Aridity promotes differences in proline and phytohormone levels in <Emphasis Type="Italic">Pinus pinaster</Emphasis> populations from contrasting environments

We analysed proline, abscisic acid, (ABA), jasmonic acid (JA), indole acetic acid (IAA) and salicylic acid (SA) accumulation after summer drought at two Pinus pinaster provenance-progeny trial sites. The aim of the study was to evaluate P. pinaster phenotypic plasticity and intraspecific variation in the endogenous concentrations of these metabolites and to determine the best stress indicators for family and population discrimination. The environmental effect was remarkable, as striking differences between the sites were obtained for all indicators except for SA, which was unaffected by the environmental conditions. The levels of proline, ABA and IAA were higher in the xeric than in the mesic site. In contrast, JA was higher in the mesic site. The higher variation displayed at the family level led to family differences for all parameters and sites. Differences in proline and ABA between populations were exclusively found in the xeric site, where the population from the wet climate showed higher accumulation. This study provides evidence for differentiation among P. pinaster populations and families in their plastic responses to drought and highlights the importance of considering intraspecific variability when evaluating biochemical stress indicators in environmental studies.     

Radial-growth and wood-anatomical changes in overaged Quercus pyrenaica coppice stands: functional responses in a new Mediterranean landscape

Recent land-use changes in intensively managed forests such as Mediterranean coppice stands might profoundly alter their structure and function. We assessed how the abandonment of traditional management practices in coppice stands, which consisted of short cutting-cycles (10–15 years), has caused overaging (stems are usually much older than when they were coppiced) and altered their wood anatomy and hydraulic architecture. We studied the recent changes of wood anatomy, radial growth, and hydraulic architecture in two stands of Quercus pyrenaica, a transitional Mediterranean oak with ring-porous wood forming coppice stands in W–NW Spain. We selected a xeric and a mesic site because of their contrasting climates and disturbance histories. The xeric site experienced an intense defoliation after the severe 1993–1994 summer drought. The mesic site was thinned in late 1994. We studied the temporal variability in width, vessel number and diameter, and predicted the hydraulic conductivities (K _h) of earlywood and latewood. In the mesic site, we estimated the vulnerability to xylem cavitation of earlywood vessels. Overaging caused a steep decline in latewood production at a cambial age of 14 years., which was close to the customary cutting cycle of Q. pyrenaica. The diameter distribution of vessels was bimodal, and latewood vessels only accounted for 4% of the K _h. Overaging, acting as a predisposing factor in the decline episode, was observed at the xeric site, where most trees did not produce latewood in 1993–1995. At the mesic site, thinned trees formed wider tree-rings, more latewood and multiseriate tree-rings than overaged trees. The growth enhancement remained 8 years after thinning. Most of the hydraulic conductivity in earlywood was lost in a narrow range of potentials, between −2.5 and −3.5 MPa. We have shown how hydraulic conductivity and radial growth are closely related in Q. pyrenaica and how aging modulates this relationship.     

Vulnerability to cavitation,hydraulic efficiency,growth and survival in an insular pine (Pinus canariensis)

Background and Aims

It is widely accepted that hydraulic failure due to xylem embolism is a key factor contributing to drought-induced mortality in trees. In the present study, an attempt is made to disentangle phenotypic plasticity from genetic variation in hydraulic traits across the entire distribution area of a tree species to detect adaptation to local environments.

Methods

A series of traits related to hydraulics (vulnerability to cavitation and hydraulic conductivity in branches), growth performance and leaf mass per area were assessed in eight Pinus canariensis populations growing in two common gardens under contrasting environments. In addition, the neutral genetic variability (F_ST) and the genetic differentiation of phenotypic variation (Q_ST) were compared in order to identify the evolutionary forces acting on these traits.

