PHOTOPERIODIC HISTORY AFFECTS THE CRITICAL DAYLENGTH OF THE SHORT-DAYPLANT ACROSYMPHYTON-PURPURIFERUM (RHODOPHYTA)

The crustose tetrasporophyte of the red alga Acrosymphyton purpuriferu m is a qualitative short-day plant in the formation of its tetrasporan gia. The critical daylength for the response was determined in plants precultured in various long-day regimes [20:4, 18:6, 16:8 and 14:10 (L :D, h)]. There was a strong influence of photoperiodic history. The sh arper the decrease in daylength the stronger and faster the plants res ponded. The critical daylength (daylength inducing 50% response) incre ased from 9-5 h for plants precultured in 14 h days to 12.5 h for plan ts precultured in 20 h days. Acrosymphyton thus responds to a change t n daylength, rather than to a fixed critical value. This is of adaptiv e significance in synchronising the onset of reproduction throughout i ts broad depth range in the subtidal region.