Elevated CO2 enhances water relations and productivity and affects gas exchange in C3 and C4 grasses of the Colorado shortgrass steppe.

Six open‐top chambers were installed on the shortgrass steppe in north‐eastern Colorado, USA from late March until mid‐October in 1997 and 1998 to evaluate how this grassland will be affected by rising atmospheric CO₂. Three chambers were maintained at current CO₂ concentration (ambient treatment), three at twice ambient CO₂, or approximately 720 μmol mol^?1 (elevated treatment), and three nonchambered plots served as controls. Above‐ground phytomass was measured in summer and autumn during each growing season, soil water was monitored weekly, and leaf photosynthesis, conductance and water potential were measured periodically on important C₃ and C₄ grasses. Mid‐season and seasonal above‐ground productivity were enhanced from 26 to 47% at elevated CO₂, with no differences in the relative responses of C₃/C₄ grasses or forbs. Annual above‐ground phytomass accrual was greater on plots which were defoliated once in mid‐summer compared to plots which were not defoliated during the growing season, but there was no interactive effect of defoliation and CO₂ on growth. Leaf photosynthesis was often greater in Pascopyrum smithii (C₃) and Bouteloua gracilis (C₄) plants in the elevated chambers, due in large part to higher soil water contents and leaf water potentials. Persistent downward photosynthetic acclimation in P. smithii leaves prevented large photosynthetic enhancement for elevated CO₂‐grown plants. Shoot N concentrations tended to be lower in grasses under elevated CO₂, but only Stipa comata (C₃) plants exhibited significant reductions in N under elevated compared to ambient CO₂ chambers. Despite chamber warming of 2.6 °C and apparent drier chamber conditions compared to unchambered controls, above‐ground production in all chambers was always greater than in unchambered plots. Collectively, these results suggest increased productivity of the shortgrass steppe in future warmer, CO₂ enriched environments.     

Growth, gas exchange, leaf nitrogen and carbohydrate concentrations in NAD‐ME and NADP‐ME C4 grasses grown in elevated CO2

Plants with the C₄ photosynthetic pathway have predominantly one of three decarboxylation enzymes in their bundle sheath cells. Within the grass family (Poaceae) bundle sheath leakiness to CO₂ is purported to be lowest in the nicotinamide adenine dinucleotide phosphate-malic enzyme (NADP-ME, EC 1.1.1.40) group, highest in the NAD-ME (EC 1.1.1.39) group and intermediate in the phosphoenolpyruvate carboxykinase (PCK, EC 4.1.1.32) group. We investigated the hypothesis that growth and photosynthesis of NAD-ME C₄ grasses would respond more to elevated CO₂ treatment than NADP-ME grasses. Plants were grown in 8-1 pots in growth chambers with ample water and fertilizer for 39 days at a continuous CO₂ concentration of either 350 or 700 µl l^?1. NAD-ME species included Bouteloua gracilis Lag. ex Steud (Blue grama), Buchloe dactyloides (Nutt.) Engelm. (Buffalo grass) and Panicum virgatum L. (Switchgrass) and the NADP-ME species were Andropogon gerardii Vittman (Big bluestem), Schizachyrium scoparium (Michx.) Nash (Little bluestem), and Sorghastrum nutans (L.) Nash (Indian grass). Contrary to our hypothesis, growth of the NADP-ME grasses was generally greater under elevated CO₂ (significant for A. gerardii and S. nutans), while none of the NAD-ME grasses had a significant growth response. Increased leaf total non-structural carbohydrate (TNC) was associated with greater growth responses of NADP-ME grasses. Decreased leaf nitrogen in NADP-ME species grown at elevated CO₂ was found to be an artifact of TNC dilution. Assimilation (A) vs intercellular CO₂ (C_i) curves revealed that leaf photosynthesis was not saturated at 350 µl l^?1 CO₂ in any of these C₄ grasses. Assimilation of elevated CO₂-grown A. gerardii was higher than in plants grown in ambient CO₂. In contrast, B. gracilis grown in elevated CO₂ displayed lower A, a trait more commonly reported in C₃ plants. Photosynthetic acclimation in B. gracilis was not related to leaf TNC or nitrogen concentrations, but A:C_i curves suggest a reduction in activity of both phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39). Some adaptation of stomatal functioning was also seen in B. gracilis and A. gerardii leaves grown in elevated CO₂. Our study shows that C₄ grasses have the capacity for increased growth and photosynthesis under elevated CO₂ even when water and nutrients are non-limiting. While it was the NADP-ME species which had significant responses in the present study, we have previously reported significant growth increases in elevated CO₂ for B. gracilis.     

