ANNALES • Scr. hist. nat. • 13 • 2003 • 2
review article	UDK 551.521.17:581.5
received 2003-02-04
THE EFFECTS OF UV-B RADIATION ON AQUATIC AND TERRESTRIAL
PRIMARY PRODUCERS
Mateja GERM National Institute of Biology, Si-i 000 Ljubljana, Večna pot 111 E-triail: mateja.germ@nib.si Alenka GABERŠČIK Department of Biology, Biotechnical faculty and National Institute of Biology, Sl-l 000 Ljubljana, Večna pot 1 H
Tadeja TROŠT-SEDEJ Department of Biology, Biotechntcal Faculty, St-1000 Ljubljana, Večna poi I t 1 Zdenka MAZE,J ER1CO, SI-3320 Velenje, Koroška 58, Slovenia Jože BAVCON
Department of Biology, Biotechnical Faculty, SI-1000 Ljubljana, Večna pot 11 i
ABSTRACT
The effect of enhanced UV-B radiation on aquatic and terrestrial primary producers is reviewed, based on the data from literature and those derived from experiments performed under UV-B doses corresponding to 17% ozone depletion. The changes of the following parameters, i.e. total contents of UV-B absorbing compounds and photo-synthetic pigments, terminal electron transport system (ETS) activity and photochemical efficiency of photosvstem li, were compared in different species. In some species, UV-B induced synthesis of UV-B absorbing compounds, while the others did not respond to enhanced UV-B or synthesized saturated amounts of these substances, with no respect to UV-B level. It was established that the production of UV-8 absorbing compounds demanded additional energy in Scenedesmus quadricauda, Selenastrum capricornutum and Ceratophyllum dernersum, since it was correlated to ETS activity. Generally, no effect on potential and actual photochemical efficiencies of photosystem II was observed.
Key words: UV-B radiation, primary producers, UV-B absorbing compounds, photochemical efficiency of PS li,
terminal electron transport activity
EFFETTI DEILA RADIAZIONE UV-B SU PRODUTTORI PRIMARI ACQUATICI E TERRESTRI
SINTESI
L'articolo presenta una revisions degli effetti di un'aumentala radiazione UV-B su produttori primari acquatici e terrestri, basata su dati di letteratura e su risultati di esperimenti condotti con dosi di UV-B corrispondenti ad un irn-poverimento in ozono pari al 17%. Le variazioni net seguenti parametri sono state confrontate per diverse specie: contenuto totale di cotnposli e pigmenti fotosintetici assorbenti raggi UV, attivita terminale di traspoito elettroni (EES) ed eflicienza fotochimica del fotosistema II. In alcune specie la radiazione UV-B ha indotto la sintesi di composti assorbenti UV-B, mentre altre specie non hanno manifestato risposta all'aumentata radiazione UV-B o hanno sintetizzato quantita sature di tali sostanze, a prescindere dal liveilo di UV-B. in Scenedesmus quadricauda, Selenastrum capricornutum e Ceratophyllum dernersum, la produzione di composti assorbenti UV-B ha richiesto tassi piu elevari di energia, in quanlo conelata all'attivita ETS. Gli autori non hanno osservato effetti sull'efficienza fotochimica potenziah o effettiva del fotosistema 1!.
Parole chiave: radiazione UV-B, produttori primari, composti assorbenti UV-B, efficienza fotochimica di PS II,
attivita terminale di trasporto elettroni.