Key Results

The variability for hydraulic traits was largely due to phenotypic plasticity. Nevertheless, the vulnerability to cavitation displayed a significant genetic variability (approx. 5 % of the explained variation), and a significant genetic × environment interaction (between 5 and 19 % of the explained variation). The strong correlation between vulnerability to cavitation and survival in the xeric common garden (r = –0·81; P < 0·05) suggests a role for the former in the adaptation to xeric environments. Populations from drier sites and higher temperature seasonality were less vulnerable to cavitation than those growing at mesic sites. No trade-off between xylem safety and efficiency was detected. Q_ST of parameters of the vulnerability curve (0·365 for P₅₀ and the slope of the vulnerability curve and 0·452 for P₈₈) differed substantially from F_ST (0·091), indicating divergent selection. In contrast, genetic drift alone was found to be sufficient to explain patterns of differentiation for xylem efficiency and growth.

Conclusions

The ability of P. canariensis to inhabit a wide range of ecosystems seemed to be associated with high phenotypic plasticity and some degree of local adaptations of xylem and leaf traits. Resistance to cavitation conferred adaptive potential for this species to adapt successfully to xeric conditions.     

Phenotypic plasticity of desiccation resistance in Glossina puparia: are there ecotype constraints on acclimation responses?

Phenotypic plasticity allows organisms to cope with environmental variation and may aid in the evolution of novel traits. However, whether phenotypic plasticity is beneficial, or if acclimation responses might be constrained to particular ecotypes is generally poorly explored. Here we test the beneficial acclimation hypothesis (BAH) and its alternatives for desiccation resistance to atmospheric moisture in mesic‐ and xeric‐adapted Glossina species. Highly significant interactions among acclimation and test humidity were detected for water loss rates indicative of significant phenotypic plasticity. Ordered‐factor anova was unable to reject predictions of the ‘drier is better’ acclimation hypothesis in xeric Glossina morsitans and mesic G. austeni. Evidence for the ‘deleterious acclimation hypothesis’ was found for mesic G. palpalis as expected from the moist habitats it typically occupies. By contrast, support for the ‘optimal acclimation hypothesis’ was found in xeric G. pallidipes. Little support for BAH was obtained in the present study, although other hypotheses, which might enhance fitness within the environments these species are typically exposed to, were supported. However, acclimation responses were not necessarily constrained to xeric/mesic ecotypes which might be expected if adaptation to a particular environment arose as a trade‐off between plastic responses and living in a particular habitat. These results highlight the complexity of acclimation responses and suggest an important role for phenotypic plasticity in moderating environmental effects on evolutionary fitness in Glossina.     

Tests of the contribution of acclimation to geographic variation in water loss rates of the West Indian lizard <Emphasis Type="Italic">Anolis cristatellus</Emphasis>

Phenotypic plasticity can contribute to the process of adaptive radiation by facilitating population persistence in novel environments. West Indian Anolis lizards provide a classic example of an adaptive radiation, in which divergence has occurred along two primary ecological axes: structural microhabitat and climate. Adaptive plasticity in limb morphology is hypothesized to have facilitated divergence along the structural niche axis in Anolis, but very little work has explored plasticity in physiological traits. Here, we experimentally ask whether Puerto Rican Anolis cristatellus from mesic and xeric habitats differ in desiccation rates, and whether these lizards exhibit an acclimation response to changes in relative humidity. We first present microclimatic data collected at lizard perch sites that demonstrate that abiotic conditions experienced by lizards differ between mesic and xeric habitat types. In Experiment 1, we measured desiccation rates of lizards from both habitats maintained under identical laboratory conditions. This experiment demonstrated that desiccation rates differ between populations; xeric lizards lose water more slowly than mesic lizards. In Experiment 2, lizards from each habitat were either maintained under the conditions of Experiment 1, or under extremely low relative humidity. Desiccation rates did not differ between lizards from the same habitat maintained under different treatments and xeric lizards maintained lower desiccation rates than mesic lizards within each treatment. Our results demonstrate that A. cristatellus does not exhibit an acclimation response to abrupt changes of hydric conditions, and suggest that tropical Anolis lizards might be unable to exhibit physiological plasticity in desiccation rates in response to varying climatic conditions.     

