Efi Levizou

The effects of UV-B radiation on European heathland species

The effects of enhanced UV-B radiation on three examples of European shrub-dominated vegetation were studied in situ. The experiments were in High Arctic Greenland, northern Sweden and Greece, and at all sites investigated the interaction of enhanced UV-B radiation (simulating a 15% reduction in the ozone layer) with artificially increased precipitation. The Swedish experiment also involved a study of the interaction between enhanced UV-B radiation and elevated CO2 (600 ppm). These field studies were supported by an outdoor controlled environment study in the United Kingdom involving modulated enhancement of UV-B radiation in combination with elevated CO2 (700 ppm). Effects of the treatments on plant growth, morphology, phenology and physiology were measured. The effects observed were species specific, and included both positive and negative responses to the treatments. In general the negative responses to UV-B treatments of up to three growing seasons were small, but included reductions in shoot growth and premature leaf senescence. Positive responses included a marked increase in flowering in some species and a stimulation of some photosynthetic processes. UV-B treatment enhanced the drought tolerance of Pinus pinea and Pinus halepensis by increasing leaf cuticle thickness. In general, there were few interactions between the elevated CO2 and enhanced UV-B treatments. There was evidence to suggest that although the negative responses to the treatments were small, damage may be increasing with time in some long-lived woody perennials. There was also evidence in the third year of treatments for effects of UV-B on insect herbivory in Vaccinium species. The experiments point to the necessity for long-term field investigations to predict the likely ecological consequences of increasing UV-B radiation.

Beneficial effects of enhanced UV-B radiation under field conditions: improvement of needle water relations and survival capacity of Pinus pinea L. seedlings during the dry Mediterranean summer

The possible mechanism(s) by which supplemental UV-B radiation alleviates the adverse effects of summer drought in Mediterranean pines (Petropoulou et al. 1995) were investigated with seedlings of Pinus pinea. Plants received ambient or ambient plus supplemental UV-B radiation (biologically equivalent to a 15% ozone depletion over Patras, 38.3° N, 29.1° E) and natural precipitation or additional irrigation. Treatments started on 1 February, 1994 and lasted up to the end of the dry period (29 September). In well-watered plants, UV-B radiation had no influence on photosystem II photochemical efficiency and biomass accumulation. Water stressed plants suffered from needle loss and reduced photosystem II photochemical efficiency during the summer. These symptoms, however, were less pronounced in plants receiving supplemental UV-B radiation, resulting in higher total biomass at plant harvest. Laboratory tests showed that enhanced UV-B radiation did not improve the tolerance of photosystem II against drought, high light, high temperature and oxidative stress. Enhanced UV-B radiation, however, improved the water economy of water stressed plants, as judged by measurements of needle relative water content. In addition, it caused an almost two-fold increase of cuticle thickness. No such UV-B radiation effects were observed in well-watered pines. The results indicate that the combination of water stress and UV-B radiation may trigger specific responses, enabling the plants to avoid excessive water loss and, thereby, maintain a more efficient photosynthetic apparatus during the summer. The extent of this apparently positive UV-B radiation effect would depend on the amount of summer precipitation. Abbreviations: DW – dry weight, Fv/Fm – ratio of variable to maximum fluorescence, A 300 – absorbance at 300 nm, PAR – photosynthetically active radiation, PS II – photosystem II, RWC – relative water content, TCA – trichloroacetic acid, UV-BBE – biologically effective ultraviolet-B radiation

Combined effects of enhanced UV-B radiation and additional nutrients on growth of two Mediterranean plant species

Seedlings of two Mediterranean plants, the slow-growing, evergreen sclerophyll Ceratonia siliqua L. and the fast growing drought semi-deciduous Phlomis fruticosa L., were grown for one year in the field at ambient or ambient plus supplemental UV-B radiation (equivalent to a 15% ozone depletion) and two levels of applied fertilizers (NPK). The effects on growth, morphological, anatomical and physiological parameters were measured at final plant harvest. Additional nutrients increased leaf nitrogen, improved growth and reduced the root/shoot ratio in both plants, yet these effects were more pronounced in the fast growing P. fruticosa. A nutrient-induced increase in chlorophyll content was also observed in this plant. The growth responses to UV-B radiation were different for the two species. Growth in C. siliqua was not affected by UV-B radiation at both nutrient levels and the same was true for P. fruticosa at low nutrients. However, at the high nutrient level, supplemental UV-B radiation improved growth in P. fruticosa, indicating a strong interaction between the treatments. Photosystem II (PSII) photochemical efficiency, methanol-extractable UV-B absorbing capacity, total phenolics and tannins were not affected by either treatment in both plants. It is concluded that nutrient levels can strongly modify the UV-B radiation effects on growth of P. fruticosa. We presume that this may be correlated to the fast growing habit of this species.