2B1
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■v.aieja GERM«al.THE EFFECTS OF UV-B RADIATION ON AQUATIC AND TERRESTRIAL PRIMARY PRODUCERS. 381-J8S
INTRODUCTION
Tiie intensive UV research cfLiring recent years is (he result of our concern regarding the thinning of ozone layer in the stratosphere and consequently increasing ultraviolet (UV) radiation levels that may influence terrestrial as well as aquatic ecosystems (Rozema et ai, 1997; Hader et ai, 1998; Trost & Gaberséik, 2001; Gaberscik et a/v 2001; Gaberscik et a!., 2002a, b; Germ ei ai, 2002a, b; Rozema et ai., 2002). UV-B radiation causes damage to nucleic acids by absorption of UV-B photons by DNA and formation of cyciobutane dimmers as well as by formation of free radicals. Membrane damages occur as a consequence of photoabsorption, peroxidation and changes in the membrane lipid composition. UV-B radiation affects photosynthesis by damaging photosystem Ii (Björn, 1999; Xiong, 2001), disruption of thylakoide membrane, reduction in chlorophyll content, disturbance of membrane permeability and damaging RuBP carboxylase (r ¡bulóse-1 -5-bisphospha-te). It has been established that the activity of respiratory electron transport system (ETS) is enhanced by UV-B (Ferreyra et al., 1997: Gaberscik et a!., 2002a). UV-B radiation also affected the activity of phytochormones by influencing the synthesis or by inactivation. Fiant mor-phogenetic responses to enhanced solar UV B radiation are decreases in height, leaf length, leaf area, increases in leaf thickness, altered leaf angle, plant architecture, canopy structure, altered emergence, phenology, senescence, and seed production (Newsharn et ai, 1996; Rozema et ai, 1997; Gaberscik er al., 2002b). Enhanced UV-B results in the disturbance of motility and orientation of phytoplankton (Gullen et ai, 1992) decrease cell wall thickness, inhibited enzyme activity and metabolism of nitrogen (Hader, 1996; Nielsen, 1996).
Protection against UV-B radiation is of primary importance for photosynthetic organisms, which depend on solar radiation as the primary source of energy. Organisms have evolved different strategies and mechanisms to cope with UV-B stress. The general response found in the majority of primary producers is enhanced production of UV-absorbing compounds, which provide a protection screen filtering olí! harmful UV radiation (Sommaruga, 2001; Xiong, 2001; Gaberscik et ai, 2002b, Germ et ai, 2002a). The concentration and type of these compoLinds generally depends on the group of organisms and the level of UV-B radiation (Hotm-Hansen et ai, 1993; Hannach & Sigleo, 1998; Sommaruga & Garcia-Pichel, 1999). Defence mechanisms of higher plants against UV-B damage also include scattering and reflection of UV-B radiation by epidermal and cuticular structures, photoreactivation enzymes, excision of DNA damage and scavenging of radicals, while polyamines may additionally ameliorate UV-B damage to membranes (Stapfeton, 1992; Mitchell & Karentz, 1993; Runeckles & Krupa, 1994). Phytopiankton pro-
tects itself by forming cenobia or relative larger ceil size and shading vital cellular structure (Nielsen, 1996; Xiong et ai, 1999; Xiong 2001). The net effect of UV-B on organisms is the result of damage and repair processes and depends on the type of the environment.
Aquatic and terrestrial environments differ in many parameters essential for plant survival. Terrestrial plants have evolved structures like cuticle and stoinata, which on the one hand reduce the loss of water, while on the other hand they limit Liptake of carbon dioxide (CO?) from the air. The important role of epidermal and cuticular structures and other leaf properties, such as waxy layer, leaf hairs and leaf bladders, is also scattering and reflection of UV-B radiation. The main factors limiting growth and development of aquatic plants are variable light intensify and slow diffusion of CO; (Frost-Christensen & Sand-jensen, 1992? Covering et ai, 1996; Vad-strup & Mads en, 1996). Plants in aquatic and terrestrial environment are exposed to different radiation conditions, including those in the UV range. The UV-B penetration in water may vary from only few centimetres in highly liLimic lakes to dozen of meters in clear oceanic waters (Smith & Baker, 1931). Optical properties of water depend on water itself, dissolved organic matter (DOM), the photosynthetic biota and particulate matter (Nielsen, 1996; Williamson ei a!., 1996; Sommaruga & Psenner, 1997; HLtovinen et ai, 2003). Aquatic plants could be therefore partly protected from direct UV-B radiation by water filter. Phytop/ankton populafrons are exposed to high solar UV-B level, when they are dose to the water surface and when the water transparency for UV-B is high. Higher aquatic plants thriving in the littoral are exposed to UV-B when water level decreases. Amphibious plants deserve special attention in UV-B research, since they thrive at the water/land interface and therefore in contrasting environments regarding availability of water, gas and radiation (Madsen & Breinholt, 1995).
