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Summary It has been 60 years since the discovery of C₄photosynthesis, an event that rewrote our understanding of plant adaptation, ecosystem responses to global change, and global food security. Despite six decades of research, one aspect of C₄photosynthesis that remains poorly understood is how the pathway fits into the broader context of adaptive trait spectra, which form our modern view of functional trait ecology. The C₄CO₂‐concentrating mechanism supports a general C₄plant phenotype capable of fast growth and high resource‐use efficiencies. The fast‐efficient C₄phenotype has the potential to operate at high productivity rates, while allowing for less biomass allocation to root production and nutrient acquisition, thereby providing opportunities for the evolution of novel trait covariances and the exploitation of new ecological niches. We propose the placement of the C₄fast‐efficient phenotype near the acquisitive pole of the world‐wide leaf economic spectrum, but with a pathway‐specific span of trait space, wherein selection shapes both acquisitive and conservative adaptive strategies. A trait‐based perspective of C₄photosynthesis will open new paths to crop improvement, global biogeochemical modeling, the management of invasive species, and the restoration of disturbed ecosystems, particularly in grasslands. 

Summary Nonstructural carbohydrate (NSC) concentrations might reflect the strategies described in the leaf economic spectrum (LES) due to their dependence on photosynthesis and respiration.We examined if NSC concentrations correlate with leaf structure, chemistry, and physiology traits for 114 species from 19 sites and 5 biomes around the globe.Total leaf NSC concentrations varied greatly from 16 to 199 mg g⁻¹dry mass and were mostly independent of leaf gas exchange and the LES traits. By contrast, leaf NSC residence time was shorter in species with higher rates of photosynthesis, following the fast‐slow strategies in the LES. An average leaf held an amount of NSCs that could sustain one night of leaf respiration and could be replenished in just a few hours of photosynthesis under saturating light, indicating that most daily carbon gain is exported.Our results suggest that NSC export is clearly linked to the economics of return on resource investment. 

Abstract Photosynthetic acclimation to both warming and elevated CO₂of boreal trees remains a key uncertainty in modelling the response of photosynthesis to future climates. We investigated the impact of increased growth temperature and elevated CO₂on photosynthetic capacity (V_cmaxandJ_max) in mature trees of two North American boreal conifers, tamarack and black spruce. We show thatV_cmaxandJ_maxat a standard temperature of 25°C did not change with warming, whileV_cmaxandJ_maxat their thermal optima (T_opt) and growth temperature (T_g) increased. Moreover,V_cmaxandJ_maxat either 25°C,T_optorT_gdecreased with elevated CO₂. TheJ_max/V_cmaxratio decreased with warming when assessed at bothT_optandT_gbut did not significantly vary at 25°C. TheJ_max/V_cmaxincreased with elevated CO₂at either reference temperature. We found no significant interaction between warming and elevated CO₂on all traits. If this lack of interaction between warming and elevated CO₂on theV_cmax,J_maxandJ_max/V_cmaxratio is a general trend, it would have significant implications for improving photosynthesis representation in vegetation models. However, future research is required to investigate the widespread nature of this response in a larger number of species and biomes. 

Abstract 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 (CO₂) conditions. It is unclear whether shifts inT_optAof field-grown trees will keep pace with the temperatures predicted for the 21^stcentury under elevated atmospheric CO₂concentrations. Here, using a whole-ecosystem warming controlled experiment under either ambient or elevated CO₂levels, we show thatT_optAof mature boreal conifers increased with warming. However, shifts inT_optAdid not keep pace with warming asT_optAonly 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 CO₂spruce, while remaining constant in ambient CO₂spruce and in both ambient CO₂and elevated CO₂tamarack with warming. Although shifts inT_optAof 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. 

Abstract. Elevated atmospheric CO2 concentration is expectedto increase leaf CO2 assimilation rates, thus promoting plant growthand increasing leaf area. It also decreases stomatal conductance, allowingwater savings, which have been hypothesized to drive large-scale greening,in particular in arid and semiarid climates. However, the increase in leafarea could reduce the benefits of elevated CO2 concentration through soilwater depletion. The net effect of elevated CO2 on leaf- andcanopy-level gas exchange remains uncertain. To address this question, wecompare the outcomes of a heuristic model based on the Partitioning ofEquilibrium Transpiration and Assimilation (PETA) hypothesis and three modelvariants based on stomatal optimization theory. Predicted relative changes in leaf-and canopy-level gas exchange rates are used as a metric of plant responsesto changes in atmospheric CO2 concentration. Both model approaches predictreductions in leaf-level transpiration rate due to decreased stomatalconductance under elevated CO2, but negligible (PETA) or no(optimization) changes in canopy-level transpiration due to the compensatoryeffect of increased leaf area. Leaf- and canopy-level CO2 assimilationis predicted to increase, with an amplification of the CO2fertilization effect at the canopy level due to the enhanced leaf area. Theexpected increase in vapour pressure deficit (VPD) under warmer conditions isgenerally predicted to decrease the sensitivity of gas exchange toatmospheric CO2 concentration in both models. The consistentpredictions by different models that canopy-level transpiration varieslittle under elevated CO2 due to combined stomatal conductancereduction and leaf area increase highlight the coordination ofphysiological and morphological characteristics in vegetation to maximizeresource use (here water) under altered climatic conditions. 

Abstract Warming nights are correlated with declining wheat growth and yield. As a key determinant of plant biomass, respiration consumes O2 as it produces ATP and releases CO2 and is typically reduced under warming to maintain metabolic efficiency. We compared the response of respiratory O2 and CO2 flux to multiple night and day warming treatments in wheat leaves and roots, using one commercial (Mace) and one breeding cultivar grown in controlled environments. We also examined the effect of night warming and a day heatwave on the capacity of the ATP-uncoupled alternative oxidase (AOX) pathway. Under warm nights, plant biomass fell, respiratory CO2 release measured at a common temperature was unchanged (indicating higher rates of CO2 release at prevailing growth temperature), respiratory O2 consumption at a common temperature declined, and AOX pathway capacity increased. The uncoupling of CO2 and O2 exchange and enhanced AOX pathway capacity suggest a reduction in plant energy demand under warm nights (lower O2 consumption), alongside higher rates of CO2 release under prevailing growth temperature (due to a lack of down-regulation of respiratory CO2 release). Less efficient ATP synthesis, teamed with sustained CO2 flux, could thus be driving observed biomass declines under warm nights. 

Abstract The ability to transport water through tall stems hydraulically limits stomatal conductance (g_s), thereby constraining photosynthesis and growth. However, some plants are able to minimize this height‐related decrease ing_s, regardless of path length. We hypothesized that kudzu (Pueraria lobata) prevents strong declines ing_swith height through appreciable structural and hydraulic compensative alterations. We observed only a 12% decline in maximumg_salong 15‐m‐long stems and were able to model this empirical trend. Increasing resistance with transport distance was not compensated by increasing sapwood‐to‐leaf‐area ratio. Compensating for increasing leaf area by adjusting the driving force would require water potential reaching −1.9 MPa, far below the wilting point (−1.2 MPa). The negative effect of stem length was compensated for by decreasing petiole hydraulic resistance and by increasing stem sapwood area and water storage, with capacitive discharge representing 8–12% of the water flux. In addition, large lateral (petiole, leaves) relative to axial hydraulic resistance helped improve water flow distribution to top leaves. These results indicate thatg_sof distal leaves can be similar to that of basal leaves, provided that resistance is highest in petioles, and sufficient amounts of water storage can be used to subsidize the transpiration stream.