Uncoupling of various measures of growth in Ulva rotundata (Chlorophyta) following a large decrease in irradiance

A vegetative clone of the chlorophyte macroalga Ulva rotundata Blid. was maintained in an outdoor continuous flow system and subjected to a large decrease in irradiance. Specific growth rates based on changes in fresh ( mu sub(FW)) and dry weight ( mu sub(DW)) and surface area ( mu sub(SA)) were determined using precut disks over the 24 h following a post-sunset transfer from full sunlight (100% I sub(o)) to 9% I sub(o). All three measures of growth rate were approximately equivalent for untransferred control plants at either limiting (9%) or saturating (100%) I sub(o). Transferred disks exhibited mu sub(FW) and mu sub(SA) slightly lower than 100% I sub(o) controls and much higher than 9% I sub(o) controls; mu sub(DW) was nearly identical for transferred disks and 9% I sub(o) controls. Cell division therefore continued at a high rate for one day following transfer of U. rotundata) to irradiances which are subsaturating for photosynthesis (indicated by mu sub(DW)). Starch reserves were largely depleted, and the C/N ratio decreased during this period.