The publications on UV-B research experiments are numerous (Rozema et ai, 1997, 2002), but in many cases the results are not comparable due to different methodological approaches. The major problem was the radiation conditions with unrealistic UV-B doses and low ratio of photosynthetic active radiation. The aim of the present article is to compare selected responses of different primary producers exposed to the level of UV-B radiation doses based on expected future scenarios,
MATERIAL AND METHODS
The data on the following plant species were reviewed: Scenedesmus quadricauda (Turp.) Breb., Selenastrum capricornutum Prinz. Sphagnum mageHanicum Brid., Ranunculus trichophyllus Chaix /Batxachium trichophylium (Chaix) vat! den Boschj, Myosotis scotpi-oides L. ¡M. palustris (L.) Hiflj, Ceratophyllum demetsum L„ Mynophyllum spicaturn I., Potamogeton alpinus Bal-
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bis. Picea abies (l.) Karst, Fagopyrum esculentum Mo-erich fF. vulgare T. Nees, Polygonum fagopyrum L.J, Pultnonaria officinalis L. [P. officinalis L. subsp. macu-losa (I iayne) Gams!, Tropaeolum majus L. and Picea abies (L.) Karst. Plants were treated under similar conditions in outdoor and indoor experiments.
Higher plants were exposed to enhanced level of U VB radiation in the semi-controlled conditions in field and indoor experiments. Phytoplankton was treated with enhanced level of UV-B in indoor experiments. Plants from the natural environment were transplanted into natural sediment in semi-controlled conditions in the Botanical Garden of Ljubljana University (46°35'N, 14°55'E), Slovenia. An UV-B supplement system was designed as described by Björn & Teramura (1993). Three different treatments were applied: simulation of 17% ozone depletion <UV-B<+)) using Q-Panel UV-B 313 lamps, filtered with cellulose diacetate filters, which cut the UV-C range (wavelengths lower than 280 nm). The second treatment reduced the ambient level of UV-B radiation (UV-B(-)) for 50% using Mylar foil, which blocks wavelengths below about 320 nm (Gehrke et a/., 1996). Finally, control treatment was ambient radiation and Q-Panel UV-B 31 3 lamps, filtered with Mylar foil, to correct for effects of the UV-A radiation (control). The doses simulating 17% ozone depletion were calculated and adjusted weekly using the program published by Bjorn & Murphy (1985), based on the generalized plant action spectrum (Caldwell, 1968). Ambient UV-B radiation was measured using the European Light Dosimeter Network (EIDONET, Real Time Computer, Möhrendorf, Germany) measuring system, which also monitors UV-A radiation and PAR.
Cell suspensions of 5. quadricauda and S. capricor-nutum were cultured in polyethylene (PE) open-top (200 ml, less than 4 cm suspension layer) vessels in laworski medium at 23±2°C. Light sources were GROLUX lamps, which provided 200 pmo! mV of PAR (12/12 hours, light/dark). The UV-B doses varied from 0.8 to 12.3 kj mVday, using the lamps and filters mentioned above,
Photosynthetk: pigments and UV-B absorbing substances
Carotenoids and chlorophyll a and b were determined according to Jeffrey & Humphrey (1975), The procedure for the extraction of UV-B absorbing substances followed the method described by Caldwell (1968). UV-6 absorbing substances were extracted with an extraction medium (methanokdistilled water: HCl -79:20:1) and centrifuged. The supernatants of the samples were scanned in the range from 280-320 nm. The extinction values were corrected for dry weight of the sample.
Physiological parameters
The quantum efficiency of PS 11 (photosystem II) was
measured using the fluorometer OS-500 (Opti-Science, USA). The optimal quantum efficiency was calculated as Fv/Fm. Plants were kept in cuvettes for dark adaptation for 15 min before the measurements at ambient temperature. The effective quantum efficiency of PS li (yield - Y) was measured under actual light conditions, described by the expression V = (Fm' - F)/FnV. The yield was measured under full light conditions (from 1500 to 2000 pmol tri'" s'1) at the prevailing ambient temperature (Schreiber eta/., 1995).
