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Open Access Publications from the University of California

Scholarly Works (8 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2014)

There has been increasing worldwide recognition of the importance of hydraulic physiology--the transport of water through the plant--in explaining plant growth and drought tolerance. By combining physiology and anatomy within an evolutionary framework, we can discover the mechanisms underlying species differences in hydraulic function, especially those of the leaf, the central organ in plant metabolism. I refined and developed new methods to investigate leaf water transport and its decline during drought, focusing on a critical measure of the capacity for water movement (leaf hydraulic conductance, Kleaf). I found that species most tolerant of Kleaf decline had small leaves with dense major veins, providing pathways for the water to bypass embolized conduits during drought giving a new, direct explanation to the fact that species of dry areas have small leaves. I also developed a new method to investigate the role of leaf shrinkage on water movement. As leaves shrink with dehydration, mesophyll cells lose connectivity, physically impacting water movement outside the xylem. I found that species most sensitive to Kleaf decline were those with strongest shrinkage in thickness. I then developed a new method to measure xylem hydraulic decline in leaves to test for a possible artifact of cutting leaf petioles under tension while under water. Such artifact has been recently found to occur in stems, and has put into question measurements of Kleaf. Across four diverse species, I found no sign of such an artifact in leaves, likely due to the lesser mechanical stress imposed when cutting a petiole vs. stem. Finally, I took an evolutionary perspective. I quantified the anatomical and physiological plasticity in leaves of six species of endemic Hawaiian lobeliads grown under different light regimes and found a high degree of plasticity in Kleaf with light, relating to leaf anatomical changes. Across 30 species of Viburnum I have identified the evolutionary shifts of leaf anatomy, water transport and drought tolerance. This work provides new techniques, clarity and applications toward understanding leaf water transport and its role in plant performance and drought tolerance, with applications for ecology, paleobiology and the conservation of species and ecosystems.

