Nitrogen nutrition of C3 plants at elevated atmospheric CO2 concentrations

The atmospheric CO₂ concentration has risen from the preindustrial level of approximately 290 μl l⁻¹ to more than 350 μl l⁻¹ in 1993. The current rate of rise is such that concentrations of 420 μl l⁻¹ are expected in the next 20 years. For C₃ plants, higher CO₂ levels favour the photosynthetic carbon reduction cycle over the photorespiratory cycle, resulting in higher rates of carbohydrate production and plant productivity. The change in balance between the two photosynthetic cycles appears to alter nitrogen and carbon metabolism in the leaf, possibly causing decreases in nitrogen concentrations in the leaf. This may result from increases in the concentration of storage carbohydrates of high molecular weight (soluble or insoluble) and/or changes in distribution of protein or other nitrogen containing compounds. Uptake of nitrogen may also be reduced at high CO₂ due to lower transpiration rates. Decreases in foliar nitrogen levels have important implications for production of crops such as wheat, because fertilizer management is often based on leaf chemical analysis, using standards estimated when the CO₂ levels were considerably lower. These standards will need to be re-evaluated as the CO₂ concentration continues to rise. Lower levels of leaf nitrogen will also have implications for the quality of wheat grain produced, because it is likely that less nitrogen would be retranslocated during grain filling.     

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.     

C4 photosynthesis in a single C3 cell is theoretically inefficient but may ameliorate internal CO2 diffusion limitations of C3 leaves

Attempts are being made to introduce C₄ photosynthetic characteristics into C₃ crop plants by genetic manipulation. This research has focused on engineering single‐celled C₄‐type CO₂ concentrating mechanisms into C₃ plants such as rice. Herein the pros and cons of such approaches are discussed with a focus on CO₂ diffusion, utilizing a mathematical model of single‐cell C₄ photosynthesis. It is shown that a high bundle sheath resistance to CO₂ diffusion is an essential feature of energy‐efficient C₄ photosynthesis. The large chloroplast surface area appressed to the intercellular airspace in C₃ leaves generates low internal resistance to CO₂ diffusion, thereby limiting the energy efficiency of a single‐cell C₄ concentrating mechanism, which relies on concentrating CO₂ within chloroplasts of C₃ leaves. Nevertheless the model demonstrates that the drop in CO₂ partial pressure, pCO₂, that exists between intercellular airspace and chloroplasts in C₃ leaves at high photosynthetic rates, can be reversed under high irradiance when energy is not limiting. The model shows that this is particularly effective at lower intercellular pCO₂. Such a system may therefore be of benefit in water‐limited conditions when stomata are closed and low intercellular pCO₂ increases photorespiration.     

Stomatal acclimation over a subambient to elevated CO2 gradient in a C3/C4 grassland

An investigation to determine whether stomatal acclimation to [CO₂] occurred in C₃/C₄ grassland plants grown across a range of [CO₂] (200–550 µmol mol^?1) in the field was carried out. Acclimation was assessed by measuring the response of stomatal conductance (g_s) to a range of intercellular CO₂ (a g_s–C_i curve) at each growth [CO₂] in the third and fourth growing seasons of the treatment. The g_s–C_i response curves for Solanum dimidiatum (C₃ perennial forb) differed significantly across [CO₂] treatments, suggesting that stomatal acclimation had occurred. Evidence of non–linear stomatal acclimation to [CO₂] in this species was also found as maximum g_s (g_smax; g_s measured at the lowest C_i) increased with decreasing growth [CO₂] only below 400 µmol mol^?1. The substantial increase in g_s at subambient [CO₂] for S. dimidiatum was weakly correlated with the maximum velocity of carboxylation (V_cmax; r² = 0·27) and was not associated with CO₂ saturated photosynthesis (A_max). The response of g_s to C_i did not vary with growth [CO₂] in Bromus japonicus (C₃ annual grass) or Bothriochloa ischaemum (C₄ perennial grass), suggesting that stomatal acclimation had not occurred in these species. Stomatal density, which increased with rising [CO₂] in both C₃ species, was not correlated with g_s. Larger stomatal size at subambient [CO₂], however, may be associated with stomatal acclimation in S. dimidiatum. Incorporating stomatal acclimation into modelling studies could improve the ability to predict changes in ecosystem water fluxes and water availability with rising CO₂ and to understand their magnitudes relative to the past.     