Ancient latitudinal gradients of C3/C4 grasses interpreted from stable isotopes of New World Pleistocene horse (Equus) teeth

Carbon and oxygen isotopic data are reported from 116 Pleistocene Equus teeth from sixty-six localities in the New World ranging from 68°N (Alaska, Canada) to 35°S (Argentina). Equus species have been predominantly grazers, and as such, carbon isotopic values of their tooth enamel provide evidence of the Pleistocene distribution of C₃ and C₄ grasses. The carbon data presented here indicate a gradient (δ¹³C range of 10 parts/mil) in the relative proportion of C₃ and C₄ grasses between high latitude and equatorial Equus samples. The largest amount of change from C₃ to C₄ grasses during the Pleistocene occurred in the mid-latitudes between about 30 to 40°. The oxygen data, which vary proportionately with temperature, indicate a latitudinal gradient (δ¹⁸O range of 20 parts/mil) between high-latitude and equatorial Equus samples. The basic pattern of latitudinal gradients of C₃/C₄ grass distribution and temperature as interpreted from these Pleistocene data is similar to the modern-day. The use of stable isotopes of fossil herbivore teeth represents a new means to interpret Pleistocene climates and terrestrial ecology.     

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.     

Photosynthesis, immediate export and carbon partitioning in source leaves of C3, C3-C4 intermediate, and C4Panicum and Flaveria species at ambient and elevated CO2 levels

Immediate export in leaves of C₃‐C₄ intermediates were compared with their C₃ and C₄ relatives within the Panicum and Flaveria genera. At 35 Pa CO₂, photosynthesis and export were highest in C₄ species in each genera. Within the Panicum, photosynthesis and export in ‘type I’ C₃‐C₄ intermediates were greater than those in C₃ species. However, ‘type I’ C₃‐C₄ intermediates exported a similar proportion of newly fixed ¹⁴C as did C₄ species. Within the Flaveria, ‘type II’ C₃‐C₄ intermediate species had the lowest export rather than the C₃ species. At ambient CO₂, immediate export was strongly correlated with photosynthesis. However, at 90 Pa CO₂, when photosynthesis and immediate export increased in all C₃ and C₃‐C₄ intermediate species, proportionally less C was exported in all photosynthetic types than that at ambient CO₂. All species accumulated starch and sugars at both CO₂ levels. There was no correlation between immediate export and the pattern of ¹⁴C‐labelling into sugars and starch among the photosynthetic types within each genus. However, during CO₂ enrichment, C₄Panicum species accumulated sugars above the level of sugars and starch normally made at ambient CO₂, whereas the C₄Flaveria species accumulated only additional starch.     

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.     

Photosynthetic oxygen exchange in C4 grasses: the role of oxygen as electron acceptor

C₄ grasses of the NAD‐ME type (Astrebla lappacea, Eleusine coracana, Eragrostis superba, Leptochloa dubia, Panicum coloratum, Panicum decompositum) and the NADP‐ME type (Bothriochloa bladhii, Cenchrus ciliaris, Dichanthium sericeum, Panicum antidotale, Paspalum notatum, Pennisetum alopecuroides, Sorghum bicolor) were used to investigate the role of O₂ as an electron acceptor during C₄ photosynthesis. Mass spectrometric measurements of gross O₂ evolution and uptake were made concurrently with measurements of net CO₂ uptake and chlorophyll fluorescence at different irradiances and leaf temperatures of 30 and 40 °C. In all C₄ grasses gross O₂ uptake increased with increasing irradiance at very high CO₂ partial pressures (pCO₂) and was on average 18% of gross O₂ evolution. Gross O₂ uptake at high irradiance and high pCO₂ was on average 3.8 times greater than gross O₂ uptake in the dark. Furthermore, gross O₂ uptake in the light increased with O₂ concentration at both high CO₂ and the compensation point, whereas gross O₂ uptake in the dark was insensitive to O₂ concentration. This suggests that a significant amount of O₂ uptake may be associated with the Mehler reaction, and that the Mehler reaction varies with irradiance and O₂ concentration. O₂ exchange characteristics at high pCO₂ were similar for NAD‐ME and NADP‐ME species. NAD‐ME species had significantly greater O₂ uptake and evolution at the compensation point particularly at low irradiance compared to NADP‐ME species, which could be related to different rates of photorespiratory O₂ uptake. There was a good correlation between electron transport rates estimated from chlorophyll fluorescence and gross O₂ evolution at high light and high pCO₂.     

