A CO2-Flux Mechanism Operating via pH-Polarity in Hydrilla Verticillata Leaves With C3 and C4 Photosynthesis

The aquatic angiosperm Hydrilla verticillata lacks Kranz anatomy, but has an inducible, C₄-based, CO₂ concentrating mechanism (CCM) that concentrates CO₂ in the chloroplasts. Both C₃ and C₄ Hydrilla leaves showed light-dependent pH polarity that was suppressed by high dissolved inorganic carbon (DIC). At low DIC (0.25 mol m⁻³), pH values in the unstirred water layer on the abaxial and adaxial sides of the leaf were 4.2 and10.3, respectively. Abaxial apoplastic acidification served as a CO₂ flux mechanism (CFM), making HCO₃ ⁻ available for photosynthesis by conversion to CO₂. DIC at 10 mol m⁻³ completely suppressed acidification and alkalization. The data, along with previous results, indicated that inhibition was specific to DIC, and not a buffer effect. Acidification and alkalization did not necessarily show 1:1 stoichiometry; their kinetics for the apolar induction phase differed, and alkalization was less inhibited by 2.5 mol m⁻³ DIC. At low irradiance (50 μmol photons m⁻² s⁻¹), where CCM activity in C₄ leaves is minimized, both leaf types had similar DIC inhibition of pH polarity. However, as irradiance increased, DIC inhibition of C₃ leaves decreased. In C₄ leaves the CFM and CCM seemed to compete for photosynthetic ATP and/or reducing power. The CFM may require less, as at low irradiance it still operated maximally, if [DIC] was low. Iodoacetamide (IA), which inhibits CO₂ fixation in Hydrilla, also suppressed acidification and alkalization, especially in C₄ leaves. IA does not inhibit the C₄ CCM, which suggests that the CFM and CCM can operate independently. It has been hypothesized that irradiance and DIC regulate pH polarity by altering the chloroplastic [DIC], which effects the chloroplast redox state and subsequently redox regulation of a plasma-membrane H⁺-ATPase. The results lend partial support to a down-regulatory role for high chloroplastic [DIC], but do not exclude other sites of DIC action. IA inhibition of pH polarity seems inconsistent with the chloroplast NADPH/NADP⁺ ratio being the redox transducer. The possibility that malate and oxaloacetate shuttling plays a role in CFM regulation requires further investigation. This revised version was published online in June 2006 with corrections to the Cover Date.     

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.     

Regulation of the induction of bicarbonate uptake by dissolved CO2 in the marine diatom, Phaeodactylum tricornutum

Physiological properties of photosynthesis were determined in the marine diatom, Phaeodactylum tricornutum UTEX640, during acclimation from 5% CO₂ to air and related to H₂CO₃ dissociation kinetics and equilibria in artificial seawater. The concentration of dissolved inorganic carbon at half maximum rate of photosynthesis (K_0·5[DIC]) value in high CO₂‐grown cells was 1009 mmol m ^{? 3} but was reduced three‐fold by the addition of bovine carbonic anhydrase (CA), whereas in air‐grown cells K_0·5[DIC] was 71 mmol m ^{? 3}, irrespective of the presence of CA. The maximum rate of photosynthesis (P_max) values varied between 300 and 500 μ mol O₂ mg Chl ^{? 1} h ^{? 1} regardless of growth pCO₂. Bicarbonate dehydration kinetics in artificial seawater were re‐examined to evaluate the direct HCO₃ ^? uptake as a substrate for photosynthesis. The uncatalysed CO₂ formation rate in artificial seawater of 31·65°/_oo of salinity at pH 8·2 and 25 °C was found to be 0·6 mmol m ^{? 3} min ^{? 1} at 100 mmol m ^{? 3} DIC, which is 53·5 and 7·3 times slower than the rates of photosynthesis exhibited in air‐ and high CO₂‐grown cells, respectively. These data indicate that even high CO₂‐grown cells of P. tricornutum can take up both CO₂ and HCO₃ ^? as substrates for photosynthesis and HCO₃ ^? use improves dramatically when the cells are grown in air. Detailed time courses were obtained of changes in affinity for DIC during the acclimation of high CO₂‐grown cells to air. The development of high‐affinity photosynthesis started after a 2–5 h lag period, followed by a steady increase over the next 15 h. This acclimation time course is the slowest to be described so far. High CO₂‐grown cells were transferred to controlled DIC conditions, at which the concentrations of each DIC species could be defined, and were allowed to acclimate for more than 36 h. The K_0·5[DIC] values in acclimated cells appeared to be correlated only with [CO_2(aq)] in the medium but not to HCO₃ ^? , CO₃^{2 ?} , total [DIC] or the pH of the medium and indicate that the critical signal regulating the affinity of cells for DIC in the marine diatom, P. tricornutum, is [CO_2(aq)] in the medium.     