C3 grasses have higher nutritional quality than C4 grasses under ambient and elevated atmospheric CO2

Grasses with the C₃ photosynthetic pathway are commonly considered to be more nutritious host plants than C₄ grasses, but the nutritional quality of C₃ grasses is also more greatly impacted by elevated atmospheric CO₂ than is that of C₄ grasses; C₃ grasses produce greater amounts of nonstructural carbohydrates and have greater declines in their nitrogen content than do C₄ grasses under elevated CO₂. Will C₃ grasses remain nutritionally superior to C₄ grasses under elevated CO₂ levels? We addressed this question by determining whether levels of protein in C₃ grasses decline to similar levels as in C₄ grasses, and whether total carbohydrate : protein ratios become similar in C₃ and C₄ grasses under elevated CO₂. In addition, we tested the hypothesis that, among the nonstructural carbohydrates in C₃ grasses, levels of fructan respond most strongly to elevated CO₂. Five C₃ and five C₄ grass species were grown from seed in outdoor open‐top chambers at ambient (370 ppm) or elevated (740 ppm) CO₂ for 2 months. As expected, a significant increase in sugars, starch and fructan in the C₃ grasses under elevated CO₂ was associated with a significant reduction in their protein levels, while protein levels in most C₄ grasses were little affected by elevated CO₂. However, this differential response of the two types of grasses was insufficient to reduce protein in C₃ grasses to the levels in C₄ grasses. Although levels of fructan in the C₃ grasses tripled under elevated CO₂, the amounts produced remained relatively low, both in absolute terms and as a fraction of the total nonstructural carbohydrates in the C₃ grasses. We conclude that C₃ grasses will generally remain more nutritious than C₄ grasses at elevated CO₂ concentrations, having higher levels of protein, nonstructural carbohydrates, and water, but lower levels of fiber and toughness, and lower total carbohydrate : protein ratios than C₄ grasses.     

Comparative ecophysiology of C3 and C4 plants

Abstract. In this review we relate the physiological significance of C₄ photosynthesis to plant performance in nature. We begin with an examination of the physiological consequences of the C₄ pathway on photosynthesis, then discuss the ecophysiological performance of C₄ plants in contrasting environments. We then compare the performance of C₃ and C₄ plants when they occur together in similar habitats, and finally discuss the distribution of C₄ photosynthesis with respect to the physical environment, phylogeny, and life form.     

Insects and fungi on a C3 sedge and a C4 grass exposed to elevated atmospheric CO2 concentrations in open-top chambers in the field

The effects of elevated atmospheric CO₂ concentration on plant-fungi and plant-insect interactions were studied in an emergent marsh in the Chesapeake Bay. Stands of the C₃ sedge Scirpus olneyi Grey, and the C₄ grass Spartina patens (Ait.) Muhl. have been exposed to elevated atmospheric CO₂ concentrations during each growing season since 1987. In August 1991 the severities of fungal infections and insect infestations were quantified. Shoot nitrogen concentration ([N]) and water content (WC) were determined. In elevated concentrations of atmospheric CO₂, 32% fewer S. olneyi plants were infested by insects, and there was a 37% reduction in the severity of a pathogenic fungal infection, compared with plants grown in ambient CO₂ concentrations. S. olneyi also had reduced [N], which correlated positively with the severities of fungal infections and insect infestations. Conversely, S. patens had increased WC but unchanged [N] in elevated concentrations of atmospheric CO₂ and the severity of fungal infection increased. Elevated atmospheric CO₂ concentration increased or decreased the severity of fungal infection depending on at least two interacting factors, [N] and WC; but it did not change the number of plants that were infected with fungi. In contrast, the major results for insects were that the number of plants infected with insects decreased, and that the amount of tissue that each insect ate also decreased.     

