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Phytohormones are pivotal signaling compounds in higher plants, in which they exert their roles intracellularly, but are also released for cell-to-cell communication. In unicellular organisms, extracellularly released phytohormones can be involved in chemical crosstalk with other organisms. However, compared to higher plants, hardly any knowledge is available on the roles of phytohormones in green algae. Here, we studied phytohormone composition and extracellular release in aero-terrestrial Trebouxiophyceae. We investigated (a) which phytohormones are produced and if they are released extracellularly, and if extracellular phytohormone levels are (b) affected by environmental stimuli, and (c) differ between lichen-forming and non-lichen-forming species. Three free-living microalgae (Apatococcus lobatus, Chloroidium ellipsoideum, and Myrmecia bisecta) and three lichen-forming microalgae (Asterochloris glomerata, Trebouxia decolorans, and Trebouxia sp.) were studied. Algae were grown on solid media and the following cellular phytohormones were identified by ultra-high-performance liquid chromatography coupled with tandem mass spectrometry (UHPLC-MS/MS): indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), abscisic acid (ABA), gibberellin A4 (GA4 ), and zeatin (ZT). Furthermore, IAA, IBA, ABA, jasmonic acid (JA), gibberellin A3 (GA3 ), and GA4 were found to be released extracellularly. IAA and ABA were released by all six species, and IAA was the most concentrated. Phytohormone release was affected by light and water availability, especially IAA in A. glomerata, Trebouxia sp., and C. ellipsoideum. No clear patterns were observed between lichen-forming and non-lichen-forming species. The results are envisaged to contribute valuable baseline information for further studies into the roles of phytohormones in microalgae.

Summary Urban trees help towns to cope with climate warming by cooling both air and surfaces. The challenges imposed by the urban environment, with special reference to low water availability due to the presence of extensive pavements, result in high rates of mortality of street trees, that can be increased by climatic extremes. We investigated the water relations and xylem hydraulic safety/efficiency of Quercus ilex trees growing at urban sites with different percentages of surrounding impervious pavements. Seasonal changes of plant water potential and gas exchange, vulnerability to cavitation and embolism level, and morpho-anatomical traits were measured. We found patterns of increasing water stress and vulnerability to drought at increasing percentages of impervious pavement cover, with a consequent reduction in gas exchange rates, decreased safety margins toward embolism development, and increased vulnerability to cavitation, suggesting the occurrence of stress-induced hydraulic deterioration. The amount of impermeable surface and chronic exposure to water stress influence the site-specific risk of drought-induced dieback of urban trees under extreme drought. Besides providing directions for management of green spaces in towns, our data suggest that xylem hydraulics is key to a full understanding of the responses of urban trees to global change.

SUMMARY The photosynthetic activity of calcicolous endo- and epilithic lichens from the Trieste Karst area (NE Italy) was investigated. The data consist of: (a) gas exchange rates of Acrocordia conoidea, Petractis clausa, Rinodina immersa (endolithic), and Aspicilia calcarea (epilithic), recorded at different combinations of temperature, water content and radiation flux values, (b) chlorophyll content values of nine species, and (c)δ13C values measured in 24 lichen species with different growth-forms. The results show that: (1) the maximum photosynthetic rates of endolithic lichens are rather small, ranging between 0·2 and 1·5μmol CO2 m-2 h-1 at optimal conditions; (2) the relation between thallus water content and photosynthesis differs from that of foliose and fruticose lichens; the optimum water content of endolithic lichens is particularly small, when expressed in g H2O cm-2; (3) the algae of endolithic lichens, which belong to different systematic groups, are light-saturated at a small radiation flux; (4) chlorophyll contents of endolithic lichens are rather large, being similar to those of some parmelioid lichens. The ecological implications of the endolithic growth-form are briefly discussed. Endolithic lichens should be regarded as slow-growing, stress-tolerant organisms, which are rather similar in their physiology to epilithic crustose lichens; they have a high resistance to CO2 diffusion, saturation being reached only at a very large CO2 concentration.

When preparing the Lobaria treatment for the Flora of the Iberian Peninsula (Burgaz & Martinez, in press), a problem arose concerning the typification of Lobaria amplissima (Scop.) Forssell, a well-known lichen of humid, nemoral habitats of Europe, northern Africa and eastern Asia. As already done by Yoshimura (1971, 1999) and Laundon (1984) for other Lobaria species described in pre-Linnean times, it is necessary to select a lectotype because Scopoli (1772) did not designate any type material among his collections ("in fago et abiete, circa Idriam in Weichental"). His herbarium was lost (Saccardo, 1895, 1901), and only two illustrations are available for lectotypification: an unpublished drawing, which is part of a series of colour plates now in W, drawn by Thomas Horman for the first edition of Scopoli's Flora Carniolica (Piltaver, 1997), and the plate n. 46 published by Micheli (1729), cited by Scopoli in the protologue of Lichen amplissimus. Because the validity of selecting as lectotype an unpublished drawing might be questioned, lectotypification is restricted in this case to Micheli's plate, which was judged "optime" by the same Scopoli (1772: 386), and was based exclusively on Lichen Pulmonarius, arboribus adnascens, maximus, inferne obscurus, desuper e glauco-cinereus, receptaculis florum rubris, feminibus nigricantibus (Micheli