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

Abstract

ABSTRACT: Gametophytic and sporophytic microthalli of Dumontia contorta from Isle of Man appear to persist indefinitely in the vegetative state at photoperiods of 14 h and longer, but form erect. branched, tubular macrothalli at photoperiods of 12 h or less. Responses of microthalli kept under different daylengths at 2,000 lux, 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 approxin~ately 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 (2,000 or 180 lux); 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 first step (induction of macrothallus initials) depends entirely on daylength, the second (development of initials into macrothalli) on both daylength and temperature. This was deduced from the fact that macrothallus initials were formed under short- day conditions irrespective of temperature (4 to 24'), whereas macrothalli were formed from initials under short-day conditions but only at temperatures of 16" (critical temperature) or lower. At 26" microthalli died gradually. Like microthalli, macrothalli grow optimally at 16 to 18". It follows, therefore, that macrothalli cannot be expected to grow from macrothallus initials above 16" and under short-day conditions, since elongation of short cells in the f~laments 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 degrees.