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The ultrastructural changes observed under an electron microscope, growth and biomass production expressed as the dry matter:cell volume ratio (DM:CV), as well as the photosynthetic activity characterized by chlorophyll fluorescence, of Desmodesmus armatus have been monitored in a synchronized light:dark (14 h:10 h) regime cell cycle. The timing of the commitment points triggering the reproductive processes and their termination were characterized by the commitment and autospore release curves, respectively. During the light period, each cell attained three commitment points in the 3rd, 6th and 11th hour, which resulted in the formation of eight autospores released from the parent cell during the dark period of the cell cycle. The successive commitment points occurred at times when DM:CV reached extreme values. The maximum DM:CV in the middle of the light period (5-6 h) coincided with the maximum effective quantum efficiency of PS II (ÈPS II). At this time, bands of several thylakoids could be seen in some areas of chloroplast, which were closer to each other and more contrasted than in other phases of the cell cycle. The highest efficiency of PS II antennae (F (qP) increased slightly, whereas non-photochemical quenching (NPQ) rapidly decreased during the first 3 h of the cell cycle. The main contributor to NPQ was its energydependent component (qE). In the 11 th hour, cells started to accumulate starch grains and lipid droplets, which were partly (starch) or completely (lipids) utilized during the first few hours of the next cell cycle. In conjunction with the ultrastructural changes, two functionally different phases of the cell cycle could be distinguished: the vegetative (about 10 h) and the reproductive (about 4 h) phases, and the relationships between growth, reproduction and photosynthesis in the Desmodesmus cell cycle are discussed.

Conditioned medium (CM) is a general term describing media in which cells have already been cultivated for some time. Such media, usually clarified by filtration, have been used by plant biotechnologists as additives supporting the growth of cell suspensions, organs and whole plants. This study examined the effect of CM obtained from green alga Desmodesmus subspicatus on the growth and functioning of the photosynthetic apparatus of Nicotiana tabacum and Arabidopsis thaliana in culture in vitro. Plants where cultured on CM diluted 1.25-, 2-and 5-fold with MS medium. The increase in fresh and dry weight was highest in tobacco and Arabidopsis cultured on CM/2 and CM/1.25 media. Those two concentrations also increased the amount of chlorophylls in both plants tested. CM improved parameter PI (reflecting the photosynthetic "vitality" of the organism) and electron transport efficiency, and increased the fraction of active reaction centers. Analysis of chlorophyll fluorescence in vivo suggests that the improvement of these plants grown in the presence of algal CM may result from stimulation of photosynthesis. Algal CM offers a convenient, cheap, universal supplement for stimulating the growth of higher plants in vitro.K Ke ey y w wo or rd ds s: : Algal exudates, in vitro culture, growth improvement, photosynthesis.

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