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.     

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.     

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.     

The impact of elevated carbon dioxide on the growth and gas exchange of three C4 species differing in CO2 leak rates

Recent work has suggested that the photosynthetic rate of certain C₄ species can be stimulated by increasing CO₂ concentration, [CO₂], even under optimal water and nutrients. To determine the basis for the observed photosynthetic stimulation, we tested the hypothesis that the CO₂ leak rate from the bundle sheath would be directly related to any observed stimulation in single leaf photosynthesis at double the current [CO₂]. Three C₄ species that differed in the reported degree of bundle sheath leakiness to CO₂, Flaveria trinervia, Panicum miliaceum, and Panicum maximum, were grown for 31–48 days after sowing at a [CO₂] of 350 μl l^?1 (ambient) or 700 μl l^?1 (elevated). Assimilation as a function of increasing [CO₂] at high photosynthetic photon flux density (PPFD, 1 600 μmol m^?2 s^?1) indicated that leaf photosynthesis was not saturated under current ambient [CO₂] for any of the three C₄ species. Assimilation as a function of increasing PPFD also indicated that the response of leaf photosynthesis to elevated [CO₂] was light dependent for all three C₄ species. The stimulation of leaf photosynthesis at elevated [CO₂] was not associated with previously published values of CO₂ leak rates from the bundle sheath, changes in the ratio of activities of PEP-carboxylase to RuBP carboxylase/oxgenase, or any improvement in daytime leaf water potential for the species tested in this experiment. In spite of the simulation of leaf photosynthesis, a significant increase in growth at elevated [CO₂] was only observed for one species, F. trinervia. Results from this study indicate that leaf photosynthetic rates of certain C₄ species can respond directly to increased [CO₂] under optimal growth conditions, but that the stimulation of whole plant growth at elevated carbon dioxide cannot be predicted solely on the response of individual leaves.     

Interactions of tropospheric CO2 and O3 enrichments and moisture variations on microbial biomass and respiration in soil

Soil microbial biomass C (C_mic) is a sensitive indicator of trends in organic matter dynamics in terrestrial ecosystems. This study was conducted to determine the effects of tropospheric CO₂ or O₃ enrichments and moisture variations on total soil organic C (C_org), mineralizable C fraction (C_Min), C_mic, maintenance respiratory (qCO₂) or C_mic death (qD) quotients, and their relationship with basal respiration (BR) rates and field respiration (FR) fluxes in wheat‐soybean agroecosystems. Wheat (Triticum aestivum L.) and soybean (Glycine max. L. Merr) plants were grown to maturity in 3‐m dia open‐top field chambers and exposed to charcoal‐filtered (CF) air at 350 μL CO₂ L^?1; CF air + 150 μL CO₂ L^?1; nonfiltered (NF) air + 35 nL O₃ L^?1; and NF air + 35 nL O₃ L^?1 + 150 μL CO₂ L^?1 at optimum (? 0.05 MPa) and restricted soil moisture (? 1.0 ± 0.05 MPa) regimes. The + 150 μL CO₂ L^?1 additions were 18 h d^?1 and the + 35 nL O₃ L^?1 treatments were 7 h d^?1 from April until late October. While C_org did not vary consistently, C_Min, C_mic and C_mic fractions increased in soils under tropospheric CO₂ enrichment (500 μL CO₂ L^?1) and decreased under high O₃ exposures (55 ± 6 nL O₃ L^?1 for wheat; 60 ± 5 nL O₃ L^?1 for soybean) compared to the CF treatments (25 ± 5 nL O₃ L^?1). The qCO₂ or qD quotients of C_mic were also significantly decreased in soils under high CO₂ but increased under high O₃ exposures compared to the CF control. The BR rates did not vary consistently but they were higher in well‐watered soils. The FR fluxes were lower under high O₃ exposures compared to soils under the CF control. An increase in C_mic or C_mic fractions and decrease in qCO₂ or qD observed under high CO₂ treatment suggest that these soils were acting as C sinks whereas, reductions in C_mic or C_mic fractions and increase in qCO₂ or qD in soils under elevated tropospheric O₃ exposures suggest the soils were serving as a source of CO₂.     

