes from the photo-refractory to the photo-sensitive phase. Three kinds of light responses were observed, however. Each relates to the treatment given the embryos prior to light exposure.

In one so called internal state the embryo requires one long day or two pulses of light. Scant evidence suggests that the first pulse may be interpreted as either dawn or sunset. In this state there are obviously two photo-inductive phases in each inductive cycle. The embryo may change, however, by "losing" one of the photo-inductive phases. In this condition a single two-hour pulse of light, if given at the appropriate time of a thermocycle, may terminate the diapause.

A third internal state is introduced when CO₂ is employed to break the refractory phase of diapause. The diapause is terminated by a single two-hour pulse of light, and the embryo is continuously sensitive to the light stimulus. However, the pattern of hatching indicates that activation may occur at restricted phases within the induction cycle.

Several roles of CO₂ may be deduced from the experimental results. An elevated CO₂ tension breaks photo-refractoriness. It also induces a "dark" reaction which may need to be completed before the light may stimulate termination of the diapause. Withdrawal of CO₂ after 10 hours prevents activation unless a second light pulse is given immediately thereafter. The apparent deferral of light stimulation suggests a third function of CO₂, that of "storing" the light signal until the embryo becomes photo-inductive.

The potential roles of CO₂ are described in a model of photoperiodism that requires photo-induction at two phases in each daily cycle.