Functional coordination between branch hydraulic properties and leaf functional traits in miombo woodlands: implications for water stress management and species habitat preference

We investigated functional coordination between branch hydraulic properties and leaf functional traits among nine miombo woodlands canopy tree species differing in habitat preference and phenology. Specifically, we were seeking to answer the question: are branch hydraulic properties coordinated with leaf functional traits linked to plant drought tolerance in seasonally dry tropical forests and what are the implications for species habitat preference? The hydraulic properties investigated in this study were stem area specific hydraulic conductivity (K _S), Huber value (H _v), and xylem cavitation vulnerability (??₅₀). The leaf functional traits measured were specific leaf area (SLA), leaf dry matter content (LDMC), and mean leaf area (MLA). Generalists displayed significantly (P?<?0.05) higher cavitation resistance (??₅₀) and SLA, but lower sapwood specific hydraulic conductivity (K _S), leaf specific conductivity (K _L), MLA, and LDMC than mesic specialists. Although MLA was uncorrelated with ??₅₀, we found significant (P?<?0.05) positive and negative correlations between plant hydraulic properties and leaf functional traits linked to plant drought tolerance ability, indicating that the interactions between branch hydraulics and leaf functional traits related to plant drought tolerance ability may influence tree species habitat preference in water-limited ecosystems.     

Intraspecific variation in Pinus pinaster PSII photochemical efficiency in response to winter stress and freezing temperatures

As part of a program to select maritime pine (Pinus pinaster Ait.) genotypes for resistance to low winter temperatures, we examined variation in photosystem II activity by chlorophyll fluorescence. Populations and families within populations from contrasting climates were tested during two consecutive winters through two progeny trials, one located at a continental and xeric site and one at a mesic site with Atlantic influence. We also obtained the LT₅₀, or the temperature that causes 50% damage, by controlled freezing and the subsequent analysis of chlorophyll fluorescence in needles and stems that were collected from populations at the continental trial site. P. pinaster showed sensitivity to winter stress at the continental site, during the colder winter. The combination of low temperatures, high solar irradiation and low precipitation caused sustained decreases in maximal photochemical efficiency (F_v/F_m), quantum yield of non-cyclic electron transport (Φ_PSII) and photochemical quenching (qP). The variation in photochemical parameters was larger among families than among populations, and population differences appeared only under the harshest conditions at the continental site. As expected, the environmental effects (winter and site) on the photochemical parameters were much larger than the genotypic effects (population or family). LT₅₀ was closely related to the minimum winter temperatures of the population''s range. The dark-adapted F_v/F_m ratio discriminated clearly between interior and coastal populations.In conclusion, variations in F_v/F_m, Φ_PSII, qP and non-photochemical quenching (NPQ) in response to winter stress were primarily due to the differences between the winter conditions and the sites and secondarily due to the differences among families and their interactions with the environment. Populations from continental climates showed higher frost tolerance (LT₅₀) than coastal populations that typically experience mild winters. Therefore, LT₅₀, as estimated by F_v/F_m, is a reliable indicator of frost tolerance among P. pinaster populations.     

Xylem cavitation vulnerability influences tree species’ habitat preferences in miombo woodlands

Although precipitation plays a central role in structuring Africa’s miombo woodlands, remarkably little is known about plant-water relations in this seasonally dry tropical forest. Therefore, in this study, we investigated xylem vulnerability to cavitation for nine principal tree species of miombo woodlands, which differ in habitat preference and leaf phenology. We measured cavitation vulnerability (Ψ₅₀), stem-area specific hydraulic conductivity (K _S), leaf specific conductivity (K _L), seasonal variation in predawn water potential (Ψ_PD) and xylem anatomical properties [mean vessel diameter, mean hydraulic diameter, mean hydraulic diameter accounting for 95 % flow, and maximum vessel length (V _L)]. Results show that tree species with a narrow habitat range (mesic specialists) were more vulnerable to cavitation than species with a wide habitat range (generalists). Ψ₅₀ for mesic specialists ranged between ?1.5 and ?2.2 MPa and that for generalists between ?2.5 and ?3.6 MPa. While mesic specialists exhibited the lowest seasonal variation in Ψ_PD, generalists displayed significant seasonal variations in Ψ_PD suggesting that the two miombo habitat groups differ in their rooting depth. We observed a strong trade-off between K _S and Ψ₅₀ suggesting that tree hydraulic architecture is one of the decisive factors setting ecological boundaries for principal miombo species. While vessel diameters correlated weakly (P > 0.05) with Ψ₅₀, V _L was positively and significantly correlated with Ψ₅₀. Ψ_PD was significantly correlated with Ψ₅₀ further reinforcing the conclusion that tree hydraulic architecture plays a significant role in species’ habitat preference in miombo woodlands.     

