Guard cell photosynthesis is critical for stomatal turgor production,yet does not directly mediate CO2‐ and ABA‐induced stomatal closing

Stomata mediate gas exchange between the inter‐cellular spaces of leaves and the atmosphere. CO₂ levels in leaves (Ci) are determined by respiration, photosynthesis, stomatal conductance and atmospheric [CO₂]. [CO₂] in leaves mediates stomatal movements. The role of guard cell photosynthesis in stomatal conductance responses is a matter of debate, and genetic approaches are needed. We have generated transgenic Arabidopsis plants that are chlorophyll‐deficient in guard cells only, expressing a constitutively active chlorophyllase in a guard cell specific enhancer trap line. Our data show that more than 90% of guard cells were chlorophyll‐deficient. Interestingly, approximately 45% of stomata had an unusual, previously not‐described, morphology of thin‐shaped chlorophyll‐less stomata. Nevertheless, stomatal size, stomatal index, plant morphology, and whole‐leaf photosynthetic parameters (PSII, qP, qN, F_V′/F_M′) were comparable with wild‐type plants. Time‐resolved intact leaf gas‐exchange analyses showed a reduction in stomatal conductance and CO₂‐assimilation rates of the transgenic plants. Normalization of CO₂ responses showed that stomata of transgenic plants respond to [CO₂] shifts. Detailed stomatal aperture measurements of normal kidney‐shaped stomata, which lack chlorophyll, showed stomatal closing responses to [CO₂] elevation and abscisic acid (ABA), while thin‐shaped stomata were continuously closed. Our present findings show that stomatal movement responses to [CO₂] and ABA are functional in guard cells that lack chlorophyll. These data suggest that guard cell CO₂ and ABA signal transduction are not directly modulated by guard cell photosynthesis/electron transport. Moreover, the finding that chlorophyll‐less stomata cause a ‘deflated’ thin‐shaped phenotype, suggests that photosynthesis in guard cells is critical for energization and guard cell turgor production.     

New insights into the cellular mechanisms of plant growth at elevated atmospheric carbon dioxide concentrations

Rising atmospheric carbon dioxide concentration ([CO₂]) significantly influences plant growth, development, and biomass. Increased photosynthesis rate, together with lower stomatal conductance, has been identified as the key factors that stimulate plant growth at elevated [CO₂] (e[CO₂]). However, variations in photosynthesis and stomatal conductance alone cannot fully explain the dynamic changes in plant growth. Stimulation of photosynthesis at e[CO₂] is always associated with post‐photosynthetic secondary metabolic processes that include carbon and nitrogen metabolism, cell cycle functions, and hormonal regulation. Most studies have focused on photosynthesis and stomatal conductance in response to e[CO₂], despite the emerging evidence of e[CO₂]'s role in moderating secondary metabolism in plants. In this review, we briefly discuss the effects of e[CO₂] on photosynthesis and stomatal conductance and then focus on the changes in other cellular mechanisms and growth processes at e[CO₂] in relation to plant growth and development. Finally, knowledge gaps in understanding plant growth responses to e[CO₂] have been identified with the aim of improving crop productivity under a CO₂ rich atmosphere.     

Elevated CO2 affects photosynthetic responses in canopy pine and subcanopy deciduous trees over 10 years: a synthesis from Duke FACE

