Environmental Factors Stimulate Synthesis of Protective ... - CiteSeerX

Proceedings ofthe 9th International Symposium on Buckwheat, Prague 2004

Environmental Factors Stimulate Synthesis of Protective Substances in Buckwheat Mateja Germ Nationallnstitute ofBiology, SI-IOOO Ljubljana, Vecna pot 111, Slovenij'a

ABSTRACT Common buckwheat was exposed to reduced, ambient, and enhanced UV-B radiation, corresponding to 17% reduction of ozone layer. The total amounts of UV-absorbing compounds were lower in buckwheat cultivated under reduced UV-B compared to those grown under ambient and enhanced UV-B levels. Rutin, quercetin, quercitrin and other flavonoids are synthesized in higher plants in order to protect the plants against UV radiation, diseases and predators. Ability to synthesize rutin and related compounds might be an evolutionary plant response to the extreme growing conditions. Buckwheat is because of rutin and other valuable constituents of the grains an important material for functional food. Keywords: buckwheat, UV-8 radiation, polyphenols, rutin INTRODUCTION

Buckwheat is an important agricultural plant thriving at higher altitudes, which are not suitable for all cultural plants. OHNISHI (2002) and OHNISHI (2003) report that Eastern Tibet and adjacent areas of Sichuan and Yunnan are probable sites of origin of cultivated common and tartary buckwheat. Villages and towns in Eastern Tibet are located from of 2500 to 3500 m above sea level (OHNISHI AND KONISHI, 2001). The history of cultivating buckwheat dates back to ancient times (SHIRATORl AND NAGATA, 1986) and today it obtains importance of food source again. It is annual plant, with short vegetation period. The amount and quality of grains in buckwheat depends on environmental conditions during growth. Favorable conditions are needed for successful growth and development of seeds, but any stress demands additional energy for the establishment of protective and repair mechanisms and could therefore result in lower yield and reproduction potential. Since buckwheat is an agricultural plant, it has been likely subjected to intensive breeding in order to improve the yield (TAHIR AND FAROOQ, 1988) thus the potential of buckwheat to cope different stresses is probably lowered. One of the important environmental factors, affecting plant metabolism is UV-B radiation (ROZEMAET AL., 1997; JANSEN ET AL., 1998; BJORN, 1999), the consequence of thinning of ozone layer. Protection against UV-B radiation is very important in photosynthetic organisms that depend on solar radiation as the primary source of energy. Rutin, quercetin, quercitrin and other flavonoids are secondary plant metabolites. They are synthesized in higher plants in order to protect the plants against UV radiation, diseases and predators. Ability to synthesize rutin and related compounds might be an evolutionary plant response to the extreme growing conditions. Rutin is synthesized mainly in the herb, especially in the inflorescence. Buckwheat is among field crops the only one, which contains rutin. Buckwheat is because of rutin and other valuable constituents of the grains an important material for functional food.

UV screening substances UV absorbing phenolic compounds had an important role in the evolution of land plants. From algae to charophycean algae, bryophytes, pteridophytes and higher plants, there is an increasing degree of polymerization and complexity of polyphenolic substances (ROZEMA ET AL., 1997). Beside the many functions of phenolics, like signal transduction, flower colour, defence against microorganisms and herbivores, as well as structural rigidity of plants (DAVIES ET AL., 1993), they absorb harmful UV radiation and protect vulnerable targets, while transmit photosynthetically active radiation, needed for plant photosynthesis

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(CUADRA ET AI., 1997). The concentration and type of these compounds depend on the group of organisms (DAY ET AI., 1992) and the level of UV-B radiation (HOLM-HANSEN ET AL" 1993). UV-B radiation induces activity of genes, which accelerate metabolism of phenylpropanoid pathway and synthesis of flavonoids. They are conjugated aromatic compounds absorbing strongly in the UV region of the spectrum (OLSSON, 1999). UV absorbing compounds are located mainly in the vacuole in adaxial epidermis, but may also be call-wall bound (DAY ET AI., 1994). Flavonoids have varying ability to scavenge free radicals, which are induced by UV-B radiation. Research show, that flavonoids scavenge singlet oxygen, superoxide and peroxy radicals due to their structure. It is known that flavonoids, which possess additional hydroxy groups on the B-ring of the flavonoid skeleton, are better scavengers of free radicals, UV-B radiation induces the production of flavonols with more hydroxy groups, like quercetin, in many plant species. Beside direct absorption of UV-B radiation, dihydroxylated flavonols may protect plants from UV-B damage also indirectly by scavenging of free radicals (OLSSON, 1999).

