Plant growth regulators (PGRs) play an important role in mediating growth and stress responses in plants. Light influences PGRs concentrations in vascular plants. The effect of light on growth and endogenous PGR concentrations in microalgae was investigated in the present study. Chlorella minutissima MACC 360 was grown in 14:10 h light:dark (L:D), continuous dark (CD) and continuous dark with the addition of 5 g L(-1) glucose (CD + G) for 48 h. Cultures were synchronized in the L:D cultures, increasing in size during the light period and dividing during the dark period. C. minutissima cells did not increase in size or undergo cell division in CD cultures. In CD + G conditions, the cultures were no longer synchronized but did continue to increase in cell size and constantly underwent cell division although fewer cells divided than in the L:D cultures. Endogenous auxin and cytokinin concentrations increased and gibberellin concentrations decreased over time in the actively growing cultures (L:D and CD + G) but did not increase in the CD cultures. The largest increase in indole content was in the CD + G cultures while the L:D cultures had the largest cytokinin increase. Brassinosteroid concentrations decreased over time in all the cultures including those grown in CD conditions. Abscisic acid (ABA) concentrations were low and only increased in the CD cultures. These results show that endogenous PGRs were affected by the light regime and/or culture growth.

Hungarian Academy of Sciences Biological Research Centre Institute of Biochemistry Temeszári krt 62 H 6726 Szeged Hungary

Institute of Plant Biology Faculty of Agricultural and Food Sciences University of West Hungary H 9200 Mosonmagyaróvár Hungary

Laboratory of Growth Regulators Centre of the Region Haná for Biotechnological and Agricultural Research Institute of Experimental Botany ASCR and Palacký University Šlechtitelů 11 CZ 783 71 Olomouc Czech Republic

Laboratory of Growth Regulators Centre of the Region Haná for Biotechnological and Agricultural Research Institute of Experimental Botany ASCR and Palacký University Šlechtitelů 11 CZ 783 71 Olomouc Czech Republic; Umeå Plant Science Centre Department of Forest Genetics and Plant Physiology Swedish University of Agricultural Sciences SE 90183 Umeå Sweden

Research Centre for Plant Growth and Development School of Life Sciences University of KwaZulu Natal Pietermaritzburg P Bag X01 Scottsville 3209 South Africa

Research Centre for Plant Growth and Development School of Life Sciences University of KwaZulu Natal Pietermaritzburg P Bag X01 Scottsville 3209 South Africa; Institute of Plant Biology Faculty of Agricultural and Food Sciences University of West Hungary H 9200 Mosonmagyaróvár Hungary

Umeå Plant Science Centre Department of Forest Genetics and Plant Physiology Swedish University of Agricultural Sciences SE 90183 Umeå Sweden

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