Micro-evolutionary patterns of juvenile wood density in a pine species

Wood density can be considered an adaptive trait, because it ensures the safe and efficient transport of water from the roots to the leaves, mechanical support for the body of the plant and the storage of biological chemicals. Its variability has been extensively described in narrow genetic backgrounds and in wide ranges of forest tree species, but little is known about the extent of natural genetic and phenotypic variability within species. This information is essential to our understanding of the evolutionary forces that have shaped this trait, and for the evaluation of its inclusion in breeding programs. We assessed juvenile wood density, leaf area, total aboveground biomass, and growth in six Pinus pinaster populations of different geographic origins (France, Spain, and Morocco) growing in a provenance-progeny trial. No genetic differentiation was found for wood density, whereas all other traits significantly differed between populations. Heritability of this trait was moderate, with a low additive genetic variance. For retrospective identification of the evolutionary forces acting on juvenile wood density, we compared the distribution of neutral markers (F _ST) and quantitative genetic differentiation (Q _ST). We found that Q _ST was significantly lower than F _ST, suggesting evolutionary stasis. Furthermore, we did not detect any relationship between juvenile wood density and drought tolerance (resistance to cavitation), suggesting that this trait could not be used as a proxy for drought tolerance at the intraspecific level.     

Ammonia volatilization during drought in perennial C4 grasses of tallgrass prairie

We measured foliar NH₃ volatilization as part of our study of the decrease (up to 40%) in shoot N concentration during drought in three perennial C₄ grasses of tallgrass prairie. Volatilization of recently expanded leaves was quantified using cuvettes and acid traps for Spartina pectinata, Andropogon gerardii, and Schizachyrium scoparium, a mesic, intermediate, and xeric species, respectively. In general, volatilization decreased during drought, approaching zero as stomates closed, and increased with plant N status and drought tolerance. Prior to drought, NH₃ volatilization was greater in xeric than mesic species (179 and 131 vs. 115 ng m^-2 s^-1 for individual leaves of S. scoparium and A. gerardii vs. Sp. pectinata). During a 2–3 week drought, whole-shoot volatile N losses can exceed 5% of total plant N in these species, accounting for 2–10% of the decrease in shoot percent N (again, xeric > mesic). Drought-induced N retranslocation of shoot N to roots and rhizomes is responsible for c. 63% of the decrease in percent N in Sp. pectinata, 28% in A. gerardii, and 8% in S. scoparium. The remainder of the decrease in percent N is attributable to growth dilution of existing shoot N, accounting for 34, 65, and 87% of the decrease in shoot percent N during drought in Sp. pectinata, A. gerardii, and S. scoparium, respectively. Thus, the relative importance of volatilization, retranslocation, and dilution in decreasing foliar percent N during drought in prairie grasses is species dependent and related to drought tolerance.     

Photosynthetic pathway alters hydraulic structure and function in woody plants

Xylem structure and function is proposed to reflect an evolutionary balance between demands for efficient movement of water to the leaf canopy and resistance to cavitation during high xylem tension. Water use efficiency (WUE) affects this balance by altering the water cost of photosynthesis. Therefore species of greater WUE, such as C₄ plants, should have altered xylem properties. To evaluate this hypothesis, we assessed the hydraulic and anatomical properties of 19 C₃ and C₄ woody species from arid regions of the American west and central Asia. Specific conductivity of stem xylem (K_s ) was 16%–98% lower in the C₄ than C₃ shrubs from the American west. In the Asian species, the C₃ Nitraria schoberi had similar and Halimodendron halodendron higher K_s values compared with three C₄ species. Leaf specific conductivity (K_L ; hydraulic conductivity per leaf area) was 60%–98% lower in the C₄ than C₃ species, demonstrating that the presence of the C₄ pathway alters the relationship between leaf area and the ability of the xylem to transport water. C₄ species produced similar or smaller vessels than the C₃ shrubs except in Calligonum, and most C₄ shrubs exhibited higher wood densities than the C₃ species. Together, smaller conduit size and higher wood density indicate that in most cases, the C₄ shrubs exploited higher WUE by altering xylem structure to enhance safety from cavitation. In a minority of cases, the C₄ shrubs maintained similar xylem properties but enhanced the canopy area per branch. By establishing a link between C₄ photosynthesis and xylem structure, this study indicates that other phenomena that affect WUE, such as atmospheric CO₂ variation, may also affect the evolution of wood structure and function.     

