Investigations concerning cavitation and frost fatigue in clonal 84K poplar using high-resolution cavitron measurements

A systems genetic analysis identifies putative mechanisms and candidate genes regulating vessel traits in poplar wood

Wood is the water conducting tissue of tree stems. Like most angiosperm trees, poplar wood contains water-conducting vessel elements whose functional properties affect water transport and growth rates, as well as susceptibility to embolism and hydraulic failure during water stress and drought. Here we used a unique hybrid poplar pedigree carrying genomically characterized chromosomal insertions and deletions to undertake a systems genomics analysis of vessel traits. We assayed gene expression in wood forming tissues from clonal replicates of genotypes covering dosage quantitative trait loci with insertions and deletions, genotypes with extreme vessel trait phenotypes, and control genotypes. A gene co-expression analysis was used to assign genes to modules, which were then used in integrative analyses to identify modules associated with traits, to identify putative molecular and cellular processes associated with each module, and finally to identify candidate genes using multiple criteria including dosage responsiveness. These analyses identified known processes associated with vessel traits including stress response, abscisic acid and cell wall biosynthesis, and in addition identified previously unexplored processes including cell cycle and protein ubiquitination. We discuss our findings relative to component processes contributing to vessel trait variation including signaling, cell cycle, cell expansion, and cell differentiation.

Adaptive strategies to freeze-thaw cycles in branch hydraulics of tree species coexisting in a temperate forest

Freeze-thaw cycles (FTCs) limit the distribution and survival of temperate tree species. Tree species with different wood types coexist in temperate forests and are subjected to the same FTCs. It is essential to understand how these trees differentially cope with xylem hydraulic failure induced by FTCs in the field. The branch hydraulic traits and nonstructural carbohydrate concentration of six coexisting tree species in a temperate forest were measured from mid-winter to early spring when the FTCs occurred from January to April. The percentage loss of hydraulic conductivity (PLC) was lower, and the water potential inducing a 50% loss of hydraulic conductivity (P50) was more negative in tracheid trees than in ring- and diffuse-porous trees, suggesting tracheid trees with narrow tracheid diameters showed less vulnerable to embolism and provided a lower degree of hydraulic failure during FTCs (stronger resistance). Ring-porous trees always showed lower hydraulic conductivity and higher PLC and P50, and these traits did not change during FTCs, suggesting that they might lose the hydraulic functions in winter and abandon the last year xylem. The P50 in diffuse-porous increased after several FTCs (frost fatigue), but that in tracheid species continued to increase (or even decrease) until the end of FTCs (69 cycles), suggesting that tracheid trees were less sensitive to frost fatigue than diffuse-porous trees. Soluble sugar concentration in deciduous trees negatively correlated with PLC at the end of FTCs, indicating that the effect of soluble sugar on refilling embolism occurred in early spring. While the soluble sugar concentration of deciduous trees decreased, that of two evergreen tracheid trees gradually increased, possibly due to the winter photosynthesis of evergreen leaves. Our results suggest temperate trees adopt different strategies to cope with the same FTCs. These findings enrich the understanding of plant hydraulics and carbon physiology in winter and provide insights into the response of different species coexisting in temperate forests under climate change.

Xylem resistance to cavitation increases during summer in Pinus halepensis

Cavitation resistance has often been viewed as a relatively static trait, especially for stems of forest trees. Meanwhile, other hydraulic traits, such as turgor loss point (Ψ_tlp ) and xylem anatomy, change during the season. In this study, we hypothesized that cavitation resistance is also dynamic, changing in coordination with Ψ_tlp . We began with a comparison of optical vulnerability (OV), microcomputed tomography (µCT) and cavitron methods. All three methods significantly differed in the slope of the curve,Ψ₁₂ and Ψ₈₈ , but not in Ψ₅₀ (xylem pressures that cause 12%, 88%, 50% cavitation, respectively). Thus, we followed the seasonal dynamics (across 2 years) of Ψ₅₀ in Pinus halepensis under Mediterranean climate using the OV method. We found that Ψ₅₀ is a plastic trait with a reduction of approximately 1 MPa from the end of the wet season to the end of the dry season, in coordination with the dynamics of the midday xylem water potential (Ψ_midday ) and the Ψ_tlp . The observed plasticity enabled the trees to maintain a stable positive hydraulic safety margin and avoid cavitation during the long dry season. Seasonal plasticity is vital for understanding the actual risk of cavitation to plants and for modeling species' ability to tolerate harsh environments.

