Eustaquio Gil-Pelegrín

Effects of a severe drought on Quercus ilex radial growth and xylem anatomy

We assessed the response of Quercus ilex subsp. ballota to the severe summer drought recorded in 1994 in NE Spain through the study of changes in radial growth and wood anatomy. We selected a coppice stand in the Iberian Peninsula, which is characterized by a Mediterranean climate under continental influence. We measured internode length, tree-ring width, mean and maximum vessel diameter, and vessel density for 1981–1997. The annual predicted hydraulic conductance (Kh) was calculated following Hagen-Poisseuilles law. We compared the tree-ring width, vessel diameter and Kh of Q. ilex subsp. ballota and co-existing ring-porous oaks (Q. faginea, Q. pyrenaica) for a dry summer (1994) and a wet summer (1997). To evaluate the drought-resistance of xylem for Q. ilex subsp. ballota (dominant under continental conditions) and Q. ilex subsp. ilex (dominant in mild areas) we determined vulnerability curves. Dimensionless indices of internode length, tree-ring width, and vessel density were compared with climatic data (monthly total precipitation and mean temperature) using correlation analyses. Internode length, tree-ring width, Kh, and mean and maximum vessel diameter declined in 1994. According to vulnerability curves, Q. ilex subsp. ballota showed a greater drought resistance than Q. ilex subsp. ilex. During the year of growth, we found a positive influence of January and June–August precipitation on the internode length, tree-ring width, and vessel density. The response of Q. ilex subsp. ballota radial-growth to summer drought was comparable to that of Q. faginea latewood. Overall, growth and wood anatomy of Q. ilex subsp. ballota showed a plastic response to drought.

Functional groups in Quercus species derived from the analysis of pressure-volume curves

Abstract. Oaks that occupy Mediterranean phytoclimates share common leaf features (evergreenness, high leaf dry mass per unit area, LMA). Due to this phytoclimatic, morphological, and phenological convergence it has been suggested that they might constitute a coherent functional group. To confirm this hypothesis, some physiological parameters were determined after calculating pressure–volume curves (P–V curves) using the free-transpiration method. Seventeen Quercus species from contrasting phytoclimates were studied: six Mediterranean evergreen species (Q. agrifolia, Q. chrysolepis, Q. coccifera, Q. ilex ssp. ballota, Q. ilex ssp. ilex, and Q. suber); seven nemoral deciduous species (Q. alba, Q. laurifolia, Q. nigra, Q. petraea, Q. robur, Q. rubra and Q.velutina), and four nemoro-Mediterranean deciduous species (Q. cerris, Q. faginea, Q. frainetto, and Q. pyrenaica). Two-year-old seedlings growing under homogeneous environmental conditions (no water restrictions, uniform light, and nutrient supply) were used. The statistical analyses (correlation, mean-value comparisons, principal component analysis) of leaf features and parameters derived from the P–V curves confirmed the functional homogeneity of the three phytoclimatic groups, which were characterized by their contrasting ecophysiological response to water stress. The Mediterranean oak species developed mechanisms to avoid an excessive loss of cell water (e.g. high cell-wall rigidity). Conversely, the nemoral oaks showed the opposite. The nemoro-Mediterranean oaks perform better than nemoral oaks under water-stress conditions, but they cannot perform as well as the Mediterranean oaks on relatively dry soils.

EFFECTS OF A SEVERE DROUGHT ON GROWTH AND WOOD ANATOMICAL PROPERTIES OF QUERCUS FAGINEA

We studied the growth response to drought of a Quercus faginea Lam. stand in a xeric site in NE Spain, that experienced an intense defoliation in 1993–94. This event coincided with very low precipitation from November to February, the period when total monthly precipitation exceeds evapotranspiration. We evaluated the effects of November–February precipitation (recharge precipitation, RP) on internode length, radial growth, and wood anatomy. Quercus faginea showed reduced longitudinal and radial growth during the years with low RP, and most sampled trees did not produce latewood in 1993–94 but showed wide earlywood vessels. We observed the reverse for years with a high RP. Radial growth was enhanced by increased precipitation during January and May of the growth year. If severe droughts become more frequent, due to a greater climatic variability, extensive dieback of marginal Q. faginea populations may be expected.