Respiratory potential was estimated via measuring the potential electron transport system (l-TS) activity of mitochondria as reported in detail by Packard (1971! and modified by Kenner & Ahmed (1975). The material was homogenized in ice-cold homogenizing buffer and centrifuged in a refrigerated ultracentrifuge (500 g, 4 min, 0°Q. The supernatant of the homogenate was mixed with substrate solution, (NT solution and incubated for 40 min at room temperature. During incubation, the INT (instead of oxygen) was reduced to for-mazan. The absorption of formazan was measured with UV/ViS Spectrometer System (Lambda 12, Perkin-Elmer, USA) at 490 nm. The absorption was converted to the amount of oxygen utilized per dry mass per tirne.
Statistical analysis
All measurements were conducted on 4-10 parallel samples respectively. The significance of the differences was indicated as follows: ('+) stands for positive trend, (++) indicates significant positive effect, (x) indicates no clear effect, and (-) negative effect. Differences were tested by two-way Student's t-test.
DISCUSSION
According to data from many researches, UV-B induces the bleaching of photosynthetic pigments (Strid et at.. 1990; Holm-Hansen el a I., 1993; Bischof et al., 1998), Even though accessory pigments appear to be more sensitive than chlorophylls (Tevini, 1993), the destructive effect on chlorophyll has been reported by Ian-sen eta/. (1996), Olsson (1999) and Oemmig-Adams & Adams (1992). The fatter suggest that the decrease in chlorophyll a is related to a kind of excess-light stress avoidance mechanism. The effect of UV-B on the content of chlorophyll a seems to be species specific. Several researchers (Tosserams & Rozema, 1996; Antonelli el at1997; Garde & Cailiiau, 1998) support the results obtained in our laboratory under the 17% depletion of ozone layer (Tab. 1), which showed no effect of UV-B radiation on chlorophyll a content. In some cases, the contents of chlorophyll a even increase under UV-B radiation (Liu etai, 1995). Beardail et al. (1997) and De Lange et al. (2000) report a negligible effect on chlorophyll a in UV-B treated cells of A. fios-aquae and Sele-
ANNALES • Ser. hist. rial. - 13 • 2003 • 2
Mateja CE«M «i »i„- THE EFFECTS OF UV-B RADIATION ON AQUATIC AND TERRFSTRJAL PRIMARY PRODUCERS. 28:-258
nastrum, respectively. Veen et al. {1997} find a stimulation of chlorophyll a and b production in green alga S. capricomutum. The same effect was also found in 5. quadricauda and S. 'capricomutum in out laboratory (Germ ef al., 2002a). The increase of chlorophyll a in P. abies was observed in emergent needles in spring only, when the protective mechanisms were not fully developed (Trost & Gabersfik, 2001). The fact that the production of chlorophyll a was not depressed but slightly stimulated could also be explained as a protective strategy of cells. By "multiplication" of target sites, an organism avoids disturbances in the activity. Karentz ef al. (1991) also point out the protective role of chloroplasts in the cell. Their position in the cell could provide the protection of nucleus against strong radiation.