Leaf-branch vulnerability segmentation occurs all year round for three temperate evergreen tree species

Vulnerability segmentation (VS) and Hydraulic segmentation (HS) hypotheses propose higher hydraulic resistance and vulnerability to embolism in leaves than in branches, respectively. The VS and HS are suggested as an acclimation strategy of trees to drought stress, but whether they occur during freezing stress has rarely been explored. We measured the leaf and branch hydraulic traits of three temperate evergreen tree species [Picea koraiensis (Korean spruce), Pinus koraiensis (Korean pine), and Pinus sylvestris var. mongolica (Mongolian pine)] during four seasons (winter, spring, summer, and autumn) across the year. We assessed the applicability of VS and HS all year round, particularly in winter. The water potential at which leaf hydraulic conductance lost 50% (P_50L), was more negative in winter than in summer, while higher leaf mass per area was obtained in winter. These results suggest that these species invest more carbon into leaf (including hydraulic systems) to acclimate to winter frost drought. Leaf and branch hydraulic conductance (K_mL and K_mB) were lower, and the percentage loss of branch hydraulic conductance (PLC_B) was higher in spring than in autumn. These results were probably because of more freeze-thaw cycles in spring (69 cycles) than in autumn (37 cycles). The water potential at which branch hydraulic conductance lost 50%, P_50B, was more negative than P_50L across the year. The values of VS (P_50L minus P_50B) were positive, i.e. leaf was more vulnerable than the branch in all species and across seasons, with higher values occurring in spring or autumn. However, K_mL positively correlated with K_mB, suggesting hydraulic coordination between leaf and branch, but did not support HS. Our findings indicate that leaf-branch vulnerability segmentation can occur all year round, including freezing stress, to protect branches from hydraulic failure in temperate evergreen conifers.

The JASMONATE ZIM-domain-OPEN STOMATA1 cascade integrates jasmonic acid and abscisic acid signaling to regulate drought tolerance by mediating stomatal closure in poplar

Drought, which directly affects the yield of crops and trees, is a natural stress with a profound impact on the economy. Improving water use efficiency (WUE) and drought tolerance are relatively effective strategies to alleviate drought stress. OPEN STOMATA1 (OST1), at the core of abscisic acid (ABA) signaling, can improve WUE by regulating stomatal closure and photosynthesis. Methyl jasmonate (MeJA) and ABA crosstalk is considered to be involved in the response to drought stress, but the detailed molecular mechanism is insufficiently known. Here, Populus euphratica, which naturally grows in arid and semiarid regions, was selected as the species for studying MeJA and ABA crosstalk under drought. A yeast two-hybrid assay was performed using PeOST1 as bait and a nucleus-localized factor, JASMONATE ZIM-domain protein 2 (PeJAZ2), was found to participate in MeJA signaling by interacting with PeOST1. Overexpression of PeJAZ2 in poplar notably increased water deficit tolerance and WUE in both severe and mild drought stress by regulating ABA signaling rather than ABA synthesis. Furthermore, a PeJAZ2 overexpression line was shown to have greater ABA-induced stomatal closure and hydrogen peroxide (H2O2) production. Collectively, this evidence establishes a mechanism in which PeJAZ2 acts as a positive regulator in response to drought stress via ABA-induced stomatal closure caused by H2O2 production. Our study presents a new insight into the crosstalk of ABA and jasmonic acid signaling in regulating WUE and drought stress, providing a basis of the drought tolerance mechanism of P. euphratica.

