Low CO2 Compensation Point Research Articles

The carbon-concentrating mechanism of the extremophilic red microalga Cyanidioschyzon merolae

Cyanidioschyzonmerolae is an extremophilic red microalga which grows in low-pH, high-temperature environments. The basis of C. merolae’s environmental resilience is not fully characterized, including whether this alga uses a carbon-concentrating mechanism (CCM). To determine if C. merolae uses a CCM, we measured CO2 uptake parameters using an open-path infra-red gas analyzer and compared them to values expected in the absence of a CCM. These measurements and analysis indicated that C. merolae had the gas-exchange characteristics of a CCM-operating organism: low CO2 compensation point, high affinity for external CO2, and minimized rubisco oxygenation. The biomass δ13C of C. merolae was also consistent with a CCM. The apparent presence of a CCM in C. merolae suggests the use of an unusual mechanism for carbon concentration, as C. merolae is thought to lack a pyrenoid and gas-exchange measurements indicated that C. merolae primarily takes up inorganic carbon as carbon dioxide, rather than bicarbonate. We use homology to known CCM components to propose a model of a pH-gradient-based CCM, and we discuss how this CCM can be further investigated.

Salsola laricifolia, another C3-C 4 intermediate species in tribe Salsoleae s.l. (Chenopodiaceae).

This study identifies Salsola laricifolia as a C3-C4 intermediate in tribe Salsoleae s.l., Chenopodiaceae, and compares S. laricifolia with the previously described C3-C4 intermediates in Salsoleae. Photosynthetic pathway characteristics were studied in four species of this tribe including S. laricifolia, C3 Sympegma regelii, C3-C4 S. arbusculiformis, and C4 S. arbuscula, using the approaches of leaf anatomy and ultrastructure, activities of ribulose 1-5-bisphosphate carboxylase/oxygenase (Rubisco) and PEP carboxylase (PEPC), CO2 compensation point, and immunolocalization of Rubisco, PEPC, and the P-subunit of glycine decarboxylase (GDC). Salsola laricifolia has intermediate features, with near continuous and distinctive Kranz-like cells (KLCs) compared with the C3-Sympegmoid anatomical type and the C3-C4 intermediate S. arbusculiformis, a relatively low CO2 compensation point (30.4μmolmol(-1)) and mesophyll (M)-to KLC tissue ratio, mitochondria in KLCs primarily occurring along the centripetal wall, and specific localization of P-protein GDC in the KLCs. The C3-type isotope value (-22.4‰), the absence of the clear labeling for PEPC in M cells, and the low activity of the PEPC enzyme (61.5μmolmg(-1)chlorophyll(-1)h(-1)) support the identification of S. laricifolia as a type I C3-C4 intermediate. Although these C3-C4 intermediate species have different structural features, one with discontinuous KL cells and the other with continuous, they have similar characteristics in physiology and biochemistry.

Ecophysiology of gelatinous Nostoc colonies: unprecedented slow growth and survival in resource-poor and harsh environments.

The cyanobacterial genus Nostoc includes several species forming centimetre-large gelatinous colonies in nutrient-poor freshwaters and harsh semi-terrestrial environments with extended drought or freezing. These Nostoc species have filaments with normal photosynthetic cells and N2-fixing heterocysts embedded in an extensive gelatinous matrix of polysaccharides and many other organic substances providing biological and environmental protection. Large colony size imposes constraints on the use of external resources and the gelatinous matrix represents extra costs and reduced growth rates. The objective of this review is to evaluate the mechanisms behind the low rates of growth and mortality, protection against environmental hazards and the persistence and longevity of gelatinous Nostoc colonies, and their ability to economize with highly limiting resources. Simple models predict the decline in uptake of dissolved inorganic carbon (DIC) and a decline in the growth rate of spherical freshwater colonies of N. pruniforme and N. zetterstedtii and sheet-like colonies of N. commune in response to a thicker diffusion boundary layer, lower external DIC concentration and higher organic carbon mass per surface area (CMA) of the colony. Measured growth rates of N. commune and N. pruniforme at high DIC availability comply with general empirical predictions of maximum growth rate (i.e. doubling time 10-14 d) as functions of CMA for marine macroalgae and as functions of tissue thickness for aquatic and terrestrial plants, while extremely low growth rates of N. zetterstedtii (i.e. doubling time 2-3 years) are 10-fold lower than model predictions, either because of very low ambient DIC and/or an extremely costly colony matrix. DIC uptake is limited by diffusion at low concentrations for all species, although they exhibit efficient HCO3(-) uptake, accumulation of respiratory DIC within the colonies and very low CO2 compensation points. Long light paths and light attenuation by structural substances in large Nostoc colonies cause lower quantum efficiency and assimilation number and higher light compensation points than in unicells and other aquatic macrophytes. Extremely low growth and mortality rates of N. zetterstedtii reflect stress-selected adaptation to nutrient- and DIC-poor temperate lakes, while N. pruniforme exhibits a mixed ruderal- and stress-selected strategy with slow growth and year-long survival prevailing in sub-Arctic lakes and faster growth and shorter longevity in temperate lakes. Nostoc commune and its close relative N. flagelliforme have a mixed stress-disturbance strategy not found among higher plants, with stress selection to limiting water and nutrients and disturbance selection in quiescent dry or frozen stages. Despite profound ecological differences between species, active growth of temperate specimens is mostly restricted to the same temperature range (0-35 °C; maximum at 25 °C). Future studies should aim to unravel the processes behind the extreme persistence and low metabolism of Nostoc species under ambient resource supply on sediment and soil surfaces.

