Photoassimilation of CO2 by Isolated Bundle-Sheath Strands of Zea mays I. Stimulation of CO2 Assimilation by Adding Various Intermediates of the Photosynthetic Cycle; Evidence for a Deficient Photosystem II Activity

Isolated bundle-sheath (BS) strands from leaves of mature maize plants show enhanced rates of CO₂ fixation in the presence of reduced intermediates of the photosynthetic cycle (R5P, DHAP, FruDP.) 3PGA is the major labelled product of ¹⁴CO₂ fixation whatever the substrate added. CO₂ fixation is much lower with PGA than with reduced intermediates, suggesting a limited capacity of the cells to regenerate RuDP (the CO₂-acceptor) from PGA. These two experimental facts, which are characteristic features of bundle-sheath photosynthesis for maize (a species with agranal bundle-sheath chloroplasts) indicate that phaotosystem II activity is a limiting factor for the evolution of the bundle-sheath photosynthetic process. Nevertheless, a reducing capacity arises as proved by sensitivity of CO₂ fixation to DCMU, particularly when PGA is added to the bundle-sheath. PGA synthesis occurs, in the presence of non-limiting amounts of CO₂, according to the equation: RuDP + CO₂→ 2 PGA; the oxygen effect on ¹⁴CO₂ fixation, at lower CO₂ concentration, is interpreted as a dilution effect of the internal pool of ¹⁴CO₂ by unlabelled CO₂ generated by photorespiration.     

The temperature acclimation of photosynthetic responses to CO2 in Zea mays and its relationship to the activities of photosynthetic enzymes and the CO2-concentrating mechanism of C4 photosynthesis

Abstract Associations between photosynthetic responses to CO₂ at rate-saturating light and photosynthetic enzyme activities were compared for leaves of maize grown under constant air temperatures of 19, 25 and 31°C. Key photosynthetic enzymes analysed were ribulose bisphosphatc (RuBP) carboxylase, phosphoenolpyruvate (PEP) carboxylase, NADP-malic enzyme and pyruvate, P_i dikinasc. Rates of CO₂-saturated photosynthesis were similar in leaves developed at 19°C and 25°C but were decreased significantly by growth at 31°C. In contrast, carboxylation efficiency differed significantly between all three temperature regimes. Carboxylation efficiency was greatest in leaves developed at 19°C and decreased with increasing temperature during growth. The changes of carboxylation efficiency were highly correlated with changes in the activity of pyruvate, P_i dikinase (r= 0.95), but not with other photosynthetic enzyme activities. The activities of these latter enzymes, including that of RuBP carboxylase, were relatively insensitive to temperature during growth. The sensitivity of quantum yield to O₂ concentration was lower in leaves grown at 19°C than in leaves grown at 31°C. These observations support the novel hypothesis that variation in the capacity for CO₂ delivery to the bundle sheath by the C₄ cycle, relative to the capacity for net assimilation by the C₂ cycle, can be a principal determinant of C₄ photosynthetic responses to CO₂.     

Sensing of atmospheric CO2 by plants

Abstract. Despite recent interest in the effects of high CO₂ on plant growth and physiology, very little is known about the mechanisms by which plants sense changes in the concentration of this gas. Because atmospheric CO₂ concentration is relatively constant and because the conductance of the cuticle to CO₂ is low, sensory mechanisms are likely to exist only for intercellular CO₂ concentration. Therefore, responses of plants to changes in atmospheric CO₂ will depend on the effect of these changes on intercellular CO₂ concentration. Although a variety of plant responses to atmospheric CO₂ concentration have been reported, most of these can be attributed to the effects of intercellular CO₂ on photosynthesis or stomatal conductance. Short-term and long-term effects of CO₂ on photosynthesis and stomatal conductance are discussed as sensory mechanisms for responses of plants to atmospheric CO₂. Available data suggest that plants do not fully realize the potential increases in productivity associated with increased atmospheric CO₂. This may be because of genetic and environmental limitations to productivity or because plant responses to CO₂ have evolved to cope with variations in intercellular CO₂ caused by factors other than changes in atmospheric 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.     

CO2 and intracellular pH

Abstract The experimental determination of cytoplasmic and vacuolar pH values is discussed. Despite variation in these values evidence indicates that intracellular pH values are normally regulated within narrow limits. The regulatory mechanisms proposed involve the metabolic consumption of OH^& and the active efflux of H ⁺. The evidence for intracellular pH modification in response to CO₂ hydration and the production of HCO^?₃ and H⁺ is examined. Theoretical calculations and experimental data indicate that CO₂ concentrations as high as 5% will lower intracellular pH. Conversely, variation in CO₂ levels around atmospheric concentrations is unlikely to perturb intracellular pH. High CO₂ levels are found in bulky tissues, and flooded root systems. Evidence is presented that the slow diffusion of dissolved CO₂ compared to gaseous CO₂ results in its accumulation. It is proposed that the accumulation of respiratory CO₂ may reduce intracellular pH values when plant tissues, cells or protoplasts are maintained in a liquid culture medium. Finally, the possible role of dark CO₂ fixation and organic acid synthesis in the regulation of intracellular pH is examined.     

