Similarities and differences in the balances between leaf, xylem and phloem structures in Fraxinus ornus along an environmental gradient

The plant carbon balance depends on the coordination between photosynthesis and the long-distance transport of water and sugars. How plants modify the allocation to the different structures affecting this coordination under different environmental conditions has been poorly investigated. In this study, we evaluated the effect of soil water availability on the allocation to leaf, xylem and phloem structures in Fraxinus ornus L. We selected small individuals of F. ornus (height ~2 m) from sites contrasting in soil water availability (wet vs dry). We measured how the leaf (LM) and stem + branch biomass (SBM) are cumulated along the stem. Moreover, we assessed the axial variation in xylem (XA) and phloem tissue area (PA), and in lumen area of xylem vessels (CAxy) and phloem sieve elements (CAph). We found a higher ratio of LM:SBM in the trees growing under drier conditions. The long-distance transport tissues of xylem and phloem followed axial patterns with scaling exponents (b) independent of site conditions. PA scaled isometrically with XA (b ~ 1). While CAxy was only marginally higher at the wet sites, CAph was significantly higher at the drier sites. Our results showed that under reduced soil water availability, F. ornus trees allocate relatively more to the leaf biomass and produce more conductive phloem, which is likely to compensate for the drought-related hydraulic limitations to the leaf gas exchanges and the phloem sap viscosity.

Above and below ground carbohydrate allocation differs between ash (Fraxinus excelsior L.) and beech (Fagus sylvatica L.)

We investigated soluble carbohydrate transport in trees that differed in their phloem loading strategies in order to better understand the transport of photosynthetic products into the roots and the rhizosphere as this knowledge is needed to better understand the respiratory processes in the rhizosphere. We compared beech, which is suggested to use mainly passive loading of transport sugars along a concentration gradient into the phloem, with ash that uses active loading and polymer trapping of raffinose family oligosaccharides (RFOs). We pulse-labeled 20 four-year old European beech and 20 four-year old ash trees with 13CO2 and tracked the fate of the label within different plant compartments. We extracted soluble carbohydrates from leaves, bark of stems and branches, and fine roots, measured their amount and isotopic content and calculated their turnover times. In beech one part of the sucrose was rapidly transported into sink tissues without major exchange with storage pools whereas another part of sucrose was strongly exchanged with unlabeled possibly stored sucrose. In contrast the storage and allocation patterns in ash depended on the identity of the transported sugars. RFO were the most important transport sugars that had highest turnover in all shoot compartments. However, the turnover of RFOs in the roots was uncoupled from the shoot. The only significant relation between sugars in the stem base and in the roots of ash was found for the amount (r2 = 0.50; p = 0.001) and isotopic content (r2 = 0.47; p = 0.01) of sucrose. The negative relation of the amounts suggested an active transport of sucrose into the roots of ash. Sucrose concentration in the root also best explained the concentration of RFOs in the roots suggesting that RFO in the roots of ash may be resynthesized from sucrose. Our results interestingly suggest that in both tree species only sucrose directly entered the fine root system and that in ash RFOs are transported indirectly into the fine roots only. The direct transport of sucrose might be passive in beech but active in ash (sustained active up- and unloading to co-cells), which would correspond to the phloem loading strategies. Our results give first hints that the transport of carbohydrates between shoot and root is not necessarily continuous and involves passive (beech) and active (ash) transport processes, which may be controlled by the phloem unloading.

Root tip morphology, anatomy, chemistry and potential hydraulic conductivity vary with soil depth in three temperate hardwood species

Root traits in morphology, chemistry and anatomy are important to root physiological functions, but the differences between shallow and deep roots have rarely been studied in woody plants. Here, we selected three temperate hardwood species, Juglans mandshurica Maxim., Fraxinus mandschurica Rupr. and Phellodendron amurense Rupr., in plantations in northeastern China and measured morphological, anatomical and chemical traits of root tips (i.e., the first-order roots) at surface (0-10 cm) and subsurface (20-30 cm) soil layers. The objectives of this study were to identify how those traits changed with soil depth and to reveal potential functional differences. The results showed that root diameters in deep root tips were greater in J. mandshurica and F. mandschurica, but smaller in P. amurense. However, root stele diameter and the ratio of stele to root diameter in the subsurface layer were consistently greater in all three species, which may enhance their abilities to penetrate into soil. All deep roots exhibited lower tissue nitrogen concentration and respiration rate, which were possibly caused by lower nutrient availability in the subsurface soil layer. Significant differences between shallow and deep roots were observed in xylem structure, with deep roots having thicker stele, wider maximum conduit and greater number of conduits per stele. Compared with shallow roots, the theoretical hydraulic conductivities in deep roots were enhanced by 133% (J. mandshurica), 78% (F. mandschurica) and 217% (P. amurense), respectively, indicating higher efficiency of transportation. Our results suggest that trees' root tip anatomical structure and physiological activity vary substantially with soil environment.

