Tete Severien Barigah

Does sample length influence the shape of xylem embolism vulnerability curves? A test with the Cavitron spinning technique.

The Cavitron spinning technique is used to construct xylem embolism vulnerability curves (VCs), but its reliability has been questioned for species with long vessels. This technique generates two types of VC: sigmoid s-shaped and exponential, levelling-off r-shaped curves. We tested the hypothesis that r-shaped VCs were anomalous and caused by the presence of vessels cut open during sample preparation. A Cavitron apparatus was used to construct VCs from samples of different lengths in species with contrasting vessel lengths. The results were compared with VCs obtained using other independent techniques. When vessel length exceeded sample length, VCs were r-shaped and anomalous. Filling vessels cut open at both ends with air before measurement produced more typical s-shaped VCs. We also found that exposing segments of 11 woody species in a Cavitron at the pressure measured in planta before sampling considerably increased the degree of embolism above the native state level for species with long vessels. We concluded that open vessels were abnormally more vulnerable to cavitation than intact vessels. We recommend restricting this technique to species with short conduits. The relevance of our conclusions for other spinning techniques is discussed.

Genotypic variability and phenotypic plasticity of cavitation resistance in Fagus sylvatica L. across Europe

Xylem cavitation resistance is a key physiological trait correlated with species tolerance to extreme drought stresses. Little is known about the genetic variability and phenotypic plasticity of this trait in natural tree populations. Here we measured the cavitation resistance of 17 Fagus sylvatica populations representative of the full range of the species in Europe. The trees were grown in three field trials under contrasting climatic conditions. Our findings suggest that the genotypic variability of cavitation resistance is high between genotypes of a given population. By contrast, no significant differences were found for this trait across populations, the mean population cavitation resistance being remarkably constant in each trial. We found a significant site effect and a significant siteu2009×u2009population interaction, suggesting that cavitation resistance has a high phenotypic plasticity and that this plasticity is under genetic control. The implications of our findings for beech forest management in a context of climate change are discussed.

Water stress-induced xylem hydraulic failure is a causal factor of tree mortality in beech and poplar

BACKGROUND AND AIMSnExtreme water stress episodes induce tree mortality, but the physiological mechanisms causing tree death are still poorly understood. This study tests the hypothesis that a potted trees ability to survive extreme monotonic water stress is determined by the cavitation resistance of its xylem tissue.nnnMETHODSnTwo species were selected with contrasting cavitation resistance (beech and poplar), and potted juvenile trees were exposed to a range of water stresses, causing up to 100 % plant death.nnnKEY RESULTSnThe lethal dose of water stress, defined as the xylem pressure inducing 50 % mortality, differed sharply across species (1·75 and 4·5 MPa in poplar and beech, respectively). However, the relationships between tree mortality and the degree of cavitation in the stems were similar, with mortality occurring suddenly when >90 % cavitation had occurred.nnnCONCLUSIONSnOverall, the results suggest that cavitation resistance is a causal factor of tree mortality under extreme drought conditions.

Common trade‐offs between xylem resistance to cavitation and other physiological traits do not hold among unrelated Populus deltoides ×Populus nigra hybrids

We examined the relationships between xylem resistance to cavitation and 16 structural and functional traits across eight unrelated Populus deltoides x Populus nigra genotypes grown under two contrasting water regimes. The xylem water potential inducing 50% loss of hydraulic conductance (Psi(50)) varied from -1.60 to -2.40 MPa. Drought-acclimated trees displayed a safer xylem, although the extent of the response was largely genotype dependent, with Psi(50) being decreased by as far as 0.60 MPa. At the tissue level, there was no clear relationship between xylem safety and either xylem water transport efficiency or xylem biomechanics; the only structural trait to be strongly associated with Psi(50) was the double vessel wall thickness, genotypes exhibiting a thicker double wall being more resistant. At the leaf level, increased cavitation resistance was associated with decreased stomatal conductance, while no relationship could be identified with traits associated with carbon uptake or bulk leaf carbon isotope discrimination, a surrogate of intrinsic water-use efficiency. At the whole-plant level, increased safety was associated with higher shoot growth potential under well-irrigated regime only. We conclude that common trade-offs between xylem resistance to cavitation and other physiological traits that are observed across species may not necessarily hold true at narrower scales.

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 x 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 (P(12), P(50) and P(88)). Vulnerability to cavitation was found to acclimate to growing conditions under different levels of soil water content, with P(50) 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 P(12), the xylem tension at which cavitation begins, was correlated with the lowest value of midday leaf water potential (psi m) 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.

Hydraulic efficiency and coordination with xylem resistance to cavitation, leaf function, and growth performance among eight unrelated Populus deltoides×Populus nigra hybrids