Photosynthetic pathway variation among C4 grasses along a precipitation gradient in Argentina

Aim Based on the biochemical and physiological attributes of C₄ grasses, and on the close association between decarboxylation pathways and the taxa in which they evolved, the hypotheses tested were: (1) that C₄ grasses would become progressively more abundant as precipitation decreased, with grasses of the NADP‐me subtype more abundant in wetter sites and those of the NAD‐me subtype more common in arid regions; and (2) that the distribution of grass subfamilies would also be correlated with annual precipitation. Location The study was conducted along a precipitation gradient in central Argentina, from the eastern Pampas (>1000 mm year^?1) to the western deserts and semi‐deserts near the Andes (<100 mm year^?1). Methods Percentage of species and relative cover of C₃ and C₄ grasses (including C₄ subtypes) in local floras from 15 lowland sites of central Argentina were obtained from our own unpublished data and from recently published floristic surveys. Pearson correlation coefficients were obtained between grass distribution parameters and the available climatic data. Results The percentage of C₄ grasses increased towards the arid extreme and showed a strong negative correlation with annual rainfall (r = ?0.74, P < 0.01). Within the C₄ subtypes, the NADP‐me species showed a higher proportional representation at the wetter extreme, whereas the representation of NAD‐me species increased towards the more arid extreme. The relationship of PEP‐ck species with climatic parameters in central Argentina was less evident. The distributions of the Panicoideae and Chloridoideae subfamilies along the precipitation gradient were diametrically opposed, with the Panicoideae positively (r = 0.86, P < 0.001) and the Chloridoideae negatively (r = ?0.87, P < 0.001) correlated with annual precipitation. Main conclusions Our data are consistent with the broad observation that C₄ grasses tend to dominate in areas where the wet season falls in the warmer summer months. In agreement with previously reported results for Africa, Asia, Australia and North America, we describe here for the first time a significant relationship between annual precipitation and the prevalence of the NADP‐me and NAD‐me photosynthetic pathways along climatic gradients for the Neotropics. We also report for the first time that correlations between C₄ species and annual rainfall are stronger when the relative cover of grass species is considered. The association of grass subfamilies Panicoideae and Chloridoideae with rainfall is as strong as that recorded for the NADP‐me and NAD‐me variants, respectively, suggesting that characteristics other than decarboxylation type may be responsible for the geographic patterns described in this study.     

Responses of a C3 and a C4 perennial grass to elevated CO2 and temperature under different water regimes

An experiment was carried out to determine the effects of elevated CO₂, elevated temperatures, and altered water regimes in native shortgrass steppe. Intact soil cores dominated by Bouteloua gracilis, a C₄ perennial grass, or Pascopyrum smithii, a C₃ perennial grass, were placed in growth chambers with 350 or 700 μL L^?1 atmospheric CO₂, and under either normal or elevated temperatures. The normal regime mimicked field patterns of diurnal and seasonal temperatures, and the high-temperature regime was 4 °C warmer. Water was supplied at three different levels in a seasonal pattern similar to that observed in the field. Total biomass after two growing seasons was 19% greater under elevated CO₂, with no significant difference between the C₃ and C₄ grass. The effect of elevated CO₂ on biomass was greatest at the intermediate water level. The positive effect of elevated CO₂ on shoot biomass was greater at normal temperatures in B. gracilis, and greater at elevated temperatures in P. smithii. Neither root-to-shoot ratio nor production of seed heads was affected by elevated CO₂. Plant tissue N and soil inorganic N concentrations were lower under elevated Co₂, but no more so in the C₃ than the C₄ plant. Elevated CO₂ appeared to increase plant N limitation, but there was no strong evidence for an increase in N limitation or a decrease in the size of the CO₂ effect from the first to the second growing season. Autumn samples of large roots plus crowns, the perennial organs, had 11% greater total N under elevated CO₂, in spite of greater N limitation.     

