THE MECHANISM OF DAYLENGTH PERCEPTION IN THE RED ALGA ACROSYMPHYTON PURPURIFERUM 1

ABSTRACT The red alga Acrosymphyton purpuriferum (J. Ag.) Sjöst. (Dumontiaceae) is a short day plant in the formation of its tetrasporangia. Tetrasporogenesis was not inhibited by 1 h night‐breaks when given at any time during the long (16 h) dark period (tested at 2 h intervals). However, tetrasporogenesis was inhibited when short (8 h) main photoperiods were extended beyond the critical daylength with supplementary light periods (8 h) at an irradiance below photosynthetic compensation. The threshold irradiance for inhibition of tetrasporogenesis was far lower when supplementary light periods preceded the main photoperiod than when they followed it (<0.05 μmol·m −2 ·s −1 vs. 3 μmol·m −2 ·s −1 ). The threshold level also depended on the irradiance given during the main photoperiod and was higher after a main photoperiod in bright light than after one in dim light (threshold at 3 μmol·m −2 ·s −1 after a main photoperiod at ca. 65 μmol·m −2 ·s −1 vs. threshold at <0.5 μmol·m −2 ·s −1 after a main photoperiod at ca. 35 μmol·m −2 ·s −1 ). The spectral dependence of the response was investigated in day‐extensions (supplementary light period (8 h) after main photoperiod (8 h) at 48 μmol·m −2 ·s −1 ) with narrow band coloured light. Blue light (λ= 420 nm) was most effective, with 50% inhibition at a quantum‐dose of 2.3 mmol·m −2 . However, yellow (λ= 563 nm) and red light (λ= 600 nm; λ= 670 nm) also caused some inhibition, with ca. 30% of the effectiveness of blue light. Only far‐red light (λ= 710 nm; λ= 730 nm) was relatively ineffective with no significant inhibition of tetrasporogenesis at quantum‐doses of up to 20 mmol·m −2 .