| Summary: | Shade tolerance is a key adaptive strategy displayed by heliophytic photosynthetic organisms in response to limited light. Although generalized morphological and physiological traits associated with shade tolerance exist, the interest in shade tolerance has been expanding over the past few years due primarily to the controversies that have emerged on classical hypotheses of shade tolerance. In this paper the shade responses of unicellular excavate <em>Euglena gracilis</em> is discussed. Euglena was photoautotrophically grown under three different light intensities; 28, 84 and 210 μmol m<sup>-2</sup> s<sup>-1</sup>. Results revealed that <em>E. gracilis</em> is a shade tolerant species which exhibits some typical shade tolerant responses such as decrease in growth rate, light saturation point, light compensation point and dark respiration rate, and increased chlorophyll content. Most importantly, it is reported for the first time that the shade tolerance of this organism is also characterized by the increased chlorophyll a:b ratio, contradicting the generally accepted hypothesis of decreased chlorophyll a:b in shade tolerance response. The probable reasons for increased chlorophyll a:b ratio in E. gracilis under shade are also discussed. <br /><br /><strong>Key words:</strong> Chlorophyll; Fluorescence; Photosynthesis; PSI; PSII. <br /><br />DOI: 10.4038/tar.v22i1.2666<br /><br /><em>Tropical Agricultural Research</em> Vol. 22 (1): 12-25 (2010)
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