& Key message The relationship between relative water loss (RWL) and hydraulic conductivity loss (PLC) in sapwood is robust across conifer species. We provide an empirical model (conifer-curve) for predicting PLC from simple RWL measurements. The approach is regarded as a new relevant phenotyping tool for drought sensitivity and offers reliable and fast prediction of diurnal, seasonal, or drought-induced changes in PLC. & Context For conifer species drought is one of the main climate risks related to loss of hydraulic capacity in sapwood inducing dieback or mortality. More frequently occurring drought waves call for fast and easily applicable methods to predict drought sensitivity. & Aims We aimed at developing a fast and reliable method for determination of the percent loss of hydraulic conductivity (PLC) and eventually the drought sensitivity trait P 50 , i.e., the water potential that causes 50% conductivity loss. & Methods We measured the loss of water transport capacity, defined as the relative water loss (RWL) together with PLC in trunk wood, branches, and saplings of eight different conifer species. Air injection was used to induce specific water potentials. & Results The relationship between RWL and PLC was robust across species, organs, and age classes. The equation established allows fast prediction of PLC from simple gravimetrical measurements and thus post hoc calculation of P 50 (r 2 = 0.94). & Conclusion The approach is regarded as a relevant new phenotyping tool. Future potential applications are screening conifers for drought sensitivity and a fast interpretation of diurnal, seasonal, or drought-induced changes in xylem water content upon their impact on conductivity loss.
This article is a Commentary on Harwood et al. (2020), 225: 2567–2578.
Leaves grown at different light intensities exhibit considerable differences in physiology, morphology, and anatomy. Because plant leaves develop over three dimensions, analyses of the leaf structure should account for differences in lengths, surfaces, as well as volumes. In this manuscript, we set out to disentangle the mesophyll surface area available for diffusion per leaf area (Sm,LA) into underlying one-, two-, and three-dimensional components. This allowed us to estimate the contribution of each component to Sm,LA, a whole-leaf trait known to link structure and function. We introduce the novel concept of a “stomatal vaporshed,” i.e. the intercellular airspace unit most closely connected to a single stoma, and use it to describe the stomata-to-diffusive-surface pathway. To illustrate our new theoretical framework, we grew two cultivars of Vitis vinifera L. under high and low light, imaged 3D leaf anatomy using microcomputed tomography (microCT), and measured leaf gas exchange. Leaves grown under high light were less porous and thicker. Our analysis showed that these two traits and the lower Sm per mesophyll cell volume (Sm,Vcl) in sun leaves could almost completely explain the difference in Sm,LA. Further, the studied cultivars exhibited different responses in carbon assimilation per photosynthesizing cell volume (AVcl). While Cabernet Sauvignon maintained AVcl constant between sun and shade leaves, it was lower in Blaufränkisch sun leaves. This difference may be related to genotype-specific strategies in building the stomata-to-diffusive-surface pathway.
Research Highlights: novel fast and easily assessable proxies for vulnerability to cavitation of conifer sapwood are proposed that allow reliable estimation at the species level. Background and Objectives: global warming calls for fast and easily applicable methods to measure hydraulic vulnerability in conifers since they are one of the most sensitive plant groups regarding drought stress. Classical methods to determine P12, P50 and P88, i.e., the water potentials resulting in 12, 50 and 88% conductivity loss, respectively, are labour intensive, prone to errors and/or restricted to special facilities. Vulnerability proxies were established based on empirical relationships between hydraulic traits, basic density and sapwood anatomy. Materials and Methods: reference values for hydraulic traits were obtained by means of the air injection method on six conifer species. Datasets for potential P50 proxies comprised relative water loss (RWL), basic density, saturated water content as well as anatomical traits such as double wall thickness, tracheid lumen diameter and wall/lumen ratio. Results: our novel proxy P25W, defined as 25% RWL induced by air injection, was the most reliable estimate for P50 (r = 0.95) and P88 (r = 0.96). Basic wood density (r = −0.92), tangential lumen diameters in earlywood (r = 0.88), wall/lumen ratios measured in the tangential direction (r = −0.86) and the number of radial cell files/mm circumference (CF/mm, r = −0.85) were also strongly related to P50. Moreover, CF/mm was a very good predictor for P12 (r = −0.93). Conclusions: the proxy P25W is regarded a strong phenotyping tool for screening conifer species for vulnerability to cavitation assuming that the relationship between RWL and conductivity loss is robust in conifer sapwood. We also see a high potential for the fast and easily applicable proxy CF/mm as a screening tool for drought sensitivity and for application in dendroecological studies that investigate forest dieback.
<p><span>Plant physiologists have used microscopy to study how leaf anatomy is related to photosynthetic performance and how this relation is affected by environmental conditions. However, leaf anatomy is not invariant over time: small pores on the leaf surface (stomata) open and close within minutes in response to the availability of water, CO</span><sub><span>2</span></sub><span> and light. Within tens of minutes following a water deficit, cells in </span><span>many</span><span> leaves also shrink significantly in volume and the leaf undergoes structural changes as a result of wilting. Gas-exchange setups can monitor changes in photosynthesis and transpiration under such conditions, but classical microscopy techniques are not well-suited to capture the concomitant changes in leaf anatomy </span><span>for two main reasons</span><span>. </span><span>First, available non-destructive microscopy techniques are limited in resolution and imaging depth, making it difficult to analyze changes in anatomy to the required detail. Second, u</span><span>sing sectioned fixated samples is known to be associated with tissue shrinkage, swelling or deformation, making estimates of cellular volumes and surfaces prone to art</span><span>i</span><span>facts. </span><span>Moreover, t</span><span>he destructive nature of </span><span>these</span><span> techniques makes it impossible to monitor changes in leaf anatomy during ongoing gas-exchange measurements. These limitations hinder advancing our understanding of the relation between leaf anatomy and photosynthesis </span><span>or</span><span> transpiration.</span></p><p><span>Here, we present a novel gas-exchange setup that combines synchrotron-base</span><span>d </span><span>high-resolution computed tomography (microCT) with </span><span>c</span><span>oncurrent measurements of gas-exchange using an commercially available infra-red gas analyzer. We designed </span><span>and constructed </span><span>a novel gas-exchange cuvette with CO</span><sub><span>2</span></sub><span> and H</span><sub><span>2</span></sub><span>O control that allow</span><span>s</span> <span>for</span><span> non-invasive monitoring of leaf anatomy in a microCT setup. Custom-built sensors were used to </span><span>measure</span><span> light intensity and leaf temperature. At given time points </span><span>during gas-exchange measurements</span><span>, 300-500 X-ray projections </span><span>(</span><span>100 ms</span><span>)</span><span> were taken while the chamber rotated 180&#176;. </span><span>From this data, a</span><span> leaf volume corresponding to 0.5 mm</span><sup><span>2</span></sup><span> leaf surface was reconstructed </span><span>at high resolution (</span><span>0.325 &#181;m </span><span>per </span><span>voxel edge). </span></p><p><span>The setup provides 3D images that can be used to measure the aperture of multiple stomata and the volumes, shapes and surface areas of cells and airspaces within the leaf. We found that the same leaf section can be scanned several times without measurable radiation damage, allowing for the combination of </span><span>three</span><span> spatial dimensions with time to create a 4D analysis of the leaf structure. Using poplar, willow and </span><span><em>Arabidopsis</em></span><span> leaves we studied how leaf anatomy rapidly adjusts after limiting water availability and show that such effects are not limited to the stomatal pore alone. We discuss the issues and pitfalls with the methodology and suggest avenues for future improvement.</span></p>
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