Summary• Woody species in Mediterranean ecosystems form intra-annual density fluctuations (IADFs) in tree rings in response to changes in environmental conditions, especially water availability.• Dendrochronology, quantitative wood anatomy and high-resolution isotopic analysis (using a laser ablation technique) were used to characterize IADFs in Arbutus unedo shrubs grown on two sites with different water availability on the island of Elba (Italy).• Our findings show that IADF characterization can provide information about the relationship between environmental factors and tree growth at the seasonal level. At the more xeric site, IADFs mainly located in the early and middle parts of the annual ring, showed a decrease in vessel size and an increase in d 13 C as a result of drought deficit. Opposite trends were found at the more mesic site, with IADFs located at the end of the ring and associated with a lower d 13 C. Moreover, at the first site, IADFs are induced by drought deficit, while at the second site IADFs are linked with the regrowth in the last part of the growing season triggered by favourable wet conditions.• This combined approach is a promising way for dating problematic wood samples and interpreting the phenomena that trigger the formation of IADFs in the Mediterranean environment.

Plant resistance to drought relies on adaptive strategies based on the timing of phenophases and on the presence of structural traits mainly related to:(1) increase of water uptake and storage; (2) reduction of water loss during dry periods; and (3) mechanical reinforcement of tissues to prevent wilting that may lead to irreversible collapse and damage of cells. In this chapter, after a few evolutionary considerations, we focus on the adaptive value of the main phenological, morphological and anatomical properties. We report the common existence of such traits in both desert and semiarid environments, especially in Mediterranean-type ecosystems. All morpho-anatomical characteristics are interpreted considering that plant resistance to drought also depends on the ability to respond to multiple stressors. We conclude that various combinations of anatomical features can contribute in different degrees to the adaptive capacity of plants to drought.

Erica arborea (L) is a widespread Mediterranean species, able to cope with water stress and colonize semiarid environments. The eco-physiological plasticity of this species was evaluated by studying plants growing at two sites with different soil moistures on the island of Elba (Italy), through dendrochronological, wood-anatomical analyses and stable isotopes measurements. Intra-annual density fluctuations (IADFs) were abundant in tree rings, and were identified as the key parameter to understand site-specific plant responses to water stress. Our findings showed that the formation of IADFs is mainly related to the high temperature, precipitation patterns and probably to soil water availability, which differs at the selected study sites. The recorded increase in the 13 C-derived intrinsic water use efficiency at the IADFs level was linked to reduced water loss rather than to increasing C assimilation. The variation in vessel size and the different absolute values of δ 18O among trees growing at the two study sites underlined possible differences in stomatal control of water loss and possible differences in sources of water uptake. This approach not only helped monitor seasonal environmental differences through tree-ring width, but also added valuable information on E. arborea responses to drought and their ecological implications for Mediterranean vegetation dynamics.

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We investigated the variation of wood anatomical characteristics and carbon isotopic composition of tree rings showing intra-annual density fluctuations (IADFs) in plants of Pinus pinaster Ait. growing at a coastal plantation in Tuscany (Italy). IADFs are regions of the tree ring where wood density changes abruptly due to a sudden change of environmental conditions, particularly of water availability. Dendrochronological analyses allowed dating of the rings and four regions were considered in each tree ring: earlywood, IADF, late-earlywood and latewood. Although IADF commonly has been classified as latewood-like tissue in the literature, we found differences in anatomical characteristics and carbon isotopic composition between tracheids of the two regions. The lumen area of tracheids in IADF was significantly larger than in latewood, while still smaller than in earlywood and late-earlywood. Latewood and IADF had a greater proportion of narrow tracheids than both earlywood and late-earlywood. Although latewood and IADF were characterized by tracheids with lumina lengthened in the tangential direction, while earlywood tracheids were elongated in the radial direction, some differences were found also between latewood and IADF. Moreover, IADF tracheids had a higher 13C/12C ratio than any other region and showed isotopic values significantly different from the latewood. The quantification of anatomical features of tracheids within rings was useful to discriminate between latewood and IADFs, as well as helpful for the identification of tree-ring boundaries. The overall interpretation of dendrochronological, wood anatomical and carbon isotopic data seems to be a promising approach for the dating and the ecological interpretation of tree rings in Mediterranean ecosystems and for gaining climatic information with intra-annual resolution.