Uptake of CO2 by aquatic vegetation

Abstract Photosynthesis by aquatic plants based on the supply of CO₂ from air-equilibrated solutions may be limited by the low diffusion coefficient of CO₂ in water. For plants in which the transport of CO₂ from the bulk medium is by diffusion, and the initial carboxylation uses RUBISCO, CO₂ supply can be increased by growth in habitats with fast water flow over the surface (reducing unstirred layer thickness), or with heterotrophically-augmented CO₂ levels, including the direct use of sediment CO₂. Many aquatic plants using RUBISCO as their initial carboxylase counter the limitations on CO₂ supply via the operation of biophysical CO₂ concentrating mechanisms which are based on active transport of HCO^?₃, CO₂ or H⁺ at the plasmalemma, and use bulk-phase HCO^?₃ or CO₂ as the C source. A final group of aquatic plants use biochemical CO₂ concentrating mechanisms based on auxiliary carboxylation by PEPc: C₄-like and Crassulacean Acid Metabolism–like processes are involved. These various mechanisms for increasing CO₂ supply to RUBISCO also help to offset the low specific reaction rate of aquatic plant RUBISCOs at low [CO₂] and low [CO₂]: [CO₂]. In addition to overcoming restrictions on CO₂ supply, the various methods of increasing inorganic C availability may also be important in alleviating shortages of nitrogen or photons.     

Carbonic anhydrase and C4 photosynthesis: a transgenic analysis

Carbonic anhydrase (CA, EC 4.2.1.1) catalyses the first reaction in the C₄ photosynthetic pathway, the conversion of atmospheric CO₂ to bicarbonate in the mesophyll cytosol. To examine the importance of the enzyme to the functioning of the C₄ photosynthetic pathway, Flaveria bidentis (L.) Kuntze, a C₄ dicot, was genetically transformed with an antisense construct in which the cDNA encoding a putative cytosolic CA (CA3) was placed under the control of a constitutive promoter. Some of the primary transformants had impaired CO₂ assimilation rates and required high CO₂ for growth. The T₁ progeny of four primary transformants were used to examine the quantitative relationship between leaf CA activity and CO₂ assimilation rate. CA activity was determined in leaf extracts with a mass spectrometric technique that measured the rate of ¹⁸O exchange from doubly labelled ¹³C¹⁸O₂. Steady‐state CO₂ assimilation rates were unaffected by a decrease in CA activity until CA activity was less than 20% of wild type when they decreased steeply. Transformants with less than 10% of wild‐type CA activity had very low CO₂ assimilation rates and grew poorly at ambient CO₂ partial pressure. Reduction in CA activity also increased the CO₂ partial pressure required to saturate CO₂ assimilation rates. The present data show that CA activity is essential for the functioning of the C₄ photosynthetic pathway.     

Interannual climatic variation mediates elevated CO2 and O3 effects on forest growth

We analyzed growth data from model aspen (Populus tremuloides Michx.) forest ecosystems grown in elevated atmospheric carbon dioxide ([CO₂]; 518 μL L^?1) and ozone concentrations ([O₃]; 1.5 × background of 30–40 nL L^?1 during daylight hours) for 7 years using free‐air CO₂ enrichment technology to determine how interannual variability in present‐day climate might affect growth responses to either gas. We also tested whether growth effects of those gasses were sustained over time. Elevated [CO₂] increased tree heights, diameters, and main stem volumes by 11%, 16%, and 20%, respectively, whereas elevated ozone [O₃] decreased them by 11%, 8%, and 29%, respectively. Responses similar to these were found for stand volume and basal area. There were no growth responses to the combination of elevated [CO₂+O₃]. The elevated [CO₂] growth stimulation was found to be decreasing, but relative growth rates varied considerably from year to year. Neither the variation in annual relative growth rates nor the apparent decline in CO₂ growth response could be explained in terms of nitrogen or water limitations. Instead, growth responses to elevated [CO₂] and [O₃] interacted strongly with present‐day interannual variability in climatic conditions. The amount of photosynthetically active radiation and temperature during specific times of the year coinciding with growth phenology explained 20–63% of the annual variation in growth response to elevated [CO₂] and [O₃]. Years with higher photosynthetic photon flux (PPF) during the month of July resulted in more positive growth responses to elevated [CO₂] and more negative growth responses to elevated [O₃]. Mean daily temperatures during the month of October affected growth in a similar fashion the following year. These results indicate that a several‐year trend of increasingly cloudy summers and cool autumns were responsible for the decrease in CO₂ growth response.     

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.     

