Understanding the traits that drive stomatal responses to environmental stress and change, particularly water stress
Stomata are central to how plants respond to environmental stress, but research into the integrated control of plant responses has long been hampered by reliance on empirical models for stomatal conductance, which do not allow insights about how different traits and processes interact in various species to produce observed responses. One of my major career goals is to fill this critical gap by providing a mechanistic model for stomatal conductance and demonstrating its application in crops and natural systems. We are extending this model to guide continuing research to understand the array of traits that control drought and other stress responses, to accommodate recent discoveries about the mechanisms of leaf drought response, to predict stomatal behavior in darkness and to understand mechanisms behind variation in nocturnal water loss in crops. [Funded by Australian Research Council Discovery Project 150103863.]
Combining optimization theory with mechanistic modeling to understand the traits that limit the efficacy of water and N use in crops
Plants typically diverge systematically and greatly from theoretically optimal spatiotemporal distributions of photosynthetic inputs (water and nitrogen) across canopies, and this has potentially large implications for crop yield. We are using theoretical modeling to understand these deviations and to identify the traits or assemblages of traits that are responsible. The end goal is to inform breeding efforts and management of irrigation and fertilization. We are also developing similar analyses in relation to whole-plant structural acclimation to elevated atmospheric carbon dioxide and other aspects of global climate change.
In one set of studies, we are quantifying the divergence of actual and optimal spatio-temporal patterns of water use in grapevine and almond canopies and using mechanistic models of the regulation of water loss to identify the traits responsible for this divergence. In another study, we are doing the same thing for the distribution of photosynthetic nitrogen in wheat canopies: theoretical modeling of canopy photosynthetic N partitioning predicts that whole-plant net photosynthesis would be maximized if plants put more N towards the top of the canopy, and less towards the bottom, than is actually observed in nature. The potential benefits are huge — on the order of 20% improvements in photosynthesis. We are screening a wide and diverse range of germplasm in wheat and its wild relatives to find lines with more-nearly optimal N distribution, and exploring the underlying mechanisms to help guide breeding efforts. [Funded by the International Wheat Yield Partnership, via the Grains Research and Development Corporation, award US00082.]
Understanding the mechanisms of adaptation to drought and high temperatures in native trees
In another project, we are performing a common garden experiment to understand what assemblages of physiological and structural traits help different species of native eucalypts compete for light and thrive across a rainfall transect in Victoria, Australia. This work is informed by optimization modeling of carbon partitioning at the whole-tree scale. [Funded by NSF Award 1557906.]
Improving inference of canopy transpiration from sap flow measurements, and studying the role of soil and tree water storage in the dynamics of catchment runoff in relation to drought and fire
It is common to use sap flow methods to quantify water loss in crop trees and natural forests, for the purpose of informing irrigation strategies and estimating large-scale fluxes. These methods have serious limitations, however: for one, they measure water flow at the base of trees, which can be substantially decoupled from canopy water loss by trunk water storage, and for another, they are incapable of measuring both high- and low-flow conditions accurately with a single method.
We have developed and validated a new method that can simultaneously measure low, negative and high flow rates and trunk water storage, and have recently installed an array of sensors in field sites in subalpine forests in Australia. The data from these new sensors will inform a study into what controls the response of water yield from forested catchments to drought and fire. [Funded by Australian Research Council Linkage Project 130101183.]
Effects of environmental conditions on leaf water transport, and impacts on regulation of gas exchange
Historically, most research in leaf and plant water transport has focused on the xylem network, but recent work highlights the importance, both qualitatively and quantitatively, of water transport outside of the xylem in leaves. With collaborators at UCLA, I have built a spatially explicit model of outside-xylem coupled heat and water transport, and we’re using this model to understand how differences in leaf anatomy across species and genotypes drives plant responses to soil and atmospheric drought, light and temperature. I am also contributing to a study applying this modeling approach to conifer leaves and expanding the analysis to address impacts of leaf hydraulic properties on carbon-water balance across scales [Funded by NSF Award 1146514.]
My colleagues Dan Johnson and Craig Brodersen were recently awarded an NSF grant, on which I am a collaborator, to explore the mechanisms of leaf hydraulic responses to water stress and their role in carbon-water relations in conifer leaves. This work will combine field and lab measurements, x-ray micro-computed tomography for high resolution visualization of leaf internal structure, and advanced three-dimensional modeling of water and heat transport within leaves to understand what drives hydraulic function and dysfunction in conifers. [Funded by NSF Award
Understanding leaf respiration in the light
Leaves respire both in darkness and in the light, but since respiration is typically accompanied by much larger photosynthetic and photorespiratory fluxes in the light, respiration is difficult to measure in the light. I created a model based on stoichiometric flux balance of energy carriers that couples photosynthesis, photorespiration and non-photorespiratory CO2 release in leaves, and am using this model to understand mechanisms of suppression of respiration by light. We are carrying out careful measurements of leaf gas exchange at low light intensities to test hypotheses about how the shift from oxidative to reductive carbon flow through the pentose phosphate pathway at low light may help explain the “Kok effect,” a change in the apparent quantum yield of CO2 exchange that occurs at very low light.