Seasonal water availability predicts the relative abundance of C3 and C4 grasses in Australia

Aim Numerous studies have examined the climatic factors that influence the abundance of C₄ species within the grass flora (C₄ relative species richness) in various regions throughout the world, but very few have examined the relative abundance of C₄ vs. C₃ grasses (C₄ relative abundance). We sought to determine the climatic factors that influence C₄ relative abundance throughout Australia. Location Australia (including Tasmania). Methods We measured C₄ relative abundance at 168 locations and measured δ¹³C (the abundance of ¹³C relative to ¹²C) of the bone collagen of 779 kangaroos collected throughout Australia, as bone collagen δ¹³C was assumed to be proportional to the relative abundance of C₄ grasses in the diet. Results Both C₄ relative abundance and kangaroo bone collagen δ¹³C were found to have a strong positive relationship with seasonal water availability, i.e. the distribution of rainfall in the C₄ vs. C₃ growing seasons (76% and 69% of deviance explained, respectively). There was clear evidence that seasonal water availability was a better predictor of both C₄ relative abundance and bone collagen δ¹³C than other climate variables such as mean annual temperature and January daily minimum temperature. However, seasonal water availability appeared to be a relatively poor predictor of C₄ relative species richness, which was most closely related to January daily minimum temperature (90% of deviance explained). Main conclusions Our results highlight the relatively poor relationship between C₄ relative abundance and C₄ relative species richness, and suggest that these two variables may be related to different climatic factors. They also suggest that caution is required when using C₄ relative species richness to infer the relative biomass and productivity of C₄ grasses on a global scale.     

The photosynthesis of young Panicum C4 leaves is not C3-like

Evidence is presented contrary to the suggestion that C₄ plants grow larger at elevated CO₂ because the C₄ pathway of young C₄ leaves has C₃-like characteristics, making their photosynthesis O₂ sensitive and responsive to high CO₂. We combined PAM fluorescence with gas exchange measurements to examine the O₂ dependence of photosynthesis in young and mature leaves of Panicum antidotale (C₄, NADP-ME) and P. coloratum (C₄, NAD-ME), at an intercellular CO₂ concentration of 5 Pa. P. laxum (C₃) was used for comparison. The young C₄ leaves had CO₂ and light response curves typical of C₄ photosynthesis. When the O₂ concentration was gradually increased between 2 and 40%, CO₂ assimilation rates (A) of both mature and young C₄ leaves were little affected, while the ratio of the quantum yield of photosystem II to that of CO₂ assimilation (Φ_PSII/Φ_CO2) increased more in young (up to 31%) than mature (up to 10%) C₄ leaves. A of C₃ leaves decreased by 1·3 and Φ_PSII/Φ_CO2 increased by 9-fold, over the same range of O₂ concentrations. Larger increases in electron transport requirements in young, relative to mature, C₄ leaves at low CO₂ are indicative of greater O₂ sensitivity of photorespiration. Photosynthesis modelling showed that young C₄ leaves have lower bundle sheath CO₂ concentration, brought about by higher bundle sheath conductance relative to the activity of the C₄ and C₃ cycles and/or lower ratio of activities of the C₄ to C₃ cycles.     

