John Richard Evans

Summary: Mesophyll conductance ( g _m ) limits CO ₂ delivery to Rubisco, yet its responses to temperature, heat and water deficit are poorly resolved in cotton ( Gossypium hirsutum ). We characterised photosynthetic and g _m temperature responses in five Gossypium species, then examined cultivated G. hirsutum and the Australian native Gossypium bickii under combined elevated temperature and soil water deficit to link physiological responses with mesophyll anatomy. Plants were grown under controlled glasshouse conditions. We quantified g _m using simultaneous gas exchange and online stable carbon isotope discrimination (Δ, ‰) and measured leaf anatomy. Across species, photosynthesis and g _m exhibited broad thermal optima (33–36°C). In G. hirsutum , water deficit at control temperature reduced g _m and induced thicker, more porous mesophyll and thicker cell walls, while assimilation declined only under combined heat and water deficit. In G. bickii , g _m and assimilation remained stable under combined stress, and g _m was stimulated by elevated temperature without anatomical adjustment. g _m emerged as a stress‐sensitive trait in cultivated cotton, whereas G. bickii exhibited stress resilience, with responses incompletely explained by anatomical acclimation. Further research on g _m responses to environmental stresses and on the mechanisms underpinning these responses is required to inform model development and identify traits for climate resilient crops.

Summary: Mesophyll conductance ( g _m ) governs CO ₂ diffusion to Rubisco and is a key determinant of photosynthetic performance, yet the mechanisms underlying its sensitivity to heat and water stress remain unresolved. We quantified g _m temperature responses across diverse Gossypium species and examined anatomical drivers of g _m plasticity in cultivated cotton ( G. hirsutum ) and the wild Australian species G. bickii under elevated temperature and soil water deficit. Species exhibited contrasting g _m strategies: G. hirsutum exhibited high g _m and carbon assimilation near thermal optima but showed greater sensitivity under combined heat and water deficit, whereas G. bickii maintained comparatively stable g _m and photosynthesis across stress conditions. Under water deficit, structural adjustments in G. hirsutum (increased leaf porosity, cell wall thickness and mesophyll surface exposure to intercellular airspaces) were insufficient to sustain g _m , suggesting that liquid-phase resistances impose dominant constraints on CO ₂ diffusion under extreme climatic stress. These results identify g _m as a dynamic, multi-component trait and a key physiological vulnerability in cotton, shaped by coordinated anatomical characteristics and potentially cell wall properties and membrane-associated processes, with major implications for mechanistic photosynthesis modelling and improving climate resilience in cotton and other C ₃ species.

Purpose: This work reports for the first time on the implementation and application of cardiac diffusion‐weighted MRI on a Connectom MR scanner with a maximum gradient strength of 300 mT/m. It evaluates the benefits of the increased gradient performance for the investigation of the myocardial microstructure.Methods: Cardiac diffusion‐weighted imaging (DWI) experiments were performed on 10 healthy volunteers using a spin‐echo sequence with up to second‐ and third‐order motion compensation ( and ) and , and 1000 (twice the commonly used on clinical scanners). Mean diffusivity (MD), fractional anisotropy (FA), helix angle (HA), and secondary eigenvector angle (E2A) were calculated for b = [100, 450] and b = [100, 1000] for both and .Results: The MD values with are slightly higher than with with for and for . A reduction in MD is observed by increasing the from 450 to 1000 ( for and for ). The difference between FA, E2A, and HA was not significant in different schemes ().Conclusion: This work demonstrates cardiac DWI in vivo with higher b‐value and higher order of motion compensated diffusion gradient waveforms than is commonly used. Increasing the motion compensation order from to and the maximum b‐value from 450 to 1000 affected the MD values but FA and the angular metrics (HA and E2A) remained unchanged. Our work paves the way for cardiac DWI on the next‐generation MR scanners with high‐performance gradient systems.