Integrated Transcriptome and Metabolome Analyses Reveal the Anthocyanin Biosynthesis Pathway in AmRosea1 Overexpression 84K Poplar

Populus alba × Populus glandulosa (84K poplar) is model material with excellent genetic engineering resource and ornamental value. In our study, AmRosea1 (Antirrhinum majus) was overexpressed in 84K poplar, and the transgenic 84K (AM) poplar with high content of anthocyanin exhibited red pigmentation leaves. The transcriptome analysis between wild type (WT) and AM showed that 170 differentially expressed genes (DEGs) (86 up-regulated and 84 down-regulated) were found, and some DEGs were involved in flavone and flavonol biosynthesis, flavonoid biosynthesis and anthocyanin biosynthesis. The metabolome analysis showed that 13 anthocyanins-related differentially accumulated metabolites (DAMs) were detected in AM. The correlation analysis between DEGs and DAMs were performed, and the results revealed that 18 DEGs, including 11 MYB genes, two BZ1 genes, one FG2 gene, one ANS gene, and three IF7MAT genes, were negatively or positively correlated with 13 DAMs. The phylogenetic analysis demonstrated that there was high homology between AmRosea1 and PagMYB113, and MYB113 co-expressed with BZ1, ANS and DFR directly. Our results elucidated the molecular mechanism of plant color change mediated by anthocyanin biosynthesis pathway, which laid the foundation for the development and utilization of colorful woody plant.

Narrow vessels cavitate first during a simulated drought in Eucalyptus camaldulensis

Establishing drying-limits for mortality of different tree species and understanding the anatomical and physiological traits involved is crucial to predict forests' responses to climate change. The xylem of Eucalyptus camaldulensis presents a complex of solitary vessels surrounded by different imperforate tracheary elements and parenchyma that influence, in a poorly known way, its hydraulic functioning. We aimed at describing the dynamics of embolism propagation in this type of xylem, seeking any vessel-size pattern, and unraveling the threshold of xylem embolism leading to nonrecovery after drought in E. camaldulensis. We assigned potted saplings to a protracted water-stress for 70 days. We relied on colorimetric and hydraulic methods to test for links between xylem anatomy and embolism propagation in the main stem. On average, the occurrence of embolism was randomly distributed in the stem xylem, but the probability of embolized vessels was higher than predicted by chance in the narrowest vessels of individuals that experienced low to moderate water-stress. The saplings could recover from severe water-stress if their percentage loss of conductance (PLC) was <77%, but not when the PLC was ˃ 85%. We concluded that, contrary to results reported for most species, the narrowest vessels are the most vulnerable to cavitation in E. camaldulensis, suggesting a lack of tradeoff between xylem efficiency and safety (in response to drought) at the tissue level. These results challenge the well-established paradigm of the effect of vessel size on cavitation, which states that the widest conduits are the most vulnerable to both freeze-thaw and drought-induced cavitation.

Cavitation fatigue in conifers: a study on eight European species

After drought-induced embolism and repair, tree xylem may be weakened against future drought events (cavitation fatigue). As there are few data on cavitation fatigue in conifers available, we quantified vulnerability curves (VCs) after embolism/repair cycles on eight European conifer species. We induced 50% and 100% loss of conductivity (LC) with a cavitron, and analyzed VCs. Embolism repair was obtained by vacuum infiltration. All species demonstrated complete embolism repair and a lack of any cavitation fatigue after 50% LC . After 100% LC, European larch (Larix decidua), stone pine (Pinus cembra), Norway spruce (Picea abies), and silver fir (Abies alba) remained unaffected, while mountain pine (Pinus mugo), yew (Taxus baccata), and common juniper (Juniperus communis) exhibited 0.4-0.9 MPa higher vulnerability to embolism. A small cavitation fatigue observed in Scots pine (Pinus sylvestris) was probably biased by incomplete embolism repair, as indicated by a correlation of vulnerability shifts and conductivity restoration. Our data demonstrate that cavitation fatigue in conifers is species-specific and depends on the intensity of preceding LC. The lack of fatigue effects after moderate LC, and relevant effects in only three species after high LC, indicate that conifers are relatively resistant against cavitation fatigue. This is remarkable considering the complex and delicate conifer pit architecture and may be important considering climate change projections.

