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.     

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.     

An examination of the advantages of C3-C4 intermediate photosynthesis in warm environments

Abstract. The photosynthetic responses to temperature in C₃, C₃-C₄ intermediate, and C₄ species in the genus Flaveria were examined in an effort to identify whether the reduced photorespiration rates characteristic of C₃-C₄ intermediate photosynthesis result in adaptive advantages at warm leaf temperatures. Reduced photorespiration rates were reflected in lower CO₂ compensation points at all temperatures examined in the C₃-C₄ intermediate, Flaveria floridana, compared to the C₃ species, F. cronquistii. The C₃-C₄ intermediate, F. floridana, exhibited a C₃-like photosynthetic temperature dependence, except for relatively higher photosynthesis rates at warm leaf temperatures compared to the C₃ species, F. cronquistii. Using models of C₃ and C₃-C₄ intermediate photosynthesis, it was predicted that by recycling photorespired CO₂ in bundle-sheath cells, as occurs in many C₃-C₄ intermediates, photosynthesis rates at 35°C could be increased by 28%, compared to a C₃ plant. Without recycling photorespired CO₂, it was calculated that in order to improve photosynthesis rates at 35°C by this amount in C₃ plants, (1) intercellular CO₂ partial pressures would have to be increased from 25 to 31 Pa, resulting in a 57% decrease in water-use efficiency, or (2) the activity of RuBP carboxylase would have to be increased by 32%, resulting in a 22% decrease in nitrogen-use efficiency. In addition to the recycling of photorespired CO₂, leaves of F. floridana appear to effectively concentrate CO₂ at the active site of RuBP carboxylase, increasing the apparent carboxylation efficiency per unit of in vitro RuBP carboxylase activity. The CO₂-concentrating activity also appears to reduce the temperature sensitivity of the carboxylation efficiency in F. floridana compared to F. cronquistii. The carboxylation efficiency per unit of RuBP carboxylase activity decreased by only 38% in F. floridana, compared to 50% in F. cronquistii, as leaf temperature was raised from 25 to 35°C. The C₃-C₄ intermediate, F. ramosissima, exhibited a photosynthetic temperature temperature response curve that was more similar to the C₄ species, F. trinervia, than the C₃ species, F. cronquistii. The C₄-like pattern is probably related to the advanced nature of C₄-like biochemical traits in F. ramosissima The results demonstrate that reductions in photorespiration rates in C₃-C₄ intermediate plants create photosynthetic advantages at warm leaf temperatures that in C₃ plants could only be achieved through substantial costs to water-use efficiency and/or nitrogen-use efficiency.     

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.     

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.     

Low-temperature photosynthetic performance of a C4 grass and a co-occurring C3 grass native to high latitudes

The photosynthetic performance of C₄ plants is generally inferior to that of C₃ species at low temperatures, but the reasons for this are unclear. The present study investigated the hypothesis that the capacity of Rubisco, which largely reflects Rubisco content, limits C₄ photosynthesis at suboptimal temperatures. Photosynthetic gas exchange, chlorophyll a fluorescence, and the in vitro activity of Rubisco between 5 and 35 °C were measured to examine the nature of the low‐temperature photosynthetic performance of the co‐occurring high latitude grasses, Muhlenbergia glomerata (C₄) and Calamogrostis canadensis (C₃). Plants were grown under cool (14/10 °C) and warm (26/22 °C) temperature regimes to examine whether acclimation to cool temperature alters patterns of photosynthetic limitation. Low‐temperature acclimation reduced photosynthetic rates in both species. The catalytic site concentration of Rubisco was approximately 5.0 and 20 µmol m^?2 in M. glomerata and C. canadensis, respectively, regardless of growth temperature. In both species, in vivo electron transport rates below the thermal optimum exceeded what was necessary to support photosynthesis. In warm‐grown C. canadensis, the photosynthesis rate below 15 °C was unaffected by a 90% reduction in O₂ content, indicating photosynthetic capacity was limited by the capacity of P_i‐regeneration. By contrast, the rate of photosynthesis in C. canadensis plants grown at the cooler temperatures was stimulated 20–30% by O₂ reduction, indicating the P_i‐regeneration limitation was removed during low‐temperature acclimation. In M. glomerata, in vitro Rubisco activity and gross CO₂ assimilation rate were equivalent below 25 °C, indicating that the capacity of the enzyme is a major rate limiting step during C₄ photosynthesis at cool temperatures.     