Comparative ultrastructure and gas exchange characteristics of the C3–C4 intermediate Neurachne minor S. T. Blake (Poaceae)

Abstract Ultrastructural and physiological characteristics of the C₃-C₄ intermediate Neurachne minor S. T. Blake (Poaceae) are compared with those of C₃ and C₄ relatives, and C₃-C₄Panicum milioides Nees ex Trin. N. minor consistently exhibits very low CO₂ compensation points (τ: 1.0, usually 0.3–0.6 Pa) yet has C₃-like δ¹³C values. CO₂ assimilation rates (A) respond like those of C₃ plants to a decrease in O₂ partial pressure (2 × 104–1.9 × 103 Pa) at ambient CO₂ levels, but this response is progressively attenuated until negligible at very low CO₂. By contrast, other species of the Neurachneae are clearly either C₄ (two spp.) or C₃ (seven spp.). For plants grown and measured at different photon flux densities (PFDs), τ for N. minor and P. milioides increases from 0.5 to 1.0, and from 1.0 to 2.1 Pa, respectively, as PFD is decreased from 1860 to 460 μmol m^?2s^?1. In N. minor, the O₂ response of τ is either biphasic as in P. milioides, but much diminished and with a higher transition point, or is very C₄-like. As in C₄ relatives, inner sheath cells contain numerous chloroplasts. Their walls possess a suberized lamella, which may make them more CO₂-tight than bundle sheath cells of P. milioides, contributing to the almost C₄-like τ characteristics of N. minor. The biochemical basis of C₃-C₄ intermediacy is considered.     

Elevated CO2 stimulates associative N2 fixation in a C3 plant of the Chesapeake Bay wetland

In this study, the response of N₂ fixation to elevated CO₂ was measured in Scirpus olneyi, a C₃ sedge, and Spartina patens, a C₄ grass, using acetylene reduction assay and ¹⁵N₂ gas feeding. Field plants grown in PVC tubes (25 cm long, 10 cm internal diameter) were used. Exposure to elevated CO₂ significantly (P < 0·05) caused a 35% increase in nitrogenase activity and 73% increase in ¹⁵N incorporated by Scirpus olneyi. In Spartina patens, elevated CO₂ (660 ± 1 μ mol mol ^{− 1}) increased nitrogenase activity and ¹⁵N incorporation by 13 and 23%, respectively. Estimates showed that the rate of N₂ fixation in Scirpus olneyi under elevated CO₂ was 611 ± 75 ng ¹⁵N fixed plant ^{− 1} h ^{− 1} compared with 367 ± 46 ng ¹⁵N fixed plant ^{− 1} h ^{− 1} in ambient CO₂ plants. In Spartina patens, however, the rate of N₂ fixation was 12·5 ± 1·1 versus 9·8 ± 1·3 ng ¹⁵N fixed plant ^{− 1} h ^{− 1} for elevated and ambient CO₂, respectively. Heterotrophic non-symbiotic N₂ fixation in plant-free marsh sediment also increased significantly (P < 0·05) with elevated CO₂. The proportional increase in ¹⁵N₂ fixation correlated with the relative stimulation of photosynthesis, in that N₂ fixation was high in the C₃ plant in which photosynthesis was also high, and lower in the C₄ plant in which photosynthesis was relatively less stimulated by growth in elevated CO₂. These results are consistent with the hypothesis that carbon fixation in C₃ species, stimulated by rising CO₂, is likely to provide additional carbon to endophytic and below-ground microbial processes.     

Fire and the Miocene expansion of C4 grasslands

C₄ photosynthesis had a mid‐Tertiary origin that was tied to declining atmospheric CO₂, but C₄‐dominated grasslands did not appear until late Tertiary. According to the ‘CO₂‐threshold’ model, these C₄ grasslands owe their origin to a further late Miocene decline in CO₂ that gave C₄ grasses a photosynthetic advantage. This model is most appropriate for explaining replacement of C₃ grasslands by C₄ grasslands, however, fossil evidence shows C₄ grasslands replaced woodlands. An additional weakness in the threshold model is that recent estimates do not support a late Miocene drop in pCO₂. We hypothesize that late Miocene climate changes created a fire climate capable of replacing woodlands with C₄ grasslands. Critical elements were seasonality that sustained high biomass production part of year, followed by a dry season that greatly reduced fuel moisture, coupled with a monsoon climate that generated abundant lightning‐igniting fires. As woodlands became more open from burning, the high light conditions favoured C₄ grasses over C₃ grasses, and in a feedback process, the elevated productivity of C₄ grasses increased highly combustible fuel loads that further increased fire activity. This hypothesis is supported by paleosol data that indicate the late Miocene expansion of C₄ grasslands was the result of grassland expansion into more mesic environments and by charcoal sediment profiles that parallel the late Miocene expansion of C₄ grasslands. Many contemporary C₄ grasslands are fire dependent and are invaded by woodlands upon cessation of burning. Thus, we maintain that the factors driving the late Miocene expansion of C₄ were the same as those responsible for maintenance of C₄ grasslands today.     