Leaf responses to elevated atmospheric CO₂ concentration (C_a) are central to models of forest CO₂ exchange with the atmosphere and constrain the magnitude of the future carbon sink. Estimating the magnitude of primary productivity enhancement of forests in elevated C_a requires an understanding of how photosynthesis is regulated by diffusional and biochemical components and up‐scaled to entire canopies. To test the sensitivity of leaf photosynthesis and stomatal conductance to elevated C_a in time and space, we compiled a comprehensive dataset measured over 10 years for a temperate pine forest of Pinus taeda, but also including deciduous species, primarily Liquidambar styraciflua. We combined over one thousand controlled‐response curves of photosynthesis as a function of environmental drivers (light, air C_a and temperature) measured at canopy heights up to 20 m over 11 years (1996–2006) to generate parameterizations for leaf‐scale models for the Duke free‐air CO₂ enrichment (FACE) experiment. The enhancement of leaf net photosynthesis (A_net) in P. taeda by elevated C_a of +200 μmol mol^?1 was 67% for current‐year needles in the upper crown in summer conditions over 10 years. Photosynthetic enhancement of P. taeda at the leaf‐scale increased by two‐fold from the driest to wettest growing seasons. Current‐year pine foliage A_net was sensitive to temporal variation, whereas previous‐year foliage A_net was less responsive and overall showed less enhancement (+30%). Photosynthetic downregulation in overwintering upper canopy pine needles was small at average leaf N (N_area), but statistically significant. In contrast, co‐dominant and subcanopy L. styraciflua trees showed A_net enhancement of 62% and no A_net–N_area adjustments. Various understory deciduous tree species showed an average A_net enhancement of 42%. Differences in photosynthetic responses between overwintering pine needles and subcanopy deciduous leaves suggest that increased C_a has the potential to enhance the mixed‐species composition of planted pine stands and, by extension, naturally regenerating pine‐dominated stands.     

Plant responses to atmospheric carbon dioxide enrichment: interactions with some soil and atmospheric conditions

In general, C₃ plant species are more responsive to atmospheric carbon dioxide (CO₂) enrichment than C₄-plants. Increased relative growth rate at elevated CO₂ primarily relates to increased Net Assimilation Rate (NAR), and enhancement of net photosynthesis and reduced photorespiration. Transpiration and stomatal conductance decrease with elevated CO₂, water use efficiency and shoot water potential increase, particularly in plants grown at high soil salinity. Leaf area per plant and leaf area per leaf may increase in an early growth stage with increased CO₂, after a period of time Leaf Area Ratio (LAR) and Specific Leaf Area (SLA) generally decrease. Starch may accumulate with time in leaves grown at elevated CO₂. Plants grown under salt stress with increased (dark) respiration as a sink for photosynthates, may not show such acclimation to increased atmospheric CO₂ levels. Plant growth may be stimulated by atmospheric carbon dioxide enrichment and reduced by enhanced UV-B radiation but the limited data available on the effect of combined elevated CO₂ and ultraviolet B (280–320 nm) (UV-B) radiation allow no general conclusion. CO₂-induced increase of growth rate can be markedly modified at elevated UV-B radiation. Plant responses to elevated atmospheric CO₂ and other environmental factors such as soil salinity and UV-B tend to be species-specific, because plant species differ in sensitivity to salinity and UV-B radiation, as well as to other environmental stress factors (drought, nutrient deficiency). Therefore, the effects of joint elevated atmospheric CO₂ and increased soil salinity or elevated CO₂ and enhanced UV-B to plants are physiologically complex.     

Winners always win: growth of a wide range of plant species from low to future high CO2

Evolutionary adaptation to variation in resource supply has resulted in plant strategies that are based on trade‐offs in functional traits. Here, we investigate, for the first time across multiple species, whether such trade‐offs are also apparent in growth and morphology responses to past low, current ambient, and future high CO₂ concentrations. We grew freshly germinated seedlings of up to 28 C₃ species (16 forbs, 6 woody, and 6 grasses) in climate chambers at 160 ppm, 450 ppm, and 750 ppm CO₂. We determined biomass, allocation, SLA (specific leaf area), LAR (leaf area ratio), and RGR (relative growth rate), thereby doubling the available data on these plant responses to low CO₂. High CO₂ increased RGR by 8%; low CO₂ decreased RGR by 23%. Fast growers at ambient CO₂ had the greatest reduction in RGR at low CO₂ as they lost the benefits of a fast‐growth morphology (decoupling of RGR and LAR [leaf area ratio]). Despite these shifts species ranking on biomass and RGR was unaffected by CO₂, winners continued to win, regardless of CO_2. Unlike for other plant resources we found no trade‐offs in morphological and growth responses to CO₂ variation, changes in morphological traits were unrelated to changes in growth at low or high CO₂. Thus, changes in physiology may be more important than morphological changes in response to CO₂ variation.     