The effect of enhanced UV-B radiation on synthesis of protective substanes in common buckwheat The increased transmission of solar ultraviolet (UV) radiation as a consequence of stratospheric ozone depletion (MADRONICH ETAI., 1998) could have harmful effects on living organisms (SULLIVAN ET AI., 1996; JOHANSON, 1997; ROZEMAET AL" 1997; GABERSCIK ET AL., 2001; GERM ET AL., 2002). The direct dependence of primary producers on solar energy, that includes also harmful UV-B rays, results in disturbances in photosynthesis and lower production. The UV influences the susceptibility of the plant to disease, cold hardiness, on attractiveness to herbivores and on the decomposition of plant materials by microorganisms (CALDWELL AND FLINT, 1994). In crop plants they also alter the quality of food for humans and animals. The response of buckwheat to enhanced UV-B radiation on biochemical, and physiological level was studied. Experiments were conductiong in Botanical garden, University of Ljubljana (320 m above see level, 46°35'N, l4°55'E) Slovenia. Common buckwheat was exposed to three levels of UV-B radiation: enhanced UV-B radiation corresponing to 17% reduction of ozone layer, reduced, and ambient radiation. We studied the effect of enhanced UV-B radiation on the production of UV-B and UV-A absorbing compounds, as well as some other parameters (photochemical efficiency of photosystem II (PSII), photosynthesis, transpiration and ETS activity, as well as seed productivity of buckwheat plants) were measured. UV-B absorbing compounds were accumulated in assimilation tissue during the plant development. The total amounts in buckwheat plants cultivated under reduced UV-B were lower compared to those grown under ambient and enhanced UV-B levels (GABERSCIK ET AL., 2002). Increased amount of screening substances improves the UV-B shielding ofUV-B susceptible tissues and increases UV-B tolerance of plants (FENG ET AL., 2003). DAY AND DEMCHIK (1996) report about the experiments with Brassica rapa where plants were treated with similar level of UV-B (16% ozone reduction); results showed no significant increase of production of UV-B absorbing compounds and only doubled doses exerted an effect In our case the absorption spectra of methanol extracts of treated buckwheat leaves in the UV range revealed an evident increase in UV-B and UV-A range, the rutin being included in latter (GABERSCIK ET AL., 2002). The results of the research on Pisum sativum conducted under relatively high UV-B doses support our results (DAY AND VOGELMAN, 1995).

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Proceedings a/the 9th International Symposium on Buckwheat, Prague 2004

Importance of rutin in human nutrition GRIFFITH ET AI. (1955) report about health effects of rutin and related flavonoids. Many authors evidence the role of rutin in the prevention of cardiovascular diseases (KOSCIELNY ET AI., 1996) and in lowering the risk for arteriosclerosis (WOJCICKI ET AI., 1995). According to GRIFFITH ET AL. (1944) and SCHILCHER ET AL. (1990), rutin prevents capillary fragility and high blood pressure. Antioxidative effect of buckwheat was studied by WATANABE (1998), PARK ET AI. (2000), HOLASOVA ET AL. (2002) and MORISHITA ET AI. (2002). Chemical structure of the flavonoid molecules, including the number, location and kind of substitution on the basic flavan molecule, is important for the antioxidative effect of flavonoids (HElM ET AL., 2002). Antioxidative effects of buckwheat are based on the interaction of genetic and environmental factors (OOMAH AND MAZZA, 1996). NAGAI ET AI. (2001) and GHELDOF AND ENGESETH (2002) investigated the antioxidative effects of buckwheat honey. Their research evidence, that it had higher antioxidative ability than honey from some other sources. MANACH ET AI. (1997) studied bioavailability of rutin and quercetin. LUTHAR (1992), LUTHAR AND KREFT (1993) and HAGELS (1999 a, b) discussed content of tannin and other polyphenols in buckwheat. It is not yet confirmed if anti-allergic action of buckwheat grain extract (KIM ET AI., 2003) is connected with rutin and related substances or not. According to AURA ET AI. (2002) quercetin derivatives are in the colon decomposed to other molecules. KIM ET AI. (1999) proved that the result of their decomposition is mainly quercetin. As reported by SIMMERING ET AL. (2002), when the food consumed is rich in flavonoids, the development of flavonoid degrading microorganisms is in the colon more abundant. HOLLMAN ET AL. (1995), Gee (2001), GRAEFE ET AL. (2001), WOLFFRAM ET AI. (2002) and BLASIAK ET AI. (2002) studied the absorption of quercetin and its glycosides from the small intestine; in this transportation it seems to be of special importance the sugar part of the complex glycoside molecule. After the consumption of buckwheat tea, quercetin content in blood arises for four hours, and then is slowly diminishing. Quercetin may have along with the vitamin C an important role in the protection of DNA molecules from damages, and thus for preventing the appearance of mutations and cancer.

Rutin and other flavonoids are secondary plant metabolites. They are synthesized in higher plants in order to protect the plants against UV radiation, microorganisms and herbivores. Ability to synthesize rutin and related compounds might be an evolutionary plant response to the extreme growing conditions, like higher flux of UV-B. Since buckwheat contains rutin and other valuable constituents of the grains it is an important source for functional food.

ACKNOWLEDGEMENTS This research is a part of the projects "The role of UV-B radiation on aquatic and terrestrial ecosystems: an experimental and functional analysis of the evolution of protective and adaptive mechanisms in plants, environment and climate" (pL 970637) and projects financed by Ministry of Education, Science and Sport, Republic of Slovenia: Physiological indicators of stress in cultivated plants (J4-6428-0105-04/4.03), and Impact of selenium on the yield of vegetables and crop plants (J46476-0481~04/4.03). The financial support is gratefully acknowledged.

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