George K. Psaras

Chlorophyll distribution pattern in inner stem tissues: evidence from epifluorescence microscopy and reflectance measurements in 20 woody species

The occurrence of functional chloroplasts in internal stem tissues and their distribution profiles in 20 woody species have been investigated. Chloroplasts were identified from the red chlorophyll auto-fluorescence using epi-fluorescence microscopy. Chloroplasts were detected in the cortex of all species examined, in the xylem rays of 19 and in the perimedullar and the pith cells of 16 out of the 20 investigated species. Chloroplast containing cell clusters in the pith were identified in some species. In addition, we report on the semi-quantitative distribution of chlorophylls in various internal stem tissues. Chlorophyll level was estimated by reflectance measurements at specific wave bands. Although decreasing chlorophyll gradients from cortex to pith were observed in half of the species, chlorophyll distribution in the remaining species was irregular with occasionally high levels in the pith. According to our data, chlorophyll occurrence in stem internal tissues is quite widespread, even in the light remote, deeply shaded central compartments like pith, provided that corresponding cells are viable. The species-specific tissue distribution of chlorophyll levels may be used to select suitable plants to investigate further this neglected area of photosynthesis research.

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.

Seasonal photosynthetic changes in the green-stemmed Mediterranean shrub Calicotome villosa: a comparison with leaves

Some photosynthetic attributes of leaves and stems were seasonally followed in the small-leaved, summer-deciduous, green-stemmed Mediterranean shrub Calicotome villosa. Both leaves and stems displayed similar photon energy-saturated photosystem 2 (PS2) efficiencies with a minimum during winter. A second minimum in stems during the leafless summer period could be ascribed to sustained photoinhibition. Yet, stems were slightly inferior in photon capture, resulting partly from lower chlorophyll (Chl) contents and partly from higher reflectance due to pubescence. As a result, photon energy-saturated linear electron transport rates were slightly higher in leaves. However, when the total leaf and stem areas were taken into account, this superiority was abolished during autumn and winter and more than overturned during spring. Given that during summer the stems were the only photosynthetic organs, the yearly photosynthetic contribution of stems was much higher. Chl contents in stems displayed a transient and considerable summer drop, accompanied by an increase in the carotenoid to Chl ratio, indicating a photo-protective adaptation to summer drought through a decrease of photo-selective capacity, typical for leaves of many Mediterranean plants.

Erinea formation on Quercus ilex leaves: anatomical, physiological and chemical responses of leaf trichomes against mite attack.

Structures on the surfaces of leaves, such as dense layers of non-glandular trichomes, strongly affect phylloplane mite activities. On the other hand the feeding of eriophyoid mites on leaf surfaces can cause hyperplasia of leaf trichomes (erinea formation). In many cases the hyperplasia is accompanied by the accumulation of pigments within trichome cells, causing an impressive red-brown colouration of the erineum. There is no information, however, on the structure of these pigments as well as on the chemical alterations in the phenolic content of plant trichomes in response to mite attack. Erinea formation on the abaxial surface of Quercus ilex leaves upon Aceria ilicis (Acari: Eriophyoidea) attack provides an excellent model on this topic. Differences in the structure and chemical composition of isolated trichomes derived either from healthy (normal trichomes) or mite attacked (hypertrophic trichomes) leaves were examined. Carbon investment was comparable between the two different trichome types, but the cell walls of the hypertrophic trichomes appeared thinner and did not contain microcrystalline cellulose. Observations under the fluorescence microscope showed that the emitted fluorescence was different between the two trichome types, indicating a different composition in fluorescencing phenolic compounds. The chemical analyses confirmed that hypertrophic trichomes contained higher concentrations of the feeding deterrents proanthocyanidin B3 and catechin, as well as of quercetin-3-O-glucoside, but lower concentrations of acylated flavonoid glycosides, than the normal ones. The results showed that the structural and functional changes in leaf trichomes upon mite attack may be an effort of the leaf to compensate the damage caused by the pest.

