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1. Stomatal heterogeneity in responses to humidity and temperature: Testing a mechanistic model

   https://doi.org/10.1111/pce.13051  

   Sweet, Kathryn J. ; Peak, David ; Mott, Keith A. ( October 2017 , Plant, Cell & Environment)

   The role of stomatal heterogeneity in the response of stomatal conductance (g_s) to the mole fraction difference in water vapour between the inside of the leaf and the ambient air (Δw) was determined using thermography and gas exchange for 3 species. The value of Δwfor the leaf was varied in 2 different ways: first by varying air humidity while holding leaf temperature constant and second by varying leaf temperature while holding air humidity constant. Stomatal heterogeneity was explored by examining the response ofg_sin small areas of the leaf (as determined by thermography) and comparing them to each other and to the average value ofg_s(as determined by gas exchange). These analyses show that despite substantial heterogeneity ing_svalues, the response ofg_sto Δwwas qualitatively similar in all areas of the leaf, and all responses ofg_sto Δwwere well predicted by a recently proposed, vapour‐phase mechanism for stomatal responses to temperature and humidity. Remarkably, the 2 model parameters, Θ andZ, that depend on leaf anatomy were constant for a given species, and only the maximum conductance varied in different regions of the leaf.

    

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2. Aquaporin regulation in roots controls plant hydraulic conductance, stomatal conductance, and leaf water potential in Pinus radiata under water stress

   https://doi.org/10.1111/pce.13460  

   Rodríguez‐Gamir, Juan ; Xue, Jianming ; Clearwater, Michael J. ; Meason, Dean F. ; Clinton, Peter W. ; Domec, Jean‐Christophe ( November 2018 , Plant, Cell & Environment)

   Stomatal regulation is crucial for forest species performance and survival on drought‐prone sites. We investigated the regulation of root and shoot hydraulics in threePinus radiataclones exposed to drought stress and its coordination with stomatal conductance (g_s) and leaf water potential (Ψ_leaf). All clones experienced a substantial decrease in root‐specific root hydraulic conductance (K_root‐r) in response to the water stress, but leaf‐specific shoot hydraulic conductance (K_shoot‐l) did not change in any of the clones. The reduction inK_root‐rcaused a decrease in leaf‐specific whole‐plant hydraulic conductance (K_plant‐l). Among clones, the larger the decrease inK_plant‐l, the more stomata closed in response to drought. Rewatering resulted in a quick recovery ofK_root‐randg_s. Our results demonstrated that the reduction inK_plant‐l, attributed to a down regulation of aquaporin activity in roots, was linked to the isohydric stomatal behaviour, resulting in a nearly constant Ψ_leafas water stress started. We concluded that higherK_plant‐lis associated with water stress resistance by sustaining a less negative Ψ_leafand delaying stomatal closure.

    

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3. CO2, nitrogen deposition and a discontinuous climate response drive water use efficiency in global forests

   https://doi.org/10.1038/s41467-021-25365-1  

   Adams, Mark A. ; Buckley, Thomas N. ; Binkley, Dan ; Neumann, Mathias ; Turnbull, Tarryn L. ( August 2021 , Nature Communications)

   Reduced stomatal conductance is a common plant response to rising atmospheric CO₂and increases water use efficiency (W). At the leaf-scale,Wdepends on water and nitrogen availability in addition to atmospheric CO₂. In hydroclimate modelsWis a key driver of rainfall, droughts, and streamflow extremes. We used global climate data to derive Aridity Indices (AI) for forests over the period 1965–2015 and synthesised those with data for nitrogen deposition andWderived from stable isotopes in tree rings. AI and atmospheric CO₂account for most of the variance inWof trees across the globe, while cumulative nitrogen deposition has a significant effect only in regions without strong legacies of atmospheric pollution. The relation of aridity andWdisplays a clear discontinuity.Wand AI are strongly related below a threshold value of AI ≈ 1 but are not related where AI > 1. Tree ring data emphasise that effective demarcation of water-limited from non-water-limited behaviour of stomata is critical to improving hydrological models that operate at regional to global scales.