Salinity stress constrains photosynthesis in Fraxinus ornus more when growing in partial shading than in full sunlight: consequences for the antioxidant defence system

BACKGROUND AND AIMS

A major challenge in plant ecophysiology is understanding the effects of multiple sub-optimal environmental conditions on plant performance. In most Mediterranean areas soil salinity builds up during the summer because of low availability of soil water coupled with hot temperatures. Although sunlight and soil salinity may strongly interact in determining a plant's performance, this has received relatively little attention.

METHODS

Two-year-old seedlings of Fraxinus ornus were grown outdoors in pots during a Mediterranean summer in either 45 % (shaded plants) or 100 % (sun plants) sunlight irradiance and were supplied with either deionized water or deionized water plus 75 mm NaCl. Morpho-anatomical traits, water and ionic relations, gas exchange and photosystem II performance, concentrations of individual carotenoids, activity of antioxidant enzymes, concentrations of ascorbic acid and individual polyphenols were measured in leaves. Leaf oxidative stress and damage were estimated by in vivo analysis of stable free radicals and ultrastructural analyses.

KEY RESULTS

Leaf concentrations of potentially toxic ions did not markedly differ in shaded or sun plants in response to salinity. Leaves of sun plants displayed superior water use efficiency compared with leaves of shaded plants, irrespective of salinity treatment, and had both better stomatal control and higher CO2 carboxylation efficiency than leaves of shaded plants. In the salt-treated groups, the adverse effects of excess midday irradiance were greater in shade than in sun plants. The activity of enzymes responsible for detoxifying hydrogen peroxide decreased in shaded plants and increased in sun plants as a result of salinity stress. In contrast, the activity of guaiacol peroxidase and the concentration of phenylpropanoids increased steeply in response to salinity in shaded plants but were unaffected in sun plants.

CONCLUSIONS

It is concluded that salinity may constrain the performance of plants growing under partial shading more severely than that of plants growing under full sun during summer. The results suggest co-ordination within the antioxidant defence network aimed at detoxifying salt-induced generation of reactive oxygen species.

Stem water storage in five coexisting temperate broad-leaved tree species: significance, temporal dynamics and dependence on tree functional traits

The functional role of internal water storage is increasingly well understood in tropical trees and conifers, while temperate broad-leaved trees have only rarely been studied. We examined the magnitude and dynamics of the use of stem water reserves for transpiration in five coexisting temperate broad-leaved trees with largely different morphology and physiology (genera Fagus, Fraxinus, Tilia, Carpinus and Acer). We expected that differences in water storage patterns would mostly reflect species differences in wood anatomy (ring vs. diffuse-porous) and wood density. Sap flux density was recorded synchronously at five positions along the root-to-branch flow path of mature trees (roots, three stem positions and branches) with high temporal resolution (2 min) and related to stem radius changes recorded with electronic point dendrometers. The daily amount of stored stem water withdrawn for transpiration was estimated by comparing the integrated flow at stem base and stem top. The temporal coincidence of flows at different positions and apparent time lags were examined by cross-correlation analysis. Our results confirm that internal water stores play an important role in the four diffuse-porous species with estimated 5-12 kg day(-1) being withdrawn on average in 25-28 m tall trees representing 10-22% of daily transpiration; in contrast, only 0.5-2.0 kg day(-1) was withdrawn in ring-porous Fraxinus. Wood density had a large influence on storage; sapwood area (diffuse- vs. ring-porous) may be another influential factor but its effect was not significant. Across the five species, the length of the time lag in flow at stem top and stem base was positively related to the size of stem storage. The stem stores were mostly exhausted when the soil matrix potential dropped below -0.1 MPa and daily mean vapor pressure deficit exceeded 3-5 hPa. We conclude that stem storage is an important factor improving the water balance of diffuse-porous temperate broad-leaved trees in moist periods, while it may be of low relevance in dry periods and in ring-porous species.

