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.

Summary• Reflectance indices are frequently used for the nondestructive assessment of leaf chemistry, especially pigment content, in environmental or developmental studies. Since reflectance spectra are influenced by trichome density, and trichome density displays a considerable phenotypic plasticity, we asked whether this structural parameter could be a source of variation in the values of the most commonly used indices.• Trichome density was manipulated in detached leaves of three species having either peltate ( Olea europaea and Elaeagnus angustifolius ) or tubular ( Populus alba ) trichomes by successive removal of hairs. After each dehairing step, trichome density was determined by light or scanning electron microscopy and reflectance spectra were obtained with a diode-array spectrometer.• Although species-specific differences were evident, most of the indices were considerably affected even at low trichome densities. In general, the less-affected indices were those using wavebands within the visible spectral region. The index that could be safely used even at very high hair densities in all species was the red edge index ( λ RE ) for chlorophyll.• The results indicate that changes in reflectance indices should be interpreted cautiously when concurrent changes in trichome density are suspected. In this case, the red edge for chlorophyll content may be the index of choice.

Diurnal and seasonal fluctuations in water potential (Ψ), stomatal conductance (g s ), transpiration rate (E), and net photosynthetic rate (P N ) 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, P N 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 (F v /F m ) 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 CO 2 per m 2 during its growth period (May to October) and up to 1.8 times more on an annual basis.

Seedlings of Nerium oleander L. were grown in the field under ambient or ambient plus supplemental UV-B radiation (simulating a 15% ozone depletion over Patras), and received natural precipitation or additional irrigation during the summer dry period. Two experiments, in 1994 and 1995, were performed. Natural precipitation during summer in 1994 was near the mean for the area but summer 1995 was exceptionally wet. During 1994, supplemental UV-B radiation reduced by ca 25% the above-ground biomass under natural summer precipitation but had no effect on the plants receiving additional irrigation. During the wet 1995, UV-B radiation caused a 5%, non-significant reduction in biomass under natural summer precipitation, and a 16%, non-significant increase under ample water availability. No effects of UV-B radiation were observed on PSII photochemical efficiency (assessed from in vivo Chl fluorescence), photosynthetic pigments, UV-B absorbing compounds, leaf relative water content, leaf specific mass or leaf thickness. However, cuticle thickness and cuticle mass (on a unit area basis) were increased by supplemental UV-B radiation, on both leaf surfaces, independent of water availability. We conclude that UV-B radiation effects on growth depend on the extent of summer precipitation. The increase in cuticle thickness may counteract UV-B radiation damage, through attenuation of UV-B radiation and/or reduction of cuticular transpiration during the dry summer.

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