Effect of light and CO2 on inorganic carbon uptake in the invasive aquatic CAM-plant Crassula helmsii

Crassula helmsii (T. Kirk) Cockayne is an invasive aquatic plant in Europe that can suppress many native species because it can grow at a large range of dissolved inorganic carbon concentrations and light levels. One reason for its ecological success may be the possession of a regulated Crassulacean Acid Metabolism (CAM), which allows aquatic macrophytes to take up CO2 in the night in addition to the daytime. The effect of light and CO2 on the regulation of CAM and photosynthesis in C. helmsii was investigated to characterise how physiological acclimation may confer this ecological flexibility. After 3 weeks of growth at high light (230 µmol photon m–2 s–1), C. helmsii displayed 2.8 times higher CAM at low compared with high CO2 (22 v. 230 mmol m–3). CAM was absent in plants grown at low light (23 µmol photon m–2 s–1) at both CO2 concentrations. The observed regulation patterns are consistent with CAM acting as a carbon conserving mechanism. For C. helmsii grown at high light and low CO2, mean photosynthetic rates were relatively high at low concentrations of CO2 and were on average 80 and 102 µmol O2 g–1 DW h–1 at CO2 concentrations of 3 and 22 mmol m–3 CO2, which, together with mean final pH values of 9.01 in the pH drift, indicate a low CO2 compensation point (<3 mmol m–3) but do not indicate use of bicarbonate as an additional source of exogenous inorganic carbon. The relatively high photosynthetic rates during the entire daytime were caused by internally derived CAM-CO2 and uptake from the external medium. During decarboxylation, CO2 generated from CAM contributed up to 29% to photosynthesis, whereas over a day the contribution to the carbon balance was ≤13%. The flexible adjustment of CAM and the ability to maintain photosynthesis at very low external CO2 concentrations, partly by making use of internally generated CO2 via CAM, may contribute to the broad ecological niche of C. helmsii.

Physiological, anatomical and biochemical characterisation of photosynthetic types in genus Cleome (Cleomaceae)

C4 photosynthesis has evolved many times in 18 different families of land plants with great variation in leaf anatomy, ranging from various forms of Kranz anatomy to C4 photosynthesis occurring within a single type of photosynthetic cell. There has been little research on photosynthetic typing in the family Cleomaceae, in which only one C4 species has been identified, Cleome gynandra L. There is recent interest in selecting and developing a C4 species from the family Cleomaceae as a model C4 system, since it is the most closely related to Arabidopsis, a C3 model system (Brown et al. 2005). From screening more than 230 samples of Cleomaceae species, based on a measure of the carbon isotope composition (δ13C) in leaves, we have identified two additional C4 species, C. angustifolia Forssk. (Africa) and C. oxalidea F.Muell. (Australia). Several other species have δ13C values around -17‰ to -19‰, suggesting they are C4-like or intermediate species. Eight species of Cleome were selected for physiological, anatomical and biochemical analyses. These included C. gynandra, a NAD-malic enzyme (NAD-ME) type C4 species, C. paradoxa R.Br., a C3-C4 intermediate species, and 6 others which were characterised as C3 species. Cleome gynandra has C4 features based on low CO2 compensation point (Γ), C4 type δ13C values, Kranz-type leaf anatomy and bundle sheath (BS) ultrastructure, presence of C4 pathway enzymes, and selective immunolocalisation of Rubisco and phosphoenolpyruvate carboxylase. Cleome paradoxa was identified as a C3-C4 intermediate based on its intermediate Γ (27.5 μmol mol-1), ultrastructural features and selective localisation of glycine decarboxylase of the photorespiratory pathway in mitochondria of BS cells. The other six species are C3 plants based on Γ, δ13C values, non-Kranz leaf anatomy, and levels of C4 pathway enzymes (very low or absent) typical of C3 plants. The results indicate that this is an interesting family for studying the genetic basis for C4 photosynthesis and its evolution from C3 species.