Enhancing the expression of ARK1 genes in poplar leads to multiple branches and transcriptomic changes

The ARBORKNOX1 (ARK1) gene is an important gene for regulating plant growth and development; however, transcriptomic responses of enhancing expression of ARK1 gene in poplar are poorly investigated. To provide insight into the gene function of the ARK1 gene in poplar, the ARK1 transgenic poplar '717' and '84 K' lines were obtained, the morphology of transgenic plants was observed, and transcriptome profiles were compared. The results showed that there were multiple branches in ARK1 transgenic seedlings compared with non-transgenic seedlings. The results of transcriptome analysis showed that there were significant differences in transcriptome profiles between the transgenic lines of '717' and '84 K', and between non-transgenic lines (CK) and transgenic plants. The real-time quantitative polymerase chain reaction (RT-qPCR) analysis confirmed the expression levels of the genes involved in the pathway of zeatin biosynthesis and brassinosteroid biosynthesis. The increase in expression levels of AHP and CYCD3 was related to multiple branches. Enhancing the expression of the ARK1 gene in poplar seedlings leads to multiple branches and transcriptomic changes.

Die hard: timberline conifers survive annual winter embolism

During winter, timberline trees are exposed to drought and frost, factors known to induce embolism. Studies indicated that conifers cope with winter embolism by xylem refilling. We analysed the loss of hydraulic conductivity (LC) in Picea abies branch xylem over 10 years, and correlated winter embolism to climate parameters. LC was investigated by direct X-ray micro-computer tomography (micro-CT) observations and potential cavitation fatigue by Cavitron measurements. Trees showed up to 100% winter embolism, whereby LC was highest, when climate variables indicated frost drought and likely freeze-thaw stress further increased LC. During summer, LC never exceeded 16%, due to hydraulic recovery. Micro-CT revealed homogenous embolism during winter and that recovery was based on xylem refilling. Summer samples exhibited lower LC in outermost compared to older tree rings, although no cavitation fatigue was detected. Long-term data and micro-CT observations demonstrate that timberline trees can survive annual cycles of pronounced winter-embolism followed by xylem refilling. Only a small portion of the xylem conductivity cannot be restored during the first year, while remaining conduits are refilled without fatigue during consecutive years. We identify important research topics to better understand the complex induction and repair of embolism at the timberline and its relevance to general plant hydraulics.

Frost fatigue and its spring recovery of xylem conduits in ring-porous, diffuse-porous, and coniferous species in situ

Frost-induced embolism and frost fatigue are two major aspects of frost damage to xylem water transport in trees. In this study, three species of each ring-porous, diffuse-porous, and coniferous trees growing in situ were used to explore their differences in winter embolism and frost fatigue. Changes in predawn water potential, predawn native embolism, maximal specific conductivity (K_max), and cavitation resistance (P₅₀, xylem water potential at 50% loss of conductivity) of current-year branches were measured from autumn to spring. Maximum native embolism of late winter was near 100% for ring-porous species, approximately 80% for diffuse-porous species, and below 50% for conifers. In early spring, there was no significant reduction of native embolism until formation of new vessels in ring-porous trees, while diffuse-porous trees and conifers exhibited a reduction in native embolism before development of new xylem. There was a significant decrease in P₅₀ of ring- and diffuse-porous species over winter; however, in May P₅₀ was markedly reduced along with formation of new vessels. K_max of ring- and diffuse-porous species significantly decreased from autumn to late winter. The results revealed that vulnerability to cavitation and frost fatigue was related to conduit diameter. The strategies for coping with winter embolism differed among the three wood types: in ring-porous species there was no active embolism refilling; in diffuse-porous species there was refilling associated with positive xylem pressure; and in conifers there was refilling without positive xylem pressure. New vessels could completely restore stem hydraulic conductivity but only partially restore xylem cavitation resistance in spring.