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.     

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.     

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).     

Utilization of O2 in the metabolic optimization of C4 photosynthesis

The combined effects of O₂ on net rates of photosynthesis, photosystem II activity, steady‐state pool size of key metabolites of photosynthetic metabolism in the C₄ pathway, C₃ pathway and C₂ photorespiratory cycle and on growth were evaluated in the C₄ species Amaranthus edulis and the C₃ species Flaveria pringlei. Increasing O₂ reduced net CO₂ assimilation in F. pringlei due to an increased flux of C through the photorespiratory pathway. However, in A. edulis increasing O₂ up to 5–10% stimulated photosynthesis. Analysis of the pool size of key metabolites in A. edulis suggests that while there is some O₂ dependent photorespiration, O₂ is required for maximizing C₄ cycle activity to concentrate CO₂ in bundle sheath cells. Therefore, the response of net photosynthesis to O₂ in C₄ plants may result from the balance of these two opposing effects. Under 21 versus 5% O₂, growth of A. edulis was stimulated about 30% whereas that of F. pringlei was inhibited about 40%.     

Environmental and Evolutionary Preconditionsfor the Origin and Diversification of the C4 PhotosyntheticSyndrome

Abstract: C₄ photosynthesis is an evolutionary solution to high rates of photorespiration and low kinetic efficiency of Rubisco in CO₂‐depleted atmospheres of recent geologic time. About 7500 plant species are C₄, in contrast to 30 000 CAM and 250 000 C₃ species. All C₄ plants occur in approximately 90 genera from 18 angiosperm families. In all of these families, the C₄ pathway evolved independently. In many, multiple independent origins have occurred, such that over 30 distinct evolutionary origins of the C₄ pathway are recognized. Fossil and carbon isotope evidence show that the C₄ syndrome is at least 12 to 15 million years old, although estimates based on molecular sequence comparisons indicate it is over 20 million years old. The evolutionary radiation of herbaceous angiosperms may have been required for C₄ plant evolution. All C₄ species occur in advanced angiosperm families that appeared in the fossil record in the past 70 million years. Most of these families diversified in terms of genera and species numbers between 20 to 40 million years ago, during a period of global cooling, atmospheric CO₂ reduction and aridification. During the period of diversification, numerous traits arose in the C₃ flora that enhanced their performance in arid environments and atmospheres of reduced CO₂. Some of these traits may have predisposed certain taxa to develop the C₄ pathway once atmospheric CO₂ levels declined to a point where the ability to concentrate CO₂ had a selective advantage. Leading traits in C₃ plants that may have facilitated the initial transition to C₄ photosynthesis include close vein spacing and an enlargement of the bundle sheath cell layer to form a Kranz‐like anatomy. Ecological factors not directly connected with photosynthesis probably also played a role. For example, extensive ecological disturbance may have been needed to convert C₃‐dominated woodlands into open, high‐light habitats where herbaceous C₄ plants could succeed. Disturbances in the form of fire, and browsing by large mammals, increase during the time of C₄ plant evolution and diversification. Fire increased because of the drying climate, while browsing increased with the evolutionary diversification of the mammalian megafauna in the Oligocene and Miocene epochs. In summary, the origin of C₄ plants is hypothesized to have resulted from a novel combination of environmental and phylogenetic developments that, for the first time, established the preconditions required for C₄ plant evolution.     

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.     

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.     

