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Plant carbon metabolism is impacted by rising CO2 concentrations and temperatures, but also feeds back onto the climate system to help determine the trajectory of future climate change. Here we review how photosynthesis, photorespiration and respiration are affected by increasing atmospheric CO2 concentrations and climate warming, both separately and in combination. We also compile data from the literature on plants grown at multiple temperatures, focusing on net CO2 assimilation rates and leaf dark respiration rates measured at the growth temperature (A growth and R growth, respectively). Our analyses show that the ratio of A growth to R growth is generally homeostatic across a wide range of species and growth temperatures, and that species that have reduced A growth at higher growth temperatures also tend to have reduced R growth, while species that show stimulations in A growth under warming tend to have higher R growth in the hotter environment. These results highlight the need to study these physiological processes together to better predict how vegetation carbon metabolism will respond to climate change.

Most plants show considerable capacity to adjust their photosynthetic characteristics to their growth temperatures (temperature acclimation). The most typical case is a shift in the optimum temperature for photosynthesis, which can maximize the photosynthetic rate at the growth temperature. These plastic adjustments can allow plants to photosynthesize more efficiently at their new growth temperatures. In this review article, we summarize the basic differences in photosynthetic reactions in C 3 , C 4 , and CAM plants. We review the current understanding of the temperature responses of C 3 , C 4 , and CAM photosynthesis, and then discuss the underlying physiological and biochemical mechanisms for temperature acclimation of photosynthesis in each photosynthetic type. Finally, we use the published data to evaluate the extent of photosynthetic temperature acclimation in higher plants, and analyze which plant groups (i.e., photosynthetic types and functional types) have a greater inherent ability for photosynthetic acclimation to temperature than others, since there have been reported interspecific variations in this ability. We found that the inherent ability for temperature acclimation of photosynthesis was different: (1) among C 3 , C 4 , and CAM species; and (2) among functional types within C 3 plants. C 3 plants generally had a greater ability for temperature acclimation of photosynthesis across a broad temperature range, CAM plants acclimated day and night photosynthetic process differentially to temperature, and C 4 plants was adapted to warm environments. Moreover, within C 3 species, evergreen woody plants and perennial herbaceous plants showed greater temperature homeostasis of photosynthesis (i.e., the photosynthetic rate at high-growth temperature divided by that at low-growth temperature was close to 1.0) than deciduous woody plants and annual herbaceous plants, indicating that photosynthetic acclimation would be particularly important in perennial, long-lived species that would experience a rise in growing season temperatures over their lifespan. Interestingly, across growth temperatures, the extent of temperature homeostasis of photosynthesis was maintained irrespective of the extent of the change in the optimum temperature for photosynthesis ( T opt ), indicating that some plants achieve greater photosynthesis at the growth temperature by shifting T opt , whereas others can also achieve greater photosynthesis at the growth temperature by changing the shape of the photosynthesis–temperature curve without shifting T opt . It is considered that these differences in the inherent stability of temperature acclimation of photosynthesis would be reflected by differences in the limiting steps of photosynthetic rate.

While interest in photosynthetic thermal acclimation has been stimulated by climate warming, comparing results across studies requires consistent terminology. We identify five types of photosynthetic adjustments in warming experiments: photosynthesis as measured at the high growth temperature, the growth temperature, and the thermal optimum; the photosynthetic thermal optimum; and leaf-level photosynthetic capacity. Adjustments of any one of these variables need not mean a concurrent adjustment in others, which may resolve apparently contradictory results in papers using different indicators of photosynthetic acclimation. We argue that photosynthetic thermal acclimation (i.e., that benefits a plant in its new growth environment) should include adjustments of both the photosynthetic thermal optimum (T ₒₚₜ) and photosynthetic rates at the growth temperature (A gᵣₒwₜₕ), a combination termed constructive adjustment. However, many species show reduced photosynthesis when grown at elevated temperatures, despite adjustment of some photosynthetic variables, a phenomenon we term detractive adjustment. An analysis of 70 studies on 103 species shows that adjustment of T ₒₚₜ and A gᵣₒwₜₕ are more common than adjustment of other photosynthetic variables, but only half of the data demonstrate constructive adjustment. No systematic differences in these patterns were found between different plant functional groups. We also discuss the importance of thermal acclimation of respiration for net photosynthesis measurements, as respiratory temperature acclimation can generate apparent acclimation of photosynthetic processes, even if photosynthesis is unaltered. We show that while dark respiration is often used to estimate light respiration, the ratio of light to dark respiration shifts in a non-predictable manner with a change in leaf temperature.