The increase of UV-8 absorbing compounds with increasing UV-B radiation dose (Shick ef al., 1996) suggests that UV-B induces synthesis of these substances for protection of the photosynthetic apparatus in primary producers (Hunt & McNeil, 1999; Karsten et a!., 1999; Tu-runen et al., 1999). Synthesis of UV-B absorbing compounds was induced by UV-B radiation in 5. quadricauda and in 5. capricorni/ium (Tab. 1) as it had been observed for many other algae (Karentz ei al., 1994; Hader, 1996; Xiong et a!., 1999.: Sommaruga, 2001; Xiong, 2001; Germ ei al., 2002a). Enhanced UV-B radiation also increased the production of UV-B absorbing compounds in C. demersumand i. esculentum (Gaberscik eta!., 2002a, b). In many cases, the production of UV-B absorbing compounds does not necessarily depend on UV-B dose (Ran & Hofmarin, 1996). No correlation with the UV-B dose and synthesis of UV-B absorbing compounds was found in R. tricbopbyllus, M. scorpioides, P. alpinus, and M. spicatum (Germ et al., 2002b). Some plants, such as those from tropical and high altitude environments, contain saturated amounts of flavonoids, and enhanced doses do not exert an increased production (Teramura & Sullivan, 1994). It is hypothesized (Sullivan ef al., 1996) that the receptors triggering the synthesis of UV-B absorbing compounds are saturated in plants growing in an open environment so that they provoke maximal synthesis at all ir-radiances. tn P. abies, the protective mechanisms also appeared to be dependent more on the developmental state of leaf than induced by enhanced LiV-B radiation. In emergent needles only, where UV-B radiation could penetrate into the mesophyli, biosynthesis of UV-B absorbing compounds was related to UV-B radiation dose (Trost & Gaberscik, 2001). It seemed that in the studied plants the amount of UV-B absorbing compounds was sufficient, since we detected no disturbances in plant physiology (Tab. 2), None of the studied species exposed to UV-B radiation corresponding to 17% ozone depletion showed decrease of Fv/Frn ratio or Y that would reflect: disturbance in photosynthesis. It is also likely that the damage caused by UV-B radiation is efficiently repaired (Rozema etal1997). On the contrary, many authors re-
port about the effects of UV-B radiation on photochemical efficiency of PS ¡1 (Schoefield etal., 1995; Haderet al., 1996; Xiong et at., 1999; Xiong, 2001,)
Tab. I: The influence of enhanced UV-B radiation content of chlorophyll a, carotonohls and UV-B absorbing compounds (UV-B AC). Legend: + indicates positive trend, ++ indicates significant positive effect, x indicates no clear effect, - stands for negative effect (n = 410, p< 0.05).
Tab. 1: Vpliv povečanega UV-B sevanja na vsebnost klorofila a, karotenoidov in UV-B absorbirajoči!) snovi (UV-B AC). Legenda: -h označuje pozitivno težnjo, ++ označuje značilno pozitiven vpliv, x označuje nejasen vpliv, - označuje negativen vpliv (n - 4-10, p< 0,05).
Species	Chi s	Carotinoids	UV-B AC	Source
S. quadricauda	4-		++	Gesm etal.. 2002a
5. capricomutum	+		++	not published
5. magel&vcum	X	X	X	not published
R. trichophylius	X	X	X	Goim etal., 2002b
C. demersum	+ /		+ + / +	not published / Gaberscik etal., 2002a
M. spicatum	X	X	X	not published
P. alpinus	X	X	X	Germ et ai, 2002b \
M. scorpioides	X	X	X	not published
F. esculentum	X	+	+	Gaberscik si al., 2002b
P. officinalis	X	X	X	(jaberscik et al., 2001
T. ma ¡us	X	X	+	not published
P. abies	+ , X		-F, X	TroSt & Gabetjiik, 2001
Tab. 2: The influence of enhanced UV-B radiation on Fv/Fm ratio> yield and £TS activity. Legend: + indicates positive trend, ++ indicates significant positive effect, x indicates no clear effect, - stands for negative effect (n = 4-10, p< 0.05).
Tab. 2: Vpliv povečanega UV-B sevanja na razmerje Fv/Fm, učinkovitost in aktivnost FIS- Legenda: + označuje pozitivno težnjo, ++ označuje značilno pozitiven vpliv, x označuje nejasen vpliv, - označuje negativen vpliv (n - 4-10, p< 0,05).