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 responses of C3, C3-C4 and C4Flaveria species to light and intercellular CO2 concentration: implications for the evolution of stomatal behaviour

Stomatal function mediates physiological trade‐offs associated with maintaining a favourable H₂O balance in leaf tissues while acquiring CO₂ as a photosynthetic substrate. The C₃ and C₄ species appear to have different patterns of stomatal response to changing light conditions, and variation in this behaviour may have played a role in the functional diversification of the different photosynthetic pathways. In the current study, we used gain analysis theory to characterize the stomatal conductance response to light intensity in nine different C₃, C₄ and C₃‐C₄ intermediate species Flaveria species. The response of stomatal conductance (g_s) to a change in light intensity represents both a direct (related to a change in incident light intensity, I) and indirect (related to a change in intercellular CO₂ concentration, C_i) response. The slope of the line relating the change in g_s to C_i was steeper in C₄ species, compared with C₃ species, with C₃‐C₄ species having an intermediate response. This response reflects the greater relative contribution of the indirect versus direct component of the g_s versus I response in the C₄ species. The C₃‐C₄ species, Flaveria floridana, exhibited a C₄‐like response whereas the C₃‐C₄ species, Flaveria sonorensis and Flaveria chloraefolia, exhibited C₃‐like responses, similar to their hypothesized position along the evolutionary trajectory of the development of C₄ photosynthesis. There was a positive correlation between the relative contribution of the indirect component of the g_s versus I response and water use efficiency when evaluated across all species. Assuming that the C₃‐C₄ intermediate species reflect an evolutionary progression from fully expressed C₃ ancestors, the results of the current study demonstrate an increase in the contribution of the indirect component of the g_s versus I response as taxa evolve toward the C₄ extreme. The greater relative contribution of the indirect component of the stomatal response occurs through both increases in the indirect stomatal components and through decreases in the direct. Increases in the magnitude of the indirect component may be related to the maintenance of higher water use efficiencies in the intermediate evolutionary stages, before the appearance of fully integrated C₄ photosynthesis.     

Natural CO2 springs in Italy: a resource for examining long-term response of vegetation to rising atmospheric CO2 concentrations

It is estimated that more than 100 geothermal CO₂ springs exist in central-western Italy. Eight springs were selected in which the atmospheric CO₂ concentrations were consistently observed to be above the current atmospheric average of 354μmol mol^-1. CO₂ concentration measurements at some of the springs are reported. The springs are described, and their major topographic and vegetational features are reported. Preliminary observations made on natural vegetation growing around the gas vents are then illustrated. An azonal pattern of vegetation distribution occurs around every CO₂ spring regardless of soil type and phytoclimatic areas. This is composed of pioneer populations of a Northern Eurasiatic species (Agrostis canina L.) which is often associated with Scirpus lacustris L. The potential of these sites for studying the long-term response of vegetation to rising atmospheric CO₂ concentrations is discussed.     

Response of wild C4 crop progenitors to subambient CO2 highlights a possible role in the origin of agriculture

The synchronous origin of agriculture in at least four independent climatic regions at the end of the last glacial period (c10 kyr bp ) points to a global limitation for crop domestication. One hypothesis proposes that a rapid carbon dioxide (CO₂) increase from 18 Pa to ～27 Pa during deglaciation caused significant increases in the growth rates of wild crop progenitors, thereby removing a productivity barrier to their successful domestication. However, early C₄ crops present a challenge to this hypothesis, because they were among the first domesticates, but have a carbon‐concentrating mechanism that offsets the limitation of photosynthesis by CO₂. We investigated the CO₂‐limitation hypothesis using the wild progenitors of five C₄ founder crops from four independent centres of domestication. Plants were grown in controlled environment chambers at glacial (18 Pa), postglacial (28 Pa) and current ambient (38 Pa) CO₂ levels, and photosynthesis, transpiration and biomass were measured. An increase in CO₂ from glacial to postglacial levels caused a significant gain in vegetative biomass of up to 40%, but the equivalent rise in CO₂ from postglacial to modern levels generally had no effect on biomass. Investigation into the underlying mechanisms showed C₄ photosynthesis to be limited more by glacial than postglacial CO₂ levels, matching theoretical expectations. Moreover, the increase in CO₂ from glacial to postglacial levels caused a reduction in the transpiration rate via decreases in stomatal conductance of ～35%. In combination, these physiological changes conferred a large improvement in water‐use efficiency at the postglacial CO₂ partial pressure compared with the glacial level. Our data, therefore, provide experimental support for the CO₂‐limitation hypothesis, suggesting that these key physiological changes could have greatly enhanced the productivity of wild crop progenitors after deglaciation.     

