G.K. Psaras

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

Exposed red (anthocyanic) leaves of Quercus coccifera display shade characteristics

Young leaves in some plants are transiently red due to the presence of anthocyanins, which disappear upon maturation. We investigated the hypothesis that light attenuation by anthocyanins may lead to a shade acclimation of the photosynthetic machinery in red leaves. We took advantage of the intra-species variation in anthocyanin levels of young, exposed leaves of Quercus coccifera. Thus, photosynthetic and photoprotective characteristics were compared in young green and red leaves of the same age, sampled from the corresponding phenotypes occupying the same habitat. Red leaves displayed several shade attributes like thinner laminae, lower Chl a/b ratios and lower levels of the xanthophyll cycle components and β-carotene. In addition, although both leaf kinds had the same area based levels of chlorophylls, these pigments were excluded from the sub-epidermic anthocyanic cell layers, leading to a further reduction of effective mesophyll thickness and an increase in chlorophyll density. Accordingly, red leaves had higher absolute chlorophyll fluorescence signals. In spite of these apparent shade characters, red leaves were less prone to photoinhibition under mild laboratory conditions and displayed slightly but significantly higher PS II photochemical efficiencies at pre-dawn in the field. No differences in all the above measured parameters were found in mature green leaves of the two phenotypes. The results confirm the light acclimation hypothesis and are also compatible with a photoprotective function of anthocyanins.

Lettuce endosperm structural changes during germination under different light, temperature, and hydration conditions

The endosperm structure of dark-germinating Grand Rapids lettuce (Lactuca sativa L.) achenes was examined by light microscopy under different germination conditions. Achenes imbibed in darkness at 25 C show significant structural modifications in a restricted area of the endosperm, opposite the radicle tip, before radicle emergence. These preemergence changes, similar to those observed in light-requiring Grand Rapids lettuce achenes after phytochrome activation, are mainly characterized by the mobilization of storage materials and the vacuolation of the cytoplasm Achenes imbibed under conditions not permitting the completion of germination (continuous far-red light, high temperature, and appropriate osmotic solution) do not show any structural change in their endosperm cells. We conclude that cytoplasmic modification of the endosperm cells in the micropylar region is necessary for the completion of germination in lettuce achenes, in addition to the necessity for decrease in embryo water potential.

Red-Light-Induced Endosperm Preparation for Radicle Protrusion of Lettuce Embryos

Light microscopic examination of red-light-treated Lactuca sativa achenes at various stages of imbibition revealed that, prior to visible germination (radicle protrusion), the endosperm cells of the micropylar end undergo rapid characteristic structural changes. The cytoplasm becomes highly vacuolated, and reserve materials are mobilized. Generally, these cells swell, while the cell walls remain PAS positive. In dark-, far-red, and red + far-red-light-treated achenes, however, the endosperm remains unchanged. The structural changes may be a prerequisite for endosperm rupture and radicle protrusion.

Phytochrome-controlled fern-spore germination: kinetics of Pfr action

Summary Spore germination is induced by well-defined periods of Pfr, established and terminated by saturating red and far-red pulses, respectively. Pf, has to be present at least 5 to 6 hours in order to induce the most sensitive (or responsive) spores, but for saturation Pfr has to act between 12 and 24 h. As derived from these «Pfr period-response curves-, Pf, is most effective between 24 and 72 h after sowing. A subthreshold Pfr pretreatment (i.e., 4 h) increases the effect of a subsequent Pf, period in a purely additive way, the two Pfr periods together have almost the same effect as an uninterrupted Pfr period of equal length, even if they are separated by an interval of more than 40 h. Thus, the internal signal produced by the action of a subthreshold Pf, period is stored and is remarkably stable. The subthreshold Pf, period was allowed to act at various times after sowing and its effect on a later Pfr period was tested. From the results it is concluded that competence of the spore to Pf, develops between 15 and 20 h after sowing, i.e., the «preirradiation effect,, of red light is no longer fully reversible by far-red. This is much earlier than the -apparent coupling point. as derived from loss of reversibility, measuring the terminal response only.

