The ability of abaxial and adaxial epidermis of sun and shade leaves to attenuate UV-A and UV-B radiation in relation to the UV absorbing capacity of the whole leaf methanolic extracts

The UV-absorbing capacity (measured as A(310) cm(-2) and A(365) cm(-2) or AUVR cm(-2)) of the shade leaves of four representative evergreen sclerophylls of the Mediterranean region (Quercus coccifera, Q. ilex, Arbutus andrachne and A. unedo) was considerably lower than the corresponding one of sun leaves of the same species. However, fibre optic microprobe measurements showed that adaxial as well as abaxial epidermis of shade leaves of all examined plants, except abaxial epidermis of A. andrachne, were almost as effective as the corresponding ones of the sun leaves in screening out most of the incident UV-B radiation. There is probably a threshold, under which the concentration of the UV-B absorbing compounds in the protective tissues is not furthermore reduced, in spite of the low levels of the stress factor (UV-B radiation) in the environment. On the other hand, the ability of both abaxial and adaxial epidermis to attenuate UV-A radiation, except of adaxial leaf epidermis of Quercus species, depended on the UV absorbing capacity of the whole-leaf extracts, with different correlation patterns between the two Quercus species and the two Arbutus species. This could be explained by the fact that shade leaves showed not only quantitative, but also qualitative differences (higher A(310)/A(365) ratio) in the absorbance of their methanolic extracts compared to these of sun leaves. The results of the present study showed that we should not always correlate the depth of penetration of UV radiation into sun and shade leaves according to the corresponding UV absorbing capacity of the whole leaf methanolic extracts, without taking into account all the anatomical, developmental and biochemical (such as different composition and distribution of the UV-absorbing compounds among the different protective tissues) peculiarities of the leaves of each species

Seasonal fluctuations in the concentration of UV-absorbing compounds in the leaves of some Mediterranean plants under field conditions

Leaves of 14 representative Mediterranean plant species were collected on a monthly basis and assayed for UV-absorbing compounds concentration, either on an area or a dry mass basis, from 1995 to 1997, Strong seasonal fluctuations were observed in eight species tall evergreens, two phrygana, one deciduous, one summer perennial and one winter perennial). Two different patterns of changing concentrations of UV-absorbing compounds were observed. In the first, concentration of these compounds was higher in young developing leaves and concentration declined during maturation, whereas in other plants, the opposite trend was observed. These differences could be attributed to the particular leaf surface morphology of each plant. The observed seasonal fluctuations of UV-absorbing compounds seem to be more correlated to developmental processes, than to seasonal fluctuations of the naturally occurring UV-B radiation. Most of the winter perennials did not show strong fluctuations during the period of development. The concentration of these compounds varied not only on a seasonal basis among the examined plants, but between different life forms as well: during winter, examination of the leaves of 13 species showed that evergreen sclerophylls and phrygana had at least two-fold higher concentration of UV-B-absorbing compounds on a leaf area basis than winter perennials. In addition, during the same season and irrespective of life form and species, the absorbance at 300 nm per unit of mature leaf area followed an asymptotic exponential decrease when specific leaf area increased. The UV-B radiation screening capacity of the leaves of these plants is discussed in relation to each adaptive strategy

Structural analysis of Castanea sativa Mill. leaves from different regions in the tree top

The aim of this work was to perform the histological characterization of the C. sativa leaves of three Portuguese cultivars to establish comparison among the leaves of the different quadrants in accord and with the cardinal points of the tree top and among different cultivars of this species, using light microscopy (OM) and scanning electron microscopy (SEM). Measurements were also carried out for the leaf tissue thickness, stomatal density, leaf area in the four tree top quadrants. The leaves turned to the North had lesser thickness of mesophyll mainly due to lower amount of palisade parenchyma. The stomatal density was significantly lower in these leaves, unlike the leaf area that has the highest expression