J.W.M. van de Staaij

Flavonoid concentrations in three grass species and a sedge grown in the field and under controlled environment conditions in response to enhanced UV-B radiation.

An investigation was carried out to find whether enhanced levels of UV-B radiation induce increased concentrations of flavonoids in the leaves of the grass species Deschampsia antarctica, Deschampsia borealis and Calamagrostis epigeios and the sedge Carex arenaria. Whether the enhanced levels of UV-B influenced the proportions of the various flavonoids in the leaves was also studied. Increased flavonoid concentrations would improve the UV-B shielding of UV-B susceptible tissues. Using HPLC analysis the flavonoids orientin and luteolin were identified in D. antarctica, orientin in D. borealis and tricin in C. arenaria. Neither flavonoid concentrations nor the proportion of the various flavonoids in climate room grown D. antarctica and D. borealis plants differed between individuals grown under 0, ambient or elevated UV-B levels. After 12 weeks of growth biomass production and shoot-to-root ratios of D. antarctica were not affected by elevated UV-B radiation. Greenhouse grown C. epigeios plants contained higher concentrations and different proportions of flavonoids grown under elevated levels of UV-B than when grown under ambient or 0 UV-B. In C. epigeios plants grown in their natural habitat in the field under ambient or elevated levels of UV-B, flavonoid concentrations and proportions were the same in plants from both treatments. In the leaves of the sedge C. arenaria grown in a greenhouse flavonoid concentrations and proportions were not affected by UV-B radiation. Leaves were harvested four times during the growing season from C. arenaria plants grown in their natural habitat in the field under ambient or elevated levels of UV-B. Leaves harvested in January contained higher concentrations of flavonoids when grown under elevated UV-B than when grown under ambient UV-B radiation. In leaves harvested in May, September and December flavonoid concentrations were the same in plants grown under ambient or elevated UV-B. The proportion of the different flavonoids was the same for both treatments in all months. These results indicate that constitutive levels of flavonoids in these grass and sedge species are adequately high to protect them against ambient and elevated levels of solar UV-B radiation.

Ultraviolet-B (280 − 320 nm) absorbing pigments in the leaves of Silene vulgaris : their role in UV-B tolerance

Summary The UV-B radiation tolerant perennial herb Silene vulgaris was tested on the influence of incident UV-B fluxes during growth on the synthesis of UV-B absorbing pigments in the leaves. Analysis of methanolic leaf extracts showed a stimulating effect of UV-B on the absorbing ability of leaf extracts. HPLC analysis made clear that UV-B radiation stimulated extractable flavonoid concentrations in leaves, but that UV-B absorption could only be partly attributed to these flavonoids. The contribution of flavonoids to UV-B absorption diminishes if plants mature. Other possible functions of flavonoids in plants growing under elevated UV-B conditions are discussed.

THE IMPACT OF ELEVATED UV-B (280-320 NM) RADIATION LEVELS ON THE REPRODUCTION BIOLOGY OF A HIGHLAND AND A LOWLAND POPULATION OF SILENE VULGARIS

A highland (altitude 1600 m) and a lowland (altitude –2 m) population of the perennial herb Silene vulgaris were tested on the effects of elevated levels of UV-B radiation on their reproductivity. Highland populations receive higher natural UV-B doses than lowland populations. Therefore adaptation to high UV-B levels of the highland population is to be expected. The lowland population showed a decrease in the number of seed producing flowers and the number of seeds produced per plant under elevated UV-B levels. The highland population increased the number of seeds per plant under elevated UV-B levels. In both populations individual seed mass as well as seed germination percentages were unaffected by the UV-B flux received by the parental plant. Possible effects of UV-B induced alterations in reproductivity on the geographical distribution of the different populations are discussed.

Effects of UV-B radiation on terrestrial plants and ecosystems: interaction with CO2 enrichment

UV-B radiation is just one of the environmental factors, that affect plant growth. It is now widely accepted that realistic assessment of plant responses to enhanced UV-B should be performed at sufficiently high Photosynthetically Active Radiation (PAR), preferably under field conditions. This will often imply, that responses of plants to enhanced UV-B in the field will be assessed under simultaneous water shortage, nutrient deficiency and variation of temperature. Since atmospheric CO2 enrichment, global warming and increasing UV-B radiation represent components of global climatic change, interactions of UV-B with CO2 enrichment and temperature are particularly relevant. Only few relevant UV-B× CO2 interaction studies have been published. Most of these studies refer to greenhouse experiments. We report a significant CO2 × UV-B interaction for the total plant dry weight and root dry weight of the C3-grass Elymus athericus. At elevated CO2 (720 μmol mol-1, plant growth was much less reduced by enhanced UV-B than at ambient atmospheric CO2 although there were significant (positive) CO2 effects and (negative) UV-B effects on plant growth. Most other CO2 × UV-B studies do not report significant interactions on total plant biomass. This lack of CO2 × UV-B interactions may result from the fact that primary metabolic targets for CO2 and UVB are different. UV-B and CO2 may differentially affect plant morphogenetic parameters: biomass allocation, branching, flowering, leaf thickness, emergence and senescence. Such more subtle interactions between CO2 and UV-B need careful and long term experimentation to be detected. In the case of no significant CO2× UV-B interactions, combined CO2 and UV-B effects will be additive. Plants differ in their response to CO2 and UV-B, they respond in general positively to elevated CO2 and negatively to enhanced UV-B. Moreover, plant species differ in their responsiveness to CO2 and UV-B. Therefore, even in case of additive CO2 and UV-B effects, plant competitive relationships may change markedly under current climatic change with simultaneous enhanced atmospheric CO2 and solar UV-B radiation.

