Drought stress modifies early effective resistance and induced chemical defences of Aleppo pine against a chewing insect herbivore

During their long lifespan, pines must cope with simultaneous abiotic and biotic stresses such as drought and herbivory. Mediterranean pines are isohydric species that rapidly close their stomata in response to drought reducing carbon fixation. In such situations, the synthesis of chemical defences could be impaired. Here, we tested the hypothesis that drought stress may constrain the capability of Mediterranean pines to defend against herbivory and to induce chemical defences. For this purpose, we subjected three contrasting populations of Aleppo pine (Pinus halepensis Mill.) to three levels of drought stress, thereafter exposing the seedlings to the herbivore Hylobius abietis L. A suite of ecophysiological and defensive traits was measured to explore the interaction between both stresses. Drought significantly affected the 13C signature and reduced starch and fatty acids concentration. Damage caused by the insect was affected by drought stress, being 75% higher at the moderate stress level but returning under severe stress to similar values as control seedlings. Seedlings responded to herbivory by decreasing the concentration of total polyphenolics and condensed tannins, increasing the concentration of total diterpenes, and modifying the profile of major terpenes. Induced responses to herbivory were, as expected, altered by drought. Inducibility of polyphenolics decreased as drought stress increased while for diterpenes it was higher at moderate stress. Moreover, a significant drought × herbivory interaction was found on the multivariate terpene profile. These results should be considered for predicting responses of pine forests to the forecasted increase of abiotic and biotic risks associated with global change.This research was supported by the grants AGL2012-40151-C03-01, AGL2015-68274-C03-02-R and AGL2015-68274-C03-03-R, founded by MINECO/FEDER. E.S-V. received financial support from the FPU grant program (Ministerio de Educación, Cultura y Deporte Gobierno de España)

Scarce population genetic differentiation but substantial spatiotemporal phenotypic variation of water‑use efficiency in Pinus sylvestris at its western distribution range

Water and carbon fluxes in forests are largely related to leaf gas exchange physiology varying across spatiotemporal scales and modulated by plant responses to environmental cues. We quantified the relevance of genetic and phenotypic variation of intrinsic water-use efficiency (WUEi, ratio of net photosynthesis to stomatal conductance of water) in Pinus sylvestris L. growing in the Iberian Peninsula as inferred from tree-ring carbon isotopes. Inter-population genetic variation, evaluated in a provenance trial comprising Spanish and German populations, was low and relevant only at continental scale. In contrast, phenotypic variation, evaluated in natural stands (at spatial level) and by tree-ring chronologies (at temporal inter-annual level), was important and ten- and threefold larger than the population genetic variance, respectively. These results points to preponderance of plastic responses dominating variability in WUEi for this species. Spatial phenotypic variation in WUEi correlated negatively with soil depth (r = − 0.66; p < 0.01), while temporal phenotypic variation was mainly driven by summer precipitation. At the spatial level, WUEi could be scaled-up to ecosystem-level WUE derived from remote sensing data by accounting for soil water-holding capacity (r = 0.63; p < 0.01). This outcome demonstrates a direct influence of the variation of leaf-level WUEi on ecosystem water and carbon balance differentiation. Our findings highlight the contrasting importance of genetic variation (negligible) and plastic responses in WUEi (large, with changes of up to 33% among sites) on determining carbon and water budgets at stand and ecosystem scales in a widespread conifer such as Pinus sylvestris.This work was supported by the Spanish Government [MINECO Grant Number AGL2015-68274-C3-3-R] and the Russian Science Foundation (Project Number 14-14-00219-P, mathematical approach). We acknowledge P. Sopeña and M.J. Pau for technical assistance and V. Muñoz, M. Sala and A. Teixidó for field sampling

Respuestas ecofiológicas a la sequía en "Eucalyptus globulus": relaciones híbridas y parámetros de intercambio gaseoso

Tesis doctoral inédita leida en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Biología. Fecha de lectura: 08-10-199

Seasonal evolution in leaf and wood δ13C in two co-occurring Mediterranean oaks (Quercus ilex subsp. ballota L. and Quercus faginea Lam.): dynamics of water-use efficiency

