Using a reaction-diffusion model to estimate day respiration and reassimilation of (photo)respired CO2 in leaves

Importance of Timing of Dark Acclimation for Estimating Light Inhibition of Leaf Respiratory CO2 Efflux

The degree of inhibition of leaf respiration by light is often studied, but the methods used and the results obtained are variable. We suggest that in the future daytime leaf respiration is measured 3 min after dark acclimation to avoid under‐estimating the degree of light inhibition of leaf respiration. This will most likely speed up future surveys and perhaps also result in less inter‐study variation in the calculated degree of light inhibition of leaf respiration.

Instantaneous Q10 of night-time leaf respiratory CO2 efflux - measurement and analytical protocol considerations

The temperature sensitivity (e.g. Q10) of night-time leaf respiratory CO2 efflux (RCO2) is a fundamental aspect of leaf physiology. The Q10 typically exhibits a dependence on measurement temperature, and it is speculated that this is due to temperature-dependent shifts in the relative control of leaf RCO2. Two decades ago, a review hypothesized that this mechanistically caused change in values of Q10 is predictable across plant taxa and biomes. Here, we discuss the most appropriate measuring protocol among existing data and for future data collection, to form the foundation of a future mechanistic understanding of Q10 of leaf RCO2 at different temperature ranges. We do this primarily via a review of existing literature on Q10 of night-time RCO2 and only supplement this to a lesser degree with our own original data. Based on mechanistic considerations, we encourage that instantaneous Q10 of leaf RCO2 to represent night-time should be measured: only at night-time; only in response to short-term narrow temperature variation (e.g. max. 10°C) to represent a given midpoint temperature at a time; in response to as many temperatures as possible within the chosen temperature range; and on still attached leaves.

Temperature responses of leaf respiration in light and darkness are similar and modulated by leaf development

Our ability to predict temperature responses of leaf respiration in light and darkness (RL and RDk ) is essential to models of global carbon dynamics. While many models rely on constant thermal sensitivity (characterized by Q10 ), uncertainty remains as to whether Q10 of RL and RDk are actually similar. We measured short-term temperature responses of RL and RDk in immature and mature leaves of two evergreen tree species, Castanopsis carlesii and Ormosia henry in an open field. RL was estimated by the Kok method, the Yin method and a newly developed Kok-iterCc method. When estimated by the Yin and Kok-iterCc methods, RL and RDk had similar Q10 (c. 2.5). The Kok method overestimated both Q10 and the light inhibition of respiration. RL /RDk was not affected by leaf temperature. Acclimation of respiration in summer was associated with a decline in basal respiration but not in Q10 in both species, which was related to changes in leaf nitrogen content between seasons. Q10 of RL and RDk in mature leaves were 40% higher than in immature leaves. Our results suggest similar Q10 values can be used to model RL and RDk while leaf development-associated changes in Q10 require special consideration in future respiration models.

Estimating leaf day respiration from conventional gas exchange measurements

Leaf day respiration (Rd ) strongly influences carbon-use efficiencies of whole plants and the global terrestrial biosphere. It has long been thought that Rd is slower than respiration in the dark at a given temperature, but measuring Rd by gas exchange remains a challenge because leaves in the light are also photosynthesizing. The Kok method and the Laisk method are widely used to estimate Rd . We highlight theoretical limitations of these popular methods, and recent progress toward their improvement by using additional information from chlorophyll fluorescence and by accounting for the photosynthetic reassimilation of respired CO2 . The latest evidence for daytime CO2 and energy release from the oxidative pentose phosphate pathway in chloroplasts appears to be important to understanding Rd .

Toward mechanistic modeling and rational engineering of plant respiration

Plant respiration not only provides energy to support all cellular processes, including biomass production, but also plays a major role in the global carbon cycle. Therefore, modulation of plant respiration can be used to both increase the plant yield and mitigate the effects of global climate change. Mechanistic modeling of plant respiration at sufficient biochemical detail can provide key insights for rational engineering of this process. Yet, despite its importance, plant respiration has attracted considerably less modeling effort in comparison to photosynthesis. In this update review, we highlight the advances made in modeling of plant respiration, emphasizing the gradual but important change from phenomenological to models based on first principles. We also provide a detailed account of the existing resources that can contribute to resolving the challenges in modeling plant respiration. These resources point at tangible improvements in the representation of cellular processes that contribute to CO2 evolution and consideration of kinetic properties of underlying enzymes to facilitate mechanistic modeling. The update review emphasizes the need to couple biochemical models of respiration with models of acclimation and adaptation of respiration for their effective usage in guiding breeding efforts and improving terrestrial biosphere models tailored to future climate scenarios.

