The Regulation of the Life History of Dumontia contorta in Comparison to that of Several Other Dumontiaceae (Rhodophyta)

Effects of daylength and temperature on the formation of erect macrothalli from crustose microthalli tf Dumontia contorta from Roscoff (49° N.lat.), the Isle of Man (54° N.lat.) and Zeeland (51.5° N.lat.) were investigated. Initiation of macrothalli in all three strains appeared to be a short-day response whose critical daylength, however, differed between the three strains. In the Isle of Man and Roscoff strains the critical daylength was c. 12-13 h, in the Zeeland strain c. 16h. This latter high value does not seem to have adaptive value to the latitude where this strain occurs. The three strains also differ in the temperature ranges in which macrothalli were formed (Roscoff strain up to 20 °C, Isle of Man strain up to 16 °C, and Zeeland strain up to 24 °C), and these could be related with the highest values of the surface seawater temperature at the three collection sites (Roscoff 16 C, Isle of Man 14 °C, Zeeland 19 °C). The response to short-day conditions was suppressed by short white light night-break, and a red and blue light nightbreak are equally effective. Far-red proved to be ineffective. The short-day response of Dumontia contorta is compared with that ofAcrosymphytonpurpuriferum and other taxonomically related species.