Phenotypic plasticity and population differentiation in seeds and seedlings of the grass Anthoxanthum odoratum

Summary Seeds of Anthoxanthum odoratum were transplanted reciprocally between a xeric and a mesic field population that were genetically differentiated in adult traits. In one experiment seeds were reciprocally buried in bags in the soil, in a second experiment seeds were reciprocally sown in small plots. For most traits, site effects were much larger than seed-source effects. Germination, emergence, mortality of buried seed and recruitment were significantly higher at the xeric site than at the mesic site, irrespective of population of origin. Seed dormancy, was significantly higher for seed originating from the mesic than from the xeric population. Seedling recruits originating from the xeric population tended to be larger at both sites. Fecundity of seedling recruits depended on the environment; fecundities of plants growing in the xeric site had more than double the fecundity of plants growing in the mesic site. Phenotypic plasticity rather than population differences determined variation in performance in the seed and seedling stages.     

Do Invasive Trees have a Hydraulic Advantage over Native Trees?

The hypothesis was tested that invasive trees have hydraulic traits that contribute to their invasive nature. Five pairs of co-occurring invasive and native trees, in mesic habitats, were selected: (1) Tamarix ramosissima and Salix amygdaloides; (2) Robinia pseudoacacia and Alnus rhombifolia (3) Schinus terebinthifolius and Myrica cerifera; (4) Ligustrum sinense and Acer negundo; and (5) Sapium sebiferum and Diospyros virginiana, respectively. Resistance to cavitation (the water potential [Ψ _x ] at 75% loss of hydraulic conductivity [Ψ₇₅]) was not consistently greater for invasive compared to native species (Ψ₇₅=−1.91 and −1.67 MPa, respectively). Xylem specific conductivity (K _s), a measure of xylem efficiency, was not different between native and invasive species (K _s = 3.50 and 3.70 kg s⁻¹ MPa⁻¹ m⁻¹, respectively). The lack of difference for resistance to cavitation among invasive and native species suggests that the sampled invaders are not more tolerant to water stress than co-occurring native species. Apparently the spread and invasive nature of the sampled species cannot be explained by hydraulic traits alone.     

Phenology constrains opportunistic growth response in <Emphasis Type="Italic">Bromus tectorum</Emphasis> L.

Seasonal resource availability may act as a constraint on plant phenology and thereby influence the range of growth responses observed among populations of annual species, especially those occupying a wide range of environments. We compared a mesic and a xeric population of the non-native, annual grass, Bromus tectorum, to examine phenology in response to interspecific competition and water availability. Using a target-neighborhood approach, we assessed how phenological patterns of the two populations affected morphological and growth responses to enhanced resource availability represented by late-season soil moisture. The xeric population exhibited a highly constrained phenology and was unable to extend the growing season despite available soil resources. Because of the low phenotypic variation, allocation to reproduction was similar across resource conditions. In contrast, the mesic population flowered later and showed a more opportunistic phenology in response to late-season water availability. The mesic population was not able to maintain consistent reproductive allocation at low resource levels. The responses of the two populations to late-season water availability were not affected by the density of neighboring plants. We suggest that post-introduction selection pressure on B. tectorum in the xeric habitat has resulted in a more fixed phenology which limits opportunistic response to unpredictable, particularly late-season resource availability. Opportunistic and fixed responses represent contrasting strategies for optimizing fitness in temporally varying environments and, while both play important roles for ensuring reproductive success, these results suggest that local adaptation to temporal resource variation may reflect a balance between flexible and inflexible phenology.     

