Aris Kyparissis

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.

Seasonal fluctuations in photoprotective (xanthophyll cycle) and photoselective (chlorophylls) capacity in eight Mediterranean plant species belonging to two different growth forms.

Xanthophyll cycle components and their pool sizes were seasonally examined in eight Mediterranean plant species from two different growth forms (drought semi-deciduals and evergreen sclerophylls), in an attempt to correlate possible fluctuations in the above parameters with potentially photoinhibitory conditions. The xanthophyll cycle was common in all species examined and was functional throughout the year. In semi-deciduals, the maximum midday values for conversion state, defined as the ratio (zeaxanthin + antheraxanthin)/(violaxanthin + zeaxanthin + antheraxanthin), and the maximum xanthophyll pool sizes, expressed on a Chl basis, were recorded during the dry summer, indicating that the period of water shortage may be the most demanding for a high photoprotective potential. However, the data suggests that sclerophylls may also need a high photoprotective potential during winter, since the xanthophyll pool sizes were equally high during both summer and winter. Corresponding spring and autumn values were low. Furthermore, winter pre-dawn conversion state values were the highest recorded amongst all species studied, indicating that the mildly low winter temperatures may be a considerable stress factor for these plants. It is assumed that by preserving high concentrations of zeaxanthin and antheraxanthin overnight, sclerophylls retain high energy dissipation activity early in the morning, when minimum temperatures, coinciding with high photon fluence rates, are likely to drive the photosynthetic apparatus to potentially harmful overexcitation. Of significance may be the fact that almost all species show a minimum in chlorophyll content during the summer. In this way, overexcitation is alleviated and the photoprotective action of carotenoids per chlorophyll molecule is enhanced.

Leaf hairs of Olea europeae protect underlying tissues against ultraviolet-B radiation damage

Abstract The photochemical efficiency of photosystem II, as measured by chlorophyll fluorescence induction, was not affected in de-haired olive leaves kept in the dark or intact leaves irradiated with a moderate (3.75 W m −2 ) ultraviolet-B (UV-B) intensity. In de-haired, UV-B-irradiated leaves, however, the ratio of variable to maximum ( F v / F m ) chlorophyll fluorescence declined significantly and irreversibly. Reduction in F v / F m was associated with an increase in instantaneous ( F 0 ) and a decrease in maximum ( F m ) fluorescence, indicating perturbation by the UV-B exposure of more than one photosynthetic site. Extensive epidermal browning in dehaired, UV-B irradiated leaves was also observed, indicating possible damage to cell membranes. The results strengthen the hypothesis that leaf hairs protect the underlying tissues against UV-B radiation damage.

Effects of UV-B radiation on cuticle thickness and nutritional value of leaves in two mediterranean evergreen sclerophylls

Summary Seedlings of Laurus nobilis L. and Ceratonia siliqua L., grown for 1 year in the field under ambient radiation conditions were subsequendy allowed to produce their new spring growth in a glasshouse in the absence or presence of UV-B radiation. The UV-B radiation dose was biologically equivalent to that which would have being received by the plants in their natural environment. Plant height, number of leaves, total and mean leaf area, the biomass allocated to both above and below ground parts, photochemical efficiency of PS II and chlorophyll content were not affected. However, UV-B radiation resulted in slighdy thicker leaves in L. nobilis and considerably thicker cuticles in both plants. UV-B absorbing compounds were increased only in L. nobilis. In addition, the leaf nutritional quality of this plant was considerably reduced by UV-B radiation, since phenolics and tannins were increased but nitrogen was decreased. No such changes were observed in C. siliqua . We may conclude that L. nobilis and C. siliqua are not only resistant against UV-B radiation damage, but the observed changes may benefit the plants through their anti-transpirant and anti-herbivore functions.

The use of the portable, non-destructive, spad-502 (minolta) chlorophyll meter with leaves of varying trichome density and anthocyanin content

Summary The credibility of the portable, non-destructive SPAD (Minolta) chlorophyll meter was tested in the particular cases where the optical properties of a leaf change, irrespectively of its chlorophyll content. In greening Eukalyptus sp., Rosa sp. and Ricinus communis leaves, whose anthocyanin content declines dramatically with leaf age, the SPAD values versus actual chlorophyll concentration curves were not distorted by the different levels of anthocyanins. In Teucrium sp., Populus sp. and Buddleia sp., whose abaxial surfaces are highly reflective due to a dense indumentum, but their adaxial surfaces are glabrous, the SPAD values obtained from both surfaces were the same. It is concluded that the use of the chlorophyll meter can be extended to both highly pubescent and red leaves.