Understanding how different plants prioritize carbon gain and drought vulnerability under a variable water supply is important for predicting which trees will maximize woody biomass production under different environmental conditions. Here, Populus balsamifera (BS, isohydric genotype), P. simonii (SI, previously uncharacterized stomatal behaviour), and their cross, P. balsamifera x simonii (BSxSI, anisohydric genotype) were studied to assess the physiological basis for biomass accumulation and water-use efficiency across a range of water availabilities. Under ample water, whole plant stomatal conductance (gs), transpiration (E), and growth rates were higher in anisohydric genotypes (SI and BSxSI) than in isohydric poplars (BS). Under drought, all genotypes regulated the leaf to stem water potential gradient via changes in gs, synchronizing leaf hydraulic conductance (Kleaf) and E: isohydric plants reduced Kleaf, gs, and E, whereas anisohydric genotypes maintained high Kleaf and E, which reduced both leaf and stem water potentials. Nevertheless, SI poplars reduced their plant hydraulic conductance (Kplant) during water stress and, unlike, BSxSI plants, recovered rapidly from drought. Low gs of the isohydric BS under drought reduced CO₂ assimilation rates and biomass potential under moderate water stress. While anisohydric genotypes had the fastest growth under ample water and higher photosynthetic rates under increasing water stress, isohydric poplars had higher water-use efficiency. Overall, the results indicate three strategies for how closely related biomass species deal with water stress: survival-isohydric (BS), sensitive-anisohydric (BSxSI), and resilience-anisohydric (SI). Implications for woody biomass growth, water-use efficiency, and survival under variable environmental conditions are discussed.

Summary Accurate representation of photosynthesis in terrestrial biosphere models (TBMs) is essential for robust projections of global change. However, current representations vary markedly between TBMs, contributing uncertainty to projections of global carbon fluxes. Here we compared the representation of photosynthesis in seven TBMs by examining leaf and canopy level responses of photosynthetic CO2 assimilation (A) to key environmental variables: light, temperature, CO2 concentration, vapor pressure deficit and soil water content. We identified research areas where limited process knowledge prevents inclusion of physiological phenomena in current TBMs and research areas where data are urgently needed for model parameterization or evaluation. We provide a roadmap for new science needed to improve the representation of photosynthesis in the next generation of terrestrial biosphere and Earth system models.

C₄ photosynthesis evolved independently numerous times, probably in response to declining atmospheric CO₂ concentrations, but also to high temperatures and aridity, which enhance water losses through transpiration. Here, the environmental factors controlling stomatal behaviour of leaf-level carbon and water exchange were examined across the evolutionary continuum from C₃ to C₄ photosynthesis at current (400 μmol mol⁻¹) and low (280 μmol mol⁻¹) atmospheric CO₂ conditions. To this aim, a stomatal optimization model was further developed to describe the evolutionary continuum from C₃ to C₄ species within a unified framework. Data on C₃, three categories of C₃–C₄ intermediates, and C₄ Flaveria species were used to parameterize the stomatal model, including parameters for the marginal water use efficiency and the efficiency of the CO₂-concentrating mechanism (or C₄ pump); these two parameters are interpreted as traits reflecting the stomatal and photosynthetic adjustments during the C₃ to C₄ transformation. Neither the marginal water use efficiency nor the C₄ pump strength changed significantly from C₃ to early C₃–C₄ intermediate stages, but both traits significantly increased between early C₃–C₄ intermediates and the C₄-like intermediates with an operational C₄ cycle. At low CO₂, net photosynthetic rates showed continuous increases from a C₃ state, across the intermediates and towards C₄ photosynthesis, but only C₄-like intermediates and C₄ species (with an operational C₄ cycle) had higher water use efficiencies than C₃ Flaveria. The results demonstrate that both the marginal water use efficiency and the C₄ pump strength increase in C₄ Flaveria to improve their photosynthesis and water use efficiency compared with C₃ species. These findings emphasize that the advantage of the early intermediate stages is predominantly carbon based, not water related.