PeSTZ1, a C2H2-type zinc finger transcription factor from Populus euphratica, enhances freezing tolerance through modulation of ROS scavenging by directly regulating PeAPX2

In the present study, PeSTZ1, a cysteine-2/histidine-2-type zinc finger transcription factor, was isolated from the desert poplar, Populus euphratica, which serves as a model stress adaptation system for trees. PeSTZ1 was preferentially expressed in the young stems and was significantly up-regulated during chilling and freezing treatments. PeSTZ1 was localized to the nucleus and bound specifically to the PeAPX2 promoter. To examine the potential functions of PeSTZ1, we overexpressed it in poplar 84K hybrids (Populus alba × Populus glandulosa), which are known to be stress-sensitive. Upon exposure to freezing stress, transgenic poplars maintained higher photosynthetic activity and dissipated more excess light energy (in the form of heat) than wild-type poplars. Thus, PeSTZ1 functions as a transcription activator to enhance freezing tolerance without sacrificing growth. Under freezing stress, PeSTZ1 acts upstream of ASCORBATE PEROXIDASE2 (PeAPX2) and directly regulates its expression by binding to its promoter. Activated PeAPX2 promotes cytosolic APX that scavenges reactive oxygen species (ROS) under cold stress. PeSTZ1 may operate in parallel with C-REPEAT-BINDING FACTORS to regulate COLD-REGULATED gene expression. Moreover, PeSTZ1 up-regulation reduces malondialdehyde and ROS accumulation by activating the antioxidant system. Taken together, these results suggested that overexpressing PeSTZ1 in 84K poplar enhances freezing tolerance through the modulation of ROS scavenging via the direct regulation of PeAPX2 expression.

Expression Analysis of the NAC Transcription Factor Family of Populus in Response to Salt Stress

Research Highlights: Sequence phylogeny, genome organisation, gene structure, conserved motifs, promoter cis-element and expression profiling of poplar NACs related to salt stress were detected. In addition, expression of two salt-induced NACs was analysed. Background and Objectives: NAC transcription factor (TF) proteins are involved in a wide range of functions during plant development and stress-related endurance processes. To understand the function of Populus NAC TFs in salt stress tolerance, we characterised the structure and expression profile of a total of 289 NAC members. Materials and Methods: Sequence phylogeny, genome organisation, gene structure, motif composition and promoter cis-element were detected using bioinformatics. The expression pattern of Populus NAC TFs under salt stress was also detected using RNA-Seq and RT-qPCR. Results: Synteny analysis showed that 46 and 37 Populus NAC genes were involved in whole-genome duplication and tandem duplication events, respectively. The expression pattern of Populus NAC TFs under salt stress showed the expression of the 289 PtNACs of 84K poplar was induced. Similar expression trends of NACs were found in Populus simonii × P. nigra T. S. Hwang et Liang and Arabidopsis thaliana (L.) Heynh. Conclusions: The correlation analysis showed that the expression of two differentially expressed NAC genes PtNAC024 and PtNAC182 was significantly associated with most of the 63 differentially expressed genes tested. The expression of PtNAC024 and PtNAC182 in different tissues was also analysed in silico and different expression patterns were found. Together, this study provides a solid basis to explore stress-related NAC TF functions in Populus salt tolerance and development.

Frost fatigue response to simulated frost drought using a centrifuge in Acer mono Maxim

In the coldest part of winter, water uptake is blocked by the frozen soil and frozen stems known as 'frost drought' causing severe embolisms in woody plants. Frost drought in stems was simulated in a centrifuge by a synergy between freeze-thaw cycles and the different tensions induced by changing the rotation speed. Frost fatigue was defined as a reduction of embolism resistance after a freeze-thaw cycle and determined from 'vulnerability curves', which showed percent losses of conductivity vs tension (positive value) or xylem pressure (negative value). Different tensions combined with a controlled freeze-thaw cycle were induced to investigate the effects on frost resistance over the course of year. During the growing season, Acer mono Maxim. developed significant frost fatigue, and a significant positive correlation was found between frost fatigue response and exogenous tension. During the dormant season, A. mono showed very high embolism resistance to frost drought, even under a tension of 2 MPa. When the exogenous tension was increased to 3 MPa while the stem was frozen, significant frost fatigue occurred. Longer freezing times had more serious effects on frost fatigue in A. mono. A hypothesis was raised that at the same low temperature, the severer the drought (higher tension) when stems were frozen, the higher frost fatigue response would be induced.

