Plant responses to the projected future levels of CO₂ were first characterized in short-term experiments lasting days to weeks. However, longer term acclimation responses to elevated CO₂ were subsequently discovered to be very important in determining plant and ecosystem function. Free-Air CO₂ Enrichment (FACE) experiments are the culmination of efforts to assess the impact of elevated CO₂ on plants over multiple seasons and, in the case of crops, over their entire lifetime. FACE has been used to expose vegetation to elevated concentrations of atmospheric CO₂ under completely open-air conditions for nearly two decades. This review describes some of the lessons learned from the long-term investment in these experiments. First, elevated CO₂ stimulates photosynthetic carbon gain and net primary production over the long term despite down-regulation of Rubisco activity. Second, elevated CO₂ improves nitrogen use efficiency and, third, decreases water use at both the leaf and canopy scale. Fourth, elevated CO₂ stimulates dark respiration via a transcriptional reprogramming of metabolism. Fifth, elevated CO₂ does not directly stimulate C₄ photosynthesis, but can indirectly stimulate carbon gain in times and places of drought. Finally, the stimulation of yield by elevated CO₂ in crop species is much smaller than expected. While many of these lessons have been most clearly demonstrated in crop systems, all of the lessons have important implications for natural systems.

Photosynthetic responses to carbon dioxide concentration can provide data on a number of important parameters related to leaf physiology. Methods for fitting a model to such data are briefly described. The method will fit the following parameters: V_cmax, J, TPU, R_d and g_m[maximum carboxylation rate allowed by ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco), rate of photosynthetic electron transport (based on NADPH requirement), triose phosphate use, day respiration and mesophyll conductance, respectively]. The method requires at least five data pairs of net CO₂ assimilation (A) and [CO₂] in the intercellular airspaces of the leaf (C_i) and requires users to indicate the presumed limiting factor. The output is (1) calculated CO₂ partial pressure at the sites of carboxylation, C_c, (2) values for the five parameters at the measurement temperature and (3) values adjusted to 25 °C to facilitate comparisons. Fitting this model is a way of exploring leaf level photosynthesis. However, interpreting leaf level photosynthesis in terms of underlying biochemistry and biophysics is subject to assumptions that hold to a greater or lesser degree, a major assumption being that all parts of the leaf are behaving in the same way at each instant.

The principles, equipment and procedures for measuring leaf and canopy gas exchange have been described previously as has chlorophyll fluorescence. Simultaneous measurement of the responses of leaf gas exchange and modulated chlorophyll fluorescence to light and CO₂ concentration now provide a means to determine a wide range of key biochemical and biophysical limitations on photo synthesis in vivo. Here the mathematical frameworks and practical procedures for determining these parameters in vivo are consolidated. Leaf CO₂ uptake (A) versus intercellular CO₂ concentration (C_i) curves may now be routinely obtained from commercial gas exchange systems. The potential pitfalls, and means to avoid these, are examined. Calculation of in vivo maximum rates of ribulose‐1,5‐bisphosphate (RuBP) carboxylase/oxygenase (Rubisco) carboxylation (V_c,max), electron transport driving regeneration of RuBP (J_max), and triose‐phosphate utilization (V_TPU) are explained; these three parameters are now widely assumed to represent the major limitations to light‐saturated photosynthesis. Precision in determining these in intact leaves is improved by the simultaneous measurement of electron transport via modulated chlorophyll fluorescence. The A/C_i response also provides a simple practical method for quantifying the limitation that stomata impose on CO₂ assimilation. Determining the rate of photorespiratory release of oxygen (R_l) has previously only been possible by isotopic methods, now, by combining gas exchange and fluorescence measurements, R_l may be determined simply and routinely in the field. The physical diffusion of CO₂ from the intercellular air space to the site of Rubisco in C₃ leaves has long been suspected of being a limitation on photosynthesis, but it has commonly been ignored because of the lack of a practical method for its determination. Again combining gas exchange and fluorescence provides a means to determine mesophyll conductance. This method is described and provides insights into the magnitude and basis of this limitation.