A Changing Light Environment Induces Significant Lateral CO2 Diffusion within Maize Leaves

A leaf structure with high porosity is beneficial for lateral CO₂ diffusion inside the leaves. However, the leaf structure of maize is compact, and it has long been considered that lateral CO₂ diffusion is restricted. Moreover, lateral CO₂ diffusion is closely related to CO₂ pressure differences (ΔCO₂). Therefore, we speculated that enlarging the ΔCO₂ between the adjacent regions inside maize leaves may result in lateral diffusion when the diffusion resistance is kept constant. Thus, the leaf structure and gas exchange of maize (C₄), cotton (C₃), and other species were explored. The results showed that maize and sorghum leaves had a lower mesophyll porosity than cotton and cucumber leaves. Similar to cotton, the local photosynthetic induction resulted in an increase in the ΔCO₂ between the local illuminated and the adjacent unilluminated regions, which significantly reduced the respiration rate of the adjacent unilluminated region. Further analysis showed that when the adjacent region in the maize leaves was maintained under a steady high light, the photosynthesis induction in the local regions not only gradually reduced the ΔCO₂ between them but also progressively increased the steady photosynthetic rate in the adjacent region. Under field conditions, the ΔCO₂, respiration, and photosynthetic rate of the adjacent region were also markedly changed by fluctuating light in local regions in the maize leaves. Consequently, we proposed that enlarging the ΔCO₂ between the adjacent regions inside the maize leaves results in the lateral CO₂ diffusion and supports photosynthesis in adjacent regions to a certain extent under fluctuating light.

Drought exerts a greater influence than growth temperature on the temperature response of leaf day respiration in wheat (Triticum aestivum)

We assessed how the temperature response of leaf day respiration (R_d ) in wheat responded to contrasting water regimes and growth temperatures. In Experiment 1, well-watered and drought-stressed conditions were imposed on two genotypes; in Experiment 2, the two water regimes combined with high (HT), medium (MT) and low (LT) growth temperatures were imposed on one of the genotypes. R_d was estimated from simultaneous gas exchange and chlorophyll fluorescence measurements at six leaf temperatures (T_leaf ) for each treatment, using the Yin method for nonphotorespiratory conditions and the nonrectangular hyperbolic fitting method for photorespiratory conditions. The two genotypes responded similarly to growth and measurement conditions. Estimates of R_d for nonphotorespiratory conditions were generally higher than those for photorespiratory conditions, but their responses to T_leaf were similar. Under well-watered conditions, R_d and its sensitivity to T_leaf slightly acclimated to LT, but did not acclimate to HT. Temperature sensitivities of R_d were considerably suppressed by drought, and the suppression varied among growth temperatures. Thus, it is necessary to quantify interactions between drought and growth temperature for reliably modelling R_d under climate change. Our study also demonstrated that the Kok method, one of the currently popular methods for estimating R_d , underestimated R_d significantly.

The Impact of Photorespiratory Glycolate Oxidase Activity on Arabidopsis thaliana Leaf Soluble Amino Acid Pool Sizes during Acclimation to Low Atmospheric CO2 Concentrations

Photorespiration is a metabolic process that removes toxic 2-phosphoglycolate produced by the oxygenase activity of ribulose-1,5-bisphosphate carboxylase/oxygenase. It is essential for plant growth under ambient air, and it can play an important role under stress conditions that reduce CO₂ entry into the leaf thus enhancing photorespiration. The aim of the study was to determine the impact of photorespiration on Arabidopsis thaliana leaf amino acid metabolism under low atmospheric CO₂ concentrations. To achieve this, wild-type plants and photorespiratory glycolate oxidase (gox) mutants were given either short-term (4 h) or long-term (1 to 8 d) low atmospheric CO₂ concentration treatments and leaf amino acid levels were measured and analyzed. Low CO₂ treatments rapidly decreased net CO₂ assimilation rate and triggered a broad reconfiguration of soluble amino acids. The most significant changes involved photorespiratory Gly and Ser, aromatic and branched-chain amino acids as well as Ala, Asp, Asn, Arg, GABA and homoSer. While the Gly/Ser ratio increased in all Arabidopsis lines between air and low CO₂ conditions, low CO₂ conditions led to a higher increase in both Gly and Ser contents in gox1 and gox2.2 mutants when compared to wild-type and gox2.1 plants. Results are discussed with respect to potential limiting enzymatic steps with a special emphasis on photorespiratory aminotransferase activities and the complexity of photorespiration.

The Kok effect revisited

The Kok effect refers to the abrupt decrease around the light compensation point in the slope of net photosynthetic rate vs irradiance. Arguably, this switch arises from light inhibition of respiration, allowing the Kok method to estimate day respiration (R_d ). Recent analysis suggests that increasing proportions of photorespiration (quantified as Γ*/C_c , the ratio of CO₂ compensation point Γ* to chloroplast CO₂ concentration, C_c ) with irradiance explain much of the Kok effect. Also, the Kok method has been modified to account for the decrease in PSII photochemical efficiency (Φ₂ ) with irradiance. Using a model that illustrates how varying R_d , Γ*/C_c , Φ₂ and proportions of alternative electron transport could engender the Kok effect, we quantified the contribution of these parameters to the Kok effect measured in sunflower across various O₂ and CO₂ concentrations and various temperatures. Overall, the decreasing Φ₂ with irradiance explained c. 12%, and the varying Γ*/C_c explained c. 25%, of the Kok effect. Maximum real light inhibition of R_d was much lower than the inhibition derived from the Kok method, but still increased with photorespiration. Photorespiration had a dual contribution to the Kok effect, one via the varying Γ*/C_c and the other via its participation in light inhibition of R_d .