(147) Effect of Hypobaria, Oxygen, and Carbon Dioxide on Gas Exchange, Ethylene Evolution, and Growth of Lettuce Plants for NASA Advanced Life Support Systems

There are engineering and payload advantages in growing plants under hypobaric (reduced atmospheric pressure) conditions in biomass production for extraterrestrial base or spaceflight environments. Objectives of this research were to characterize the influence of hypobaria on growth, gas exchange, and ethylene evolution of lettuce (Lactuca sativa L. cv. Buttercrunch). Elevated levels of the plant hormone, ethylene, occur in enclosed crop production systems and in space-flight environments—leading to adverse plant growth and sterility. Lettuce plants were grown under variable total gas pressures [25 (low) or 101 kPa (ambient)]. During short growth periods of up to 10 days, growth was comparable between low and ambient pressure plants. Regardless of total pressure, plant growth was reduced at 6 kPa pO2 compared to 12 and 21 kPa pO2. At 6 kPa pO2 there was greater growth reduction and stress with ambient (101 kPa) than low (25kPa) pressure plants. Plants at 25/12 kPa pO2 had comparable CO2 assimilation and a 25% lower dark-period respiration than 101/21 kPa pO2 (ambient) plants. Greater efficiency of CO2 assimilation/dark-period respiration occurred with low pressure plants at 6 kPa pO2. Low pressure plants had a reduced CO2 saturation point (100 Pa CO2) compared with ambient (150 Pa CO2). Low pO2 lowered CO2 compensation points for both 25 and 101 kPa plants, i.e., likely due to reduced O2 competing with CO2 for Rubisco. Ethylene was 70% less under low than ambient pressure. High ethylene decreased CO2 assimilation rate of 101/12 kPa O2 plants. The higher dark-period respiration rates (higher night consumption of metabolites) of ambient pressure plants could lead to greater growth (biomass production) of low pressure plants during longer crop production cycles.

Induction of a C(4)-like mechanism of CO(2) fixation in Egeria densa, a submersed aquatic species.

The expression of phosphoenolpyruvate carboxylase (PEPC) and NADP-malic enzyme (NADP-ME) in Egeria densa leaves was studied under low temperature and light (LTL) following incubation under high temperature and light (HTL), conditions previously shown to induce high and low CO(2) compensation points, respectively. Transfer from LTL to HTL conditions induced increases in the activities and amounts of both enzymes. One NADP-ME isoform was observed in induced and uninduced samples. Two isoforms of PEPC were expressed, with the lower M(r) isoform being induced by HTL. NADP-ME showed properties similar to those of the isoform in C(3) species. The inducible PEPC isoform has a low K(m) for both substrates. PEPC kinetic and regulatory properties (V(max) and K(m) for phosphoenolpyruvate, and I(50) for L-malate) are different in samples taken in the dark from those in the light, indicating that some modification of PEPC may be occurring during the day. Finally, abscisic acid induced the expression of PEPC and NADP-ME in a manner similar to temperature induction, except that the activities of both PEPC isoforms were increased. A different signaling system may exist in this species in response to high temperature or abscisic acid, both of which induce changes in photosynthetic metabolism.

Long-term responses of the green-algal lichen Parmelia caperata to natural CO2 enrichment.