Interaction between CO2 enrichment and salinity stress in the C4 non-halophyte Andropogon glomeratus (Walter) BSP

Abstract Increasing atmospheric CO₂ may result in alleviation of salinity stress in salt-sensitive plants. In order to assess the effect of enriched CO₂ on salinity stress in Andropogon glomeratus, a C₄ non-halophyte found in the higher regions of salt marshes, plants were grown at 350, 500, and 650 cm³ m^?3 CO₂ with 0 or 100 mol m^?3 NaCl watering treatments. Increases in leaf area and biomass with increasing CO₂ were measured in salt-stressed plants, while decreases in these same parameters were measured in non-salt-stressed plants. Tillering increased substantially with increasing CO₂ in salt-stressed plants, resulting in the increased biomass. Six weeks following initiation of treatments, there was no difference in photosynthesis on a leaf area basis with increasing CO₂ in salt-stressed plants, although short-term increases probably occurred. Stomatal conductance decreased with increasing CO₂ in salt-stressed plants, resulting in higher water-use efficiency, and may have improved the diurnal water status of the plants. Concentrations of Na⁺ and Cl^? were higher in salt stressed-plants while the converse was found for K ⁺. There were no differences in leaf ion content between CO₂ treatments in the salt-stressed plants. Decreases in photosynthesis in salt-stressed plants occurred primarily as a result of decreased internal (non-stomatal) conductance.     

Carbon: terrestrial C4 plants

The carbon isotope composition of terrestrial C₄ plants depends on the primary carboxylation of phosphoenolpyruvate (PEP) and on the diffusion of CO₂ to the carboxylation sites, but is also influenced by the final carboxylation of ribulose-1,5-bisphosphate (RuBP). Several models have been used for reproducing this complex situation. In the present review, a particular model is applied as a means to interpret the effects of environmental and genetically determined factors on carbon isotope discrimination during C₄ photosynthesis. As a new feature, the model considers four types of limitation of the overall CO₂ assimilation rate. Both carboxylation reactions are assumed to be limited by either maximum enzyme activity or maximum substrate regeneration rate. The model is applied to experimental data on the effects of CO₂, irradiance and water stress on short-term discrimination by leaves of several C₄ species measured simultaneously with CO₂ gas exchange characteristics. In particular, different patterns of the influence of low irradiances on carbon isotope discrimination are interpreted as due to variations in that irradiance at which a transition from limitation by PEP regeneration rate and RuBP carboxylase activity to limitation by the regeneration rates of both substrates occurs. After discussing literature data on the effects of environmental conditions on carbon isotope discrimination by C₄ plants seasonal and developmental changes in carbon isotope composition, studies on the systematic and geographic distribution of C₄ plants, evolutionary and genetical aspects, and some ecological implications are reviewed.     

Root responses along a subambient to elevated CO2 gradient in a C3–C4 grassland

Atmospheric CO₂ (C_a) concentration has increased significantly during the last 20 000 years, and is projected to double this century. Despite the importance of belowground processes in the global carbon cycle, community‐level and single species root responses to rising C_a are not well understood. We measured net community root biomass over 3 years using ingrowth cores in a natural C₃–C₄ grassland exposed to a gradient of C_a from preglacial to future levels (230–550 μmol mol^?1). Root windows and minirhizotron tubes were installed below naturally occurring stands of the C₄ perennial grass Bothriochloa ischaemum and its roots were measured for respiration, carbohydrate concentration, specific root length (SRL), production, and lifespan over 2 years. Community root biomass increased significantly (P<0.05) with C_a over initial conditions, with linear or curvilinear responses depending on sample date. In contrast, B. ischaemum produced significantly more roots at subambient than elevated C_a in minirhizotrons. The lifespan of roots with five or more neighboring roots in minirhizotron windows decreased significantly at high C_a, suggesting that after dense root growth depletes soil resource patches, plants with carbon surpluses readily shed these roots. Root respiration in B. ischaemum showed a curvilinear response to C_a under moist conditions in June 2000, with the lowest rates at C_a<300 μmol mol^?1 and peak activity at 450 μmol mol^?1 in a quadratic model. B. ischaemum roots at subambient C_a had higher SRLs and slightly higher carbohydrate concentrations than those at higher C_a, which may be related to drier soils at low C_a. Our data emphasize that belowground responses of plant communities to C_a can be quite different from those of the individual species, and suggest that complex interactions between and among roots and their immediate soil environment influence the responses of root physiology and lifespan to changing C_a.     