Do nano-particles cause recalcitrant vulnerability curves in Robinia? Testing with a four-cuvette Cochard rotor and with water extraction curves

Cavitation resistance is a key trait for characterizing the drought adaption in plants and is usually presented in terms of vulnerability curves. Three principal techniques have been developed to produce vulnerability curves, but curves generated with centrifugation are reported to suffer from artifacts when applied to long-vesseled species. The main cause of this artifact is the issue of open vessels, resulting in a nano-particle effect that may seed premature embolism. We used two methods to test the potential mechanism behind the nano-particle effect in centrifuge-based vulnerability curves. A four-cuvette rotor system based on a traditional Cochard rotor was designed to inhibit effervescence while injecting water, but the recalcitrant vulnerability curves in Robinia could not be eliminated. There may be multiple sources, besides effervescence, of hypothetical nano-particles: they may arise from cut surfaces or they may be always present in the injected water, leading to the premature embolisms. To prevent the entry of the hypothetical nano-particles, water extraction curves in terms of PLV (percentage loss volume of extracted water from stems) vs tensions were constructed. The PLV curves of Robinia showed s-shaped characteristics after subtracting the first Weibull components from water extraction curves, which were not related to the water loss from vessels according to dye staining experiments. The differences between T50 (xylem tension at which 50% of hydraulic conductivity is lost) in mean PLV curve and T50 in percentage loss of conductivity curves determined by the four-cuvette rotor system and by the bench dehydration method were 3.9 MPa and 0.7 MPa, respectively. Hence, PLV curves may be a valid way to measure the cavitation resistance in long-vesseled species with centrifugation. Keeping bark intact in the process of measurement is recommended, otherwise it would increase evaporation from the entire system.

Embolism and mechanical resistances play a key role in dehydration tolerance of a perennial grass Dactylis glomerata L

Background and Aims

More intense droughts under climate change threaten species resilience. Hydraulic strategies determine drought survival in woody plants but have been hardly studied in herbaceous species. We explored the intraspecific variability of hydraulic and morphological traits as indicators of dehydration tolerance in a perennial grass, cocksfoot (Dactylis glomerata), which has a large biogeographical distribution in Europe.

Methods

Twelve populations of cocksfoot originating from Mediterranean, Temperate and Northern European areas were grown in a controlled environment in pots. Dehydration tolerance, leaf and stem anatomical traits and xylem pressure associated with 88 or 50 % loss of xylem conductance (P88, P50) were measured.

Key Results

Across the 12 populations of cocksfoot, P50 ranged from -3.06 to - 6.36 MPa, while P88 ranged from -5.06 to -11.6 MPa. This large intraspecific variability of embolism thresholds corresponded with the biogeographical distribution and some key traits of the populations. In particular, P88 was correlated with dehydration tolerance (r = -0.79). The dehydration-sensitive Temperate populations exhibited the highest P88 (-6.1 MPa). The most dehydration-tolerant Mediterranean populations had the greatest leaf dry matter content and leaf fracture toughness, and the lowest P88 (-10.4 MPa). The Northern populations displayed intermediate trait values, potentially attributable to frost resistance. The thickness of metaxylem vessel walls in stems was highly correlated with P50 (r = -0.92), but no trade-off with stem lignification was observed. The relevance of the linkage between hydraulic and stomatal traits is discussed for drought survival in perennial grasses.

Conclusions

Compared with woody species, the large intraspecific variability in dehydration tolerance and embolism resistance within cocksfoot has consequences for its sensitivity to climate change. To better understand adaptive strategies of herbaceous species to increasing drought and frost requires further exploration of the role of hydraulic and mechanical traits using a larger inter- and intraspecific range of species.

