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Plant and Cell Physiology, 1967, Vol. 8, No. 3 509-522
© 1967

Article

EFFECTS OF LIGHT ON THE DEOXYRIBONUCLEIC ACID FORMATION AND CELLULAR DIVISION IN CHLORELLA PROTOTHECOIDES

YOSHIHIRO SOKAWA and EKJI HASE

1Institute of Applied Microbiology, University of Tokyo Tokyo
2Tokugawa Institute for Biological Research Tokyo

  1. It has been demonstrated previously that when Chlorella protothecoides is grown in a medium rich in glucose and poor in nitrogen source (urea), chlorophyll-less cells with markedly degenerated plastids —called "glucose-bleached" cells—are produced either in the light or in darkness. When the glucose-bleached cells are incubated in a medium enriched with the nitrogen source but without added glucose, normal green cells with fully organized chloroplasts are obtained in the light, and pale green cells with partially organized chloroplasts in darkness. During these processes of chloroplast development in the glucose-bleached cells, there occurs, after a certain lag period, an active DNA formation followed by a more or less synchronous cellular division. In the present study the effects of light on the DNA formation and cellular division were investigated in the presence of CMU or under aeration of CO2-free air to exclude the intervening influence of photosynthetic process.
  2. It was revealed that light severely suppresses the DNA formation and cellular division of the glucose-bleached cells while enhancing remarkably their greening. The suppression was saturated at the light intensity of about 1,000 lux. Blue light was most effective, being followed by green, yellow and red light in the order of decreasing effectiveness.
  3. Further experiments unveiled that light exerts two apparently opposing effects on the DNA formation depending upon the time of application during the incubation of algal cells. When the algal cells were illuminated only during the lag period before the active DNA synthesis, there occurred an enhancement of the DNA synthesis occurring during the subsequent dark incubation. When, on the other hand, the cells were transferred to the light from darkness at or after the start of the DNA synthesis, it caused an almost complete abolition of the subsequent synthesis of DNA in the algal cells. No such effects of light were observed with RNA and protein (total)
  4. These findings were discussed in relation to the process of chlorophyll formation occurring concurrently in the algal cells.

(Received August 10, 1967; )
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