Exceptional photosynthetic performance of Capparis spinosa L. under adverse conditions of Mediterranean summer

Diurnal and seasonal fluctuations in water potential (Ψ), stomatal conductance (gs), transpiration rate (E), and net photosynthetic rate (PN) were monitored in Capparis spinosa L., a Mediterranean plant growing during summer, i.e. at the period considered the most stressful for local plant life. In spite of the complete absence of rain, Ψ exhibited a modest drop at midday (−2.7 MPa), but was fully recovered overnight, indicating sufficient access to water sources. The stomata remained open throughout the day and season and the high E resulted in leaf temperatures up to 3.9 °C below air temperature. Additionally, PN of the fully exposed leaves was higher than 25 μmol m−2 s−1 for more than 10 h per day throughout the summer growth period. No symptoms of photooxidative stress were shown, as judged by maximum photosystem 2 photochemical efficiency (Fv/Fm) and the function of xanthophyll cycle. Indeed, diurnal inter-conversions of the xanthophyll cycle components were modest during the summer and a more intensive function of the cycle was only evident during leaf senescence in autumn. In comparison with a semi-deciduous and an evergreen sclerophyll co-existing in the same ecosystem, C. spinosa assimilated up to 3.4 times more CO2 per m2 during its growth period (May to October) and up to 1.8 times more on an annual basis.

Inhibitory effects of water soluble leaf leachates from Dittrichia viscosa on lettuce root growth, statocyte development and graviperception

Summary The leaf epicuticular exudate of the Mediterranean ruderal Dittrichia viscosa is water soluble and easily drained to the soil by rain. As a result, soil enriched with this material is inhibitory for seed germination of lettuce, while germination of Malcolmia maritima, (a species co-occurring with D. viscosa in the same habitat) is considerably delayed. We investigated the possible mode of action of this material as a growth inhibitor by using germinating lettuce seeds in filter paper bioassays. Apart from a negative effect on final percentage of seed germination, the material reduced root length and the frequency of cell divisions in the meristematic zone, induced abundant lateral roots and completely suppressed the formation of root hairs. Moreover, the presence of statocytes was rare and their internal polarity strongly perturbed. As a result, the ability of primary roots to respond to gravity was suppressed. We suggest that the strong biological activity of the rinsate may enhance the competitive ability of D. viscosa by interfering with resource acquisition of germinating neighbors.

The influence of drought, shade and availability of mineral nutrients on exudate phenolics of Dittrichia viscosa

Summary Exudate phenolics are thought to defend leaf surfaces against microorganisms and interfere with neighboring plants if leached to the soil. We investigated the effects of resource availability on growth and accumulation of epicuticular water soluble phenolics in potted seedlings of Dittrichia viscosa grown in the field under two levels of nutrients, water and photon fluence rates. As expected, growth was enhanced under additional nutrients and suppressed under water stress. Under shade, aboveground biomass accumulation was not affected, yet the total leaf area was considerably increased at the expense of dry mass invested on area basis. Glandular hair density was not affected by any treatment. The concentration of leaf exudate phenolics was increased under water stress and decreased under additional nutrients and shade, in accordance to the prevailing hypotheses predicting environmental effects oncarbon-based defensive secondary metabolites. Yet, when we considered total epicuticular phenolic production on a plant basis (i.e. the amount leachable to the soil, which better indicates its allelopathic potential), the changes in total leaf area compensated (shade) or over-compensated(nutrients, water stress) the changes in concentration. Thus, total leachable epicuticular phenolics were higher under additional nutrients and lower under water stress. We conclude that environmental factors and the corresponding physiological constrains in the production of exudate phenolics may act in opposite directions when surface protection and interference with neighbors is considered.

Carotenoid composition of peridermal twigs does not fully conform to a shade acclimation hypothesis

The photosynthetic pigments of twigs in five tree and shrub species possessing chlorenchyma under a well developed, stomata-less, and highly photon absorptive periderm were analysed and compared to those of the corresponding canopy leaves. We asked whether the unavoidable shade acclimation of corticular chlorenchyma results in photosynthetic pigment complements typically found in shade leaves. As expected, chlorophyll (Chl) a/b ratios in twigs were consistently low. However, carotenoid (Car) analysis did not confirm the initial hypothesis, since twigs generally contained increased Chl-based pool sizes of the xanthophyll cycle components. The contents of photo-selective neoxanthin and lutein were high as well. Yet, β-carotene content was extraordinarily low. In addition, twigs retained high pre-dawn ratios of the deepoxidized antheraxanthin and zeaxanthin, although environmental conditions were not pre-disposing for such a state. The unexpected Car composition allows the conclusion that other micro-environmental conditions within twigs (hypoxia, increased red to blue photon ratios, and extremely high CO2 concentrations) are more important than shade in shaping the Car profiles.

Induction of Ageotropic Response in Lettuce Radicle Growth by Epicuticular Flavonoid Aglycons of Dittrichia viscosa

Thirteen flavonoid aglycons, contained in the strongly allelopathic epicuticular exudates of Dittrichia viscosa, were investigated for their effects on lettuce seedling radicle growth. Concerning radicle length and mass, variable results were obtained, with most of the substances having no effect, some being inhibitory and some even promotive. Shoot mass was slightly reduced in four cases. Seed germination rates, root hair and lateral root formation were not affected either. Three of the compounds (namely quercetin 3,3-dimethylether, naringenin and eriodictyol) induced a strong ageotropic response in radicle growth.

Total Carotenoid Amount in Crude Twig Extracts May Be Overestimated Due to Interference by High Contents of Co-Extracted Phenolics

Total carotenoids assessed spectrophotometrically in crude extracts may be considerably overestimated when high contents of phenolic compounds are co-extracted. In this case, the absorbance tails of phenolics extend well into the blue part of the spectrum, interfering with carotenoid estimation. Extracts of phenolic-rich organs, with a low ratio of photosynthetic to heterotrophic and/or supportive cells (for example, stems or twigs) are vulnerable to such pitfalls and may need chromatographic separation of carotenoids.