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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. [ABSTRACT FROM AUTHOR]

Leaves of Mediterranean evergreen tree species experience a reduction in net CO2 assimilation (A N) and mesophyll conductance to CO2 (g m) during aging and senescence, which would be influenced by changes in leaf anatomical traits at cell level. Anatomical modifications can be accompanied by the dismantling of photosynthetic apparatus associated to leaf senescence, manifested through changes at the biochemical level (i.e. lower nitrogen investment in photosynthetic machinery). However, the role of changes in leaf anatomy at cell level and nitrogen content in g m and A N decline experienced by old non-senescent leaves of evergreen trees with long leaf lifespan is far from being elucidated. We evaluated age-dependent changes in morphological, anatomical, chemical and photosynthetic traits in Quercus ilex subsp. rotundifolia Lam. an evergreen oak with high leaf longevity. All photosynthetic traits decreased with increasing leaf age. The relative change in cell wall thickness (T cw) was less than in chloroplast surface area exposed to intercellular air space (S c/ S), and S c/ S was a key anatomical trait explaining variations in g m and A N among different age classes. The reduction of S c/ S was related to ultrastructural changes in chloroplasts associated to leaf aging, with a concomitant reduction in cytoplasmic nitrogen. Changes in leaf anatomy and biochemistry were responsible for the age-dependent modifications in g m and A N. These findings revealed a gradual physiological deterioration related to the dismantling of the photosynthetic apparatus in older leaves of Q. ilex subsp. rotundifolia. [ABSTRACT FROM AUTHOR]

Nowadays, evergreen sclerophyllous and winter-deciduous malacophyllous oaks with different paleogeographical origins coexist under Mediterranean-type climates, such as the mixed forests of the evergreen Quercus ilex subsp. rotundifolia Lam. and the winter-deciduous Quercus faginea Lam. Both Mediterranean oaks constitute two examples of contrasting leaf habit, so it could be expected that they would have different functional strategies to cope with summer drought. In this study, we analysed photosynthetic, photochemical and hydraulic traits of different organs for Q. faginea and Q. ilex subsp. rotundifolia under well-watered conditions and subjected to very severe drought. The coordinated response between photosynthetic and hydraulic traits explained the higher photosynthetic capacity of Q. faginea under well-watered conditions, which compensated its shorter leaf life span at the expense of higher water consumption. The progressive imposition of water stress evidenced that both types of Mediterranean oaks displayed different functional strategies to cope with water limitations. Specifically, the decrease in mesophyll conductance associated with edaphic drought seems to be the main factor explaining the differences found in the dynamics of net CO2 assimilation throughout the drought period. The sharp decline in photosynthetic traits of Q. faginea was coupled with a strong decrease in shoot hydraulic conductance in response to drought. This fact probably avoided extensive xylem embolism in the stems (i.e. 'vulnerability segmentation'), which enabled new leaf development after drought period in Q. faginea. By contrast, leaves of Q. ilex subsp. rotundifolia showed effective photoprotective mechanisms and high resistance to drought-induced cavitation, which would be related with the longer leaf life span of the evergreen Mediterranean oaks. The co-occurrence of both types of Mediterranean oaks could be related to edaphic conditions that ensure the maintenance of soil water potential above critical values for Q. faginea , which can be severely affected by soil degradation and climate change. [ABSTRACT FROM AUTHOR]

Resprouting is an ancestral trait in angiosperms that confers resilience after perturbations. As climate change increases stress, resprouting vigor is declining in many forest regions, but the underlying mechanism is poorly understood. Resprouting in woody plants is thought to be primarily limited by the availability of non‐structural carbohydrate reserves (NSC), but hydraulic limitations could also be important. We conducted a multifactorial experiment with two levels of light (ambient, 2–3% of ambient) and three levels of water stress (0, 50 and 80 percent losses of hydraulic conductivity, PLC) on two Mediterranean oaks (Quercus ilex and Q. faginea) under a rain‐out shelter (n = 360). The proportion of resprouting individuals after canopy clipping declined markedly as PLC increased for both species. NSC concentrations affected the response of Q. ilex, the species with higher leaf construction costs, and its effect depended on the PLC. The growth of resprouting individuals was largely dependent on photosynthetic rates for both species, while stored NSC availability and hydraulic limitations played minor and non‐significant roles, respectively. Contrary to conventional wisdom, our results indicate that resprouting in oaks may be primarily driven by complex interactions between hydraulics and carbon sources, whereas stored NSC play a significant but secondary role. Here we show that root non‐structural carbohydrate reserves only play a minor role as drivers of basal resprouting in Mediterranean oaks. The probability of resprouting was mostly driven by the degree of antecedent hydraulic limitations and the growth of the resprouts depended largely on photosynthetic C uptake. [ABSTRACT FROM AUTHOR]