PeCHYR1, a ubiquitin E3 ligase from Populus euphratica, enhances drought tolerance via ABA-induced stomatal closure by ROS production in Populus

Drought, a primary abiotic stress, seriously affects plant growth and productivity. Stomata play a vital role in regulating gas exchange and drought adaptation. However, limited knowledge exists of the molecular mechanisms underlying stomatal movement in trees. Here, PeCHYR1, a ubiquitin E3 ligase, was isolated from Populus euphratica, a model of stress adaptation in forest trees. PeCHYR1 was preferentially expressed in young leaves and was significantly induced by ABA (abscisic acid) and dehydration treatments. To study the potential biological functions of PeCHYR1, transgenic poplar 84K (Populus alba × Populus glandulosa) plants overexpressing PeCHYR1 were generated. PeCHYR1 overexpression significantly enhanced H2 O2 production and reduced stomatal aperture. Transgenic lines exhibited increased sensitivity to exogenous ABA and greater drought tolerance than that of WT (wild-type) controls. Moreover, up-regulation of PeCHYR1 promoted stomatal closure and decreased transpiration, resulting in strongly elevated WUE (water use efficiency). When exposed to drought stress, transgenic poplar maintained higher photosynthetic activity and biomass accumulation. Taken together, these results suggest that PeCHYR1 plays a crucial role in enhancing drought tolerance via ABA-induced stomatal closure caused by hydrogen peroxide (H2 O2 ) production in transgenic poplar plants.

Divergent hydraulic strategies to cope with freezing in co-occurring temperate tree species with special reference to root and stem pressure generation

Some temperate tree species mitigate the negative impacts of frost-induced xylem cavitation by restoring impaired hydraulic function via positive pressures, and may therefore be more resistant to frost fatigue (the phenomenon that post-freezing xylem becomes more susceptible to hydraulic dysfunction) than nonpressure-generating species. We test this hypothesis and investigate underlying anatomical/physiological mechanisms. Using a common garden experiment, we studied key hydraulic traits and detailed xylem anatomical characteristics of 18 sympatric tree species. These species belong to three functional groups, that is, one generating both root and stem pressures (RSP), one generating only root pressure (RP), and one unable to generate such pressures (NP). The three functional groups diverged substantially in hydraulic efficiency, resistance to drought-induced cavitation, and frost fatigue resistance. Most notably, RSP and RP were more resistant to frost fatigue than NP, but this was at the cost of reduced hydraulic conductivity for RSP and reduced resistance to drought-induced cavitation for RP. Our results show that, in environments with strong frost stress: these groups diverge in hydraulic functioning following multiple trade-offs between hydraulic efficiency, resistance to drought and resistance to frost fatigue; and how differences in anatomical characteristics drive such divergence across species.

Seasonality of cavitation and frost fatigue in Acer mono Maxim

Although cavitation is common in plants, it is unknown whether the cavitation resistance of xylem is seasonally constant or variable. We tested the changes in cavitation resistance of Acer mono before and after a controlled cavitation-refilling and freeze-thaw cycles for a whole year. Cavitation resistance was determined from 'vulnerability curves' showing the percent loss of conductivity versus xylem tension. Cavitation fatigue was defined as a reduction of cavitation resistance following a cavitation-refilling cycle, whereas frost fatigue was caused by a freeze-thaw cycle. A. mono developed seasonal changes in native embolisms; values were relatively high during winter but relatively low and constant throughout the growing season. Cavitation fatigue occurred and changed seasonally during the 12-month cycle; the greatest fatigue response occurred during summer and the weakest during winter, and the transitions occurred during spring and autumn. A. mono was highly resistant to frost damage during the relatively mild winter months; however, a quite different situation occurred during the growing season, as the seasonal trend of frost fatigue was strikingly similar to that of cavitation fatigue. Seasonality changes in cavitation resistance may be caused by seasonal changes in the mechanical properties of the pit membranes.

