EFFECT OF IRRADIANCE ON GROWTH AND REPRODUCTIVE PROCESSES DURING THE CELL-CYCLE IN SCENEDESMUS-ARMATUS (CHLOROPHYTA)

Synchronized populations of the chlorococcal alga Scenedesmus armatus (Chod.) Chod. were grown under five irradiance levels. During the cell cycles of these population, reproductive processes such as DNA replic ation, nuclear division, protoplast fission, and daughter cell release and growth processes such as RNA and protein accumulation were follow ed. The amount of RNA and proteins increased stepwise with a short tim e interval between individual steps during which the rate of RNA and p rotein accumulation decreased. At each of the steps, the amount of RNA and protein approximately doubled and the number of steps increased w ith irradiance. At the end of each of the growth steps, a commitment t o trigger the sequence of reproductive events (DNA replication, nuclea r division, protoplast fission) was attained. After attaining the comm itment point, the cells were able to trigger and terminate the whole r eproductive sequence without any further growth, that is, even in the dark when the external supply of energy was cut off. With increasing i rradiance, the number of commitment points attained during one cell cy cle increased from one to four. Consequently, one to four sequences of the reproductive steps were triggered, and each of them ended by doub ling the reproductive structures, which resulted by doubling the repro ductive structures, which resulted in the formation of 2, 4, 8, or 16 daughter cells. The length of the precommitment periods shortened with increasing irradiance as the result of an increasing rate in growth. The length of postcommitment periods showed light independence and rem ained constant at the range of irradiances at which the number of grow th steps and, consequently, the number of sequences of reproductive ev ents did not change. At higher irradiances, the number of sequences of reproductive events increased, which caused a prolongation of postcom mitment periods. The length of the cell cycle varied as a result of th is distinct effect of irradiance on pre- and postcommitment periods.