Asymmetric responses of adaxial and abaxial stomata to elevated CO2: impacts on the control of gas exchange by leaves

The response of adaxial and abaxial stomatal conductance in Rumex obtusifolius to growth at elevated atmospheric concentrations of CO₂ (250 μmol mol^?1 above ambient) was investigated over two growing seasons. The conductance of both the adaxial and abaxial leaf surfaces was found to be reduced by elevated concentrations of CO₂. Elevated CO₂ caused a much greater reduction in conductance for the adaxial surface than for the abaxial surface. The absence of effects upon stomatal density indicated that the reductions were probably the result of changes in stomatal aperture. Partitioning of gas exchange between the leaf surfaces revealed that increased concentrations of CO₂ caused increased rates of photosynthesis only via the abaxial surface. Additionally, leaf thickness was found to increase during growth at elevated concentrations of CO₂. The tendency for these amphistomatous leaves to develop a distribution of conductance approaching that of hypostomatous leaves clearly reduced their maximum photosynthetic potential. This conclusion was supported by measurements of stomatal limitation, which showed greater values for the adaxial surfaces, and greater values at elevated CO₂. This reduction in photosynthesis may in part be caused by higher diffusive limitations imposed because of increased leaf thickness. In an uncoupled canopy, asymmetrical stomatal responses of the kind identified here may appreciably reduce transpiration. Species which show symmetrical responses are less likely to show reduced transpirational rates, and a redistribution of water loss between species may occur. The implications of asymmetrical stomatal responses for photosynthesis and canopy transpiration are discussed.     

Elevated atmospheric CO2 alters stomatal responses to variable sunlight in a C4 grass

Native tallgrass prairie in NE Kansas was exposed to elevated (twice ambient) or ambient atmospheric CO₂ levels in open-top chambers. Within chambers or in adjacent unchambered plots, the dominant C₄ grass, Andropogon gerardii, was subjected to fluctuations in sunlight similar to that produced by clouds or within canopy shading (full sun > 1500 μmol m⁻² s⁻¹ versus 350 μmol m⁻² s⁻¹ shade) and responses in gas exchange were measured. These field experiments demonstrated that stomatal conductance in A. gerardii achieved new steady state levels more rapidly after abrupt changes in sunlight at elevated CO₂ when compared to plants at ambient CO₂. This was due primarily to the 50% reduction in stomatal conductance at elevated CO₂, but was also a result of more rapid stomatal responses. Time constants describing stomatal responses were significantly reduced (29–33%) at elevated CO₂. As a result, water loss was decreased by as much as 57% (6.5% due to more rapid stomatal responses). Concurrent increases in leaf xylem pressure potential during periods of sunlight variability provided additional evidence that more rapid stomatal responses at elevated CO₂ enhanced plant water status. CO₂-induced alterations in the kinetics of stomatal responses to variable sunlight will likely enhance direct effects of elevated CO₂ on plant water relations in all ecosystems.     

Contemporary evolution of an invasive grass in response to elevated atmospheric CO2 at a Mojave Desert FACE site

Elevated atmospheric CO₂ has been shown to rapidly alter plant physiology and ecosystem productivity, but contemporary evolutionary responses to increased CO₂ have yet to be demonstrated in the field. At a Mojave Desert FACE (free‐air CO₂ enrichment) facility, we tested whether an annual grass weed (Bromus madritensis ssp. rubens) has evolved in response to elevated atmospheric CO₂. Within 7 years, field populations exposed to elevated CO₂ evolved lower rates of leaf stomatal conductance; a physiological adaptation known to conserve water in other desert or water‐limited ecosystems. Evolution of lower conductance was accompanied by reduced plasticity in upregulating conductance when CO₂ was more limiting; this reduction in conductance plasticity suggests that genetic assimilation may be ongoing. Reproductive fitness costs associated with this reduction in phenotypic plasticity were demonstrated under ambient levels of CO₂. Our findings suggest that contemporary evolution may facilitate this invasive species' spread in this desert ecosystem.     