    

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4. A stomatal safety-efficiency trade-off constrains responses to leaf dehydration

   https://doi.org/10.1038/s41467-019-11006-1  

   Henry, Christian ; John, Grace P. ; Pan, Ruihua ; Bartlett, Megan K. ; Fletcher, Leila R. ; Scoffoni, Christine ; Sack, Lawren ( July 2019 , Nature Communications)

   Stomata, the microvalves on leaf surfaces, exert major influences across scales, from plant growth and productivity to global carbon and water cycling. Stomatal opening enables leaf photosynthesis, and plant growth and water use, whereas plant survival of drought depends on stomatal closure. Here we report that stomatal function is constrained by a safety-efficiency trade-off, such that species with greater stomatal conductance under high water availability (g_max) show greater sensitivity to closure during leaf dehydration, i.e., a higher leaf water potential at which stomatal conductance is reduced by 50% (Ψ_gs50). Theg_max- Ψ_gs50trade-off and its mechanistic basis is supported by experiments on leaves of California woody species, and in analyses of previous studies of the responses of diverse flowering plant species around the world. Linking the two fundamental key roles of stomata—the enabling of gas exchange, and the first defense against drought—this trade-off constrains the rates of water use and the drought sensitivity of leaves, with potential impacts on ecosystems.

    

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5. Integrated metabolic strategy: A framework for predicting the evolution of carbon‐water tradeoffs within plant clades

   https://doi.org/10.1111/1365-2745.13204  

   Goud, Ellie M. ; Sparks, Jed P. ; Fishbein, Mark ; Agrawal, Anurag A. ; Shefferson, ed., Richard ( May 2019 , Journal of Ecology)

   The fundamental tradeoff between carbon gain and water loss has long been predicted as an evolutionary driver of plant strategies across environments. Nonetheless, challenges in measuring carbon gain and water loss in ways that integrate over leaf lifetime have limited our understanding of the variation in and mechanistic bases of this tradeoff. Furthermore, the microevolution of plant traits within species versus the macroevolution of strategies among closely related species may not be the same, and accordingly, the latter must be addressed using comparative phylogenetic analyses.

   Here we introduce the concept of ‘integrated metabolic strategy’ (IMS) to describe the ratio between carbon isotope composition (δ¹³C) and oxygen isotope composition above source water (Δ¹⁸O) of leaf cellulose. IMS is a measure of leaf‐level conditions that integrate several mechanisms contributing to carbon gain (δ¹³C) and water loss (Δ¹⁸O) over leaf lifespan, with larger values reflecting higher metabolic efficiency and hence less of a tradeoff. We tested how IMS evolves among closely related yet ecologically diverse milkweed species, and subsequently addressed phenotypic plasticity in response to water availability in species with divergent IMS.

   Integrated metabolic strategy varied strongly among 20Asclepiasspecies when grown under controlled conditions, and phylogenetic analyses demonstrate species‐specific tradeoffs between carbon gain and water loss. Larger IMS values were associated with species from dry habitats, with larger carboxylation capacity, smaller stomatal conductance and smaller leaves; smaller IMS was associated with wet habitats, smaller carboxylation capacity, larger stomatal conductance and larger leaves. The evolution of IMS was dominated by changes in species’ demand for carbon (δ¹³C) more so than water conservation (Δ¹⁸O). Although some individual physiological traits showed phylogenetic signal, IMS did not.

   In response to experimental decreases in soil moisture, three species maintained similar IMS across levels of water availability because of proportional increases inδ¹³C and Δ¹⁸O (or little change in either), while one species increased IMS due to disproportional changes inδ¹³C relative to Δ¹⁸O.

   Synthesis.IMS is a broadly applicable mechanistic tool; IMS variation among and within species may shed light on unresolved questions relating to the evolution and ecology of plant ecophysiological strategies.

    

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