C3 photosynthesis

  • Motivation: From my graduate studies up to present, I have been working to understand photosynthesis. Over the years, I have focused on multiple aspects of this topic and undertaken projects involving field observations, instrument development, laboratory experiments, and mathematical modeling. These cumulative investments have come together in the paper described on this page [J. E. Johnson and J. A. Berry. 2021. Photosynthesis Research 148: 101-136].

  • Background: In this paper, I ask the question: What controls how the overall photosynthetic process responds to light? I grew interested in this question during my graduate studies, when I was trying to understand photosynthesis in plants adapted to extreme environments. At that time, the conventional approach for analysis of light response measurements was based on empirical functions. While these functions could be fit to my measurements, they were unsatisfying to me because they didn’t provide insight into how the photosynthetic light response actually works.

  • Approach: To tackle this question, I developed new experimental methods, applied them in laboratory studies, and worked out the relationship between the dynamics of photosynthesis in intact leaves and the biochemical properties of a cryptic enzyme called the Cytochrome b6f complex (Cyt b6f). My key insight was that Cyt b6f controls light-limited photosynthesis in a way that is analogous to how Rubisco controls light-saturated photosynthesis. This allowed me to formulate a new model of the light response that is unique in that it is fully mechanistic, yet still has a simple structure.

  • 1. Grand Canyon

    This story starts in the Grand Canyon, a place where you can see with unusual clarity the ways the Earth has changed over the last two billion years, and the role photosynthesis has had in driving these changes.

  • 2. Colorado River

    The Colorado River has sliced a path back through time that exposes the history of the oxygenation of the atmosphere and the resulting accumulation of organic matter in silts, muds, and clays.

  • 3. Grand Canyon Youth

    A partnership with a unique NGO called Grand Canyon Youth gave us the opportunity to travel down the Colorado River in search of a plant that is one of the missing links in the history of photosynthesis.

  • 4. Taking the lab to the field

    With support from Grand Canyon Youth, the USGS Grand Canyon Monitoring and Research Center, and Grand Canyon National Park, we floated a mobile photosynthesis laboratory down the Colorado River.

  • 5. Flaveria macdougallii

    Our goal was to study the photosynthetic performance of a plant called Flaveria macdougallii that is only found around groundwater seeps and springs that emerge from the walls of the Grand Canyon.

  • 6. My motivation

    Flaveria macdougalli was particularly intriguing to me because it is the basal species in the genus that has most recently evolved from C3, through C3-C4 intermediate, to C4 photosynthesis.

  • 7. The evolutionary play

    To understand the evolution of C4 photosynthesis in the Flaveria, we need to have an understanding of the properties of the basal C3 photosynthetic system that evolution had to work with.

  • 8. The ecological theater

    We also need to have an understanding of the ecology of the places where the C3 species in Flaveria are successful - in G. E. Hutchinson’s words, “the ecological theater for the evolutionary play.”

  • 9. Lights, camera, action!

    Finally, we need to understand the physiology of the plants in these environments. Most physiological studies have only focused on plants grown under controlled conditions and measured in the laboratory.

  • 10. Photosynthesis under extreme conditions

    We discovered that Flaveria macdougallii used groundwater from the seeps and springs to drive photosynthesis under short photoperiods, very high temperatures, and strong vapor pressure deficits.

  • 11. We needed the right model

    How exactly did Flaveria macdougallii do this? To answer this question, we needed a model that could help us differentiate between the environmental and biochemical controls on photosynthesis.

  • 12. We decided to build a new model

    Since the classic model of photosynthesis that was developed by Graham Farquhar, Susanne von Caemmerer, and Joe Berry had never resolved the electron transport system, we decided to build a new model that had this capability.

  • 13. We figured out it!

    The new model is described in: J. E. Johnson and J. A. Berry (2021) The role of Cytochrome b6f in the control of steady-state photosynthesis: a conceptual and quantitative model. Photosynthesis Research 148: 101-136.

The full story, and its significance

  • Full story: The details of this study can be found in the following reference:

  • Significance: This model can replace the empirical models of the light response that have been used in areas ranging from plant physiology to remote sensing. It creates a range of exciting new opportunities for studying and simulating photosynthesis across scales.