Acclimation to elevated CO2 was investigated in Parmelia caperata originating from the vicinity of a natural CO2 spring, where the average daytime CO2 concentration was 729 ± 39 μmol mol-1 dry air. Thalli showed no evidence of a down-regulation in photosynthetic capacity following long-term exposure to CO2 enrichment in the field; carboxylation efficiency, total Ribulose bisphosphate carboxylase/oxygenase (Rubisco) content, apparent quantum yield of CO2 assimilation, and the light-saturated rate of CO2 assimilation (measured under ambient and saturating CO2 concentrations) were similar in thalli from the naturally CO2 enriched site and an adjacent control site where the average long-term CO2 concentration was about 355 μmol mol-1. Thalli from both CO2 environments exhibited low CO2 compensation points and early saturation of CO2 uptake kinetics in response to increasing external CO2 concentrations, suggesting the presence of an active carbon-concentrating mechanism. Consistent with the lack of significant effects on photosynthetic metabolism, no changes were found in the nitrogen content of thalli following prolonged exposure to elevated CO2. Detailed intrathalline analysis revealed a decreased investment of nitrogen in Rubisco in the pyrenoid of algae located in the elongation zone of thalli originating from elevated CO2, an effect associated with a reduction in the percentage of the cell volume occupied by lipid bodies and starch grains. Although these differences did not affect the photosynthetic capacity of thalli, there was evidence of enhanced limitations to CO2 assimilation in lichens originating from the CO2-enriched site. The light-saturated rate of CO2 assimilation measured at the average growth CO2 concentration was found to be significantly lower in thalli originating from a CO2-enriched atmosphere compared with that of thalli originating and measured at ambient CO2. At lower photosynthetic photon flux densities, the light compensation point of net CO2 assimilation was significantly higher in thalli originating from elevated CO2, and this effect was associated with higher usnic acid content.

Intraspecific variation in photosynthetic responses of trebouxioid lichens with reference to the activity of a carbon-concentrating mechanism.

The photosynthetic responses of a range of trebouxioid lichens were investigated to determine whether variations in net assimilation rates shown by populations of the same species collected from different habitats could be correlated with adjustments in carbon-concentrating mechanism (CCM) activity. The activity of a CCM was inferred from the high affinity for CO2 [i.e. low CO2 compensation point (Γ); low external CO2 concentration at which half-maximal assimilation rates are reached (K 0.5 CO2)], the release of a pool of accumulated dissolved inorganic carbon (Ci) during light/dark transient measurements of CO2 exchange and values for carbon isotope discrimination intermediate between those characteristic of C3 and C4 terrestrial plants. Higher net and gross assimilation rates were expressed by lichens collected from shaded woodland habitats. The higher rates were not accounted for by variations in chlorophyll content. Lichens with high assimilation rates also showed an increased affinity for CO2 as demonstrated by low CO2 compensation points and K 0.5 values and the magnitude of the Ci pool accumulated upon illumination and released after darkening of the thalli. However, there was no correlation between assimilation rates and organic matter or instantaneous carbon isotope discrimination measurements, with the latter remaining roughly consistent whatever the provenance or species of the lichen material. The data are discussed with reference to significant environmental factors which are likely to control photosynthesis in the habitats studied.

733 PB 023 PHOTOSYNTHESIS AND MANNITOL/SUCROSE PARTITIONING UNDER DIFFERENT CO, ASSIMILATION CONDITIONS IN CELERY

Photosynthetic rates (A) in celery-(Apium graveolens L.) and other polyol-synthesizers are sometimes high for C, species. In celery such rates have been related to a low CO2 compensation point typical of C4 and C3-C4 intermediate spp, although other data show celery photosynthesis as typically C3 Therefore, celery gas exchange was here reanalyzed, and while A was high (CO2 assimilation rates were 21.2 and 27.6 μ mol m-2s-1, average and maximum, photosynthesis was otherwise C,: CO, comp pt of 3.5-5.0 Pa, carboxylation efficiency of 0.99 μmol CO2m-2s-1Pa-1, light comp pt of 8-36 μ mol photon m-1s-1, optimum temp of 22-27°C for Amax. High A may relate to a capacity to synthesize both mannitol and sucrose. 14C pulse-chase studies, with different A obtained by imposing light gradients across opposite leaflets, showed 1-10% increases in mannitoll sucrose labelling. Higher A may reflect carbon partitioning into mannitol, agreeing with a hypothesis that polyol synthesis effectively recycles reductant in the cytosol.