Genetic variation of drought-induced cavitation resistance among Pinus hartwegii populations from an altitudinal gradient

Hydraulic failure can cause massive die-back of forest trees during drought. With extreme climatic events set to become more frequent and severe due to climatic change, it is essential to study resistance to water stress-induced cavitation. We investigated the genetic differentiation for cavitation resistance among Pinus hartwegii populations, the pine species growing at the treeline in México. Open-pollinated seeds were collected from seven natural populations along an altitudinal gradient (3,150–3,650 masl) from Pico de Tancítaro, Michoacán, western México. Seedlings were raised in a nursery and then established in a randomized complete block design in a common garden experiment. Resistance to cavitation (P ₅₀, xylem pressure inducing 50 % loss of hydraulic conductance and S, slope of the vulnerability curve) and specific hydraulic conductivity (k _s), were evaluated on branches of 5-year-old seedlings using the Cavitron technique. Mean P ₅₀ was ?3.42 ± 0.05 MPa, indicating that Pinus hartwegii is one of the more vulnerable pine species to cavitation. No significant genetic differentiation was detected between populations for cavitation resistance traits (P ₅₀ and S), but a significant altitudinal cline was found for S. In contrast, k _s exhibited a significant differentiation among populations and a significant decline with increasing altitude. The lack of genetic differentiation among P. hartwegii populations for cavitation resistance is likely to represent a limitation for adapting to the warmer and drier climates that are expected to occur in México under climatic change. Finally, a worldwide comparison within the Pinus genus showed that pines growing at the treeline were on average more vulnerable to cavitation than those from lowland. This might reflect an adaptation to dry environmental conditions at low elevation.     

Measured and modelled leaf and stand‐scale productivity across a soil moisture gradient and a severe drought

Environmental controls on carbon dynamics operate at a range of interacting scales from the leaf to landscape. The key questions of this study addressed the influence of water and nitrogen (N) availability on Pinus palustris (Mill.) physiology and primary productivity across leaf and canopy scales, linking the soil‐plant‐atmosphere (SPA) model to leaf and stand‐scale flux and leaf trait/canopy data. We present previously unreported ecophysiological parameters (e.g. V_cmax and J_max) for P. palustris and the first modelled estimates of its annual gross primary productivity (GPP) across xeric and mesic sites and under extreme drought. Annual mesic site P. palustris GPP was ～23% greater than at the xeric site. However, at the leaf level, xeric trees had higher net photosynthetic rates, and water and light use efficiency. At the canopy scale, GPP was limited by light interception (canopy level), but co‐limited by nitrogen and water at the leaf level. Contrary to expectations, the impacts of an intense growing season drought were greater at the mesic site. Modelling indicated a 10% greater decrease in mesic GPP compared with the xeric site. Xeric P. palustris trees exhibited drought‐tolerant behaviour that contrasted with mesic trees' drought‐avoidance behaviour.     

Uniform selection as a primary force reducing population genetic differentiation of cavitation resistance across a species range

Background

Cavitation resistance to water stress-induced embolism determines plant survival during drought. This adaptive trait has been described as highly variable in a wide range of tree species, but little is known about the extent of genetic and phenotypic variability within species. This information is essential to our understanding of the evolutionary forces that have shaped this trait, and for evaluation of its inclusion in breeding programs.

Methodology

We assessed cavitation resistance (P ₅₀), growth and carbon isotope composition in six Pinus pinaster populations in a provenance and progeny trial. We estimated the heritability of cavitation resistance and compared the distribution of neutral markers (F _ST) and quantitative genetic differentiation (Q _ST), for retrospective identification of the evolutionary forces acting on these traits.

Results/Discussion

In contrast to growth and carbon isotope composition, no population differentiation was found for cavitation resistance. Heritability was higher than for the other traits, with a low additive genetic variance (h² _ns = 0.43±0.18, CV_A = 4.4%). Q _ST was significantly lower than F _ST, indicating uniform selection for P ₅₀, rather than genetic drift. Putative mechanisms underlying Q_STST are discussed.     

