Effects of Photoperiod and Temperature on Macrothallus Initiation in Dumontia contorta (Rhodophyta)

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Abstract
Gametophytic and sporophytic microthalli of D. contorta from Isle of Man [UK] appear to persist indefinitely in the vegetative state at photoperiods of .gtoreq. 14 h but form erect, branched, tubular macrothalli at photoperiods of .gtoreq. 12 h. Responses of microthalli kept under different daylengths at 2000 lx, and those of microthalli kept under daylength-adjusted light intensities, giving equal daily light doses, did not differ from one another. The critical daylength for this short-day response is .apprx. 12 h. The response to short-day conditions was inhibited by a short white night-break of 0.25 h given in the middle of a 16-h dark period, irrespective of light intensity (2000 or 180 lx); this suggests a genuine photoperiodic response as known from flowering plants. However, a relatively large number of short-day cycles (at least 31) are required to produce some effect. Macrothalli formation involves 2 separate steps. The 1st step (induction of macrothallus initials) depends entirely on daylength, the 2nd (development of initials into macrothalli) on body daylength and temperature. This was deduced from the fact that macrothallus initials were formed under short-day conditions irrespective of temperature (4-24.degree. C), whereas macrothalli were formed from initials under shrot-day conditions but only at temperatures of 16.degree. C (critical temperature) or lower. At 26.degree. C microthalli died gradually. Like microthalli, macrothalli grow optimally at 16-18.degree. C. Macrothalli cannot be expected to grow from macrothallus initials above 16.degree. C and under short-day conditions, since elongation of short cells in the filaments of macrothallus initials (start of macrothallus growth) would be blocked. Elongation of these cells was probably also blocked by long-day conditions, because macrothallus initials did not grow out after transfer of microthalli from short-day to long-day conditions at 12.degree. C.

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