Foliar gas exchange responses of two deciduous hardwoods during 3 years of growth in elevated CO2: no loss of photosynthetic enhancement

Responses of photosynthesis and stomatal conductance were monitored throughout a 3-year field exposure of Liriodendron tulipifera (yellow-poplar) and Quercus alba (white oak) to elevated concentrations of atmospheric CO₂. Exposure to atmospheres enriched with +150 and +300 umol mol^-1 CO₂ increased net photosynthesis by 12–144% over the course of the study. Net photosynthesis was consistently higher at +300 than at +150 umol mol^-1 CO₂. The effect of CO₂ enrichment on stomatal conductance was limited, but instantaneous leaf-level water use efficiency increased significantly. No decrease in the responsiveness of photosynthesis to CO₂ enrichment over time was detected, and the responses were consistent throughout the canopy and across successive growth flushes and seasons. The relationships between internal CO₂ concentration and photosynthesis (e.g. photosynthetic capacity and carboxylation efficiency) were not altered by growth at elevated concentrations of CO₂. No alteration in the timing of leaf senescence or abscission was detected, suggesting that the seasonal duration of effective gas-exchange was unaffected by CO₂ treatment. These results are consistent with data previously reported for these species in controlled-environment studies, and suggest that leaf-level photosynthesis does not down-regulate in these species as a result of acclimation to CO₂ enrichment in the field. This sustained enhancement of photosynthesis provides the opportunity for increased growth and carbon storage by trees as the atmospheric concentration of CO₂ rises, but many additional factors interact in determining whole-plant and forest responses to global change.     

Gas exchange and water‐use efficiency in plant canopies

In this review, I first address the basics of gas exchange, water‐use efficiency and carbon isotope discrimination in C₃ plant canopies. I then present a case study of water‐use efficiency in northern Australian tree species. In general, C₃ plants face a trade‐off whereby increasing stomatal conductance for a given set of conditions will result in a higher CO₂ assimilation rate, but a lower photosynthetic water‐use efficiency. A common garden experiment suggested that tree species which are able to establish and grow in drier parts of northern Australia have a capacity to use water rapidly when it is available through high stomatal conductance, but that they do so at the expense of low water‐use efficiency. This may explain why community‐level carbon isotope discrimination does not decrease as steeply with decreasing rainfall on the North Australian Tropical Transect as has been observed on some other precipitation gradients. Next, I discuss changes in water‐use efficiency that take place during leaf expansion in C₃ plant leaves. Leaf phenology has recently been recognised as a significant driver of canopy gas exchange in evergreen forest canopies, and leaf expansion involves changes in both photosynthetic capacity and water‐use efficiency. Following this, I discuss the role of woody tissue respiration in canopy gas exchange and how photosynthetic refixation of respired CO₂ can increase whole‐plant water‐use efficiency. Finally, I discuss the role of water‐use efficiency in driving terrestrial plant responses to global change, especially the rising concentration of atmospheric CO₂. In coming decades, increases in plant water‐use efficiency caused by rising CO₂ are likely to partially mitigate impacts on plants of drought stress caused by global warming.     