PHOTOSYNTHETIC CHARACTERISTICS OF C3‐C4 INTERMEDIATE FLAVERIA FLORIDANA (ASTERACEAE) IN NATURAL HABITATS: EVIDENCE OF ADVANTAGES TO C3‐C4 PHOTOSYNTHESIS AT HIGH LEAF TEMPERATURES

Measurements of leaf gas exchange were conducted in situ for the C3‐C4 intermediate plant Flaveria floridana. Leaves exhibited measurable CO2 assimilation at atmospheric CO2 concentrations as low as 20 μmol/mol. This result demonstrates that the low CO2 compensation points observed in past studies of greenhouse‐grown C3‐C4 intermediate plants also exist in plants growing in their natural habitat. Photosynthesis rates in F. floridana were near their maximum at intercellular CO2 concentrations as low as 112 μmol/mol. The existence of near‐maximum photosynthesis rates at such low intercellular CO2 concentrations is interpreted as evidence for the existence of a CO2‐concentrating mechanism in F. floridana. Such a mechanism would also explain the observed lack of response in photosynthesis rates to reductions in stomatal conductance and intercellular CO2 concentration as the leaf‐to‐air water vapor concentration gradient is increased. Photosynthetic rates were relatively high at leaf temperatures between 35 and 40 C, compared to most C3 plants. At midday during May, when leaf temperatures were between 35 and 42 C, F. floridana leaves exhibited photosynthesis rates that were four times higher than a sympatric C3 species (Eustoma exaltatum) of similar growth form and ecological habit. The high photosynthesis rates at high leaf temperatures in F. floridana were not due to higher leaf nitrogen contents, but rather to its reduced rate of photorespiration. These results confirm that C3‐C4 intermediate photosynthesis can provide plants with an advantage at high leaf temperatures, compared to C3 plants.

Photosynthetic Characteristics of C 3 -C 4 Intermediate Flaveria floridana (Asteraceae) in Natural Habitats: Evidence of Advantages to C 3 -C 4 Photosynthesis at High Leaf Temperatures

Measurements of leaf gas exchange were conducted in situ for the C3-C4 intermediate plant Flaveria floridana. Leaves exhibited measurable CO2 assimilation at atmospheric CO2 concentrations as low as 20 μmol/mol. This result demonstrates that the low CO2 compensation points observed in past studies of greenhouse-grown C3-C4 intermediate plants also exist in plants growing in their natural habitat. Photosynthesis rates in F. floridana were near their maximum at intercellular CO2 concentrations as low as 112 μmol/mol. The existence of near-maximum photosynthesis rates at such low intercellular CO2 concentrations is interpreted as evidence for the existence of a CO2-concentrating mechanism in F. floridana. Such a mechanism would also explain the observed lack of response in photosynthesis rates to reductions in stomatal conductance and intercellular CO2 concentration as the leaf-to-air water vapor concentration gradient is increased. Photosynthetic rates were relatively high at leaf temperatures between 35 and 40 C, compared to most C3 plants. At midday during May, when leaf temperatures were between 35 and 42 C, F. floridana leaves exhibited photosynthesis rates that were four times higher than a sympatric C3 species (Eustoma exaltatum) of similar growth form and ecological habit. The high photosynthesis rates at high leaf temperatures in F. floridana were not due to higher leaf nitrogen contents, but rather to its reduced rate of photorespiration. These results confirm that C3-C4 intermediate photosynthesis can provide plants with an advantage at high leaf temperatures, compared to C3 plants.

The acquisition of inorganic carbon by marine macroalgae

Marine macroalgae inhabit three different environments: the eulittoral zone, rock pools, and the sublittoral zone. Many macroalgae exhibit C4 gas exchange characteristics, i.e., low CO2 compensation point, high pH compensation point, photosynthesis insensitive to changes in oxygen changes below 21 kPa, and a high affinity for inorganic carbon. It is concluded that in general eulittoral and rock-pool species are more C4-like than the subtidal species though there are interesting exceptions. Experimental evidence points to the following mechanisms being involved in inorganic carbon acquisition by macroalgae. The role of β-carboxylation as the primary step in carbon fixation is only convincing in Udotea flabellum, while PGA is the first product of 14C fixation in most species. Direct evidence of inorganic carbon accumulation is only available for Ulva fasciata, whereas Chondrus crispus does not have this ability. Studies show that carbonic anhydrase is prominent in the mechanism of carbon acquisition by Ascophyllum nodosum, and when inhibited the alga is dependent on CO2, whereas U. fasciata retains some ability to use bicarbonate ions. It is concluded that macroalgae display a range of inorganic carbon assimilation mechanisms that are active to varying degrees. The relationships between these mechanisms, the different macroalgal habitats, and carbonic anhydrase is discussed. Key words: inorganic carbon concentrating mechanisms, macroalgae, carbonic anhydrase.