Relationships between root respiration rate and root morphology, chemistry and anatomy in Larix gmelinii and Fraxinus mandshurica

Tree roots are highly heterogeneous in form and function. Previous studies revealed that fine root respiration was related to root morphology, tissue nitrogen (N) concentration and temperature, and varied with both soil depth and season. The underlying mechanisms governing the relationship between root respiration and root morphology, chemistry and anatomy along the root branch order have not been addressed. Here, we examined these relationships of the first- to fifth-order roots for near surface roots (0-10 cm) of 22-year-old larch (Larix gmelinii L.) and ash (Fraxinus mandshurica L.) plantations. Root respiration rate at 18 °C was measured by gas phase O2 electrodes across the first five branching order roots (the distal roots numbered as first order) at three times of the year. Root parameters of root diameter, specific root length (SRL), tissue N concentration, total non-structural carbohydrates (starch and soluble sugar) concentration (TNC), cortical thickness and stele diameter were also measured concurrently. With increasing root order, root diameter, TNC and the ratio of root TNC to tissue N concentration increased, while the SRL, tissue N concentration and cortical proportion decreased. Root respiration rate also monotonically decreased with increasing root order in both species. Cortical tissue (including exodermis, cortical parenchyma and endodermis) was present in the first three order roots, and cross sections of the cortex for the first-order root accounted for 68% (larch) and 86% (ash) of the total cross section of the root. Root respiration was closely related to root traits such as diameter, SRL, tissue N concentration, root TNC : tissue N ratio and stele-to-root diameter proportion among the first five orders, which explained up to 81-94% of variation in the rate of root respiration for larch and up to 83-93% for ash. These results suggest that the systematic variations of root respiration rate within tree fine root system are possibly due to the changes of tissue N concentration and anatomical structure along root branch orders in both tree species, which provide deeper understanding in the mechanism of how root traits affect root respiration in woody plants.

Testing the equi-resistance principle of the xylem transport system in a small ash tree: empirical support from anatomical analyses

In plants, water flows from roots to leaves through a complex network of xylem conduits. The xylem architecture is characterized by the conduit enlargement towards the stem base and the multiplication of conduits near the apices of lateral branches. The xylem architecture of a small ash tree was analysed by measuring the vessel hydraulic diameter (Dh) and number (N) at different heights along the stem and branches. Along the stem, Dh and N increased from the apex to the point of crown insertion. Below, Dh and N decreased and remained constant, respectively. In branches, the Dh and N of apices increased with distance from the ground (PL) (P < 0.001 and P < 0.0001, respectively), indicating that apical resistance (R(APEX)) becomes lower in the most peripheral branches (P < 0.0001). At the level of branch nodes along the stem, the total conductive area (AC) of the stem and branches just above the node was 11% higher than that of the stem just below the node (P = 0.024), whereas the conductivity (Kh) remained invariant above and below (P = 0.76). The difference in AC (ΔAC) between the branches and stem above each node increased with the distance of the node position from the stem apex (L). The xylem architecture of the analysed tree was characterized by anatomical modifications likely aimed at equilibrating the different path length effects on the hydraulic resistance of the different branches. Conduit tapering and multiplication seem to play a crucial role for the achievement of equal hydraulic resistance of all the leaves in the crown.

Ephemeral root modules in Fraxinus mandshurica

Historically, ephemeral roots have been equated with 'fine roots' (i.e. all roots of less than an arbitrary diameter, such as 2 mm), but evidence shows that 'fine roots' in woody species are complex branching systems with both rapid-cycling and slow-cycling components. A precise definition of ephemeral roots is therefore needed. Using a branch-order classification, a rhizotron method and sequential sampling of a root cohort, we tested the hypothesis that ephemeral root modules exist within the branching Fraxinus mandshurica (Manchurian ash) root system as distal nonwoody lateral branches, which show anatomical, nutritional and physiological patterns distinct from their woody mother roots. Our results showed that in F. mandshurica, distal nonwoody root branch orders die rapidly as intact lateral branches (or modules). These nonwoody branch orders exhibited highly synchronous changes in tissue nitrogen concentrations and respiration, dominated root turnover, nutrient flux and root respiration, and never underwent secondary development. The ephemeral root modules proposed here may provide a functional basis for differentiating and sampling short-lived absorptive roots in woody plants, and represent a conceptual leap over the traditional coarse-fine root dichotomies based on arbitrary size classes.

