Climate warming will increase the drought exposure of many forests world‐wide. It is not well understood how trees adapt their hydraulic architecture to a long‐term decrease in water availability. We ...examined 23 traits characterizing the hydraulic architecture and growth rate of branches and the dependent foliage of mature European beech (Fagus sylvatica) trees along a precipitation gradient (855–594 mm yr⁻¹) on uniform soil. A main goal was to identify traits that are associated with xylem efficiency, safety and growth. Our data demonstrate for the first time a linear increase in embolism resistance with climatic aridity (by 10%) across populations within a species. Simultaneously, vessel diameter declined by 7% and pit membrane thickness (Tₘ) increased by 15%. Although specific conductivity did not change, leaf‐specific conductivity declined by 40% with decreasing precipitation. Of eight plant traits commonly associated with embolism resistance, only vessel density in combination with pathway redundancy and Tₘ were related. We did not confirm the widely assumed trade‐off between xylem safety and efficiency but obtained evidence in support of a positive relationship between hydraulic efficiency and growth. We conclude that the branch hydraulic system of beech has a distinct adaptive potential to respond to a precipitation reduction as a result of the environmental control of embolism resistance.
Drought-induced forest dieback has been widely reported over the last decades, and the evidence for a direct causal link between survival and hydraulic failure (xylem cavitation) is now well known. ...Because vulnerability to cavitation is intimately linked to the anatomy of the xylem, the main objective of this study was to better understand the xylem anatomical properties associated with cavitation resistance. An extensive data set of cavitation resistance traits and xylem anatomical properties was developed for 115 conifer species, with special attention given to the micro-morphology of bordered pits. The ratio of torus to pit aperture diameter, so-called torus overlap, increased with increasing cavitation resistance, while the flexibility of the margo does not seem to play a role, suggesting that air-seeding is located at the seal between the aspirated torus and pit aperture. Moreover, punctured tori were reported in various Pinaceae species. Species resistant to cavitation had thicker tracheid walls, while their lumen diameter (conduit size) was only slightly reduced, minimizing the impact on hydraulic conductance. The results also demonstrated (i) the existence of an indirect trade-off between hydraulic safety and mechanical strength; and (ii) a consistency between species distribution and xylem anatomy: species with a wide torus overlap and high valve effects are found in arid environments such as the Mediterranean region.
Adapting agriculture to climate change is driving the need for the selection and breeding of drought-tolerant crops. The aim of this study was to identify key drought tolerance traits and determine ...the sequence of their water potential thresholds across three grapevine cultivars with contrasting water use behaviors, Grenache, Syrah, and Semillon. We quantified differences in water use between cultivars and combined this with the determination of other leaf-level traits (e.g. leaf turgor loss point, π TLP), leaf vulnerability to embolism (P50), and the hydraulic safety margin (HSM P50). Semillon exhibited the highest maximum transpiration (Emax), and lowest sensitivity of canopy stomatal conductance (Gc) to vapor pressure deficit (VPD), followed by Syrah and Grenache. Increasing Emax was correlated with more negative water potential at which stomata close (Pgs90), π TLP, and P50, suggesting that increasing water use is associated with hydraulic traits allowing gas exchange under more negative water potentials. Nevertheless, all the cultivars closed their stomata prior to leaf embolism formation. Modeling simulations demonstrated that despite a narrower HSM, Grenache takes longer to reach thresholds of hydraulic failure due to its conservative water use. This study demonstrates that the relationships between leaf hydraulic traits are complex and interactive, stressing the importance of integrating multiple traits in characterizing drought tolerance.
Photosynthetic carbon (C) isotope discrimination (ΔΑ) labels photosynthates (δA) and atmospheric CO₂ (δa) with variable C isotope compositions during fluctuating environmental conditions. In this ...context, the C isotope composition of respired CO₂ within ecosystems is often hypothesized to vary temporally with ΔΑ. We investigated the relationship between ΔΑ and the C isotope signals from stem (δW), soil (δS) and ecosystem (δE) respired CO₂ to environmental fluctuations, using novel tuneable diode laser absorption spectrometer instrumentation in a mature maritime pine forest. Broad seasonal changes in ΔΑ were reflected in δW,δS and δE. However, respired CO₂ signals had smaller short-term variations than ΔA and were offset and delayed by 2-10 d, indicating fractionation and isotopic mixing in a large C pool. Variations in δS did not follow ΔA at all times, especially during rainy periods and when there is a strong demand for C allocation above ground. It is likely that future isotope-enabled vegetation models will need to develop transfer functions that can account for these phenomena in order to interpret and predict the isotopic impact of biosphere gas exchange on the C isotope composition of atmospheric CO₂.
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.
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.