Species	Fvffm	Yield	ETS	Source
S. quadricauda			++	Germ ei al., 2002a
5. capricoriwtum			■H'	not published
j 5. magelanicur»	X	X	X	not published
R. trie hophy! his	X	X	X	Germ of a)., 2002b
C. demerswn	X/	X/	+4	not published / Gaberscik 6'f al., 2002a
M. spicatum	y.	X	X	not published
P. alpinus	X	X	X	Germ et al., 2002b i
j Vf. scùrpioides	X	X	X	not published
f. escif/entfim	X	X		Gabericir: ei at, 2002b
P. officinalis	X	X		Gaberscik et ai., 2001
iT. malus	X	X		not published
P. abies	X	X	+ , x	Trošl & Gaberšrik. 2001
The production of photosynthetic pigments and UV-B absorbing compounds demanded an additional supply of energy, which was provided by increased respiratory
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Mateja GERM a al.: THf EfFFCTS OF UV-3 RADIATION ON AQUATIC At JO IfRRFSTRlAL PRIMARY PRODUCERS, 281-286
potential. The relation between the amount of UV-B absorbing compounds and ET5 activity was significantly positive as reported for S. quadricauda and C demer-sum (Tab. 2) (Gaberscik et al., 2002a). Increased ETS activity under enhanced UV-B radiation therefore augmented the energetic cost involved in generating the internal mechanisms of photoprotection (Ferreyra et al., 1997; Scott eta I., 1999; Gaberscik eta!., 2002a).
SUMMARY
Enhanced UV-B radiation due to thinning of the stratospheric ozone layer affects primary producers. The research carried out on many species under similar experimental conditions, i.e. doses corresponding to 17% ozone depletion, showed species specific responses. Data from literature are very controversial. The contents of chlorophyll a were increased in algae 5. quadricauda and S. capricornutum and in submersed macrophyte C. demersum. The increase of chlorophyll a in R abies was observed in spring only, when the protective mechanisms were not fully developed. Other species showed no evident changes in chlorophyll content. The increase of UV-B absorbing compounds with increasing UV-B radiation dose is the most frequent reaction to UV-B radiation reported in literature, ft has been observed for many algae, as well as for 5. quadricauda and 5. capricornutum. UV-B radiation also induced production of UV-B absorbing compounds in higher plants C. demersum and F. esculentum. In many cases, the production of UV-B absorbing compounds does not necessarily depend on UV-B dose. No correlation with the UV-B dose was detected in aquatic plants R. trichcphyllus, M. scor-pioides, P. alpinus, and M. spicatum. It was hypothesized that the receptors triggering the synthesis of UV-8 absorbing compounds are saturated in plants growing in
an open environment, thus provoking maximal synthesis at all irradiarices. In P. abies, the production of UV-B absorbing compounds appeared to be dependent more on the developmental state of leaf than induced by enhanced UV-B radiation. In emergent needles only, where UV-B radiation could penetrate into the mesophyll, biosynthesis of UV-8 absorbing compounds was related to UV-B radiation. Many authors report about the effect of UV-B radiation on photochemical efficiency of PS II. None of the studied species exposed to UV-B radiation corresponding to 17% ozone depletion showed decrease of Fv/Fm ratio or Y, which would reflect disturbance in photosynthesis. The production of UV-B absorbing compounds demanded an additional supply of energy, which was provided by higher respiratory potential. The relation between the amount of UV-B absorbing compounds and ETS activity was significantly positive, as reported for S. quadricauda, S. capricornutum and C- demersum. The increased ETS activity under enhanced UV-B radiation therefore provided additional energy needed for the establishment of photoprotection and photorepair mechanisms.
ACKNOWLEDGEMENTS
This research was part of the project "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) supported by the EU (DGXII, Environmental Programme, ENV4-C! 97-0580), and part of SLO Alpe2 (3311-01-2183388) financed by Ministry of Education, Science and Sport of the Republic of Slovenia. Their financial support is gratefully acknowledged.