Strong photosynthetic acclimation and enhanced water‐use efficiency in grassland functional groups persist over 21 years of CO2 enrichment,independent of nitrogen supply

Uncertainty about long‐term leaf‐level responses to atmospheric CO₂ rise is a major knowledge gap that exists because of limited empirical data. Thus, it remains unclear how responses of leaf gas exchange to elevated CO₂ (eCO₂) vary among plant species and functional groups, or across different levels of nutrient supply, and whether they persist over time for long‐lived perennials. Here, we report the effects of eCO₂ on rates of net photosynthesis and stomatal conductance in 14 perennial grassland species from four functional groups over two decades in a Minnesota Free‐Air CO₂ Enrichment experiment, BioCON. Monocultures of species belonging to C₃ grasses, C₄ grasses, forbs, and legumes were exposed to two levels of CO₂ and nitrogen supply in factorial combinations over 21 years. eCO₂ increased photosynthesis by 12.9% on average in C₃ species, substantially less than model predictions of instantaneous responses based on physiological theory and results of other studies, even those spanning multiple years. Acclimation of photosynthesis to eCO₂ was observed beginning in the first year and did not strengthen through time. Yet, contrary to expectations, the response of photosynthesis to eCO₂ was not enhanced by increased nitrogen supply. Differences in responses among herbaceous plant functional groups were modest, with legumes responding the most and C₄ grasses the least as expected, but did not further diverge over time. Leaf‐level water‐use efficiency increased by 50% under eCO₂ primarily because of reduced stomatal conductance. Our results imply that enhanced nitrogen supply will not necessarily diminish photosynthetic acclimation to eCO₂ in nitrogen‐limited systems, and that significant and consistent declines in stomatal conductance and increases in water‐use efficiency under eCO₂ may allow plants to better withstand drought.     

The sensitivity of C3 photosynthesis to increasing CO2 concentration: a theoretical analysis of its dependence on temperature and background CO2 concentration

The atmospheric CO₂ concentration has increased from the pre-industrial concentration of about 280 μmol mol⁻¹ to its present concentration of over 350 μmol mol⁻¹, and continues to increase. As the rate of photosynthesis in C₃ plants is strongly dependent on CO₂ concentration, this should have a marked effect on photosynthesis, and hence on plant growth and productivity. The magnitude of photo-synthetic responses can be calculated based on the well-developed theory of photosynthetic response to intercellular CO₂ concentration. A simple biochemically based model of photosynthesis was coupled to a model of stomatal conductance to calculate photosynthetic responses to ambient CO₂ concentration. In the combined model, photosynthesis was much more responsive to CO₂ at high than at low temperatures. At 350 μmol mol⁻¹, photosynthesis at 35°C reached 51% of the rate that would have been possible with non-limiting CO₂, whereas at 5°C, 77% of the CO₂ non-limited rate was attained. Relative CO₂ sensitivity also became smaller at elevated CO₂, as CO₂ concentration increased towards saturation. As photosynthesis was far from being saturated at the current ambient CO₂ concentration, considerable further gains in photosynthesis were predicted through continuing increases in CO₂ concentration. The strong interaction with temperature also leads to photosynthesis in different global regions experiencing very different sensitivities to increasing CO₂ concentrations.     

Stomatal function,density and pattern,and CO2 assimilation in Arabidopsis thaliana tmm1 and sdd1‐1 mutants

• Stomata modulate the exchange of water and CO₂ between plant and atmosphere. Although stomatal density is known to affect CO₂ diffusion into the leaf and thus photosynthetic rate, the effect of stomatal density and patterning on CO₂ assimilation is not fully understood.
• We used wild types Col‐0 and C24 and stomatal mutants sdd1‐1 and tmm1 of Arabidopsis thaliana, differing in stomatal density and pattern, to study the effects of these variations on both stomatal and mesophyll conductance and CO₂ assimilation rate. Anatomical parameters of stomata, leaf temperature and carbon isotope discrimination were also assessed.
• Our results indicate that increased stomatal density enhanced stomatal conductance in sdd1‐1 plants, with no effect on photosynthesis, due to both unchanged photosynthetic capacity and decreased mesophyll conductance. Clustering (abnormal patterning formed by clusters of two or more stomata) and a highly unequal distribution of stomata between the adaxial and abaxial leaf sides in tmm1 mutants also had no effect on photosynthesis.
• Except at very high stomatal densities, stomatal conductance and water loss were proportional to stomatal density. Stomatal formation in clusters reduced stomatal dynamics and their operational range as well as the efficiency of CO₂ transport.