En el ámbito mediterráneo es frecuente la concurrencia en un mismo hábitat de especies perennifolias y caducifolias como Quercus ilex y Q. faginea. En este estudio se pretende analizar la diferencia en la respuesta ecofisiológica de las dos especies frente a la variación climática estacional mediante la composición isotópica de carbono de la materia orgánica soluble de las hojas (δ13Ch) y de la α-celulosa de la madera (δ13Cm). Se observaron patrones intra-anuales dependientes de la especie en la δ13Ch. Por un lado, Q. faginea utiliza reservas de almidón como fuente de carbono para producir nuevas hojas a principios de abril, dando lugar a valores altos de δ13Ch. Por otro, Q. ilex utiliza asimilados recientes de las hojas formadas el año anterior (aún funcionales). No obstante, los valores globales de δ13Ch en ambas especies son similares (-25,4±0,55 y -25,5±0,89, para Q. faginea y Q. ilex, respectivamente), lo que apunta a valores equivalentes de eficiencia intrínseca en el uso del agua. Como en las hojas, la δ13Cm de Q. faginea reflejó claramente el uso de reservas en la madera temprana. Tanto la δ13Ch como la δ13Cm de las dos especies registran ajustes como respuesta a las variaciones ambientales estacionales. No obstante, y como parámetro integrador de eficiencia hídrica, la menor amplitud de las variaciones en la δ13Cm de Q. ilex sugiere una menor sensibilidad de esta especie frente a las fluctuaciones climáticas, lo que puede otorgarle cierta ventaja competitiva bajo condiciones de incremento progresivo de aridez.In the Mediterranean region, the presence of mixed stands of evergreen and deciduous species such as Quercus ilex and Q. faginea is common. In this work we aim to analyze their differential response to seasonal climatic fluctuations using the carbon isotope composition in leaf soluble organic matter (δ13Cl) and in wood α-cellulose (δ13Cw) as indicators of water-use efficiency. We observed different intra-annual patterns in δ13Cl. On the one hand, Q. faginea uses stored starch as carbon source to build new leaves in the beginnings of April, leading to high δ13Cl values. On the other, Q. ilex uses recent assimilates from previous-year, but still functional, leaves. Nevertheless, overall δ13Cl values in both species were similar (-25.4±0.55 and -25.5±0.89, for Q. faginea and Q. ilex, respectively), which translates into similar intrinsic water-use efficiencies. As in leaves, δ13Cw for Q. faginea clearly showed the use of stored carbon for early-wood formation. Both δ13Cl and δ13Cw recorded functional adjustments in response to seasonal climatic variations, but the lower amplitude of δ13Cw changes registered in Q. ilex indicates a reduced sensitivity to climate changes as compared with the deciduous oak. This feature may provide this species with a competitive advantage under conditions of steady increase in aridity.Este trabajo se ha desarrollado en el marco de los proyectos PALEOISOTREE (CGL2009-13079-C02-01, MCINN) y SMARTREES (MC-ERG-246725, EU-FP7)

Short-term dynamics of evaporative enrichment of xylem water in woody stems: implications for ecohydrology

In ecohydrology, it is generally assumed that xylem water reflects the water source used by plants. Several studies have reported isotopic enrichment within woody tissues, particularly during dormancy periods or after long periods of inactivity. However, little is known about the short-term dynamics of this process. Here we assessed the magnitude and dynamics of xylem isotopic enrichment in suberized twigs of pines and oaks. We performed a series of laboratory experiments, in which we monitored hourly changes in water content and isotopic composition under two contrasting scenarios of sap flow restriction. First, we simulated the effect of extreme hydraulic failure by excising twigs to restrict sap flow, while sealing the wounds to ensure that water loss took place only through the leaves or bark, as would be the case for evaporation in attached stems. Second, we studied the effect of reduced leaf transpiration by darkening with aluminium foil all the leaves of healthy, well-watered saplings growing in pot conditions. We found evidence of fast evaporative enrichment in metabolically active stems, as a consequence of a temporal decline in sap flow rates, and not necessarily linked to a traceable decline in stem water content. The excision experiments showed significant isotopic changes (~+1‰ in oxygen) appearing in <1 h. Similarly, the pot experiment showed a progressive increase in isotope composition (up to +8‰ in oxygen in a 3-day cycle) when the leaves were covered, and a rapid recovery to initial values when sap flow rates were re-established. We conclude that evaporative enrichment of xylem water in stems is a highly dynamic process that may have significant effects even during short periods of restricted water flow. This has important implications for the study of plant water uptake, as well as for ecosystem- and global-scale hydrological models