Temperature dependence of plant photosynthesis and respiration using the Macromolecular Rate Theory (MMRT)

Abstract

This thesis quantifies the temperature dependence of key traits that determine leaf carbon balance in C3 plants, using Sunflower as a model system, with emphasis on the photorespiratory CO2 compensation point (Γ*), the rate of CO2 release in the light (DL), the maximal carboxylation capacity (Vcmax), the maximal electron transport capacity (Jmax), and the components of dark respiration (Rdark). The central objective was to identify temperature dependence models for each trait that were both parsimonious and physiologically interpretable. The research combined controlled leaf gas exchange across wide ranges of temperature and CO2 with comparative model fitting. I evaluated constant-Q10 relationships, simple and peaked Arrhenius formulations, and macro molecular rate theory (MMRT), with and without an explicit high temperature deactivation term. Γ* increased approximately exponentially across the measurement range from 4 to 42 °C, and a constant-Q10 model best captured this behaviour. The temperature dependence of DL was well represented by MMRT. For the photosynthetic capacities, I used an MMRT formulation that used inflection-point temperature (Tinf) as parameters and included explicit deactivation terms. That approach captured the curvature at cool to moderate temperatures and the decline at supra-optimal temperatures, with Jmax exhibiting a cooler inflection and a cooler optimum than Vcmax. For Rdark and its components, an MMRT anchored at the observed inflection temperature and fitted without a deactivation term captured the smooth warm side curvature expected from progressive changes in mitochondrial coupling and accelerated substrate use with increasing temperature. This thesis provides a unified framework for selecting temperature dependence models for leaf gas exchange traits. It introduces DL as the light linked composite flux of CO2 release, rather than a proxy of dark respiration alone, and it presents a published ii refinement for estimating Γ* and DL using the Laisk method based on gas-exchange measurements at low light and low intercellular CO2 concentrations. The refined methodology explicitly used photosynthetic theory in our parameter estimation and reduced systematic bias in estimating Γ* and DL. These advances improve the parameterisation of photosynthesis models and strengthens the transfer to crop and land- surface modelling under variable thermal environments. The thesis is organised in seven chapters. Chapter 3 has been published, and when it is cited elsewhere in the thesis it is referenced as Moreno-Echeverry et al., 2026. Chapter 1. General introduction. It sets the global climate and carbon cycle context, frames leaf carbon balance as the net outcome of photosynthesis and respiration, and explains why accurate parameterisation of Γ*, DL, Vcmax and Jmax matters for models. It defines temperature as a primary driver of gas-exchange rates, outlines the limit of Arrhenius and constant-Q10 functions, introduces MMRT as a thermodynamic alternative, and states the thesis aims. Chapter 2. Background. It reviews photosynthesis and respiration at the leaf scale, outlines gas exchange principles, describes the FvCB model and presents methods used to estimate Γ* and DL. It summarises temperature response models, including the constant- Q10 function, two variants of the Arrhenius equation and MMRT. The chapter also identifies the knowledge gaps that motivate the specific work described in the thesis. Chapter 3. Estimating Γ* and DL using the Laisk method combined with photosynthetic theory. It examines the classical Laisk approach, identifies sources of bias arising from linearisation of inherently curvilinear Anet-Cc responses, and formalises estimation of Γ* and DL within the FvCB framework. It uses simulations and leaf gas exchange measurements to test performance and uncertainty of the modified approach and compares it with the performance and uncertainty of the original Laisk approach. This chapter is based on the published methodological study. iii

Chapter 4. Temperature dependence of Γ* and DL. It quantifies how Γ* and DL vary with temperature over the range from 4 to 42 °C. It compares the constant-Q10, simple and peaked Arrhenius, and MMRT, with and without explicit high temperature deactivation term. It reports model selection, parameter estimates and presents relevant diagnostics. Chapter 5. Temperature dependence of Vcmax and Jmax. The chapter derives these parameters from Anet-Ci curves. It compares the Arrhenius formulations with MMRT, evaluates curvature, inflection and optimum temperatures, and contrast the thermal responses of carboxylation and electron transport. Chapter 6. Temperature dependence of the components of leaf CO2 release in the dark. It separates and analyses the different components of dark respiration, post illumination burst, light enhanced dark respiration and dark respiration in steady-state across temperature and integrates metabolite profiling across the light-dark transition to relate CO2 fluxes to substrate availability. It tests the Arrhenius and MMRT formulations and identifies the model that best capture warm side curvature without deactivation terms. Chapter 7. Conclusions and perspectives. It synthesises the finding across chapters, and temperature response models, discusses implications for photosynthesis and respiration modelling, and outlines priorities for future research.