The physiological implications of primary xylem organization in two ferns

Xylem structure and function are well described in woody plants, but the implications of xylem organization in less‐derived plants such as ferns are poorly understood. Here, two ferns with contrasting phenology and xylem organization were selected to investigate how xylem dysfunction affects hydraulic conductivity and stomatal conductance (g_s). The drought‐deciduous pioneer species, Pteridium aquilinum, exhibits fronds composed of 25 to 37 highly integrated vascular bundles with many connections, high g_s and moderate cavitation resistance (P₅₀ = ?2.23 MPa). By contrast, the evergreen Woodwardia fimbriata exhibits sectored fronds with 3 to 5 vascular bundles and infrequent connections, low g_s and high resistance to cavitation (P₅₀ = ?5.21 MPa). Xylem‐specific conductivity was significantly higher in P. aqulinium in part due to its wide, efficient conduits that supply its rapidly transpiring pinnae. These trade‐offs imply that the contrasting xylem organization of these ferns mirrors their divergent life history strategies. Greater hydraulic connectivity and g_s promote rapid seasonal growth, but come with the risk of increased vulnerability to cavitation in P. aquilinum, while the conservative xylem organization of W. fimbriata leads to slower growth but greater drought tolerance and frond longevity.     

Are microbial symbionts involved in the speciation of the gall-inducing aphid, <Emphasis Type="Italic">Slavum wertheimae</Emphasis>?

Microbial symbionts have come to be recognized as agents in the speciation of their eukaryote hosts. In this study, we asked if bacterial symbionts are, or were in the past, involved in the speciation of the gall-inducing aphid Slavum wertheimae (Hemiptera: Aphididae). This aphid is specific to the tree Pistacia atlantica, which has a fragmented distribution among mesic and xeric habitats, leading to corresponding fragmentation of the aphid population. Previous studies revealed genetic differentiation among populations of the gall-inducing aphid, suggesting cryptic allopatric speciation. Pistacia atlantica trees show no such variation. By means of diagnostic PCR, we screened several populations of S. wertheimae from mesic and xeric sites in Israel for the presence of nine known aphid symbionts: Arsenophonus, Hamiltonella, Regiella, Rickettsia, Rickettsiella, Serratia, Spiroplasma, Wolbachia, and X-type, as well as Cardinium, known to be a reproductive manipulator. Only one symbiont, Wolbachia, was detected in S. wertheimae. Wolbachia was found in all the aphids of the mesic populations, compared to 26% in the aphids from the xeric populations. Multilocus Sequence typing of Wolbachia revealed new haplotypes in the fbpA and coxA genes in both the mesic and xeric populations. Phylogenetic analysis showed that Wolbachia of S. wertheimae is closely related to Wolbachia strains from assorted hosts, mostly lepidopterans, but only distantly related to Wolbachia strains from other aphid species. We conclude that the cryptic speciation of mesic and xeric populations of S. wertheimae was likely driven by geographical isolation rather than by Wolbachia.     

Poplar vulnerability to xylem cavitation acclimates to drier soil conditions

Xylem vulnerability to cavitation differs between tree species according to their drought resistance, more xerophilous species being more resistant to xylem cavitation. Variability in xylem vulnerability to cavitation is also found within species, especially between in situ populations. The origin of this variability has not been clearly identified. Here we analyzed the response of xylem hydraulic traits of Populus tremula×Populus alba trees to three different soil water regimes. Stem xylem vulnerability was scored as the xylem water potential causing 12, 50 and 88% loss of conductivity (P₁₂, P₅₀ and P₈₈). Vulnerability to cavitation was found to acclimate to growing conditions under different levels of soil water content, with P₅₀ values of ?1.82, ?2.03 and ?2.45 MPa in well‐watered, moderately water‐stressed and severely water‐stressed poplars, respectively. The value of P₁₂, the xylem tension at which cavitation begins, was correlated with the lowest value of midday leaf water potential (ψm) experienced by each plant, the difference between the two parameters being approximately 0.5 MPa, consistent with the absence of any difference in embolism level between the different water treatments. These results support the hypothesis that vulnerability to cavitation is a critical trait for resistance to drought. The decrease in vulnerability to cavitation under growing conditions of soil drought was correlated with decreased vessel diameter, increased vessel wall thickness and a stronger bordered pit field (t/b)². The links between these parameters are discussed.