Leaf dry mass per unit area (LMA) has been suggested to negatively affect the mesophyll conductance to CO2 (gm), the most limiting factor for photosynthesis per unit leaf area (AN) in many evergreens. Several anatomical traits (i.e., greater leaf thickness and thicker cell walls) constraining gm could explain the negative scaling of gm and AN with LMA across species. However, the Mediterranean sclerophyll Quercus ilex L. shows a major within-species variation in functional traits (greater LMA associated with higher nitrogen content and AN) that might contrast the worldwide trends. The objective of this study was to elucidate the existence of variations in other leaf anatomical parameters determining gm and/or biochemical traits improving the capacity of carboxylation (Vc,max) that could modulate the relationship of AN with LMA across this species. The results revealed that gm was the most limiting factor for AN in all the studied Q. ilex provenances from Spain and Italy. The within-species differences in gm can be partly attributed to the variation in several leaf anatomical traits, mainly cell-wall thickness (Tcw), chloroplast thickness (Tchl) and chloroplast exposed surface area facing intercellular air spaces (Sc/S). A positive scaling of gm and AN with Vc,max was also found, associated with an increased nitrogen content per area. A strong correlation of maximum photosynthetic electron transport (Jmax) with AN further indicated a coordination between the carboxylase activity and the electron transport chain. In conclusion, we have confirmed the strong ecotypic variation in the photosynthetic performance of individual provenances of Q. ilex. Thus, the withinspecies increases found in AN for Q. ilex with increasing foliage robustness can be explained by a synergistic effect among anatomical (at the subcellular and cellular level) and biochemical traits, which markedly improved gm and Vc,max. [ABSTRACT FROM AUTHOR]

• High vapor pressure deficit severely decreases holm oak stomatal conductance. • Atmospheric and edaphic water stress additively reduce transpiration. • Mediterranean summers restrict maximum carbon gain to a short environmental window. • Vapor pressure deficit modulate plant physiological performance and distribution. High rates of vapor pressure deficit (VPD) can severely decrease plant productivity by reducing stomatal conductance, which might be exacerbated during Mediterranean summers due to soil water deficit. In this study, we monitored the response of holm oak, the archetype of Mediterranean trees, to changes in VPD during a summer drought period to evaluate the effects and consequences on gas exchange of the two water stresses (atmospheric and soil). Measurements were performed on trees growing in an experimental plantation over two summers with moderate drought stress by using three different methods: at the leaf level with an infrared gas analyzer, using a whole-plant chamber for short-term monitoring at the tree level, and measuring the canopy temperature for long-term monitoring. The three methods provided negative relationships between leaf conductance and VPD but with discrepancies probably associated with the measurement scale. Overall, the results showed that atmospheric and soil water stress had an additive effect. Under well-watered conditions, an increase in VPD was partially compensated by a reduction in stomatal conductance, resulting in a slight increase in the transpiration rates. With soil water deficit, the response to VPD resulted in a further decrease in stomatal conductance, reducing transpiration as a water saving strategy. The decrease in conductance in response to VPD was transitory, recovering to initial values as soon as the VPD decreased, both under well-watered and drought conditions. Due to this high sensitivity to atmospheric drought, the maximum carbon gain rates of holm oak were restricted to a short environmental window, which might modulate its physiological performance and natural distribution. [ABSTRACT FROM AUTHOR]

Plant trichomes play important protective functions and may have a major influence on leaf surface wettability. With the aim of gaining insight into trichome structure, composition, and function in relation to water-plant surface interactions, we analyzed the adaxial and abaxial leaf surface of holm oak (Quercus ilex) as a model. By measuring the leaf water potential 24 h after the deposition of water drops onto abaxial and adaxial surfaces, evidence for water penetration through the upper leaf side was gained in young and mature leaves. The structure and chemical composition of the abaxial (always present) and adaxial (occurring only in young leaves) trichomes were analyzed by various microscopic and analytical procedures. The adaxial surfaces were wettable and had a high degree of water drop adhesion in contrast to the highly unwettable and water-repellent abaxial holm oak leaf sides. The surface free energy and solubility parameter decreased with leaf age, with higher values determined for the adaxial sides. All holm oak leaf trichomes were covered with a cuticle. The abaxial trichomes were composed of 8% soluble waxes, 49% cutin, and 43% polysaccharides. For the adaxial side, it is concluded that trichomes and the scars after trichome shedding contribute to water uptake, while the abaxial leaf side is highly hydrophobic due to its high degree of pubescence and different trichome structure, composition, and density. Results are interpreted in terms of water-plant surface interactions, plant surface physical chemistry, and plant ecophysiology. [ABSTRACT FROM AUTHOR]