Leaf gas exchange measurements are an important tool for inferring a plant's photosynthetic biochemistry. In most cases, the responses of photosynthetic CO₂ assimilation to variable intercellular CO₂ concentrations (A/C_i response curves) are used to model the maximum (potential) rate of carboxylation by ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco, V_cmax) and the rate of photosynthetic electron transport at a given incident photosynthetically active radiation flux density (PAR; J_PAR). The standard Farquhar‐von Caemmerer‐Berry model is often used with default parameters of Rubisco kinetic values and mesophyll conductance to CO₂ (g_m) derived from tobacco that may be inapplicable across species. To study the significance of using such parameters for other species, here we measured the temperature responses of key in vitro Rubisco catalytic properties and g_m in cotton (Gossypium hirsutum cv. Sicot 71) and derived V_cmax and J₂₀₀₀ (J_PAR at 2000 µmol m⁻² s⁻¹ PAR) from cotton A/C_i curves incrementally measured at 15°C–40°C using cotton and other species‐specific sets of input parameters with our new automated fitting R package ‘OptiFitACi’. Notably, parameterisation by a set of tobacco parameters produced unrealistic J₂₀₀₀:V_cmax ratio of V_cmax above 15°C, up to 2.3‐fold higher estimates of J₂₀₀₀ and more variable estimates of V_cmax and J₂₀₀₀, for our cotton data compared to model parameterisation with cotton‐derived values. We determined that errors arise when using a g_m,25 of 2.3 mol m⁻² s⁻¹ MPa⁻¹ or less and Rubisco CO₂‐affinities in 21% O₂ (K_C^21%O2) at 25°C outside the range of 46–63 Pa to model A/C_i responses in cotton. We show how the A/C_i modelling capabilities of ‘OptiFitACi’ serves as a robust, user‐friendly, and flexible extension of ‘plantecophys’ by providing simplified temperature‐sensitivity and species‐specificity parameterisation capabilities to reduce variability when modelling V_cmax and J₂₀₀₀.

Leaf gas exchange measurements provide an important tool for inferring a plant’s photosynthetic biochemistry. In most cases, the responses of photosynthetic CO ₂ assimilation to variable intercellular CO ₂ concentrations ( A / C _i response curves) are used to model the maximum rate of carboxylation by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, V _cmax ) and the rate of electron transport at a given photosynthetically active radiation (PAR; J _PAR ). The standard Farquhar-Von Caemmerer-Berry model is typically used with default parameters of Rubisco kinetic values and mesophyll conductance to CO ₂ ( g _m ) derived from tobacco that impairs analytical reliability across species. To study this, here we measured the temperature responses of key in vitro Rubisco catalytic properties and g _m in cotton ( Gossypium hirsutum cv. Sicot 71) and derived V _cmax and J ₂₀₀₀ ( J at 2000 µmol m ^-2 s ^-1 PAR) from cotton A / C _i curves incrementally measured at 15°C to 40°C using cotton and tobacco parameters with our new automated fitting R package ‘OptiFitACi’. When applied to cotton, the tobacco parameters produced unrealistic J ₂₀₀₀ : V _cmax ratio of V _cmax , approximately 50% higher estimates of J ₂₀₀₀ and more variable estimates of V _cmax and J ₂₀₀₀ , compared to model parameterisation with cotton-derived values. We determined that errors arise when using a g _m of 0.23 mol m ^-2 s ^-1 bar ^-1 or below and Rubisco CO ₂ -affinities under ambient O ₂ ( K _C ^21%O2 ) outside 461 µbar to 627 µbar to model A / C _i responses in cotton. We show how the multi- A / C _i modelling capabilities of ‘OptiFitACi’ serves as a robust, user-friendly extension of ‘plantecophys’ by providing simplified temperature-sensitivity and species-specificity parameterisation capabilities to enable higher accuracy estimates of V _cmax and J ₂₀₀₀ .

Net photosynthetic CO₂ assimilation rate (A_n) decreases at leaf temperatures above a relatively mild optimum (T_opt) in most higher plants. This decline is often attributed to reduced CO₂ conductance, increased CO₂ loss from photorespiration and respiration, reduced chloroplast electron transport rate (J), or deactivation of Ribulose-1,5-bisphosphate Carboxylase Oxygenase (Rubisco). However, it is unclear which of these factors can best predict species independent declines in A_n at high temperature. We show that independent of species, and on a global scale, the observed decline in A_n with rising temperatures can be effectively accounted for by Rubisco deactivation and declines in J. Our finding that A_n declines with Rubisco deactivation and J supports a coordinated down-regulation of Rubisco and chloroplast electron transport rates to heat stress. We provide a model that, in the absence of CO₂ supply limitations, can predict the response of photosynthesis to short-term increases in leaf temperature.