     

Steady-state stomatal responses of C3 and C4 species to blue light fraction: Interactions with CO2 concentration

Blue light induced stomatal opening has been studied by applying a short pulse (~5 to 60 s) of blue light to a background of saturating photosynthetic red photons, but little is known about steady-state stomatal responses. Here we report stomatal responses to blue light at high and low CO₂ concentrations. Steady-state stomatal conductance (g_s) of C₃ plants increased asymptotically with increasing blue light to a maximum at 20% blue (120 μmol m⁻² s⁻¹). This response was consistent from 200 to 800 μmol mol⁻¹ atmospheric CO₂ (C_a). In contrast, blue light induced only a transient stomatal opening (~5 min) in C₄ species above a C_a of 400 μmol mol⁻¹. Steady-state g_s of C₄ plants generally decreased with increasing blue intensity. The net photosynthetic rate of all species decreased above 20% blue because blue photons have lower quantum yield (moles carbon fixed per mole photons absorbed) than red photons. Our findings indicate that photosynthesis, rather than a blue light signal, plays a dominant role in stomatal regulation in C₄ species. Additionally, we found that blue light affected only stomata on the illuminated side of the leaf. Contrary to widely held belief, the blue light-induced stomatal opening minimally enhanced photosynthesis and consistently decreased water use efficiency.     

Adaptive temperature regulation in the little bird in winter: predictions from a stochastic dynamic programming model

Stomatal CO₂ responsiveness and photosynthetic capacity vary greatly among plant species, but the factors controlling these physiological leaf traits are often poorly understood. To explore if these traits are linked to taxonomic group identity and/or to other plant functional traits, we investigated the short-term stomatal CO₂ responses and the maximum rates of photosynthetic carboxylation (V _cmax) and electron transport (J _max) in an evolutionary broad range of tropical woody plant species. The study included 21 species representing four major seed plant taxa: gymnosperms, monocots, rosids and asterids. We found that stomatal closure responses to increased CO₂ were stronger in angiosperms than in gymnosperms, and in monocots compared to dicots. Stomatal CO₂ responsiveness was not significantly related to any of the other functional traits investigated, while a parameter describing the relationship between photosynthesis and stomatal conductance in combined leaf gas exchange models (g ₁) was related to leaf area-specific plant hydraulic conductance. For photosynthesis, we found that the interspecific variation in V _cmax and J _max was related to within leaf nitrogen (N) allocation rather than to area-based total leaf N content. Within-leaf N allocation and water use were strongly co-ordinated (r ² = 0.67), such that species with high fractional N investments into compounds maximizing photosynthetic capacity also had high stomatal conductance. We conclude that while stomatal CO₂ responsiveness of tropical woody species seems poorly related to other plant functional traits, photosynthetic capacity is linked to fractional within-leaf N allocation rather than total leaf N content and is closely co-ordinated with leaf water use.     

Responses of leaf nitrogen concentration and specific leaf area to atmospheric CO2 enrichment: a retrospective synthesis across 62 species