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 (h(2) (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(ST)<F(ST) are discussed.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Hydraulic failure is one of the main causes of tree mortality in conditions of severe drought. Resistance to cavitation is known to be strongly related to drought tolerance and species survival in ...conifers, but the threshold of water-stress-induced embolism leading to catastrophic xylem dysfunction in angiosperms has been little studied. We investigated the link between drought tolerance, survival and xylem cavitation resistance in five angiosperm tree species known to have contrasting desiccation resistance thresholds. We exposed seedlings in a greenhouse to severe drought to generate extreme water stress. We monitored leaf water potential, total plant water loss rate, leaf transpiration, stomatal conductance and CO2 assimilation rate during drought exposure and after rewatering (recovery phase). The time required for the recovery of 50% of the maximum value of a given ecophysiological variable after rewatering was used to determine the critical water potential corresponding to the threshold beyond which the plant failed to recover. We also investigated the relationship between this potential and stem xylem cavitation resistance, as assessed from vulnerability curves. This minimum recoverable water potential was consistent between ecophysiological variables and varied considerably between species, from -3.4 to -6.0 MPa. This minimum recoverable water potential was strongly correlated with P50 and P88, the pressures inducing 50 and 88% losses of stem hydraulic conductance, respectively. Moreover, the embolism threshold leading to irreversible drought damage was found to be close to 88%, rather than the 50% previously reported for conifers. Hydraulic failure leading to irreversible drought-induced global dysfunction in angiosperm tree species occurred at a very high level of xylem embolism, possibly reflecting the physiological characteristics of their stem water-transport system.
Summary
The long‐standing hypothesis that the isotopic composition of plant stem water reflects that of source water is being challenged by studies reporting bulk water from woody stems with an ...isotopic composition that cannot be attributed to any potential water source. The mechanism behind such source–stem water isotopic offsets is still poorly understood.
Using a novel technique to extract selectively sap water from xylem conduits, we show that, in cut stems and potted plants, the isotopic composition of sap water reflects that of irrigation water, demonstrating unambiguously that no isotopic fractionation occurs during root water uptake or sap water extraction. By contrast, water in nonconductive xylem tissues is always depleted in deuterium compared with sap water, irrespective of wood anatomy.
Previous studies have shown that isotopic heterogeneity also exists in soils at the pore scale in which water adsorbed onto soil particles is more depleted in deuterium than unbound water. Data collected at a riparian forest indicated that sap water matches best unbound soil water from depth below −70 cm, while bulk stem and soil water differ markedly.
We conclude that source–stem isotopic offsets can be explained by micrometre‐scale heterogeneity in the isotope ratios of water within woody stems and soil micro‐pores.
Context
Understanding plant resilience and adaptation to drought is a major challenge in crop and forest sciences. Several methods have been developed to assess the vulnerability to xylem embolism. ...The in situ flow centrifuge (or cavitron) is the fastest technique allowing to characterise this trait for plants having vessel lengths shorter than the rotor size.
Aims
We present (i) a series of changes to the earlier cavitron design, aimed at improving the accuracy and speed of measurement through automated operations, and (ii) a new development through the design of a large diameter rotor expanding the range of species that can be measured.
Methods
Both hardware and software modifications to the original design have been developed. In order to avoid artefacts caused by cut open vessels, a centrifuge with a large rotor (1 m) has been developed, and vulnerability curves obtained with this new device were compared with those obtained using reference methods.
Results
The new set-up expands the range of conductance measurable with a cavitron and enables it to accurately determine the absolute value of conductivity even for species having very low hydraulic conductivity. The large rotor cavitron shows good agreement with the reference techniques for conifers and diffuse-porous species but also for ring-porous species having long vessels.
Conclusion
The set-up described in this manuscript provides a faster, safer and more accurate method to construct vulnerability curves, compared to the original cavitron design, and extends the measurement capabilities to new species that are difficult to measure to date.
Key message
Recent improvements to cavitron setup enable to measure xylem vulnerability curves for an expanded number of plant species, with longer vessels or lower hydraulic conductivity.
The regulation of water loss and the spread of xylem embolism have mostly been considered separately. The development of an integrated approach taking into account the temporal dynamics and relative ...contributions of these mechanisms to plant drought responses is urgently needed. Do conifer species native to mesic and xeric environments display different hydraulic strategies and temporal sequences under drought? A dry-down experiment was performed on seedlings of four conifer species differing in embolism resistance, from drought-sensitive to extremely drought-resistant species. A set of traits related to drought survival was measured, including turgor loss point, stomatal closure, minimum leaf conductance, and xylem embolism resistance. All species reached full stomatal closure before the onset of embolism, with all but the most drought-sensitive species presenting large stomatal safety margins, demonstrating that highly drought-resistant species do not keep their stomata open under drought conditions. Plant dry-down time to death was significantly influenced by the xylem embolism threshold, stomatal safety margin, and minimum leaf conductance, and was best explained by the newly introduced stomatal margin retention index (SMRIΨ50) which reflects the time required to cross the stomatal safety margin. The SMRIΨ50 may become a key tool for the characterization of interspecific drought survival variability in trees.
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