Expression of a populus histone deacetylase gene 84KHDA903 in tobacco enhances drought tolerance

Histone deacetylases (HDACs) play a key role in regulating plant growth, development and stress responses. However, functions of HDACs in woody plants are largely unknown. In this study, a novel gene encoding a RPD3/HDA1-type histone deacetylase was cloned from 84K poplar (Populus alba×Populus glandulosa) and designated as 84KHDA903. The 84KHDA903 encodes a protein composed of 500 amino acid residues, which contains a conserved HDAC domain. Transient expression of 84KHDA903 in onion epidermal cells suggested that it was exclusively localized in nucleus. The 84KHDA903 exhibited different expression patterns under drought, salt and ABA treatments. The expression of 84KHDA903 was responsive to drought and ABA but not to salt. To understand the function of 84KHDA903 in stress responses, the 84KHDA903 gene was transformed into tobacco. The expression of 84KHDA903 in tobacco increased the tolerance of transgenic seeds to mannitol but not to salt. In adult stage, the 84KHDA903-expressing tobacco exhibited drought tolerance and showed strong capacity to recover after drought. During the recovery period, the stress-responsive genes including NtDREB4, NtDREB3 and NtLEA5 were induced to be highly expressed in the 84KHDA903 transgenic plants in contrast to wild-type plants. Taken together, for the first time, we reported a RPD3/HDA1-type histone deacetylase from poplar, 84KHDA903, which acted as a positive regulator in drought stress responses.

Acclimation of Pistacia integerrima trees to frost in semi-arid environments depends on autumn's drought

Main conclusion Cold acclimation is revealed through induced stem respiration during pre-winter frost of native Pistacia integerrima trees in continental semi-arid environments. Semi-arid environments challenge vegetation by simultaneous abiotic stresses. In this study, we examine the combined effects of water stress and frost on the physiology of Pistacia integerrima stems. This species is native to semi-arid environments where drought and frost frequently co-occur. We quantified carbohydrates and proline in P. integerrima stems responding to frost and experiencing water potentials between -0.2 and -1.8 MPa. We report that dehydrated trees (i.e., Ψ_stem <=-1 MPa) had more soluble sugars and proline than the well-watered trees (-0.2 MPa). The dehydrated trees also froze at lower temperatures and were less damaged by freezing. Interestingly, we observed a significant increase in stem CO₂ efflux at near-freezing temperatures that could be linked to frost protection. This novel finding challenges current paradigm of plant respiration-kinetics which predicts, according to Arrhenius equation, lower respiration rates during frost. Our results support the notion that drought and frost are analogous stresses that can independently activate corresponding physiological processes in trees and amplify protection. This inevitable stress response 'collaboration' may be the key to understanding how non-dormant perennial plants survive the highly variable weather patterns of early winters in semi-arid environments.

Structural determinants of increased susceptibility to dehydration-induced cavitation in post-fire resprouting chaparral shrubs

It is well established that transpiration and photosynthetic rates generally increase in resprouting shoots after fire in chaparral shrublands. By contrast, little is known about how plant hydraulic function varies during this same recovery period. We hypothesized that vascular traits, both functional and structural, would also shift in order to support this heightened level of gas exchange and growth. We examined stem xylem-specific hydraulic conductivity (K_s ) and resistance to cavitation (P₅₀ ) for eight chaparral shrub species as well as several potential xylem structural determinants of hydraulic function and compared established unburned plants and co-occurring post-fire resprouting plants. Unburned plants were generally more resistant to cavitation than resprouting plants, but the two groups did not differ in K_s . Resprouting plants had altered vessel structure compared with unburned plants, with resprouting plants having both wider diameter vessels and higher inter-vessel pit density. For biomechanics, unburned plants had both stronger and denser stem xylem tissue than resprouting plants. Shifts in hydraulic structure and function resulted in resprouting plants being more vulnerable to dehydration. The interaction between time since disturbance (i.e. resprouting versus established stands) and drought may complicate attempts to predict mortality risk of resprouting plants.