The role of phosphoenolpyruvate carboxykinase in a marine macroalga with C4-like photosynthetic characteristics.

Udotea flabellum is a marine, macroscopic green alga with C4-like photosynthetic characteristics, including little O2 inhibition of photosynthesis, a low CO2 compensation point, and minimal photorespiration; but it lacks anatomical features analogous to the Kranz compartmentation of C4 plants, and phosphoenolpyruvate carboxylase [PEPC; orthophosphate:oxaloacetate carboxy-lyase (phosphorylating), EC 4.1.1.31] activity is negligible. Phosphoenolpyruvate carboxykinase (PEPCK) activity (carboxylating) in Udotea extracts was equivalent to that of ribulose-bisphosphate carboxylase [Rubisco; 3-phospho-D-glycerate carboxy-lyase (dimerizing), EC 4.1.1.39]. When PEPCK activity was inhibited in vivo with 3-mercaptopicolinic acid (MPA), thallus photosynthesis decreased by 70% and became sensitive to O2. Codium decorticatum, a related species that lacks C4-like photosynthetic features and PEPCK activity, showed no increase in O2 inhibition upon exposure to MPA. Rubisco and PEPC activities in Udotea were not inhibited by MPA. Labeling of the early photosynthetic products malate and aspartate was reduced 66% by MPA, while intermediates of the photorespiratory carbon oxidation cycle showed a 3-fold increase. Udotea evolved O2 in the light in the absence of inorganic carbon, suggesting it had an endogenous carbon source for photosynthesis. Exogenous malate stimulated this process, while MPA inhibited it. PEPCK was not involved in Crassulacean acid metabolism or dark CO2 fixation. These MPA studies establish a direct link between PEPCK activity and the low O2 inhibition of photosynthesis and low photorespiration in Udotea. The data are consistent with carboxylation by a cytosolic PEPCK providing a C4 acid, such as malate, to the chloroplast for decarboxylation to elevate the CO2 concentration at the Rubisco fixation site. Udotea is to date the most primitive plant with a C4-like form of photosynthesis.

Photosynthesis of Cassava (Manihot esculenta)

SUMMARYIn recent studies of cassava at CIAT, net CO2uptake rates of 20 to 35 μmol CO2m−2s−1were commonly observed. Cassava photosynthesis has a high optimum temperature (35°C) and a wide plateau (25 to 35°C) corresponding to the temperature range under which cassava is cultivated. Leaf photosynthesis requires high saturation irradiance (1500 μmol m−2s−1) and the rates are greatly reduced by leaf-air vapour pressure differences above 1.5 kPa; this reduction is associated with stomatal closure. Cassava leaves have low photorespiration, low CO2compensation point, high percentage of carbon fixation in C4acids and a high PEP-carboxylase activity (15–35% of that in maize), but cassava does not have the typical ‘C4-Kranz’ anatomy. Field measurements of single leaf photosynthesis among a wide range of cultivars grown under rain-fed conditions showed that when light interception was not limiting, there were significant correlations between leaf photosynthesis, total biomass and root yield. This suggests that the use of parental materials with high photosynthetic capacity, in combination with other yield determinants, could be a successful strategy for developing high yielding cultivars. This might be done by exploiting any genetical variations in leaf anatomy and biochemistry that could enhance photosynthesis efficiency and hence productivity.

A model of photosynthetic CO2 assimilation and carbon-isotope discrimination in leaves of certain C3−C4 intermediates

A model of leaf, photosynthesis has been developed for C3-C4 intermediate species found in the generaPanicum, Moricandia, Parthenium andMollugo where no functional C4 pathway has been identified. Model assumptions are a functional C3 cycle in both mesophyll and bundle-sheath cells and that glycine formed in the mesophyll, as a consequence of the oxygenase activity of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco, EC 4.1.1.39), diffuses to the bundle sheath, where most of the photorespiratory CO2 is released. The model describes the observed gas-exchange characteristics of these C3-C4 intermediates, such as low CO2-compensation points (Γ) at an O2 pressure of 200 mbar, a curvilinear response of Γ to changing O2 pressures, and typical responses of CO2-assimilation rate to intercellular CO2 pressure. The model predicts that bundle-sheath CO2 concentration is highest at low mesophyll CO2 pressures and decreases as mesophyll CO2 pressure increases. A partitioning of 5-15% of the total leaf Rubisco into the bundle-sheath cells and a bundlesheath conductance similar to that proposed for C4 species best mimics the gas-exchange results. The model predicts C3-like carbon-isotope discrimination for photosynthesis at atmospheric levels of CO2, but at low CO2 pressures it predicts a higher discrimination than is typically found during C3 photosynthesis at lower CO2 pressures.