Seasonal and diurnal gas exchange characteristics and water relations of the drought semi-deciduous shrub Phlomis fruticosa L. under Mediterranean field conditions

Summary Diurnal and seasonal fluctuations in gas exchange and water relations were examined in the field during a 21 months period in the drought semi-deciduous shrub Phlomis fruticosa L., in an attempt to reveal environmental factors that may limit its growth and distribution. P.fruticosa is an eastern Mediterranean malacophyllous plant dominating the so called “phryganic” ecosystem in Greece (synonym to coastal sage in California and garrigue in France). Maximum photosynthetic rates (c.a. 30 μmol CO 2 m −2 s −1 ) were observed during the spring, with a secondary maximum at late autumn, a deep minimum during summer and early autumn and a secondary minimum during mid-winter. The deep minimum (c.a. 2 − 3 μmol CO 2 m −2 S −1 ) coincided with minimal shoot water potentials and a well-developed stomatal regulation of excessive water loss and CO 2 assimilation. Photosynthesis, however, was additionally limited during the dry period by a substantial drop in photosynthetic capacity, as judged by CO 2 exchange measurements at saturated (5 %) CO 2 . The secondary minimum in photosynthesis during mid-winter, occurring at a period of relatively high pre-dawn shoot water potentials and low atmospheric evaporative demand, was partly related to low temperatures in a rather indirect way. Indeed, diurnal changes in shoot water potential revealed a characteristic delay in the replacement of evaporated water at low temperatures, leading to leaf water shortage and stomatal closure, even during the wet period. We may conclude, therefore, that growth in P.fruticosa is restricted by low water potentials during the warm dry period and, possibly, by increased hydraulic resistance when the winter temperatures fall to relatively low values.

Perturbations of the normal UV-B radiation environment alter leaf growth rates in Phlomis fruticosa L. seedlings

Abstract Phlomis fruticosa L. seedlings were raised in the field under ambient UV-B radiation for 18 months. On 15 March 1994, just before the spring growth burst), seedlings were transferred in a glasshouse under three regimes of artificial UV-B radiation, i.e. 0.06, 5.22 and 8.55 kJ m −2 biologically effective daily doses. The last two doses correspond to UV-B radiation reaching the Patras area (38.3° N, 29.1° E) under clear sky and with normal column ozone thickness at mid-April and mid-July. Leaf demography was monitored for 96 days. Growth rates new leaf formation and total leaf area increase) were significantly higher under 5.22 kJ m −2 day −1 , intermediate under 8.55 kJ m −2 day −1 and severely suppressed in the absence of UV-B radiation. In addition, shedding of older leaves after mid-May, was significantly more intense in the absence of UV-B radiation. The observed differences could not be attributed to UV-B radiation effects on CO 2 assimilation, photosynthetic electron flow or leaf water relations, since net photosynthetic rates, photochemical efficiency of photosystem II and leaf relative water contents showed no significant differences between treatments. Total dry matter measured at plant harvest was highest under 5.22 kJ m −2 day −1 and lowest in the absence of UV-B radiation. The results indicate that natural UV-B radiation may be an important factor in normal leaf development of Phlomis fruticosa .

Reduction of ambient UV-B radiation does not affect growth but may change the flowering pattern of Rosmarinus officinalis L.

The effects of sub-ambient levels of UV-B radiation on the shrub Rosmarinus officinalis L. were investigated in a field filtration experiment in which the ambient UV-B was manipulated by a combination of UV-B transmitting and UV-B absorbing filters. As a result, the plants were receiving near-ambient or drastically reduced UV-B radiation doses. Drastic reduction of UV-B radiation had no effect on mean, total and maximum stem length, number of stems per plant, dry mass of leaves, stems and roots and leaf nitrogen and phenolic contents. However, flowering was more pronounced under reduced UV-B radiation during the winter period which coincides with ascending ambient UV-B radiation. In contrast, during autumn and early winter, a period which coincides with descending ambient UV-B radiation, flowering was unaffected by reduced UV-B radiation. We can conclude that natural UV-B radiation does not affect growth of Rosmarinus officinalis, but its reduction could influence the flowering pattern of the species.