Tests were carried out to determine whether variations in the hydraulic architecture of eight Populus deltoides×Populus nigra genotypes could be related to variations in leaf function and growth performance. Measurements were performed in a coppice plantation on 1-year-old shoots under optimal irrigation. Hydraulic architecture was characterized through estimates of hydraulic efficiency (the ratio of conducting sapwood area to leaf area, A(X):A(L); leaf- and xylem-specific hydraulic conductance of defoliated shoots, k(SL) and k(SS), respectively; apparent whole-plant leaf-specific hydraulic conductance, k(plant)) and xylem safety (water potential inducing 50% loss in hydraulic conductance). The eight genotypes spanned a significant range of k(SL) from 2.63 u2009kg s(-1) m(-2) MPa(-1) to 4.18u2009 kg s(-1) m(-2) MPa(-1), variations being mostly driven by k(SS) rather than A(X):A(L). There was a strong trade-off between hydraulic efficiency and xylem safety. Values of k(SL) correlated positively with k(plant), indicating that high-pressure flowmeter (HPFM) measurements of stem hydraulic efficiency accurately reflected whole-plant water transport efficiency of field-grown plants at maximum transpiration rate. No clear relationship could be found between hydraulic efficiency and either net CO(2) assimilation rates, water-use efficiency estimates (intrinsic water-use efficiency and carbon isotope discrimination against (13)C), or stomatal characteristics (stomatal density and stomatal pore area index). Estimates of hydraulic efficiency were negatively associated with relative growth rate. This unusual pattern, combined with the trade-off observed between hydraulic efficiency and xylem safety, provides the rationale for the positive link already reported between relative growth rate and xylem safety among the same eight P. deltoides×P. nigra genotypes.

Xylem Cavitation and Embolism in Plants Living in Water-Limited Ecosystems

Water deficit is considered the main limiting factor for the establishment, survival, and growth of plants mainly in water-limited ecosystems. Plants have evolved a wide range of morphologic and functional mechanisms to adapt to arid environments. However, if the tension in the xylem conduits becomes too high, thus xylem cavitation can occur i.e., water column breakage. This results in the hydraulic disconnection of leaves and above-ground parts from roots because xylem conduits are filled with air and water vapor, and this phenomenon is called embolism. Therefore, the resistance of the xylem to cavitation and embolism is of paramount importance for plant functioning. In this chapter, we will review the role of plant hydraulics and xylem cavitation in the context of water-limited ecosystems and their relationship with other plant functional traits and with survival capacity. These topics will be analyzed and discussed on the basis of current knowledge and our research experiences.

Modulation of bud survival in Populus nigra sprouts in response to water stress-induced embolism

Understanding drought tolerance mechanisms requires knowledge about the induced weakness that leads to tree death. Bud survival is vital to sustain tree growth across seasons. We hypothesized that the hydraulic connection of the bud to stem xylem structures was critical for its survival. During an artificial drastic water stress, we carried out a census of bud metabolic activity of young Populus nigra L. trees by microcalorimetry. We monitored transcript expression of aquaporins (AQPs; plasma membrane intrinsic proteins (PIPs), X intrinsic proteins (XIPs) and tonoplast membrane intrinsic proteins (TIPs)) and measured local water status within the bud and tissues in the bearer shoot node by nuclear magnetic resonance (NMR) imaging. We found that the bud respiration rate was closely correlated with its water content and decreased concomitantly in buds and their surrounding bearer tissues. At the molecular level, we observed a modulation of AQP pattern expressions (PIP, TIP and XIP subfamilies) linked to water movements in living cells. However, AQP functions remain to be investigated. Both the bud and tree died beyond a threshold water content and respiration rate. Nuclear magnetic resonance images provided relevant local information about the various water reservoirs of the stem, their dynamics and their interconnections. Comparison of pith, xylem and cambium tissues revealed that the hydraulic connection between the bud and saturated parenchyma cells around the pith allowed bud desiccation to be delayed. At the tree death date, NMR images showed that the cambium tissues remained largely hydrated. Overall, the respiration rate (Rco2) and a few AQP isoforms were found to be two suitable, complementary criteria to assess the bud metabolic activity and the ability to survive a severe drought spell. Bud moisture content could be a key factor in determining the capacity of poplar to recover from water stress.

Hydraulic properties of naturally regenerated beech saplings respond to canopy opening

Enhanced sapling growth in advance regeneration requires gaps in the canopy, but is often delayed after canopy opening, because acclimation of saplings to the new environment is gradual and may last for several years. Canopy opening is expected to result in an increased transpiration because of a larger climatic demand and a higher stomatal conductance linked to the higher rates of photosynthesis. Therefore, we focused on the changes in water relations and the hydraulic properties of beech (Fagus sylvatica L.) saplings during 2 years after canopy opening. We tested the hypothesis that an increase in leaf-specific hydraulic conductance and a decrease in vulnerability to cavitation occur to sustain an enhanced transpiration. Hydraulic conductance of defoliated shoots, vulnerability to cavitation, size and density of xylem vessels as well as stomatal conductance were recorded on saplings growing in shade (S saplings) or in gaps created by opening the canopy (shade-to-light, SL saplings). Hydraulic conductance per unit cross-sectional area (K(AS)) did not differ in the shoots of S and SL saplings. But a higher ratio stem cross-sectional area/leaf area resulted in a higher leaf-specific hydraulic conductance of the shoots (K(AL)) of SL saplings. Contrary to expectations, vulnerability to cavitation increased transitorily in stems during the first year after canopy opening and no difference was observed between the two treatments in light-saturated stomatal conductance. During the second year, vulnerability to cavitation was similar in the S and SL saplings and light-saturated stomatal conductance increased in SL saplings. These results demonstrate a release of the hydraulic constraints after canopy opening with an adjustment of the ratio stem cross-sectional area/leaf area. But the larger vulnerability to cavitation during the first year could limit stomatal opening and therefore the ability of beech saplings to use the available light for photosynthesis and could therefore partly explain why the growth increase was delayed to the second growing season after canopy opening.