Leaf anatomical properties in relation to differences in mesophyll conductance to CO2 and photosynthesis in two related Mediterranean Abies species

Abies alba and Abies pinsapo are closely related species with the same ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco) large subunit (rbcL) but contrasting hydraulic traits and mesophyll structure occurring in the Iberian Peninsula under contrasting conditions. As photosynthesis and hydraulic capacities often co-scale, we hypothesize that these species differ in mesophyll conductance to CO(2) (g(m) ). g(m) and key anatomical traits were measured in both species. Drought-adapted population of A. pinsapo has higher photosynthesis than the more mesic population of A. alba, in agreement with its higher hydraulic capacity. However, A. alba exhibits the largest stomatal conductance (g(s) ), and so water use efficiency (WUE) is much higher in A. pinsapo. The differences in photosynthesis were explained by differences in g(m) , indicating a correlation between hydraulic capacity and g(m) . We report a case where g(m) is the main factor limiting photosynthesis in one species (A. alba) when compared with the other one (A. pinsapo). The results also highlight the discrepancy between g(m) estimates based on anatomical measurements and those based on gas exchange methods, probably due to the very large resistance exerted by cell walls and the stroma in both species. Thus, the cell wall and chloroplast properties in relation to CO(2) diffusion constitute a near-future research priority.

Phenotypic plasticity in mesic populations of Pinus pinaster improves resistance to xylem embolism (P 50 ) under severe drought

The objectives of the study were to assess the phenotypic variation in the vulnerability to water stress-induced cavitation (estimated by P50, or the xylem water potential which causes a 50% loss of conductivity) and the trade-offs between P50 and related hydraulic traits, i.e., stem specific conductivity (Ks), slope of the vulnerability curve (slope), wood density and branch size. Variability was examined for six Pinus pinaster populations covering the latitudinal range of the species and plasticity was tested through two provenance-progeny trial sites (xeric/mesic). As expected, the overall values of P50, Ks and branch size decreased in the xeric site. Variation in P50 and Ks among populations was mainly the result of phenotypic plasticity, while wood density was genetically controlled and not affected by the environment. Stress conditions in the xeric site promoted a convergence in P50 and Ks as a result of the high phenotypic plasticity of the populations from mesic origins. In the mesic site, the ranking of populations for cavitation resistance and hydraulic capacity was consistent with the geographic location of the seed source. Higher resistance to cavitation was related to lower Ks, branch size and slope, mainly at the population level, but also as a general trend across individuals. In a warmer and drier climate, there could be a potential selection of Pinus pinaster populations from mesic origins, which showed a great responsiveness and adjustment to drought conditions (similar or higher P50 than the populations from dry origins), in addition to a high wood density and growth.

Wettability, Polarity, and Water Absorption of Holm Oak Leaves: Effect of Leaf Side and Age

The highly pubescent abaxial side of holm oak leaves is unwettable and water repellent, while the adaxial side is wettable and can take up water, which may be an adaptation to growing under Mediterranean conditions. 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.

Differential photosynthetic performance and photoprotection mechanisms of three Mediterranean evergreen oaks under severe drought stress

The ability of three Mediterranean oaks (Quercus coccifera L., Quercus ilex ssp. ballota (Desf.) Samp and Quercus suber L.) to cope with intense drought was investigated. Water stress reduced stomatal conductance and photosynthesis in these species. Drought-mediated changes in photosynthetic-related parameters allowed the characterisation of the specific photo-protective mechanisms. Specifically, Q. suber downregulated photosynthetic electron transport rates (ETR) closing PSII reaction centres (i.e. decreasing photochemical quenching) and through an antheraxanthin (A) + zeaxanthin (Z)-mediated diminished intrinsic PSII efficiency (Φexc.). These changes were lower in Q. coccifera and Q. ilex ssp. ballota, which decreased further ETR photo-inactivating PSII centres (evidenced by their low predawn Fv/Fm ratios at high water stress). The predawn Fv/Fm ratio decreased in Q. coccifera largely due to Fm decreases, whereas in Q. ilex ssp. ballota Fv/Fm decreases were due to F0 increases, below -4 MPa. These Fv/Fm decreases were well correlated with increases in the A + Z photo-protective pigments. An analysis of dark respiration and photorespiration as alternative electron sinks under intense drought stress also revealed interspecific differences. The largest imbalance between electrons generated and consumed increased potentially oxidative damage in Q. suber. Subsequently, only Q. suber showed loss of chlorophyll, which is one of the main targets of oxidative damage. Data suggest that Q. coccifera and Q. ilex ssp. ballota seem more able than Q. suber to withstand highly xeric conditions. Therefore, our results question the consideration of Mediterranean evergreen oaks as a homogeneous physiological group.

