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.

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Although the kinetic chlorophyll fluorescence signals are rich in information, most of the chlorophyll fluorescence related studies deal only with the quantum yield of primary photochemistry (Fv/Fm). JIP-test based OJIP fluorescence transient analysis is relatively a new technique to investigate the environmental stress responses of photosynthetic organisms. In the present study, the deleterious effects of ultraviolet (UV) radiation on the photosynthetic machinery were probed by the JIP-test in Euglena, one of the most potent organisms for the future space stations. The cells were exposed to a series of UV-C doses and immediately after exposure, survival percentage was determined with Neutral Red staining, and the chlorophyll fluorescence was measured using AquaPen AP-C 100 fluorometer. Resultant OJIP transients were analyzed according to JIP-test, and several functional and structural parameters were derived to explain the PSII behavior. Results indicated that the UV-C induced inhibition of electron transport is severely affected due to higher sensitivity of dark reactions after QA -, represented as ψo, the electron transfer probability, than of the light dependent reactions, represented as φPo, the trapping probability. The performance index (PIABS) of PSII, which is a combination of the indices of three independent parameters, decreased markedly in exponential manner in response to UV-C. Results illustrate the advantage of using a number of fluorescent parameters over the use of one parameter, often the Fv/Fm.

Abstract:In the floriculture industry of Sri Lanka, the main operations are the production of cut foliage followed by rooted cuttings and potted plants for the export market. Cut foliage species include several genera and species of flowering plants and a few species of pteridophytes. The history of collection of pteridophyte flora in Sri Lanka dates back to 1672, however at present only a few of pteridophytes are used in the domestic and international floriculture markets. Sri Lanka is blessed with a high level of diversity of pteridophyte taxa, with an enormous diversity of plant form, appearance and foliage patterns. They thrive in many habitats and are suitable to be used in the floriculture industry of the country. Several species are suggested as suitable candidates for rational domestication and use in the floriculture industry. Effective domestication of the selected pteridophyte taxa will be useful for their in situ, ex situ and circa situm conservation through utilisation.

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