CO2 uptake and transport in leaf mesophyll cells

Abstract The acquisition of inorganic carbon for photosynthetic assimilation by leaf mesophyll cells and chloroplasts is discussed with particular reference to membrane permeation of CO₂ and HCO^?₃. Experimental evidence indicates that at the apoplast pH normally experienced by leaf mesophyll cells (pH 6–7) CO₂ is the principal species of inorganic carbon taken up. Uptake of HCO^?₃ may also occur under certain circumstances (i.e. pH 8.5), but its contribution to the net flux of inorganic carbon is small and HCO^?₃ uptake does not function as a CO₂-concentrating mechanism. Similarly, CO₂ rather than HCO^?₃ appears to be the species of inorganic carbon which permeates the chloroplast envelope. In contrast to many C₃ aquatic plants and C₄ plants, C₃ terrestrial plants lack specialized mechanisms for the acquisition and transport of inorganic carbon from the intercellular environment to the site of photosynthetic carboxylation, but rely upon the diffusive uptake of CO₂.     

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.     

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%.     

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).     

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.     

Acquisition and within-plant allocation of 13C and 15N in CO2-enriched Quercus robur plants

We assessed the effects of doubling atmospheric CO₂ concentration, [CO₂], on C and N allocation within pedunculate oak plants (Quercus robur L.) grown in containers under optimal water supply. A short-term dual ¹³CO₂ and ¹⁵NO₃^? labelling experiment was carried out when the plants had formed their third growing flush. The 22-week exposure to 700 μl l^?1 [CO₂] stimulated plant growth and biomass accumulation (+53% as compared with the 350 μl l^?1 [CO₂] treatment) but decreased the root/shoot biomass ratio (-23%) and specific leaf area (-18%). Moreover, there was an increase in net CO₂ assimilation rate (+37% on a leaf dry weight basis; +71% on a leaf area basis), and a decrease in both above- and below-ground CO₂ respiration rates (-32 and -26%, respectively, on a dry mass basis) under elevated [CO₂]. ¹³C acquisition, expressed on a plant mass basis or on a plant leaf area basis, was also markedly stimulated under elevated [CO₂] both after the 12-h ¹³CO₂ pulse phase and after the 60-h chase phase. Plant N content was increased under elevated CO₂ (+36%), but not enough to compensate for the increase in plant C content (+53%). Thus, the plant C/N ratio was increased (+13%) and plant N concentration was decreased (-11%). There was no effect of elevated [CO₂] on fine root-specific ¹⁵N uptake (amount of recently assimilated ¹⁵N per unit fine root dry mass), suggesting that modifications of plant N pools were merely linked to root size and not to root function. N concentration was decreased in the leaves of the first and second growing flushes and in the coarse roots, whereas it was unaffected by [CO₂] in the stem and in the actively growing organs (fine roots and leaves of the third growth flush). Furthermore, leaf N content per unit area was unaffected by [CO₂]. These results are consistent with the short-term optimization of N distribution within the plants with respect to growth and photosynthesis. Such an optimization might be achieved at the expense of the N pools in storage compartments (coarse roots, leaves of the first and second growth flushes). After the 60-h ¹³C chase phase, leaves of the first and second growth flushes were almost completely depleted in recent ¹³C under ambient [CO₂], whereas these leaves retained important amounts of recently assimilated ¹³C (carbohydrate reserves?) under elevated [CO₂].     

Measurement of the photorespiratory activity of the submerged aquatic plant Myriophyllum spicatum L.

Abstract The CO₂ compensation concentrations (points) of leaves of the submerged vascular aquatic plant Myriophyllum spicatum L. were determined in a closed aqueous system at pH 7.0 by a gas chromatographic technique and over the range 10–30deg;C were found to range from 36 to 46 cm³m^?3 in medium equilibrated with 21% O₂ (0.03 kgm^?3), and 25 to 35 cm³m^?3 in medium equilibrated with 2% O₂ (0.03 kgm^?3). The rates of true (TPS) and apparent (APS) photosynthesis of leaves were measured in medium equilibrated with 21% O₂ and buffered at pH 7.0, at subsaturating concentrations (12.8–18.8 mmol m^?3) of dissolved inorganic carbor. (DIC) containing H¹⁴CO₃, by determining the initial rates of uptake by the leaves of DIC and ¹⁴C-activity from the medium. The rate of photorespiration, the difference between TPS and APS, was 7.0–13.3% of TPS over the range of 10–25°C and rose to 29% of TPS at 35°C. The magnitude of the compensation point of this plant is therefore similar to, but is much less O₂-sensitive than, those of C₃ plants, and the photorespiratory rate, at DIC concentrations near the CO₂ compensation point, is very low compared to that of C₃ plants.