Seed morphology in relation to infrageneric classification of Consolida (DC.) Gray (Ranunculaceae)

Summary The surface morphology of intact seeds of 37 taxa (species and subspecies) belonging to 6 different sections of the genus Consolida (Ranunculaceae) was investigated using scanning electron and light microscopy. The material studied covers the entire distribution range of the genus and represents c. 70% of the known Consolida taxa. Unlike certain species of the related genera Delphinium and Aconitum, all Consolida species have subpyramidal to subcylindrical seeds with pronounced transverse subparallel lamellae and testa cells covered with minute papillae. The crateriform hilum cavity is surrounded by well-formed or rudimentary lamellae, scales of various sizes or fringed appendages. The seeds of sect. Involutae, considered by some to represent the independent genus Aconitella, are subpyramidal and covered with low lamellae, which are usually expanded at the seed edges and proximal parts. Similar patterns are found in other Consolida species and splitting off the genus Aconitella is not supported, at least on the basis of seed micromorphology. In sect. Brevipedunculatae seeds are mostly subpyramidal and covered with scales of various sizes. Two informal groups are distinguished: the long-spurred allies of C. aucheri present a relatively shallow hilum furnished with dense, fringed appendages, while the hilum zone of the second, short-spurred group comprising the relatives of C. raveyi is surrounded by entire lamellae. The members of sect. Consolida also appear heterogeneous. Two subspecies of C. regalis (subsp. divaricata and subsp. paniculata) and C. tenuissima are unique on account of their constantly long and almost isolated scales. In contrast, the seeds of sect. Macrocarpae are uniform and do not permit distinctions among taxa. In sect. Longibracteolatae the seeds differ from other sections in having a subcylindrical shape and regular rows of complete, transverse, membranous rings. Based on similarities in seed morphology, it is proposed that the somewhat isolated C. lineolata should be included in sect. Longibracteolatae. After transferring C. barbata to sect. Involutae, sect. Parviflorae remains with a single species with exceptional floral morphology. It is concluded that seed morphology can be used in conjunction with inflorescence architecture, flower morphology and follicle characters as primary tool for infraspecific classification of Consolida. Seed morphology alone gives good support to three of the six sections of the genus.

Distribution of calcium in epidermal cell types of Mediterranean xeromorphic calcicoles

Summary Calcium distribution in the epidermis of nine pubescent xeromorphic Mediterranean calcicoles has been studied using scanning electron microscopy and x-ray microanalysis. It has been revealed that leaf trichomes contain considerable amounts of Ca, but the main sinks of the element seem to be epidermal cells away from stomata. Ca content of stomatal guard cells was always low. Provided that trichomes of Mediterranean xeromorphic species lose their protoplasts very early during their differentiation, it is assumed that they might be involved in the Ca regulatory mechanism only during the very early stages of leaf development. In mature leaves, this role is retained by epidermal cells, away from stomata complexes.

The growth, flower properties and demography of Anthemis arvensis exposed to enhanced UV-B radiation

The winter annual species Anthemis arvensis L. (Asteraceae) was grown for 3.5 months in the field under ambient or ambient plus supplemental UV-B radiation, simulating a 15% ozone depletion over Patras (38.3° N, 29.1° E). Enhanced UV-B radiation had no effect on the methanol extractable UV-B absorbing capacity of leaves, phenological and morphometric parameters of anthesis (flowering time, anthesis duration, head life span, number of heads per plant, number of tubular and ligulate florets per head, area per ligulate floret). Concerning the optical properties of heads, enhanced UV-B radiation had no significant effect on the extractable absorbance of both floret types nor on the spectral reflectance of the tubular florets. However, under UV-B supplementation the white ligulate florets exhibited a slight, statistically significant decrease of reflectance in the visible region of the spectrum. This may be due to structural changes of the floret surface, since microscopic examination under SEM revealed the papillae of the adaxial epidermal cells to be swollen. The above ground dry mass measured at plant harvest was not affected but a significant increase in root biomass (and accordingly in root/shoot ratio) was observed. We conclude that Anthemis arvensis is resistant against UV-B radiation damage. The possible consequences of UV-B induced structural changes on floret epidermis are discussed.

The influence of enhanced UV-B radiation on the surface micromorphology of the winter annual Malcolmia maritima (L.) R. Br. (Brassicaceae)

Summary Light and scanning electron microscopy were used to study some aspects of the surface micromorphology of Malcolmia maritima (L.) R. Br. grown in the field under ambient or ambient plus supplemental UV-B radiation, simulating a 15% ozone depletion over Patras (38° 03′ N, 29° 01′ E). There was no effect of the UV-B radiation on trichome density of either leaves or fruits. However, a strong reduction in the percentage of branched trichomes was evident on fruits and the adaxial leaf surface. Epidermal papillae of petals were considerably swollen leading to a disappearance of inter-papillae spaces.