Influence of elevated CO2 and nitrogen nutrition on photosynthesis and nitrate photo-assimilation in maize (Zea mays L.)

Measurements of CO₂ and O₂ gas exchange and chlorophyll a fluorescence were used to test the hypothesis that elevated atmospheric CO₂ inhibits nitrate (NO₃ ^– ) photo‐assimilation in the C₄ plant, maize (Zea mays L.). The assimilatory quotient (AQ), the ratio of net CO₂ assimilation to net O₂ evolution, decreases as NO₃ ^– photo‐assimilation increases so that the difference in AQ between the ammonium‐ and nitrate‐fed plants (ΔAQ) provided an in planta estimate of NO₃ ^– photo‐assimilation. In fully expanded maize leaves, NO₃ ^– photo‐assimilation was detectable only under high light and was not affected by CO₂ treatments. Furthermore, CO₂ assimilation and O₂ evolution were higher under NO₃ ^– than ammonia (NH₄⁺) regardless of CO₂ levels. In conclusion, NO₃ ^– photo‐assimilation in maize primarily occurred at high light when reducing equivalents were presumably not limiting. Nitrate photo‐assimilation enhanced C₄ photosynthesis, and in contrast to C₃ plants, elevated CO₂ did not inhibit foliar NO₃^– photo‐assimilation.     

N2 fixation by Acacia species increases under elevated atmospheric CO2

In the present study the effect of elevated CO₂ on growth and nitrogen fixation of seven Australian Acacia species was investigated. Two species from semi‐arid environments in central Australia (Acacia aneura and A. tetragonophylla) and five species from temperate south‐eastern Australia (Acacia irrorata, A. mearnsii, A. dealbata, A. implexa and A. melanoxylon) were grown for up to 148 d in controlled greenhouse conditions at either ambient (350 µmol mol^?1) or elevated (700 µmol mol^?1) CO₂ concentrations. After establishment of nodules, the plants were completely dependent on symbiotic nitrogen fixation. Six out of seven species had greater relative growth rates and lower whole plant nitrogen concentrations under elevated versus normal CO₂. Enhanced growth resulted in an increase in the amount of nitrogen fixed symbiotically for five of the species. In general, this was the consequence of lower whole‐plant nitrogen concentrations, which equate to a larger plant and greater nodule mass for a given amount of nitrogen. Since the average amount of nitrogen fixed per unit nodule mass was unaltered by atmospheric CO₂, more nitrogen could be fixed for a given amount of plant nitrogen. For three of the species, elevated CO₂ increased the rate of nitrogen fixation per unit nodule mass and time, but this was completely offset by a reduction in nodule mass per unit plant mass.     

Oscillatory Transpiration and CO2 Exchange of Citrus Leaves at the CO2 Compensation Concentration

CO₂ and H₂O vapor exchange were measured by enclosing citrus (Citrus sinensis cv. Sour Orange) leaves in a temperature controlled transparent leaf chamber. Introduction of dry air into the closed circuit gas flow caused cyclic oscillation in CO₂ and H₂O vapor exchange. It is suggested that oscillation in the CO₂ exchange at the CO₂ compensation concentration is due to oscillation in non-stomatal resistance to CO₂. Three types of oscillation were observed: 3–6 min (peak to peak) in young leaves, 30 min in mature leaves, and 160 min in old leaves.     

The Reducing Power and the CO2 Fixation Activity of Isolated Choroplasts

Antimycin A, which stimulates the Co₂ fixation activity of isolated intact spinach (Spinacia oleracea L) chloroplasts, was shown to stimulate also the oxygen evolution in the presence of oxaloacetate, as do the uncouplers. Some uncoupers (tetramethlethylenediamine, Nh₄CL) were shown to stimulate the CO₂ fixation activity, and to decrease the intrachloroplastic ATP levels, like antimycin A. The intrachloroplastic NADPH levels were not affected by the inhibitors studied.     