    Cover page: Leaf hydraulics and evolution
    • Article
    • Peer Reviewed
    UC Davis Previously Published Works (2016)
    As plant tissues dehydrate, water transport efficiency declines, a process typically attributed to air obstruction (embolism) in the xylem. Trifiló et al. (pages 5029-5039) dissect leaf hydraulic vulnerability and show that both xylem and living tissues may be important. If confirmed and clarified, an important role for outsidexylem hydraulic decline will change our understanding of how plants transport water and control biosphere carbon and water fluxes.
      Cover page: Why are leaves hydraulically vulnerable?
      • Article
      • Peer Reviewed
      UC Davis Previously Published Works (2017)
      The sites of evaporation within leaves are unknown, but they have drawn attention for decades due to their perceived implications for many factors, including patterns of leaf isotopic enrichment, the maintenance of mesophyll water status, stomatal regulation, and the interpretation of measured stomatal and leaf hydraulic conductances. We used a spatially explicit model of coupled water and heat transport outside the xylem, MOFLO 2.0, to map the distribution of net evaporation across leaf tissues in relation to anatomy and environmental parameters. Our results corroborate earlier predictions that most evaporation occurs from the epidermis at low light and moderate humidity but that the mesophyll contributes substantially when the leaf center is warmed by light absorption, and more so under high humidity. We also found that the bundle sheath provides a significant minority of evaporation (15% in darkness and 18% in high light), that the vertical center of amphistomatous leaves supports net condensation, and that vertical temperature gradients caused by light absorption vary over 10-fold across species, reaching 0.3°C. We show that several hypotheses that depend on the evaporating sites require revision in light of our findings, including that experimental measurements of stomatal and hydraulic conductances should be affected directly by changes in the location of the evaporating sites. We propose a new conceptual model that accounts for mixed-phase water transport outside the xylem. These conclusions have far-reaching implications for inferences in leaf hydraulics, gas exchange, water use, and isotope physiology.
        Cover page: The Sites of Evaporation within Leaves
        • Article
        • Peer Reviewed
        UC Davis Previously Published Works (2017)
        Leaf dry mass per unit leaf area (LMA) is a central trait in ecology, but its anatomical and compositional basis has been unclear. An explicit mathematical and physical framework for quantifying the cell and tissue determinants of LMA will enable tests of their influence on species, communities and ecosystems. We present an approach to explaining LMA from the numbers, dimensions and mass densities of leaf cells and tissues, which provided unprecedented explanatory power for 11 broadleaved woody angiosperm species diverse in LMA (33-262 g m-2 ; R2  = 0.94; P < 0.001). Across these diverse species, and in a larger comparison of evergreen vs. deciduous angiosperms, high LMA resulted principally from larger cell sizes, greater major vein allocation, greater numbers of mesophyll cell layers and higher cell mass densities. This explicit approach enables relating leaf anatomy and composition to a wide range of processes in physiological, evolutionary, community and macroecology.
          Cover page: The anatomical and compositional basis of leaf mass per area
          • Article
          • Peer Reviewed
          UCLA Previously Published Works (2014)
          It has been recently proposed that leaf vein length per area (VLA) is the major determinant of leaf mass per area ( MA), and would thereby determine other traits of the leaf economic spectrum (LES), such as photosynthetic rate per mass (A(mass)), nitrogen concentration per mass (N(mass)) and leaf lifespan (LL). In a previous paper we argued that this 'vein origin' hypothesis was supported only by a mathematical model with predestined outcomes, and that we found no support for the 'vein origin' hypothesis in our analyses of compiled data. In contrast to the 'vein origin' hypothesis, empirical evidence indicated that VLA and LMA are independent mechanistically, and VLA (among other vein traits) contributes to a higher photosynthetic rate per area (A(area)), which scales up to driving a higher A(mass), all independently of LMA, N(mass) and LL. In their reply to our paper, Blonder et al. (2014) raised questions about our analysis of their model, but did not address our main point, that the data did not support their hypothesis. In this paper we provide further analysis of an extended data set, which again robustly demonstrates the mechanistic independence of LMA from VLA, and thus does not support the 'vein origin' hypothesis. We also address the four specific points raised by Blonder et al. (2014) regarding our analyses. We additionally show how this debate provides critical guidance for improved modelling of LES traits and other networks of phenotypic traits that determine plant performance under contrasting environments.
            Cover page: Leaf mass per area is independent of vein length per area: avoiding pitfalls when modelling phenotypic integration (reply to )
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2019)
            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 (gmax) show greater sensitivity to closure during leaf dehydration, i.e., a higher leaf water potential at which stomatal conductance is reduced by 50% (Ψgs50). The gmax - Ψgs50 trade-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.
              Cover page: A stomatal safety-efficiency trade-off constrains responses to leaf dehydration.
              • Article
              • Peer Reviewed
              UC Davis Previously Published Works (2018)
              The influence of the dynamics of leaf hydraulic conductance (K leaf) diurnally and during dehydration on stomatal conductance and photosynthesis remains unclear. Using the model species Arabidopsis (Arabidopsis thaliana ecotype Columbia-0), we applied a multitiered approach including physiological measurements, high-resolution x-ray microcomputed tomography, and modeling at a range of scales to characterize (1) K leaf decline during dehydration; (2) its basis in the hydraulic conductances of leaf xylem and outside-xylem pathways (K ox); (3) the dependence of its dynamics on irradiance; (4) its impact on diurnal patterns of stomatal conductance and photosynthetic rate; and (5) its influence on gas exchange and survival under simulated drought regimes. Arabidopsis leaves showed strong vulnerability to dehydration diurnally in both gas exchange and hydraulic conductance, despite lack of xylem embolism or conduit collapse above the turgor loss point, indicating a pronounced sensitivity of K ox to dehydration. K leaf increased under higher irradiance in well-hydrated leaves across the full range of water potential, but no shift in K leaf vulnerability was observed. Modeling indicated that responses to dehydration and irradiance are likely attributable to changes in membrane permeability and that a dynamic K ox would contribute strongly to stomatal closure, improving performance, survival, and efficient water use during drought. These findings for Columbia-0 provide a baseline for assessing variation across genotypes in hydraulic traits and their influence on gas exchange during dehydration.
                Cover page: The Causes of Leaf Hydraulic Vulnerability and Its Influence on Gas Exchange in Arabidopsis thaliana
                • Article
                • Peer Reviewed
                UC Davis Previously Published Works (2017)
                Ecosystems worldwide are facing increasingly severe and prolonged droughts during which hydraulic failure from drought-induced embolism can lead to organ or whole plant death. Understanding the determinants of xylem failure across species is especially critical in leaves, the engines of plant growth. If the vulnerability segmentation hypothesis holds within leaves, higher order veins that are most terminal in the plant hydraulic system should be more susceptible to embolism to protect the rest of the water transport system. Increased vulnerability in the higher order veins would also be consistent with these experiencing the greatest tensions in the plant xylem network. To test this hypothesis, we combined X-ray micro-computed tomography imaging, hydraulic experiments, cross-sectional anatomy and 3D physiological modelling to investigate how embolisms spread throughout petioles and vein orders during leaf dehydration in relation to conduit dimensions. Decline of leaf xylem hydraulic conductance (Kx ) during dehydration was driven by embolism initiating in petioles and midribs across all species, and Kx vulnerability was strongly correlated with petiole and midrib conduit dimensions. Our simulations showed no significant impact of conduit collapse on Kx decline. We found xylem conduit dimensions play a major role in determining the susceptibility of the leaf water transport system during strong leaf dehydration.
                  Cover page: Leaf vein xylem conduit diameter influences susceptibility to embolism and hydraulic decline.
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