The involvement of an endogenous circadian rhythm in photoperiodic timing inAcrochaetium asparagopsis(Rhodophyta, Acrochaetiales)

Abstract

In Acrochaetium asparagopsis (Chemin) Papenfuss, tetrasporocyst formation is a genuine short-day photoperiodic response. Experiments in bi-diurnal and tri-diurnal cycles have shown that maximum inhibition of the response occurred with night-breaks given at approximately 24 h intervals and coincided with the postulated skotophile phase, while maximum stimulation was due to night-breaks in the postulated photophile phase. In cycle length experiments the fertility was stimulated in cycle lengths of 24 and 44 h and it was inhibited in cycle lengths of 36–40 and 52 h (or more). In addition the response was inhibited by light periods longer than 14 h, however long the associated dark period. Dark periods of various durations were completely ineffective in initiating tetrasporocysts and an induction phase of 5–7 short-days (8:) was necessary for the fertility. The lighting treatment following the induction phase was found to have no significant effect on the fertility and tetrasporocysts were obtained in continuous light or darkness. In resetting experiments, three more reversed short-day cycles (11 cycles, :8, photoperiod started at 24.00) were required to obtain the same maximum fertility (100%) observed under normal short-day cycles (9 cycles, 8:16, photoperiod started at 08.00). The induction, however, was completely lost with 96 h of continuous light (20 µE m-2s-1) and even 14 reversed short-day cycles were ineffective in initiating a new rhythmic mechanism.