Woody species populating the major Mediterranean ecosystems in the world are characterized by different levels of adaptation to the seasonal Mediterranean climate conditions. Many species of these ecosystems show wood features that allow high efficiency of transport when water is available, while maintaining hydraulic safety during drought periods. This study focuses on the anatomy of juvenile and mature wood of some species representative of continuous sequences of Mediterranean vegetation formations according to gradients of water availability, from xeric to relatively mesic: Cistus monspeliensis L., Rhamnus alaternus L., Myrtus communis L., Pistacia lentiscus L., Olea europaea L., Quercus ilex L., Fraxinus ornus L. and Ostrya carpinifolia L. Twigwood collected in Southern Italy was anatomically compared with the stemwood of the same species represented in the reference slide collection of the National Herbarium of the Netherlands (Lw). The ''hydraulic distance'' between the wood of main stems and twigs was estimated on the basis of suites of anatomical features related to water efficiency/safety. Although some attributes (i.e. porosity and type of imperforate tracheary elements) were similar in young twigs and older rings, other traits (i.e. vessel frequency and size) evidenced the different hydraulic properties of twig and stemwood. The difference between juvenile and mature structures was large in the species of the mesic end of the gradient while it was relatively small in those more xeric. This tendency is in agreement with the habit gradient from medium-sized trees to small evergreen/drought deciduous shrubs according to decreasing water availability in Mediterranean vegetation types.

Histological staining methods commonly used for detecting cellulose and lignin in cell walls were combined with epifluorescence microscopy to visualize differences in lignification between and within cellular elements. We tested our approach on sections of one-year-old branches of Fraxinus ornus L., Myrtus communis L., Olea europaea L., Pistacia lentiscus L. and Rhamnus alaternus L., containing both normal and tension wood. Sections were subjected to various staining techniques, viz. safranin O, safranin O/fast green FCF, and alcoholic solutions of safranin O/astra blue, according to the commonly accepted protocols. Stained and unstained sections were compared using both light and epifluorescence microscopy. Safranin O with or without counterstaining hid the strong fluorescence of vessel walls, cell corners and middle lamellae allowing the secondary wall fibers to fluoresce more clearly. Epifluorescence microscopy applied to stained sections showed more cell wall details than autofluorescence of unstained sections or white light microscopy of counterstained sections. This simple approach proved reliable and valuable for detecting differences in lignification in thick sections without the need for costly equipment.

Root tropisms are important responses of plants, allowing them to adapt their growth direction. Research on plant tropisms is indispensable for future space programs that envisage plant-based life support systems for long-term missions and planet colonization. Root tropisms encompass responses toward or away from different environmental stimuli, with an underexplored level of mechanistic divergence. Research into signaling events that coordinate tropistic responses is complicated by the consistent coincidence of various environmental stimuli, often interacting via shared signaling mechanisms. On Earth the major determinant of root growth direction is the gravitational vector, acting through gravitropism and overruling most other tropistic responses to environmental stimuli. Critical advancements in the understanding of root tropisms have been achieved nullifying the gravitropic dominance with experiments performed in the microgravity environment. In this review, we summarize current knowledge on root tropisms to different environmental stimuli. We highlight that the term tropism must be used with care, because it can be easily confused with a change in root growth direction due to asymmetrical damage to the root, as can occur in apparent chemotropism, electrotropism, and magnetotropism. Clearly, the use of Arabidopsis thaliana as a model for tropism research contributed much to our understanding of the underlying regulatory processes and signaling events. However, pronounced differences in tropisms exist among species, and we argue that these should be further investigated to get a more comprehensive view of the signaling pathways and sensors. Finally, we point out that the Cholodny-Went theory of asymmetric auxin distribution remains to be the central and unifying tropistic mechanism after 100 years. Nevertheless, it becomes increasingly clear that the theory is not applicable to all root tropistic responses, and we propose further research to unravel commonalities and differences in the molecular and physiological processes orchestrating root tropisms.