The Annual Rhythm of Cambial Activity in Two Woody Species of the Greek “Maquis”

Summary The annual rhythm of cambial activity is compared in Arbutus unedo and Quercus coccifera , two typical shrubs of the maquis in the Mediterranean-type ecosystem of Greece. The growth activity of the species is more or less focused on the favourable periods. The behaviour of Arbutus unedo revealed stronger correlation with moisture conditions, while that of Quercus coccifera seems to be more restricted in time and less controlled by moisture availability. These results indicate that both species, although growing together have different adaptive strategies to the same climatic conditions.

Chloroplast Arrangement along Intercellular Spaces in the Leaves of a Mediterranean Subshrub

Summary A close relationship between intracellular chloroplast distribution and the pattern of intercellular spaces of the palisade mesophyll of the drought deciduous subshrub Ballota acetabulosa , a mediterranean plant common in phryganic formations of Greece, is described. As seen in tangential sections, chloroplasts of palisade cells are arranged along intercellular canals. This phenomenon is discussed in relation to the CO 2 supply of chloroplasts. Is is assumed that the reported spatial distribution of chloroplasts increases the photosynthetic efficiency of the tissue and can be considered as an adaptive character.

A contribution to the root study of the evergreen sclerophylls anatomy of the primary root of Quercus coccifera L.

Summary The structure of the primary root of Quercus coccifera , an evergreen sclerophyllous species common in maquis formations in Greece, has been studied by light and electron microscopy with special reference to the differentiation of endodermis. Histochemical identification of phenolic compounds in root tissues has also been carried out. Polyphenolic compound accumulation and extremely thin radial walls of endodermal cells observed in these roots are proposed to be adaptations contributing to a more effective water uptake and transport by the plant.

Structural Alterations in Isolated Endosperms of Lactuca sativa L. Achenes

Endosperm tissue of lettuce (Lactuca sativa L.) achenes germinating in the dark was isolated from the embryo after appropriate time periods of imbibition in the dark and was incubated in water or in gibberellic acid solution in the dark. It was found that endosperms which were dissected from achenes during the first to the eighth hours of imbibition do not show any structural alteration even after 24 h of incubation either in water or in GA(3) while endosperms excised after 9 hours of imbibition show moderate modifications of micropylar area cells if incubated in water for 24 h. Endosperms isolated after 8 hours of imbibition show great structural alterations of their micropylar cells if incubated for the same period in gibberellic acid solution. These results indicate (a) that the presence of the embryo is necessary for a long period for the manifestation of structural modifications of the isolated endosperm; and (b) that the two areas of lettuce endosperm (i.e. micropylar vs lateral) are different.

SAN 9789: A Seed Dormancy Releasing Chemical

Summary The ability of the herbicide SAN 9789 to release primary as well as different forms of secondary dormancy in lettuce seeds ( Lactuca sativa cv. Grand Rapids) was examined. Primary-, thermo-, osmo-, and far-red-dormant seeds treated with 28 μg/ml SAN 9789 in the dark at 25°C, germinated to 88, 61, 52 and 31 % respectively. The promotion of dark germination in primary dormant seeds by SAN after increased periods of pre-imbibition in water was gradually decreased and after 4 days disappeared. Brief red light (R), given either before or after 4 days of incubation with SAN resulted in full induction of germination in all these dormant seed types. The dark germination time courses of the different dormant seed categories immersed in 28 μg/ml SAN at 25°C, follows the final germination level pattern: The germination of the primary dormant seeds is faster and that of the far-red-dormant seeds slower. In thermoand osmo-dormant seeds, germination rates have intermediate values. SAN-promoted germination in primary dormant seeds proceeds through structural modifications of the endosperm tissue facing the radicle tip.