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Key message Despite been grown under the same climate, oak species are able to correlate with looser, but still identifiable, leaf morphological syndromes, composed by morphological traits with an ecological role in their respective macroclimates. Context Environmental restrictions imposed by climate have been shown to modulate leaf morphology. A reduction of leaf area in hot and dry climates reduces overheating because of a thinner boundary layer. Lobed shapes enhance hydraulic conductivity and faster cooling. Elongated leaves drain more quickly under high precipitation. Trichomes may help to reduce the effects of excessive sun exposure in hot and dry environments. Leaf mass per area (LMA) increases in stressful environments. Aim To assess the influence of global climate and clade on oak leaf syndromes comprising morphological traits with ecological roles. Methods We analyzed seven morphological traits in 141 oak species grown in a botanical garden, characterized into five macroclimates, and explored the partial effects of clade and climate. Results We found significant associations between macroclimate and every morphological trait measured. Temperate species tend to have large, obovate, lobed, malacophyllous leaves. Species occurring in dry habitats usually present small, rounded, pubescent, sclerophyllous leaves. Warm and wet climates induce the development of slender, lanceolate, glabrous leaves with an acuminate apex. Conclusion The functional roles of the different morphological traits are partially confirmed in genus Quercus as a response to the different macroclimates, where different leaf syndromes can be distinguished.

Plant water status can be assessed through leaf spectral reflectance in the near-infrared (NIR), the “water bands”, considering indices that include the reflectance at a band absorbed by water over and another one as reference. We have assessed i/ the accuracy of reflectance at 1450, 1599 and 1940 nm without reference bands and ii/ the potential use of leaf water content index (LWCI) for the estimation of plant water status in holm oak, the main host plant for black truffle cultivation. We demonstrated that contact measurements of leaf reflectance in the “water bands” constitute an accurate and non-invasive estimator of relative water content (RWC) in holm oak, despite the absence of a reference wavelength, probably due to the low variation in leaf thickness under dehydration. The use of a reference wavelength, which is needed for remote sensing, diminished the accuracy of RWC estimation. Contrastingly, LWCI increased the accuracy of RWC estimation as well as a reference wavelength were used. However, LWCI required the reflectance value at full turgor, diminishing its potential for implementation at field level. In conclusion, this technique would allow the continuous monitoring of the physiological state of holm oak and intelligent water control in truffle cultivation.

Key message Among the variety of leaf defensive strategies to counteract herbivory attacks, the oak species analyzed in this study maximize investment in no more than one, with high-LMA oaks developing very tough leaves and low-LMA oaks favoring between high concentrations of condensed tannins or high growth rates. Context Plants develop a variety of defense strategies to counteract herbivory attacks, from physical and chemical defenses to tolerance strategies. Tradeoffs between strategies have been widely assessed from a resource allocation perspective, but there is a need to consider eventual interactions among them. Aim We evaluate the among-species tradeoff between three main constitutive plant defense traits, while considering the leaf mass per area ratio (LMA) as a proxy of leaf construction investment on area basis. Methods Leaf toughness measured as work of fracture, condensed tannins, and relative growth rate (RGR) were analyzed in a set of 19 Quercus L. species with contrasting LMA. Results Most species had low values either in two or in the three traits analyzed. Moreover, the highest values of work of fracture appeared in the species with high LMA; the highest values of condensed tannins were found in the species with the lowest LMA; and high values of RGR were measured in species with intermediate or lower values of LMA. Conclusion Oaks showed a trade-off among leaf defensive strategies influenced by LMA. Oaks with high LMA developed very tough leaves while oaks with low LMA presented lower values of toughness but favored between one of the two other strategies.

BACKGROUND AND AIMS: The existence of sclerophyllous plants has been considered an adaptive strategy against different environmental stresses. Given that it literally means 'hard-leaved', it is essential to quantify the leaf mechanical properties to understand sclerophylly. However, the relative importance of each leaf trait for mechanical properties is not yet well established. METHODS: Genus Quercus is an excellent system to shed light on this because it minimizes phylogenetic variation while having a wide variation in sclerophylly. We measured leaf anatomical traits and cell wall composition, analysing their relationship with leaf mass per area and leaf mechanical properties in a set of 25 oak species. KEY RESULTS: The upper epidermis outer wall makes a strong and direct contribution to the leaf mechanical strength. Moreover, cellulose plays a crucial role in increasing leaf strength and toughness. The principal component analysis plot based on leaf trait values clearly separates Quercus species into two groups corresponding to evergreen and deciduous species. CONCLUSIONS: Sclerophyllous Quercus species are tougher and stronger owing to their thicker epidermis outer wall and/or higher cellulose concentration. Furthermore, section Ilex species share common traits, although they occupy different climates. In addition, evergreen species living in mediterranean-type climates share common leaf traits irrespective of their different phylogenetic origin.

Plant-based measurements are recognized as key methods to obtain insightful data in the field. In general, they are labor-intensive and expensive. In this context, Non-Contact Resonant Ultrasonic Spectroscopy technique (NC-RUS) emerged as a powerful alternative that enabled plant water status determination in a non-destructive, non-invasive and rapid way. However, NC-RUS is not applicable to all plant species as it depends on the possibility to excite and sense thickness resonances in the leaves. In this work, we propose and test an ultrasonic technique that can be used in all leaves, regardless of the appearance of thickness resonances. This technique is based on the contactless measurement of through transmitted airborne ultrasonic pulses in the leaves at high-frequencies and in the absence of thickness resonances, to obtain the leaf ultrasonic velocity (v). It benefits from the facts that: i) at sufficiently high frequencies (typically around 1 MHz) all leaves are non-resonant (so the technique can be applied to both resonant and non-resonant leaves), ii) the use of high-frequencies allows a greater time resolution and a further miniaturization, making possible to apply the technique to small and irregular leaves. Three different signal processing techniques were used to determine the time it takes to the ultrasonic pulse to cross the leaves (time-of-flight) from the measured signals. Two of them operate in time domain: cross-correlation, and edge detection, while the third one makes use of the Fast Fourier Transform (FFT) and operates in the frequency domain: phase-slope. If leaf thickness is also measured, ultrasound velocity can then be worked out. As ultrasound velocity is determined by density and elastic modulus, it is then closely related to water content and turgor pressure. Obtained ultrasound velocities were first validated by comparing them with those obtained by well-established and standard ultrasonic methods: water immersion transmission (v) and NC