Background and Aims Several studies show apparently contradictory findings about the functional convergence within the Mediterranean woody flora. In this context, this study evaluates the variability of functional traits within holm oak (Quercus ilex) to elucidate whether provenances corresponding to different morphotypes represent different ecotypes locally adapted to the prevaling stress levels. Methods Several morphological and physiological traits were measured at leaf and shoot levels in 9-year-old seedlings of seven Q. ilex provenances including all recognized morphotypes. Plants were grown in a common garden for 9 years under the same environmental conditions to avoid possible biases due to site-specific characteristics. Key Results Leaf morphometry clearly separates holm oak provenances into ‘ilex’ (more elongated leaves with low vein density) and ‘rotundifolia’ (short and rounded leaves with high vein density) morphotypes. Moreover, these morphotypes represent two consistent and very contrasting functional types in response to dry climates, mainly in terms of leaf area, major vein density, leaf specific conductivity, resistance to drought-induced cavitation and turgor loss point. Conclusions The ‘ilex’ and ‘rotundifolia’ morphotypes correspond to different ecotypes as inferred from their contrasting functional traits. To the best of our knowledge, this is the first time that the combined use of morphological and physiological traits has provided support for the concept of these two holm oak morphotypes being regarded as two different species. [ABSTRACT FROM AUTHOR]

Holm oak (Quercus ilex L.) is a Mediterranean species that can withstand intense summer drought through a high resistance to cavitation far beyond the stomatal closure. Besides stomatal limitations, both mesophyll and biochemical limitations to CO2 uptake could increase in holm oak under drought. However, no studies have addressed how hydraulic and non-hydraulic factors may limit the recovery of photosynthesis when re-watering after inducing 50% loss of hydraulic conductivity. We measured photosynthetic traits, xylem embolism, and abscisic acid (ABA) in holm oak with increasing levels of drought stress and seven days after plant re-watering. Drought stress caused a sharp decrease in net CO2 assimilation (AN), stomatal and mesophyll conductance (gs and gm), and maximum velocity of carboxylation (Vcmax). The stomatal closure could be mediated by the rapid increase found in ABA. The high level of xylem embolism explained the strong down-regulation of gs even after re-watering. Therefore, only a partial recovery of AN was observed, in spite of non-hydraulic factors not limiting the recovery of AN, because i/ABA strongly decreased after re-watering, and ii/gm and Vcmax recovered their original values. Therefore, the hydraulic-stomatal limitation model would be involved in the partial recovery of AN, in order to prevent extensive xylem embolism under subsequent drought events that could compromise holm oak survival. [ABSTRACT FROM AUTHOR]

• Holm oak is an evergreen and sclerophyllous tree closely associated to Mediterranean-type climates. • However, the origin of this species can be found in pre-Mediterranean lineages, under tropical or subtropical climates. • To survive in Mediterranean-type climate implies to withstand dry and hot summer, cold winters or both. • The overall response of holm oak to stress factors (drought, high or low temperatures) suggests the existence of a marked within-species variability. • The current taxonomical consideration of holm oak as a complex of two different subspecies might go beyond morphological traits or geographical distribution. Q. ilex is a circum-Mediterranean species which must deal with two stressful periods throughout the year: summer and winter. However, the intensity of these abiotic stresses is very variable depending on the specific area of its distribution range. Besides, two subspecies are usually distinguished, even sometimes recognised as two different true species. Nevertheless, differences in the response to both summer and winter stresses among populations overstep the purely taxonomic question. Regarding to temperature, this species has shown a high resistance to both extremes. On the one hand, critical temperature at leaf level has been estimated in 50.2 °C for a provenance in the Iberian Peninsula (Soria, Spain), a temperature which even in the hottest areas of its distribution range is hardly ever reached. On the other hand, minimal temperature for the survival of the leaves has been estimated in −19.8 °C for Italian provenances and -26.6 °C for one of the coldest provenances (Soria, Spain). Similarly, these temperatures are quite rarely reachable in their respective provenances. Concerning to summer-drought, this may compromise xylem stability in severe cases of scarcity of water in the soil, inducing cavitation when water soil potential is too negative. In this sense, Q. ilex has demonstrated to have a high resistance to cavitation, represented by quite negative values of P 50 and P 88 when compared to other species. Besides, Iberian provenances (i.e. the most continental ones) have shown a higher resistance to cavitation than French or Italian ones. Likewise, holm oak may suffer cavitation during winter, but in this case by a completely different factor: freezing-thawing cycles. Winter-cavitation seems to be related to vessel size, with wider vessels being more vulnerable to cavitate than narrower ones. In spite of its paleotropical origin, Q. ilex has achieved to develop multiple adaptations and physiological strategies that has enabled it not only to withstand the hard climatic conditions imposed by Mediterranean climate, but to colonize and even dominate great part of the Mediterranean landscape. [ABSTRACT FROM AUTHOR]