Trichomes and photosynthetic pigment composition changes: responses of Quercus ilex subsp. ballota (Desf.) Samp. and Quercus coccifera L. to Mediterranean stress conditions

Abstract. Sun and shade leaves of two Mediterranean Quercus species, Quercus ilex subsp. ballota (Desf.) Samp. and Quercus coccifera L., were compared by measuring leaf optical properties, photosynthetic pigment composition and photosystem II efficiency. The presence of trichomes in the adaxial (upper) leaf surface of Q. ilex subsp. ballota seems to constitute an important morphological mechanism that allows this species to maintain a good photosystem II efficiency during the summer. Q. coccifera has almost no trichomes and seems instead to develop other physiological responses, including a smaller light-harvesting antenna size, higher concentrations of violaxanthin cycle pigments and a higher (zeaxanthin + antheraxanthin)/(violaxanthin + antheraxanthin + zeaxanthin) ratio. Q. coccifera was not able to maintain a good photosystem II efficiency up to the end of the summer. In Q. ilex subsp. ballota leaves, natural loss or mechanical removal of adaxial-face leaf trichomes induced short-term decreases in photosystem II efficiency. These changes were accompanied by de-epoxidation of violaxanthin cycle pigments, suggesting that the absence of trichomes would trigger physiological responses in this species. Our data have revealed different patterns of response of Q. ilex subsp. ballota and Q. coccifera facing the stress conditions prevailing in the Mediterranean area.

Phenotypic plasticity in Pinus pinaster δ13C: environment modulates genetic variation

Abstract• Carbon isotope composition (δ13C) is a complex trait involved in acclimation, adaptive processes and related to water use efficiency (WUE) and/or productivity.• To estimate the genetic variation in δ13C and growth (h), their relationship, and the genotype by environment interaction effect in both variables, we analyzed three Pinus pinaster populations and six to ten families per population, in two experimental trial sites and in two consecutive years.• δ13C increased in the drier site (from −27.15 ± 0.18 to −24.53 ± 0.22) and was more affected by environment (62% of variance) than by genotype.• All populations and families exhibited a high phenotypic plasticity in δ13C and increased WUE in the xeric site.• As expected, significant height differences between sites, years, populations and families were displayed. Smaller trees were associated to higher water use efficiency (δ13C) in both, mesic and xeric trial sites.• Aridity and continentality enhanced population differences in δ13C, therefore, in afforestation programs of arid and continental zones, the selection at the population level would be more efficient than at the family level. In the mesic site, the presence of a high genetic variation in δ13C and h between families allows the possibility of a selection for growth and water use efficiency within populations in sub-humid sites.

Relationship between ultrasonic properties and structural changes in the mesophyll during leaf dehydration

The broad-band ultrasonic spectroscopy technique allows the determination of changes in the relative water content (RWC) of leaves with contrasting structural features. Specifically, the standardized frequency associated with the maximum transmittance (f/f(o)) is strongly related to the RWC. This relationship is characterized by the existence of two phases separated by an inflexion point (associated with the turgor loss point). To obtain a better understanding of the strong relationship found between RWC and f/f(o), this work has studied the structural changes experienced by Quercus muehlenbergii leaves during dehydration in terms of ultrasounds measurements, cell wall elasticity, leaf thickness, leaf density, and leaf structure. The results suggest that the decrease found in f/f(o) before the turgor loss point can be attributed to the occurrence of changes in the estimation of the macroscopic effective elastic constant of the leaf (c(33)), mainly associated with changes in the bulk modulus of elasticity of the cell wall (ε). These changes are overriding or compensating for the thickness decreases recorded during this phase. On the other hand, the high degree of cell shrinkage and stretching found in the mesophyll cells during the second phase seem to explain the changes in the acoustic properties of the leaf beyond the turgor loss point. The formation of large intercellular spaces, which increased the irregularity in the acoustic pathway, may explain the increase of the attenuation coefficient of ultrasounds once the turgor loss point threshold is exceeded. The direct measurement of c(33) from ultrasonic measurements would allow a better knowledge of the overall biomechanical properties of the leaf further than those derived from the P-V analysis.