Knowledge of leaf responses to elevated atmospheric [CO₂] (CO₂ concentration) is integral to understanding interactions between vegetation and global change. This work deals with responses of leaf mass‐based nitrogen concentration (N_m) and specific leaf area (SLA). It assesses the statistical significance of factors perceived as influential on the responses, and quantifies how the responses vary with the significant factors identified, based on 170 data cases of 62 species compiled from the literature. Resultant equations capture about 41% of the variance in the data for percent responses of N_m and SLA, or about 95% of the variance for N_m and SLA at 57–320% normal [CO₂]; these performance statistics also hold for leaf area‐based N concentration and specific leaf weight. The equations generalize that: (i) both N_m and SLA decline as [CO₂] increases; (ii) proportional decline of N_m is greater with deciduous woody species and with plants of normally low N_m, increases with pot size in growth chamber and greenhouse settings and with temperature and photosynthetic photon flux density (PPFD), and is mitigated by N fertilization; and (iii) proportional decline of SLA depends on pot size and PPFD similarly to N_m, increases with leaf life span and water vapour pressure deficit in enclosed experiments, and decreases with prolonged exposure to elevated [CO₂] among broadleaf woody species in field conditions. The results highlight great uncertainty in the percent‐response data and reveal the potential feasibility to estimate N_m and SLA at various magnitudes of elevated [CO₂] from a few key plant and environmental factors of broad data bases.     

PHOTOSYNTHETIC CHARACTERISTICS OF C3-C4 INTERMEDIATE FLAVERIA FLORIDANA (ASTERACEAE) IN NATURAL HABITATS: EVIDENCE OF ADVANTAGES TO C3-C4 PHOTOSYNTHESIS AT HIGH LEAF TEMPERATURES

Measurements of leaf gas exchange were conducted in situ for the C₃-C₄ intermediate plant Flaveria floridana. Leaves exhibited measurable CO₂ assimilation at atmospheric CO₂ concentrations as low as 20 μmol/mol. This result demonstrates that the low CO₂ compensation points observed in past studies of greenhouse-grown C₃-C₄ intermediate plants also exist in plants growing in their natural habitat. Photosynthesis rates in F. floridana were near their maximum at intercellular CO₂ concentrations as low as 112 μmol/mol. The existence of near-maximum photosynthesis rates at such low intercellular CO₂ concentrations is interpreted as evidence for the existence of a CO₂-concentrating mechanism in F. floridana. Such a mechanism would also explain the observed lack of response in photosynthesis rates to reductions in stomatal conductance and intercellular CO₂ concentration as the leaf-to-air water vapor concentration gradient is increased. Photosynthetic rates were relatively high at leaf temperatures between 35 and 40 C, compared to most C₃ plants. At midday during May, when leaf temperatures were between 35 and 42 C, F. floridana leaves exhibited photosynthesis rates that were four times higher than a sympatric C₃ species (Eustoma exaltatum) of similar growth form and ecological habit. The high photosynthesis rates at high leaf temperatures in F. floridana were not due to higher leaf nitrogen contents, but rather to its reduced rate of photorespiration. These results confirm that C₃-C₄ intermediate photosynthesis can provide plants with an advantage at high leaf temperatures, compared to C₃ plants.     

Physiological advantages of C4 grasses in the field: a comparative experiment demonstrating the importance of drought

Global climate change is expected to shift regional rainfall patterns, influencing species distributions where they depend on water availability. Comparative studies have demonstrated that C₄ grasses inhabit drier habitats than C₃ relatives, but that both C₃ and C₄ photosynthesis are susceptible to drought. However, C₄ plants may show advantages in hydraulic performance in dry environments. We investigated the effects of seasonal variation in water availability on leaf physiology, using a common garden experiment in the Eastern Cape of South Africa to compare 12 locally occurring grass species from C₄ and C₃ sister lineages. Photosynthesis was always higher in the C₄ than C₃ grasses across every month, but the difference was not statistically significant during the wettest months. Surprisingly, stomatal conductance was typically lower in the C₃ than C₄ grasses, with the peak monthly average for C₃ species being similar to that of C₄ leaves. In water‐limited, rain‐fed plots, the photosynthesis of C₄ leaves was between 2.0 and 7.4 μmol m^?2 s^?1 higher, stomatal conductance almost double, and transpiration 60% higher than for C₃ plants. Although C₄ average instantaneous water‐use efficiencies were higher (2.4–8.1 mmol mol^?1) than C₃ averages (0.7–6.8 mmol mol^?1), differences were not as great as we expected and were statistically significant only as drought became established. Photosynthesis declined earlier during drought among C₃ than C₄ species, coincident with decreases in stomatal conductance and transpiration. Eventual decreases in photosynthesis among C₄ plants were linked with declining midday leaf water potentials. However, during the same phase of drought, C₃ species showed significant decreases in hydrodynamic gradients that suggested hydraulic failure. Thus, our results indicate that stomatal and hydraulic behaviour during drought enhances the differences in photosynthesis between C₄ and C₃ species. We suggest that these drought responses are important for understanding the advantages of C₄ photosynthesis under field conditions.     

