Chlorophyll &lt;i&gt;a&lt;/i&gt;: &lt;i&gt;b&lt;/i&gt; Ratio Increases Under Low-light in 'Shade-tolerant' &lt;i&gt;Euglena gracilis&lt;/i&gt;

Ck Beneragama,K Goto

doi:10.4038/tar.v22i1.2666

Abstract

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 Euglena gracilis is discussed. Euglena was photoautotrophically grown under three different light intensities; 28, 84 and 210 μmol m -2 s -1 . Results revealed that E. gracilis 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. Key words: Chlorophyll; Fluorescence; Photosynthesis; PSI; PSII. DOI: 10.4038/tar.v22i1.2666 Tropical Agricultural Research Vol. 22 (1): 12-25 (2010)

Highlights

Photosynthetic apparatus of organisms adapt to low light environments allowing coordinated allocation of resources to achieve and maintain optimal rates of photosynthesis, and to function efficiently under limited light (Anderson et al, 1995)
From an ecological point of view, shade tolerance refers to the capacity of a given photosynthetic organism to tolerate low light levels (Valladares and Niinemets, 2008) and it is typically characterized by a set of morphological and physiological traits such as decrease in growth rate, light compensation point, dark respiration rate, net photosynthetic rate and chlorophyllv a:b ratio, and increase in quantum yield, chlorophyll content and carbohydrate storage together with many other traits (Valladares and Niinemets, 2008)
The shading effect on physiology and photosynthetic pigment composition of Euglena gracilis grown photoautotrophically was examined by comparing the cultures grown under three different incident photon flux densities (PFDs) and gradual decrease of actual irradiance occurring within each culture by mutual shading with increasing cell titer

Summary

Introduction

Photosynthetic apparatus of organisms adapt to low light environments allowing coordinated allocation of resources to achieve and maintain optimal rates of photosynthesis, and to function efficiently under limited light (Anderson et al, 1995). Shade tolerance is a key adaptive strategy that some heliophytic photosynthetic organisms show in response to low light. Shade tolerance can be considered as a crucial life-history trait that plays a pivotal role in community dynamics of photosynthetic organisms. The success or failure in habitat selection is governed by the extent to which the species can tolerate shade. The shade tolerance of species plays a central role in the functioning of CO2-elevated future

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