The growth response of C4 plants to rising atmospheric CO2 partial pressure: a reassessment

Despite mounting evidence showing that C₄ plants can accumulate more biomass at elevated CO₂ partial pressure (p(CO₂)), the underlying mechanisms of this response are still largely unclear. In this paper, we review the current state of knowledge regarding the response of C₄ plants to elevated p(CO₂) and discuss the likely mechanisms. We identify two main routes through which elevated p(CO₂) can stimulate the growth of both well-watered and water-stressed C₄ plants. First, through enhanced leaf CO₂ assimilation rates due to increased intercellular p(CO₂). Second, through reduced stomatal conductance and subsequently leaf transpiration rates. Reduced transpiration rates can stimulate leaf CO₂ assimilation and growth rates by conserving soil water, improving shoot water relations and increasing leaf temperature. We argue that bundle sheath leakiness, direct CO₂ fixation in the bundle sheath or the presence of C₃-like photosynthesis in young C₄ leaves are unlikely explanations for the high CO₂-responsiveness of C₄ photosynthesis. The interactions between elevated p(CO₂), leaf temperature and shoot water relations on the growth and photosynthesis of C₄ plants are identified as key areas needing urgent research.     

Salinity and sea level mediate elevated CO2 effects on C3–C4 plant interactions and tissue nitrogen in a Chesapeake Bay tidal wetland

Elevated atmospheric carbon dioxide concentrations ([CO₂]) generally increase plant photosynthesis in C₃ species, but not in C₄ species, and reduce stomatal conductance in both C₃ and C₄ plants. In addition, tissue nitrogen concentration ([N]) often fails to keep pace with enhanced carbon gain under elevated CO₂, particularly in C₃ species. While these responses are well documented in many species, implications for plant growth and nutrient cycling in native ecosystems are not clear. Here we present data on 18 years of measurement of above and belowground biomass, tissue [N] and total standing crop of N for a Scirpus olneyi‐dominated (C₃ sedge) community, a Spartina patens‐dominated (C₄ grass) community and a C₃–C₄‐mixed species community exposed to ambient and elevated (ambient +340 ppm) atmospheric [CO₂] in natural salinity and sea level conditions of a Chesapeake Bay wetland. Increased biomass production (shoots plus roots) under elevated [CO₂] in the S. olneyi‐dominated community was sustained throughout the study, averaging approximately 35%, while no significant effect of elevated [CO₂] was found for total biomass in the C₄‐dominated community. We found a significant decline in C₄ biomass (correlated with rising sea level) and a concomitant increase in C₃ biomass in the mixed community. This shift from C₄ to C₃ was accelerated by the elevated [CO₂] treatment. The elevated [CO₂] stimulation of total biomass accumulation was greatest during rainy, low salinity years: the average increase above the ambient treatment during the three wettest years (1994, 1996, 2003) was 2.9 t ha⁻¹ but in the three driest years (1995, 1999, 2002), it was 1.2 t ha⁻¹. Elevated [CO₂] depressed tissue [N] in both species, but especially in the S. olneyi where the relative depression was positively correlated with salinity and negatively related with the relative enhancement of total biomass production. Thus, the greatest amount of carbon was added to the S. olneyi‐dominated community during years when shoot [N] was reduced the most, suggesting that the availability of N was not the most or even the main limitation to elevated [CO₂] stimulation of carbon accumulation in this ecosystem.     

