Phytoremediation capacity of hybrid aspen at sites affected by industry and agriculture.

Fast-growing Populus spp. are well-acknowledged to restore contaminated soils from heavy metals in industrial areas. Thus far, there is no knowledge about the phytoremediation capacity of Populus spp. plantations in hemiboreal Estonia conditions to restore industrially polluted areas. The objective of this study was to assess the soil contamination rate of heavy metals (As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb and Zn) and their uptake by mature hybrid aspen (Populus tremula × Populus tremuloides Michx.) in plantations in different industrial pollution areas (e.g. cement factory, oil shale mining). For the reference, industrially polluted plantations were compared with the low pollution area hybrid aspen plantation on former agricultural soil, which was influenced by fertilization and liming before afforestation. Twenty-one years after afforestation, soil samples were collected from the 0-10 cm topsoil layer. Aboveground biomass sampling was performed for bark and stem wood by ingrowth cores to separate wood formed during early (1-10 years) and late (11-21 years) stand development. Two decades after the afforestation of industrially polluted areas, the heavy metal concentrations in the soil were higher than the reference plantation and the standard reference for unpolluted soils in most cases. The highest concentrations of heavy metals in woody biomass were in the oil shale quarry spoil; because of poor growth, the accumulated pools in aboveground biomass were low. Cd differed from other metals and accumulated less in wood and more in bark. The concentration of heavy metals (Cd, Cr, Cu, Fe, Mn, Ni and Zn) was higher in the first decade of stand formation (1-10 years) than in the last 10 years (11-21 years). High pools of heavy metals were accumulated in aboveground biomass in the reference plantation, indicating the considerable removal of heavy metal residues from the previous fertilization and liming source with harvest. Two decades of afforestation with hybrid aspen is too short for complete ecosystem restoration from heavy metals in industrially polluted areas.

Genome-Wide Identification, Structure Characterization, and Expression Pattern Profiling of the Aquaporin Gene Family in Betula pendula.

Aquaporin water channels (AQPs) constitute a large family of transmembrane proteins present throughout all kingdoms of life. They play key roles in the flux of water and many solutes across the membranes. The AQP diversity, protein features, and biological functions of silver birch are still unknown. A genome analysis of Betula pendula identified 33 putative genes encoding full-length AQP sequences (BpeAQPs). They are grouped into five subfamilies, representing ten plasma membrane intrinsic proteins (PIPs), eight tonoplast intrinsic proteins (TIPs), eight NOD26-like intrinsic proteins (NIPs), four X intrinsic proteins (XIPs), and three small basic intrinsic proteins (SIPs). The BpeAQP gene structure is conserved within each subfamily, with exon numbers ranging from one to five. The predictions of the aromatic/arginine selectivity filter (ar/R), Froger's positions, specificity-determining positions, and 2D and 3D biochemical properties indicate noticeable transport specificities to various non-aqueous substrates between members and/or subfamilies. Nevertheless, overall, the BpePIPs display mostly hydrophilic ar/R selective filter and lining-pore residues, whereas the BpeTIP, BpeNIP, BpeSIP, and BpeXIP subfamilies mostly contain hydrophobic permeation signatures. Transcriptional expression analyses indicate that 23 BpeAQP genes are transcribed, including five organ-related expressions. Surprisingly, no significant transcriptional expression is monitored in leaves in response to cold stress (6 °C), although interesting trends can be distinguished and will be discussed, notably in relation to the plasticity of this pioneer species, B. pendula. The current study presents the first detailed genome-wide analysis of the AQP gene family in a Betulaceae species, and our results lay a foundation for a better understanding of the specific functions of the BpeAQP genes in the responses of the silver birch trees to cold stress.

Plasticity and light sensitivity of leaf hydraulic conductance to fast changes in irradiance in common hazel (Corylus avellana L.).

Forest understory species have to acclimatize to highly heterogeneous light conditions inside forest canopies in order to utilize available resources efficiently. Light sensitivity and response speed of hydraulic conductance (KL) of common hazel (Corylus avellana L.) to fast changes in irradiance was studied in leaves from three different growth light conditions-sun-exposed, moderate shade, and deep shade. The KL of sun-exposed leaves was approximately 3-fold higher when compared to deep-shade leaves, indicating a strong dependence of leaf hydraulic capacity on light conditions. The KL of sun-exposed leaves increased by a factor of nearly four from minimal values recorded in darkness to maximal values in high light compared to deep-shade leaves. Reaction speed of KL to reach maximum values in response to light was nearly five times higher for sun-exposed vs deep-shade leaves. Plasticity indices of KL for sun-exposed and deep-shade leaves were 0.44 and 0.27, respectively. Higher light sensitivity enables a faster and more plastic response of KL to variable light conditions in sun leaves and enhances the ability of plants to maximize resource utilization under more beneficial environmental conditions.

Circadian patterns of xylem sap properties and their covariation with plant hydraulic traits in hybrid aspen.

Physiological processes taking place in plants are subject to diverse circadian patterns but some of them are poorly documented in natural conditions. The daily dynamics of physico-chemical properties of xylem sap and their covariation with tree hydraulic traits were investigated in hybrid aspen (Populus tremula L.×P. tremuloides Michx) in field conditions in order to clarify which environmental drivers govern the daily variation in these parameters. K+ concentration ([K+]), electrical conductivity (σsap), osmolality (Osm) and pH of the xylem sap, as well as branch hydraulic traits, were measured in the field over 24-h cycles. All studied xylem sap properties and hydraulic characteristics including whole-branch (Kwb), leaf blade (Klb) and petiole hydraulic conductances (KP) showed clear daily dynamics. Air temperature (TA) and photosynthetic photon flux density (PPFD), but also water vapour pressure deficit (VPD) and relative humidity (RH), had significant impacts on KwbKlb, KP, [K+] and σsap. Osm varied only with light intensity, while KB varied depending on atmospheric evaporative demand expressed as TA, VPD or RH. Xylem sap pH depended inversely on soil water potential (ΨS) and during daylight also on VPD. Although soil water content was close to saturation during the study period, ΨS influenced also [K+] and σsap. The present study presents evidence of coupling between circadian patterns of xylem sap properties and plant hydraulic conductance providing adequate water supply to foliage under environmental conditions characterised by diurnal variation.

