Diurnal and circadian periodicity of mitosis and growth in marine macroalgae. ii. the green alga ulva pseudocurvata

Abstract

Nuclear division frequency in Ulva pseudocurvata was examined by staining the nuclei with the DNA-localizing fluorochrome DAPI, after separation of the two cell layers of the thallus. Thalli grown in light-dark cycles exhibited division maxima during the dark phase. Growth rate was also coupled to the light-dark cycle, with an increase starting during the dark phase and decrease during the light phase. When thalli were transferred from light-dark cycles to continuous dark, periodicity of nuclear division frequency and growth rate persisted for up to four and six cycles, respectively, with a free-running period of 21 h for growth cycles. In continuous white light, nuclear division oscillated for three or four cycles, while growth rate became immediately arhythmic. The results support the notion that both nuclear division frequency and growth rate in U. pseudocurvata are driven by circadian rhythmicity, with a fast damping of the free-running rhythms in constant light. © 1996 The British Phycological Society.