Flash-flood impacts cause changes in wood anatomy of Alnus glutinosa, Fraxinus angustifolia and Quercus pyrenaica

Flash floods may influence the development of trees growing on channel bars and floodplains. In this study, we analyze and quantify anatomical reactions to wounding in diffuse-porous (Alnus glutinosa L.) and ring-porous (Fraxinus angustifolia Vahl. and Quercus pyrenaica Willd.) trees in a Mediterranean environment. A total of 54 cross-sections and wedges were collected from trees that had been injured by past flash floods. From each of the samples, micro-sections were prepared at a tangential distance of 1.5 cm from the injury to determine wounding-related changes in radial width, tangential width and lumen of earlywood vessels, and fibers and parenchyma cells (FPC). In diffuse-porous A. glutinosa, the lumen area of vessels shows a significant (non-parametric test, P-value <0.05) decrease by almost 39% after wounding. For ring-porous F. angustifolia and Q. pyrenaica, significant decreases in vessel lumen area are observed as well by 59 and 42%, respectively. Radial width of vessels was generally more sensitive to the decrease than tangential width, but statistically significant values were only observed in F. angustifolia. Changes in the dimensions of earlywood FPC largely differed between species. While in ring-porous F. angustifolia and Q. pyrenaica the lumen of FPC dropped by 22 and 34% after wounding, we observed an increase in FPC lumen area in diffuse-porous A. glutinosa of approximately 35%. Our data clearly show that A. glutinosa represents a valuable species for flash-flood research in vulnerable Mediterranean environments. For this species, it will be possible in the future to gather information on past flash floods with non-destructive sampling based on increment cores. In ring-porous F. angustifolia and Q. pyrenaica, flash floods leave less drastic, yet still recognizable, signatures of flash-flood activity through significant changes in vessel lumen area. In contrast, the use of changes in FPC dimensions appears less feasible for the determination of past flash-flood events as these two species do not react with the same intensity and clarity as A. glutinosa.

The influence of climate and fructification on the inter-annual variability of stem growth and net primary productivity in an old-growth, mixed beech forest

The periodic production of large seed crops by trees (masting) and its interaction with stem growth has long been the objective of tree physiology research. However, very little is known about the effects of masting on stem growth and total net primary productivity (NPP) at the stand scale. This study was conducted in an old-growth, mixed deciduous forest dominated by Fagus sylvatica (L.) and covers the period from 2003 to 2007, which comprised wet, dry and regular years as well as two masts of Fagus and one mast of the co-dominant tree species Fraxinus excelsior (L.) and Acer pseudoplatanus (L.). We combined analyses of weather conditions and stem growth at the tree level (inter- and intra-annual) with fruit, stem and leaf production, and estimates of total NPP at the stand level. Finally, we compared the annual demand of carbon for biomass production with net canopy assimilation (NCA), derived from eddy covariance flux measurements, chamber measurements and modelling. Annual stem growth of Fagus was most favoured by warm periods in spring and that of Fraxinus by high precipitation in June. For stem growth of Acer and for fruit production, no significant relationships with mean weather conditions were found. Intra-annual stem growth of all species was strongly reduced when the relative plant-available water in soil dropped below a threshold of about 60% between May and July. The inter-annual variations of NCA, total NPP and leaf NPP at the stand level were low (mean values 1313, 662 and 168 g C m(-2) year(-1), respectively), while wood and fruit production varied more and contrarily (wood: 169-241 g C m(-2) year(-1); fruits: 21-142 g C m(-2) year(-1)). In all years, an annual surplus of newly assimilated carbon was calculated (on average 100 g C m(-2) year(-1)). The results suggest that stem growth is generally not limited by insufficient carbon resources; only in mast years a short-term carbon shortage may occur in spring. In contrast to common assumption, stem growth alone is not a sufficient proxy for total biomass production or the control of carbon sequestration by weather extremes.

Erratum to "Structural and physiological responses to ozone in Manna ash (Fraxinus ornus L.) leaves of seedlings and mature trees under controlled and ambient conditions"

Leaf-level microscopical symptom structure and physiological responses were investigated in seedlings experimentally exposed to ozone (O3) in indoor chambers (150 ppb, 8 hd(-1) per 7 weeks), and field trees of Manna ash (Fraxinus ornus) exposed to ambient O3 (max 93 ppb per one growing season). Ozone-induced leaf injury, including leaf reddening and stippling, was observed in both seedlings and mature trees, but the morphology of injury in the stipples differed, being hypersensitive-like (HR-like) in the chamber seedlings and accelerated cell senescence (ACS) in the field trees. In both exposure conditions, the main structural impact of O3 was on the mesophyll and especially the upper assimilating cell layers. The main physiological impact was on carbon assimilation and on stomatal sluggishness. These effects were not due to stomatal structural injury and were more severe in juvenile compared to mature trees because of environmental (water availability, light) and constitutional (gas exchange capacity) factors and differences in the cell physiology processes (HR-like vs. ACS) triggered by ozone stress. Given the plasticity of plant responses to ozone stress, dose/response relationships for tree seedlings in the indoor chambers cannot be extrapolated to mature trees unless ambient conditions are closely simulated.