Elevated air humidity affects hydraulic traits and tree size but not biomass allocation in young silver birches (Betula pendula).

As changes in air temperature, precipitation, and air humidity are expected in the coming decades, studies on the impact of these environmental shifts on plant growth and functioning are of major importance. Greatly understudied aspects of climate change include consequences of increasing air humidity on forest ecosystems, predicted for high latitudes. The main objective of this study was to find a link between hydraulic acclimation and shifts in trees' resource allocation in silver birch (Betula pendula Roth) in response to elevated air relative humidity (RH). A second question was whether the changes in hydraulic architecture depend on tree size. Two years of application of increased RH decreased the biomass accumulation in birch saplings, but the biomass partitioning among aboveground parts (leaves, branches, and stems) remained unaffected. Increased stem Huber values (xylem cross-sectional area to leaf area ratio) observed in trees under elevated RH did not entail changes in the ratio of non-photosynthetic to photosynthetic tissues. The reduction of stem-wood density is attributable to diminished mechanical load imposed on the stem, since humidified trees had relatively shorter crowns. Growing under higher RH caused hydraulic conductance of the root system (K R) to increase, while K R (expressed per unit leaf area) decreased and leaf hydraulic conductance increased with tree size. Saplings of silver birch acclimate to increasing air humidity by adjusting plant morphology (live crown length, slenderness, specific leaf area, and fine-root traits) and wood density rather than biomass distribution among aboveground organs. The treatment had a significant effect on several hydraulic properties of the trees, while the shifts were largely associated with changes in tree size but not in biomass allocation.

Rapid and long-term effects of water deficit on gas exchange and hydraulic conductance of silver birch trees grown under varying atmospheric humidity.

BACKGROUND: Effects of water deficit on plant water status, gas exchange and hydraulic conductance were investigated in Betula pendula under artificially manipulated air humidity in Eastern Estonia. The study was aimed to broaden an understanding of the ability of trees to acclimate with the increasing atmospheric humidity predicted for northern Europe. Rapidly-induced water deficit was imposed by dehydrating cut branches in open-air conditions; long-term water deficit was generated by seasonal drought. RESULTS: The rapid water deficit quantified by leaf (ΨL) and branch water potentials (ΨB) had a significant (P < 0.001) effect on gas exchange parameters, while inclusion of ΨB in models resulted in a considerably better fit than those including ΨL, which supports the idea that stomatal openness is regulated to prevent stem rather than leaf xylem dysfunction. Under moderate water deficit (ΨL≥-1.55 MPa), leaf conductance to water vapour (gL), transpiration rate and leaf hydraulic conductance (KL) were higher (P < 0.05) and leaf temperature lower in trees grown in elevated air humidity (H treatment) than in control trees (C treatment). Under severe water deficit (ΨL<-1.55 MPa), the treatments showed no difference. The humidification manipulation influenced most of the studied characteristics, while the effect was to a great extent realized through changes in soil water availability, i.e. due to higher soil water potential in H treatment. Two functional characteristics (gL, KL) exhibited higher (P < 0.05) sensitivity to water deficit in trees grown under increased air humidity. CONCLUSIONS: The experiment supported the hypothesis that physiological traits in trees acclimated to higher air humidity exhibit higher sensitivity to rapid water deficit with respect to two characteristics - leaf conductance to water vapour and leaf hydraulic conductance. Disproportionate changes in sensitivity of stomatal versus leaf hydraulic conductance to water deficit will impose greater risk of desiccation-induced hydraulic dysfunction on the plants, grown under high atmospheric humidity, in case of sudden weather fluctuations, and might represent a potential threat in hemiboreal forest ecosystems. There is no trade-off between plant hydraulic capacity and photosynthetic water-use efficiency on short time scale.

When smaller is better: leaf hydraulic conductance and drought vulnerability correlate to leaf size and venation density across four Coffea arabica genotypes.

Leaf hydraulic conductance (Kleaf) and drought vulnerability in terms of leaf water potential inducing 50% loss of Kleaf (P50), were assessed in four genotypes of Coffea arabica L. We tested three hypotheses: (1) leaf P50 is lower in small leaves with higher vein densities; (2) lower P50 translates into lower Kleaf, limiting gas exchange rates and higher leaf mass per unit area (LMA); (3) P50 values are coordinated with symplastic drought tolerance. We found partial support for Hypotheses 1 and 3, but not for Hypothesis 2. Significant correlations existed among leaf size, vein network and drought resistance. Smaller leaves displayed higher major vein density, higher Kleaf and more negative P50. Kleaf was correlated with leaf gas exchange rates. A negative relationship was observed between Kleaf and LMA, whereas P50 was found to be positively correlated with LMA. Across coffee genotypes, reduced leaf surface area and increased vein density shifts P50 towards more negative values while not translating into higher LMA or lower Kleaf. Breeding crop varieties for both increased safety of the leaf hydraulic system towards drought-induced dysfunction and high gas exchange rates per unit of leaf area is probably a feasible target for future adaptation of crops to climate change scenarios.