Summary: The rate with which crop yields per hectare increase each year is plateauing at the same time that human population growth and other factors increase food demand. Increasing yield potential () of crops is vital to address these challenges. In this review, we explore a component of that has yet to be optimised – that being improvements in the efficiency with which light energy is converted into biomass () via modifications to CO₂ fixed per unit quantum of light (α), efficiency of respiratory ATP production () and efficiency of ATP use (). For α, targets include changes in photoprotective machinery, ribulose bisphosphate carboxylase/oxygenase kinetics and photorespiratory pathways. There is also potential for to be increased via targeted changes to the expression of the alternative oxidase and mitochondrial uncoupling pathways. Similarly, there are possibilities to improve via changes to the ATP costs of phloem loading, nutrient uptake, futile cycles and/or protein/membrane turnover. Recently developed high‐throughput measurements of respiration can serve as a proxy for the cumulative energy cost of these processes. There are thus exciting opportunities to use our growing knowledge of factors influencing the efficiency of photosynthesis and respiration to create a step‐change in yield potential of globally important crops.

Photosynthetic manipulation provides new opportunities for enhancing crop yield. However, understanding and quantifying the importance of individual and multiple manipulations on the seasonal biomass growth and yield performance of target crops across variable production environments is limited. Using a state‐of‐the‐art cross‐scale model in the APSIM platform we predicted the impact of altering photosynthesis on the enzyme‐limited (A_c) and electron transport‐limited (A_j) rates, seasonal dynamics in canopy photosynthesis, biomass growth, and yield formation via large multiyear‐by‐location crop growth simulations. A broad list of promising strategies to improve photosynthesis for C₃ wheat and C₄ sorghum were simulated. In the top decile of seasonal outcomes, yield gains were predicted to be modest, ranging between 0% and 8%, depending on the manipulation and crop type. We report how photosynthetic enhancement can affect the timing and severity of water and nitrogen stress on the growing crop, resulting in nonintuitive seasonal crop dynamics and yield outcomes. We predicted that strategies enhancing A_c alone generate more consistent but smaller yield gains across all water and nitrogen environments, A_j enhancement alone generates larger gains but is undesirable in more marginal environments. Large increases in both A_c and A_j generate the highest gains across all environments. Yield outcomes of the tested manipulation strategies were predicted and compared for realistic Australian wheat and sorghum production. This study uniquely unpacks complex cross‐scale interactions between photosynthesis and seasonal crop dynamics and improves understanding and quantification of the potential impact of photosynthesis traits (or lack of it) for crop improvement research.

Summary: Mesophyll conductance (g_m) limits photosynthesis by restricting CO₂ diffusion between the substomatal cavities and chloroplasts. Although it is known that g_m is determined by both leaf anatomical and biochemical traits, their relative contribution across plant functional types (PFTs) is still unclear. We compiled a dataset of g_m measurements and concomitant leaf traits in unstressed plants comprising 563 studies and 617 species from all major PFTs. We investigated to what extent g_m limits photosynthesis across PFTs, how g_m relates to structural, anatomical, biochemical, and physiological leaf properties, and whether these relationships differ among PFTs. We found that g_m imposes a significant limitation to photosynthesis in all C₃ PFTs, ranging from 10–30% in most herbaceous annuals to 25–50% in woody evergreens. Anatomical leaf traits explained a significant proportion of the variation in g_m (R² > 0.3) in all PFTs except annual herbs, in which g_m is more strongly related to biochemical factors associated with leaf nitrogen and potassium content. Our results underline the need to elucidate mechanisms underlying the global variability of g_m. We emphasise the underestimated potential of g_m for improving photosynthesis in crops and identify modifications in leaf biochemistry as the most promising pathway for increasing g_m in these species.