Water use of two Mojave Desert shrubs under elevated CO2

Plant responses to elevated atmospheric CO₂ have been characterized generally by stomatal closure and enhanced growth rates. These responses are being increasingly incorporated into global climate models that quantify interactions between the biosphere and atmosphere, altering climate predictions from simpler physically based models. However, current information on CO₂ responses has been gathered primarily from studies of crop and temperate forest species. In order to apply responses of vegetation to global predictions, CO₂ responses in other commonly occurring biomes must be studied. A Free Air CO₂ Enrichment (FACE) study is currently underway to examine plant responses to high CO₂ in a natural, undisturbed Mojave Desert ecosystem in Nevada, USA. Here we present findings from this study, and its companion glasshouse experiment, demonstrating that field‐grown Ephedra nevadensis and glasshouse‐grown Larrea tridentata responded to high CO₂ with reductions in the ratio of transpirational surface area to sapwood area (LSR) of 33% and 60%, respectively. Thus, leaf‐specific hydraulic conductivity increased and stomatal conductance remained constant or was increased under elevated CO₂. Field‐grown Larrea did not show a reduced LSR under high CO₂, and stomatal conductance was reduced in the high CO₂ treatment, although the effect was apparent only under conditions of unusually high soil moisture. Both findings suggest that the common paradigm of 20–50% reductions in stomatal conductance under high CO₂ may not be applicable to arid ecosystems under most conditions.     

Variations in nitrogen use efficiency reflect the biochemical subtype while variations in water use efficiency reflect the evolutionary lineage of C4 grasses at inter‐glacial CO2

C₄ photosynthesis evolved multiple times in diverse lineages. Most physiological studies comparing C₄ plants were not conducted at the low atmospheric CO₂ prevailing during their evolution. Here, 24 C₄ grasses belonging to three biochemical subtypes [nicotinamide adenine dinucleotide malic enzyme (NAD‐ME), phosphoenolpyruvate carboxykinase (PCK) and nicotinamide adenine dinucleotide phosphate malic enzyme (NADP‐ME)] and six major evolutionary lineages were grown under ambient (400 μL L^?1) and inter‐glacial (280 μL L^?1) CO₂. We hypothesized that nitrogen‐related and water‐related physiological traits are associated with subtypes and lineages, respectively. Photosynthetic rate and stomatal conductance were constrained by the shared lineage, while variation in leaf mass per area (LMA), leaf N per area, plant dry mass and plant water use efficiency were influenced by the subtype. Subtype and lineage were equally important for explaining variations in photosynthetic nitrogen use efficiency (PNUE) and photosynthetic water use efficiency (PWUE). CO₂ treatment impacted most parameters. Overall, higher LMA and leaf N distinguished the Chloridoideae/NAD‐ME group, while NADP‐ME and PCK grasses were distinguished by higher PNUE regardless of lineage. Plants were characterized by high photosynthesis and PWUE when grown at ambient CO₂ and by high conductance at inter‐glacial CO₂. In conclusion, the evolutionary and biochemical diversity among C₄ grasses was aligned with discernible leaf physiology, but it remains unknown whether these traits represent ecophysiological adaptation.