Changes in net photosynthesis and growth of Pinus eldarica seedlings in response to atmospheric CO2 enrichment

Pinus eldarica L. trees, rooted in the natural soil of an agricultural field at Phoenix, Arizona, were grown from the seedling stage in clear-plastic-wall open-top enclosures maintained at four different atmospheric CO₂ concentrations for 15 months. Light response functions were determined for one tree from each treatment by means of whole-tree net CO₂ exchange measurements at the end of this period, after which rates of carbon assimilation of an ambient-treatment tree were measured across a range of atmospheric CO₂ concentrations. The first of these data sets incorporates the consequences of both the CO₂-induced enhancement of net photosynthesis per unit needle area and the CO₂-induced enhancement of needle area itself (due primarily to the production of more needles), whereas the second data set reflects only the first of these effects. Hence the division of the normalized results of the first data set by the normalized results of the second set yields a representation of the increase in whole-tree net photosynthesis due to enhanced needle production caused by atmospheric CO₂ enrichment. In the solitary trees we studied, the relative contribution of this effect increased rapidly with the CO₂ concentration of the air to increase whole-tree net photosynthesis by nearly 50% at a CO₂ concentration approximately 300 μmol mol⁻¹ above ambient.     

Models of carbon metabolism in C3-C4 intermediate plants as applied to the evolution of C4 photosynthesis

Abstract Models developed to explain the biphasic response of CO₂ compensation concentration to O₂ concentration and the C₃-like carbon isotope discrimination in C₃-C₄ intermediate species are used to characterize quantitatively the steps necessary in the evolution of C₄ photosynthesis. The evolutionary stages are indicated by model outputs, CO₂ compensation concentration and δ₁₃C value. The transition from intermediate plants to C₄ plants requires the complete formation of C₄ cycle capacity, expressed by the models as transition from C₄ cycle limitation by phosphoenolpyruvate (PEP) regeneration rate to limitation by PEP carboxylase activity. Other steps refer to CO₂ leakage from bundle sheath cells, to further augmentations of C₄ cycle components, to the repression of ribulose-1,5-bisphos-phate carboxylase in the mesophyll cells, and to a decrease in the CO₂ affinity of the enzyme. Possibilities of extending the suggested approach to other physiological characteristics, and the adaptive significance of the steps envisaged, are discussed.     

On the significance of C3—C4 intermediate photosynthesis to the evolution of C4 photosynthesis

Abstract Evidence is drawn from previous studies to argue that C₃—C₄ intermediate plants are evolutionary intermediates, evolving from fully-expressed C₃ plants towards fully-expressed C₄ plants. On the basis of this conclusion, C₃—C₄ intermediates are examined to elucidate possible patterns that have been followed during the evolution of C₄ photosynthesis. An hypothesis is proposed that the initial step in C₄-evolution was the development of bundle-sheath metabolism that reduced apparent photorespiration by an efficient recycling of CO₂ using RuBP carboxylase. The CO₂-recycling mechanism appears to involve the differential compartmentation of glycine decarboxylase between mesophyll and bundle-sheath cells, such that most of the activity is in the bundlesheath cells. Subsequently, elevated phosphoenolpyruvate (PEP) carboxylase activities are proposed to have evolved as a means of enhancing the recycling of photorespired CO₂. As the activity of PEP carboxylase increased to higher values, other enzymes in the C₄-pathway are proposed to have increased in activity to facilitate the processing of the products of C₄-assimilation and provide PEP substrate to PEP carboxylase with greater efficiency. Initially, such a ‘C₄-cycle’ would not have been differentially compartmentalized between mesophyll and bundlesheath cells as is typical of fully-expressed C₄ plants. Such metabolism would have limited benefit in terms of concentrating CO₂ at RuBP carboxylase and, therefore, also be of little benefit for improving water- and nitrogen-use efficiencies. However, the development of such a limited C₄-cycle would have represented a preadaptation capable of evolving into the leaf biochemistry typical of fully-expressed C₄ plants. Thus, during the initial stages of C₄-evolution it is proposed that improvements in photorespiratory CO₂-loss and their influence on increasing the rate of net CO₂ assimilation per unit leaf area represented the evolutionary ‘driving-force’. Improved resourceuse efficiency resulting from an efficient CO₂-concentrating mechanism is proposed as the driving force during the later stages.