Although temperature is an important driver of seasonal changes in photosynthetic physiology, photoperiod also regulates leaf activity. Climate change will extend growing seasons if temperature cues predominate, but photoperiod-controlled species will show limited responsiveness to warming. We show that photoperiod explains more seasonal variation in photosynthetic activity across 23 tree species than temperature. Although leaves remain green, photosynthetic capacity peaks just after summer solstice and declines with decreasing photoperiod, before air temperatures peak. In support of these findings, saplings grown at constant temperature but exposed to an extended photoperiod maintained high photosynthetic capacity, but photosynthetic activity declined in saplings experiencing a naturally shortening photoperiod; leaves remained equally green in both treatments. Incorporating a photoperiodic correction of photosynthetic physiology into a global-scale terrestrial carbon-cycle model significantly improves predictions of seasonal atmospheric CO2 cycling, demonstrating the benefit of such a function in coupled climate system models. Accounting for photoperiod-induced seasonality in photosynthetic parameters reduces modeled global gross primary production 2.5% (∼4 PgC y–1), resulting in a >3% (∼2 PgC y–1) decrease of net primary production. Such a correction is also needed in models estimating current carbon uptake based on remotely sensed greenness. Photoperiod-associated declines in photosynthetic capacity could limit autumn carbon gain in forests, even if warming delays leaf senescence.

Global warming and the rise in atmospheric CO₂ will increase the operating temperature of leaves in coming decades, often well above the thermal optimum for photosynthesis. Presently, there is controversy over the limiting processes controlling photosynthesis at elevated temperature. Leading models propose that the reduction in photosynthesis at elevated temperature is a function of either declining capacity of electron transport to regenerate RuBP, or reductions in the capacity of Rubisco activase to maintain Rubisco in an active configuration. Identifying which of these processes is the principal limitation at elevated temperature is complicated because each may be regulated in response to a limitation in the other. Biochemical and gas exchange assessments can disentangle these photosynthetic limitations; however, comprehensive assessments are often difficult and, for many species, virtually impossible. It is proposed that measurement of the initial slope of the CO₂ response of photosynthesis (the A/Ci response) can be a useful means to screen for Rubisco activase limitations. This is because a reduction in the Rubisco activation state should be most apparent at low CO₂ when Rubisco capacity is generally limiting. In sweet potato, spinach, and tobacco, the initial slope of the A/Ci response shows no evidence of activase limitations at high temperature, as the slope can be accurately modelled using the kinetic parameters of fully activated Rubisco. In black spruce (Picea mariana), a reduction in the initial slope above 30 °C cannot be explained by the known kinetics of fully activated Rubisco, indicating that activase may be limiting at high temperatures. Because black spruce is the dominant species in the boreal forest of North America, Rubisco activase may be an unusually important factor determining the response of the boreal biome to climate change.

Warming shifts the thermal optimum of net photosynthesis ( T optA ) to higher temperatures. However, our knowledge of this shift is mainly derived from seedlings grown in greenhouses under ambient atmospheric carbon dioxide (CO 2 ) conditions. It is unclear whether shifts in T optA of field-grown trees will keep pace with the temperatures predicted for the 21 st century under elevated atmospheric CO 2 concentrations. Here, using a whole-ecosystem warming controlled experiment under either ambient or elevated CO 2 levels, we show that T optA of mature boreal conifers increased with warming. However, shifts in T optA did not keep pace with warming as T optA only increased by 0.26–0.35 °C per 1 °C of warming. Net photosynthetic rates estimated at the mean growth temperature increased with warming in elevated CO 2 spruce, while remaining constant in ambient CO 2 spruce and in both ambient CO 2 and elevated CO 2 tamarack with warming. Although shifts in T optA of these two species are insufficient to keep pace with warming, these boreal conifers can thermally acclimate photosynthesis to maintain carbon uptake in future air temperatures. Acclimation of tree photosynthesis to warming may be affected by elevated CO 2 . Here, the authors show that mature boreal conifers may be able to maintain leaf-level C uptake under warming and elevated CO 2 even if optimum temperature of photosynthesis does not track increased temperature.

Characterizing plant responses to past, present and future changes in atmospheric carbon dioxide concentration ([CO2]) is critical for understanding and predicting the consequences of global change over evolutionary and ecological timescales. Previous CO2 studies have provided great insights into the effects of rising [CO2] on leaf-level gas exchange, carbohydrate dynamics and plant growth. However, scaling CO2 effects across biological levels, especially in field settings, has proved challenging. Moreover, many questions remain about the fundamental molecular mechanisms driving plant responses to [CO2] and other global change factors. Here we discuss three examples of topics in which significant questions in CO2 research remain unresolved: (1) mechanisms of CO2 effects on plant developmental transitions; (2) implications of rising [CO2] for integrated plant–water dynamics and drought tolerance; and (3) CO2 effects on symbiotic interactions and eco-evolutionary feedbacks. Addressing these and other key questions in CO2 research will require collaborations across scientific disciplines and new approaches that link molecular mechanisms to complex physiological and ecological interactions across spatiotemporal scales.