The hydraulic conductance of Fraxinus ornus leaves is constrained by soil water availability and coordinated with gas exchange rates

Leaf hydraulic conductance (Kleaf) is known to be an important determinant of plant gas exchange and photosynthesis. Little is known about the long-term impact of different environmental factors on the hydraulic construction of leaves and its eventual consequences on leaf gas exchange. In this study, we investigate the impact of soil water availability on Kleaf of Fraxinus ornus L. as well as the influence of Kleaf on gas exchange rates and plant water status. With this aim, Kleaf, leaf conductance to water vapour (gL), leaf water potential (Psileaf) and leaf mass per area (LMA) were measured in F. ornus trees, growing in 21 different sites with contrasting water availability. Plants growing in arid sites had lower Kleaf, gL and Psileaf than those growing in sites with higher water availability. On the contrary, LMA was similar in the two groups. The Kleaf values recorded in sites with two different levels of soil water availability were constantly different from each other regardless of the amount of precipitation recorded over 20 days before measurements. Moreover, Kleaf was correlated with gL values. Our data suggest that down-regulation of Kleaf is a component of adaptation of plants to drought-prone habitats. Low Kleaf implies reduced gas exchange which may, in turn, influence the climatic conditions on a local/regional scale. It is concluded that leaf hydraulics and its changes in response to resource availability should receive greater attention in studies aimed at modelling biosphere-atmosphere interactions.

Temporal cline in a hybrid zone population between Fraxinus excelsior L. and Fraxinus angustifolia Vahl

The two closely related ash species Fraxinus excelsior L. (common ash) and Fraxinus angustifolia Vahl (narrow-leaved ash) have a broad contact zone in France where they hybridize. However, little is known about the local structure of hybrid zone populations and the isolation mechanisms. We assessed the potential effect of floral phenology on the structure of a riparian ash hybrid zone population in central France. The distribution of flowering times was unimodal and lay between the flowering periods of the two species. Using microsatellite markers, we detected isolation by time, which has possibly originated from assortative mating. Multivariate analyses indicated that morphological variation is not distributed at random with respect to flowering times. Spatial autocorrelation analyses showed that temporal and spatial patterns were tightly linked. Interestingly, despite the fact that the population shows isolation by time, neighbourhood size and historical dispersal variance (sigma = 63 m) are similar to those detected in pure stands of F. excelsior where individuals flower rather synchronously and hermaphrodites are not the most frequent sexual type. Trees flowering at intermediate dates, which comprised the majority of the population, produced on average more flowers and fruits. We detected no significant differences in floral parasite infections relative to reproductive timing, although there was a tendency for late flowering trees to suffer from more gall attack. We discuss the impact of temporal variation in fitness traits and their possible role in the maintenance of the hybrid zone.

Differential patterns of morphological and molecular hybridization between Fraxinus excelsior L. and Fraxinus angustifolia Vahl (Oleaceae) in eastern and western France

We examined large-scale patterns of morphology, genetic structure and ecological correlates of Fraxinus excelsior and the closely related species Fraxinus angustifolia in France, in order to determine the degree of hybridization between them. We sampled 24 populations in two putative hybrid zones (Loire and Saône), and five control populations of each species. We measured foliar characteristics of adult trees and used five nuclear microsatellites as molecular markers. Canonical discriminant analysis indicated that the two species differ in morphology, but that intermediate types are common in the Loire region but less frequent in the Saône region. Bayesian population assignment identified one F. angustifolia and two F. excelsior gene pools. Most Loire individuals clustered genetically with the F. angustifolia gene pool. In contrast, the Saône region presented individuals belonging mostly to F. excelsior pools, although the F. angustifolia type was frequent in certain populations. The lowest F(ST) values were found between the Loire and F. angustifolia controls that also exhibited no significant isolation by distance. The proportion of the F. angustifolia gene pool in each locality was negatively correlated with winter temperatures, suggesting that a cold climate may be limiting. Hybridization is probably favoured by the intermediate climatic conditions in the Loire region that allow both species to occur, but is somewhat hampered by the harsher winters in the Saône area where morphological introgression has apparently not yet occurred.