Changes in K+, Cl− and P contents in stomata of Zea mays leaves exposed to different light and CO2 levels

Maize plants (Zea mays L. hybrid INRA 508) were placed under controlled conditions of light and CO₂ partial pressure. The K⁺, Cl^? and P contents were then determined by X-ray microanalysis in the bulbous end of guard cells and in the center of subsidiary cells. The results were interpreted in connection with the stomatal conductance at the time of sampling. In normal air, the K⁺ and Cl^? contents in guard cells only rose from a light threshold of about 300 μmol m^?2 s^?1 at which stomata were already largely open. At 600 μmol m^?2 s^?1, the K⁺ and Cl^? levels in guard cells attained values that were 3- and 8-fold greater, respectively, than the values observed in darkness. The K⁺ and Cl^? contents in the subsidiary cells remained quite constant irrespective of the light conditions. CO₂-free air in darkness induced a significant K⁺ influx towards guard and subsidiary cells. Under light and in CO₂-free air, the K⁺ and Cl^? contents dramatically increased in the guard cells, but slightly decreased in the subsidiary cells. Thus, when subjected to strong light in CO₂-free air, the K⁺ and Cl^? contents in the subsidiary cells were approximately equal to those measured in normal air conditions. In the guard cells, stomatal opening was associated with a marked shift of the Cl^?/K⁺ ratio – from 0.3 for closed stomata to ca 1 for fully open stomata. This could imply a slow change in the nature of the principal counterion accompanying K⁺ during stomatal opening. The content of P in guard cells appeared, in contrast to that of K⁺ and Cl^?, to be practically independent of stomatal aperture.     

The respiratory source of CO2

Abstract Approximately half of the carbon plants fix in photosynthesis is lost in dark respiration. The major pathways for dark respiration and their control are briefly discussed in the context of a growing plant. It is suggested that whole-plant respiration may be largely ADP-limited and that fine control of the respiratory network serves to select the respiratory substrate and to partition carbon between the numerous possible fates within the network. The striking stoichiometry between whole-plant growth and respiration is reviewed, and the relationships between substrate-limited growth and ADP-limited respiration are discussed.     

A new method to measure carbon isotope composition of CO2 respired by trees: stem CO2 equilibration

1.  Applying Keeling plot techniques to derive δ¹³C of respiratory input in a closed non-equilibrated chamber can lead to large errors because steady-state diffusion rules are violated in a non-steady-state environment. To avoid these errors, respiratory δ¹³C can be derived using equilibrated closed chambers.
2.  We introduce a new method to obtain stem respired CO₂δ¹³C (δ_{st - r}) with closed equilibrated stem chambers (E-SC). We present a theoretical model describing the equilibration process, test the model against field data and find excellent agreement. The method is further tested by comparing it with closed non-equilibrated stem chambers (NE-SC); we found no difference between these methods.
3.  Our theoretical model to describe CO₂ diffusion from the respiratory pool into the chamber and the equation to derive the δ¹³C of the efflux are general. They could be applied to other ecosystem components (e.g. soils).
4.  Our method is easy to implement, cost effective, minimizes sources of error and allows for rigorous leak detection. One major limitation is its inability to detect rapid change; the equilibration process requires 15 ± 2 h. A second limitation is that it cannot be used for species that produce abundant pitch at sites of stem wounding (e.g. Pseudotsuga menziesii ).
5.  Investigating δ¹³C of CO₂ respired by different ecosystem components is necessary to interpret δ¹³C of ecosystem respiration. This parameter has major implications with respect to global carbon cycle science.     

Soil acidification induced by elevated atmospheric CO2

Soil acidification is a very important process in the functioning of earth's ecosystems. A major source of soil acidity is CO₂, derived from the respiration of plant roots and microbes, which forms carbonic acid in soil waters. Because elevated atmospheric CO₂ often stimulates respiration of soil biota in experiments that test ecosystem effects of elevated atmospheric CO₂, we hypothesize that rising atmospheric CO₂ (which has increased from ～200 ppm since the interglacial and may exceed 550 ppm by the end of the 21st century) is significantly increasing acid inputs to soils. Here, using column‐leaching experiments with contrasting soils, we demonstrate that soil CO₂ is a much more potent agent of soil acidification than is generally appreciated, capable of displacing almost all exchangeable base cations in soils, and even elevating Al(III) concentrations in H₂CO₃‐acidified soil waters. The potent soil acidifying potential of soil H₂CO₃ is attributed to the low pK_a,1 of molecular H₂CO₃ (3.76 at 25°C), which contrasts greatly with that of (a convention that combines CO₂ (aq) and molecular H₂CO₃, the pK_a,1 of which is 6.36 at 25°C). This distinction is significant for soil systems because of soil's greatly elevated CO₂, their variety of sinks for H⁺, and the wide range of contact times between soil solids, water, and gas. Modelling suggests that a doubling of atmospheric CO₂ may increase acid inputs from carbonic acid leaching by up to 50%. Combined with the results of CO₂ studies in whole ecosystems, this implies that increases in atmospheric CO₂ since the interglacial have gradually acidified soils, especially poorly buffered soils, throughout the world.