CO₂ transfer conductance from the intercellular airspaces of the leaf into the chloroplast, defined as mesophyll conductance (g_m), is finite. Therefore, it will limit photosynthesis when CO₂ is not saturating, as in C3 leaves in the present atmosphere. Little is known about the processes that determine the magnitude ofg_m. The process dominatingg_m is uncertain, though carbonic anhydrase, aquaporins, and the diffusivity of CO₂ in water have all been suggested. The response ofg_m to temperature (10°C–40°C) in mature leaves of tobacco (Nicotiana tabacum L. cv W38) was determined using measurements of leaf carbon dioxide and water vapor exchange, coupled with modulated chlorophyll fluorescence. These measurements revealed a temperature coefficient (Q₁₀) of approximately 2.2 for g_m, suggesting control by a protein-facilitated process because the Q₁₀for diffusion of CO₂ in water is about 1.25. Further,g_m values are maximal at 35°C to 37.5°C, again suggesting a protein-facilitated process, but with a lower energy of deactivation than Rubisco. Using the temperature response of g_m to calculate CO₂ at Rubisco, the kinetic parameters of Rubisco were calculated in vivo from 10°C to 40°C. Using these parameters, we determined the limitation imposed on photosynthesis byg_m. Despite an exponential rise with temperature, g_m does not keep pace with increased capacity for CO₂ uptake at the site of Rubisco. The fraction of the total limitations to CO₂uptake within the leaf attributable tog_m rose from 0.10 at 10°C to 0.22 at 40°C. This shows that transfer of CO₂ from the intercellular air space to Rubisco is a very substantial limitation on photosynthesis, especially at high temperature.

As global land surface temperature continues to rise and heatwave events increase in frequency, duration, and/or intensity, our key food and fuel cropping systems will likely face increased heat-related stress. A large volume of literature exists on exploring measured and modelled impacts of rising temperature on crop photosynthesis, from enzymatic responses within the leaf up to larger ecosystem-scale responses that reflect seasonal and interannual crop responses to heat. This review discusses (i) how crop photosynthesis changes with temperature at the enzymatic scale within the leaf; (ii) how stomata and plant transport systems are affected by temperature; (iii) what features make a plant susceptible or tolerant to elevated temperature and heat stress; and (iv) how these temperature and heat effects compound at the ecosystem scale to affect crop yields. Throughout the review, we identify current advancements and future research trajectories that are needed to make our cropping systems more resilient to rising temperature and heat stress, which are both projected to occur due to current global fossil fuel emissions.

The ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) was launched to the International Space Station on 29 June 2018 by the National Aeronautics and Space Administration (NASA). The primary science focus of ECOSTRESS is centered on evapotranspiration (ET), which is produced as Level‐3 (L3) latent heat flux (LE) data products. These data are generated from the Level‐2 land surface temperature and emissivity product (L2_LSTE), in conjunction with ancillary surface and atmospheric data. Here, we provide the first validation (Stage 1, preliminary) of the global ECOSTRESS clear‐sky ET product (L3_ET_PT‐JPL, Version 6.0) against LE measurements at 82 eddy covariance sites around the world. Overall, the ECOSTRESS ET product performs well against the site measurements (clear‐sky instantaneous/time of overpass: r² = 0.88; overall bias = 8%; normalized root‐mean‐square error, RMSE = 6%). ET uncertainty was generally consistent across climate zones, biome types, and times of day (ECOSTRESS samples the diurnal cycle), though temperate sites are overrepresented. The 70‐m‐high spatial resolution of ECOSTRESS improved correlations by 85%, and RMSE by 62%, relative to 1‐km pixels. This paper serves as a reference for the ECOSTRESS L3 ET accuracy and Stage 1 validation status for subsequent science that follows using these data.