Increased solar UV-B radiation may reduce infection by arbuscular mycorrhizal fungi (AMF) in dune grassland plants: evidence from five years of field exposure

An area of coastal dune grassland, dominated by the gramineous species Calamagrostis epigeios and Carex arenaria, was exposed to enhanced levels of UV-B radiation during a five year period. These species showed reduced AM-fungal infection percentages in their roots. In C. epigeios AM infection was reduced by 18%, C. arenaria showed a reduction by 20%. The major effect of enhanced UV-B on AM associations was a reduction of the number of arbuscules. This indicates a reduction in the exchange of nutrients between the symbionts. Since the effect of UV-B on AM associations may result from altered flavonoid levels in the root exudates of the host plants, flavonoid levels in the roots were investigated. No detectable flavonoid concentrations were found in the roots of C. epigeios and C. arenaria. Less effective AM associations can have pronounced negative effects on biodiversity and nutrient dynamics of the dune grassland ecosystem. The possible mechanisms causing these indirect effects of elevated UV-B on below ground AM associations are discussed. We conclude that UV-B induced changes in plant hormone levels are more likely to be the mechanism reducing AMF infection than UV-B induced alterations in flavonoid concentrations in the root exudates of the host plant.

Expected Changes in Dutch Coastal Vegetation Resulting from Enhanced Levels of Solar UV-B

Effects of increased UV-B radiation on plants from terrestric ecosystems will be negative, but large differences exist between species in their sensitivity to (enhanced) UV-B radiation. Field studies on the effects of enhanced UV-B are scant and should be extended to make general conclusions possible. Among two species of salt marsh plants species tested, the dicot Aster tripolium showed marked reduction of growth and photosynthesis. Growth and photosynthesis were less markedly inhibited by UV-B radiation in the monocot Spartina anglica. The prediction of the ecosystem response to increased UV-B radiation needs extension of experimental field studies. In further studies of the effects of increasing UV-B, other environmental factors forming part of the global climatic change should be included.

(Poly)phenolic compounds in pollen and spores of Antarctic plants as indicators of solar UV-B

The morphology, size and characteristics of the pollen of the plant species Antarctic hairgrass (Deschampsia antarctica, Poaceae) and Antarctic pearlwort (Colobanthus quitensis, Caryophyllaceae) are described by scanning electron microscopy and light microscopy. Based on the number of pores the pollen of Colobanthus quitensis is classified as periporate or polypantorate, while that of Deschampsia antarctica is monoporate.

The effect of elevated UV-B (280-320 nm) radiation levels on Silene vulgaris : a comparison between a highland and a lowland population

Highland (altitude 1600 m above sea level) and lowland (altitude -2 m below sea level) populations of the perennial herb Silene vulgaris (Moench) Garcke, were tested on their response to elevated levels of UV-B radiation. Highland populations typically receive high natural UV-B fluxes, whereas lowland populations receive a lower natural UV-B dose. Adaptation to high UV-B levels of the highland population is to be expected. Experimental comparison of growth rates, gas exchange rates, transpiration and biochemical parameters using adult plants as well as seedlings did not show a difference in the response to elevated UV-B levels between the two populations. Individuals of both populations were relatively insensitive to elevated UV-B radiation. The response of alpine and lowland populations of Silene vulgaris is discussed in relation to the dispersal of this species after the last ice age.

Global change, the impact of the greenhouse effect (atmospheric CO2 enrichment) and the increased UV-B radiation on terrestrial plants

Atmospheric enrichment of CO2 will favour growth of C3 plant species and as a result the competitive balance between C3 and C4 plant species may markedly change. The greenhouse effect consists, however, of both an increase of atmospheric CO2 and global warming, with an expected increase of the global temperature of 1.5–4.5° C with a doubling of the atmospheric concentration of carbon dioxide. Such a rise of temperature will prove advantageous to C4 plants. It is also indicated that below a mean air temperature of 18.5° C no positive growth response to CO2 enrichment will occur.

The effect of reciprocal treatments with ozone and ultraviolet-B radiation on photosynthesis and growth of perennial grass Elymus athericus

The impact on plant growth of the simultaneously changing factors of the global climate, rising tropospheric O3 concentrations and increasing UV-B radiation fluxes, has been tested in a combined glasshouse and growth chamber experiment. The saltmarsh grass species Elymus athericus was sequentially fumigated for two weeks with O3 and for another two weeks irradiated with UV-B (vv). Exposure to elevated UV-B did not negatively affect photosynthesis or plant growth. Fumigation with O3 had a depressing effect on net photosynthesis, the number and biomass of flowers, the number of leaves and the number of shoots. O3-induced damage only was observed in plants which had been fumigated during the last two weeks of the experiment. Since interactive responses were not observed, results suggest different primary target sites for O3 and UV-B within the plant.