VPLIV UV-B SEVANJA NA VODNE IN KOPENSKE PRIMARNE PROIZVAJALCE
Mateja CERM
Nacionalni inštitut za biologijo, S1-HT0G Ljubljana, Večna pot lit E-mail: mateja.germ@nib.5i Alenka GA8ERŠČIK Oddelek za biologijo, Biotehniška fakulteta in Nacionalni inštitut za biologijo, SI-1000 L jubljana, Večna pot 111 Tadeja TROŠT-SEDEj Oddelek za biologijo, Biotehniška fakulteta, SI-1000 Ljubljana, Večna pot 111
Zdenka MAZFJ ERiCO, 51-3320 Velenje, Koroška 58 jože BAVCON
Oddelek za biologijo, Biotehniška fakulteta, SI-100G Ljubljana, Večna pot 111
POVZETEK
Povečano (JV B sevanje, ki je posledica tanjšanja ozonske plasti, vpliva na primarne proizvajalce. Raziskave
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kažejo, da so se različne vrste, izpostavljene odmerkom, ki ustrezajo približno 17% s tanjšanju ozonske plasti, odzvale različno. Rezultati o vplivu UV-B sevanja na primarne proizvajalce si pogosto nasprotujejo. Vsebnost klorofila a je pod vplivom UV-B sevanja narasla pri algah vrste Scenedesmus quadricauda in Selenastrum capricornutum ter pri podvodni rastlini navadni rogolist. Ceratophvllum demersum. Naraščanje vsebnosti klorofila a pri.navadni smreki Picea abies smo opazili samo spomladi, ko zaščitni mehanizmi še niso bili popolnoma razviti. Druge vrste, ki smo jih preučevali, niso kazale jasnega vpliva UV-B sevanja na vsebnost klorofila a. Glede na podatke v literaturi je naraščanje UV-B zaščitnih snovi najbolj pogost odziv primarnih proizvajalcev na povečano UV-B sevanje. UV-B sevanje je vplivalo na izgradnjo UV-B zaščitnih snovi tudi pri vrstah S. quadricauda in S. capricornutum in višjih rastlinah, kot sta navadni rogolist C. ciemersum in navadna ajda Fagopyrum esculentum, V mnogih primerih pa izgradnja UV-B zaščitnih snovi ni odvisna od odmerka UV-B sevanja. Korelacije med vsebnostjo UV-B zaščitnih snovi in odmerkom UV-B sevanja ni bilo pri lasastolistnr vodni zlatici Ranunculus trichophyllus, alpskem dristavcu Potamogetón alpinos in klasastem rmancu Myriophyílum spicatum, Predpostavljamo, da so receptorji, ki vplivajo na izgradnjo UV-B zaščitnih snovi, nasičeni pri rastlinah, ki rastejo na odprtih rastiščih in tako omogočajo maksimalno izgradnjo pri različni inteziteti obsevanja. Pri navadni smreki P. abies je videti, da je bila izgradnja UV-B zaščitnih snovi bolj odvisna od razvojnega stanja iglic kot od povečanega UV-B sevanja. Izgradnja UV-B zaščitnih snovi je bila povezana z UV-B sevanjam samo pri nerazvitih iglicah, kjer je UV-B sevanje prodiralo do mezofila. Mnogi avtorji so dokazali vpliv UV-B sevanja na fotokemično učinkovitost fotosistema II (FS II/. V naši raziskavi nobena od preučevanih vrst ni pokazala vpliva UV-B sevanja, ki ustreza 17% simulaciji tanjšanja ozonske plasti, na zmanjšanje potencialne biokemične učinkovitosti (Fv/Fm), ali dejanske fotokemične učinkovitosti (angl. yield), ki kažejo na motnje v procesu fotosinteze. Izgradnja UV-B zaščitnih snovi zahteva dodatno zalogo energije, ki si jo organizmi zagotovijo s povečanim dihalnim potencialom (aktivnost ETS), Razmerje med vsebnostjo UV-B zaščitnih snovi in aktivnostjo ETS je bila značilno pozitivna pri vrstah S. quadricauda, S. capricornutum in navac/nem rogolistu C. demersum. Povečana aktivnost ETS pri organizmih, ki so bili izpostavljeni povečanemu UV-B sevanju, je zagotovila dodatno energijo za vzpostavitev fotozaščite in fotopopravijalnih mehanizmov.
Ključne besede: UV-B sevanje, primarni proizvajalci, UV-B absorbirajoče snovi, fotokemična učinkovitost FS II, aktivnost terminalnega elektronskega transporta
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