Carbon isotope discrimination in C3-C4 intermediates

Carbon isotope discrimination in C₃–C₄ intermediates is determined by fractionations during diffusion and the biochemical fractionations occurring during CO₂ fixation. These biochemical fractionations in turn depend on the fractionation by Rubisco in the mesophyll, the amount of CO₂ fixation. These biochemical fractionations in turn depend on the fractionation by Rubisco in the mesophyll, the amount of CO₂ fixation occurring in the bundle sheath, the extent of bundle-sheath leakiness and the contribution which C₄-cycle activity makes to the CO₂ pool there. In most instances, carbon isotope discrimination in C₃–C₄ intermediates is C₃-like because only a small fraction of the total carbon fixed is fixed in the bundle sheath. In particular, this must be the case for Flaveria intermediates which initially fix substantial amounts of CO₂ into C₄-acids. In C₃–C₄ intermediates that refix photorespiratory CO₂ alone, it is possible for carbon isotope discrimination to be greater than in C₃-species, particularly at low CO₂ pressures or at high leaf temperatures. Short-term measurements of carbon isotope discrimination and gas exchange of leaves can be used to study the photosynthetic pathways of C₃-C₄ intermediates and their hybrids as has recently been done for C₃ and C₄ species.     

Naturally low carbonic anhydrase activity in C4 and C3 plants limits discrimination against C18OO during photosynthesis

The ¹⁸O content of CO₂ is a powerful tracer of photosynthetic activity at the ecosystem and global scale. Due to oxygen exchange between CO₂ and ¹⁸O-enriched leaf water and retrodiffusion of most of this CO₂ back to the atmosphere, leaves effectively discriminate against ¹⁸O during photosynthesis. Discrimination against ¹⁸O ( Δ ¹⁸O) is expected to be lower in C₄ plants because of low c_i and hence low retrodiffusing CO₂ flux. C₄ plants also generally show lower levels of carbonic anhydrase (CA) activities than C₃ plants. Low CA may limit the extent of ¹⁸O exchange and further reduce Δ ¹⁸O. We investigated CO₂–H₂O isotopic equilibrium in plants with naturally low CA activity, including two C₄ (Zea mays, Sorghum bicolor) and one C₃ (Phragmites australis) species. The results confirmed experimentally the occurrence of low Δ ¹⁸O in C₄, as well as in some C₃, plants. Variations in CA activity and in the extent of CO₂–H₂O isotopic equilibrium ( θ _eq) estimated from on-line measurements of Δ ¹⁸O showed large range of 0–100% isotopic equilibrium ( θ _eq = 0–1). This was consistent with direct estimates based on assays of CA activity and measurements of CO₂ concentrations and residence times in the leaves. The results demonstrate the potential usefulness of Δ ¹⁸O as indicator of CA activity in vivo. Sensitivity tests indicated also that the impact of θ _eq < 1 (incomplete isotopic equilibrium) on ¹⁸O of atmospheric CO₂ can be similar for C₃ and C₄ plants and in both cases it increases with natural enrichment of ¹⁸O in leaf water.     

Mesophyll cell properties for some C3 and C4 species with high photosynthetic rates

Leaves of twelve C₃ species and six C₄ species were examined to understand better the relationship between mesophyll cell properties and the generally high photosynthetic rates of these plants. The CO₂ diffusion conductance expressed per unit mesophyll cell surface area (g_CO2^cell) cell was determined using measurements of the net rate of CO₂ uptake, water vapor conductance, and the ratio of mesophyll cell surface area to leaf surface area (A^mes/A). A^mes/A averaged 31 for the C₃ species and 16 for the C₄ species. For the C₃ species g_CO2^cell ranged from 0.12 to 0.32 mm s^-1, and for the C₄ species it ranged from 0.55 to 1.5 mm s^-1, exceeding a previously predicted maximum of 0.5 mm s^-1. Although the C₃ species Cammissonia claviformis did not have the highest g_CO2^cell, the combination of the highest A^mes and highest stomatal conductance resulted in this species having the greatest maximum rate of CO₂ uptake in low oxygen, 93 μmol m^-2 s^-1 (147 mg dm^-2 h^-1). The high g_CO2^cell of the C₄ species Amaranthus retroflexus (1.5 mm s^-1) was in part attributable to its thin cell wall (72 nm thick).