Oxygen sensitivity of C4 photosynthesis: evidence from gas exchange and chlorophyll fluorescence analyses with different C4 subtypes

Because photosynthetic rates in C₄ plants are the same at normal levels of O₂ (c, 20 kPa) and at c, 2 kPa O₂ (a conventional test for evaluating photorespiration in C₃ plants) it has been thought that C₄ photosynthesis is O₂ insensitive. However, we have found a dual effect of O₂ on the net rate of CO₂ assimilation among species representing all three C₄ subtypes from both monocots and dicots. The optimum O₂ partial pressure for C₄ photosynthesis at 30 °C, atmospheric CO₂ level, and half full sunlight (1000 μmol quanta m^?2 s^?1) was about 5–10 kPa. Photosynthesis was inhibited by O₂ below or above the optimum partial pressure. Decreasing CO₂ levels from ambient levels (32.6 Pa) to 9.3 Pa caused a substantial increase in the degree of inhibition of photosynthesis by supra-optimum levels of O₂ and a large decrease in the ratio of quantum yield of CO₂ fixation/quantum yield of photosystem II (PSII) measured by chlorophyll a fluorescence. Photosystem II activity, measured from chlorophyll a fluorescence analysis, was not inhibited at levels of O₂ that were above the optimum for CO₂ assimilation, which is consistent with a compensating, alternative electron How as net CO₂ assimilation is inhibited. At suboptimum levels of O₂, however, the inhibition of photosynthesis was paralleled by an inhibition of PSII quantum yield, increased state of reduction of quinone A, and decreased efficiency of open PSII centres. These results with different C₄ types suggest that inhibition of net CO₂ assimilation with increasing O₂ partial pressure above the optimum is associated with photorespiration, and that inhibition below the optimum O₂ may be caused by a reduced supply of ATP to the C₄ cycle as a result of inhibition of its production photochemically.     

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.     

Dynamic photo-inhibition and carbon gain in a C4 and a C3 grass native to high latitudes

C₄ plants are rare in the cool climates characteristic of high latitudes and altitudes, perhaps because of an enhanced susceptibility to photo‐inhibition at low temperatures relative to C₃ species. In the present study we tested the hypothesis that low‐temperature photo‐inhibition is more detrimental to carbon gain in the C₄ grass Muhlenbergia glomerata than the C₃ species Calamogrostis Canadensis. These grasses occur together in boreal fens in northern Canada. Plants were grown under cool (14/10 °C day/night) and warm (26/22 °C) temperatures before measurement of the light responses of photosynthesis and chlorophyll fluorescence at different temperatures. Cool growth temperatures led to reduced rates of photosynthesis in M. glomerata at all measurement temperatures, but had a smaller effect on the C₃ species. In both species the amount of xanthophyll cycle pigments increased when plants were grown at 14/10 °C, and in M. glomerata the xanthophyll epoxidation state was greatly reduced. The detrimental effect of low growth temperature on photosynthesis in M. glomerata was almost completely reversed by a 24‐h exposure to the warm‐temperature regime. These data indicate that reversible dynamic photo‐inhibition is a strategy by which C₄ species may tolerate cool climates and overcome the Rubisco limitation that is prevalent at low temperatures in C₄ plants.     

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.     

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.     

Estimating Photosynthesis and Concurrent Export Rates in C3 and C4 Species at Ambient and Elevated CO2

The ability of 21 C₃ and C₄ monocot and dicot species to rapidly export newly fixed C in the light at both ambient and enriched CO₂ levels was compared. Photosynthesis and concurrent export rates were estimated during isotopic equilibrium of the transport sugars using a steady-state ¹⁴CO₂-labeling procedure. At ambient CO₂ photosynthesis and export rates for C₃ species were 5 to 15 and 1 to 10 μmol C m⁻² s⁻¹, respectively, and 20 to 30 and 15 to 22 μmol C m⁻² s⁻¹, respectively, for C₄ species. A linear regression plot of export on photosynthesis rate of all species had a correlation coefficient of 0.87. When concurrent export was expressed as a percentage of photosynthesis, several C₃ dicots that produced transport sugars other than Suc had high efflux rates relative to photosynthesis, comparable to those of C₄ species. At high CO₂ photosynthetic and export rates were only slightly altered in C₄ species, and photosynthesis increased but export rates did not in all C₃ species. The C₃ species that had high efflux rates relative to photosynthesis at ambient CO₂ exported at rates comparable to those of C₄ species on both an absolute basis and as a percentage of photosynthesis. At ambient CO₂ there were strong linear relationships between photosynthesis, sugar synthesis, and concurrent export. However, at high CO₂ the relationships between photosynthesis and export rate and between sugar synthesis and export rate were not as strong because sugars and starch were accumulated.     

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.