The C4-like characteristics of the intertidal macroalga Ascophyllum nodosum (L.) Le Jolis (Fucales, Phaeophyta)

Numerous methods have been employed to ascertain whether Ascophyllum nodosum exhibits the characteristics of photorespiration. Different oxygen concentrations appear to have little effect on the apparent rate of photosynthesis, the alga displays low CO2 compensation points in air and water, postillumination CO2 bursts (and O2 ‘gulps’) were not observed in either environment, and 14CO2 evolution into a CO2-free atmosphere was inhibited in the light compared with that observed in the dark. Suggestions that photosynthetic quotients can be used as an indicator of the photorespiratory state were found to be unsuitable as determinations of the total inorganic carbon concentration from pH measurements were found to be inconsistent which resulted in variable apparent photosynthetic quotient values. Thus photorespiration is suppressed in Ascophyllum nodosum and it is suggested that the mechanism is based on an inorganic carbon transport pump.

Carbonic anhydrase and CO2concentrating mechanisms in microalgae and cyanobacteria

Among the microbial phototrophs, those belonging to the cyanobacteria utilize CO2 and HCO−3 for photosynthesis. Some Chlorophyceae mainly take up CO2 in photosynthesis, and others, which have carbonic anhydrase (CA) on their cell surface can utilize HCO−3 as well as CO2. Kinetic studies revealed that most of the HCO−3 is utilized after this ion is converted to CO2 via CA located on the cell surface. Therefore, the actual molecular species which crosses the plasmalemma is mostly free CO2. There is apparent variation in the mode of utilization of dissolved inorganic carbon (DIC) for photosynthesis in microalgae in other classes. The apparent Km(CO2) values for photosynthesis in most microalgae grown in ordinary air (low-CO2 cells) are as low as in terrestrial C4 plants, although the algal cells fix CO2 via the C3 pathway. In contrast, the apparent Km(CO2) values in cells grown on CO2-enriched air (high-CO2 cells) are as high as those in the terrestrial C3 plants. Most low-CO2 cells show low photorespiration; a low CO2 compensation point, low rates of glycolate excretion and no or low O2 inhibition of photosynthesis. These results indicate that the efficiency of DIC utilization for photosynthesis in low-CO2 cells is very high. The activity of CA in low-CO2 cells is higher than that in high-CO2 cells, while no difference has been confirmed in the activities of other photosynthetic enzymes between low- and high-CO2 cells. In addition, low-CO2 cells can accumulate large amounts of DIC internally, indicating the existence of CO2-concentrating mechanisms in these cells. When CA activity or CO2 concentrating ability is reduced by inhibitors or by mutation, the apparent Km(CO2) values for photosynthesis and the rate of photorespiration increased notably. These results indicate that the high efficiency of DIC utilization in low-CO2 cells depends on both CA and a CO2-concentrating mechanism. It is concluded that CA facilitates the diffusion of DIC from outside the cells to the site(s) of the carboxylation reaction and the concentration of DIC is achieved via an active transporter.

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.

Photosynthesis, photorespiration, and productivity in Lemna minor L.1

The roles of aqueous vs. atmospheric CO2 fixation, photorespiration, dark respiration, and organic release in the primary productivity of Lemna minor L. were investigated experimentally in field populations and in the laboratory. The mean rate of net photosynthesis through the growth season was 2.48 mg C g−1 h−1 during which an average of 86% of carbon fixed was from aqueous inorganic carbon. Seasonal variations in photosynthesis correlated principally with light intensity and temperature. Presence of photorespiration was suggested by early afternoon depressions of photosynthesis, by experimental enhancement and inhibition of photosynthesis with low and high oxygen, respectively, and by enhancement of light:dark respiration ratios with high oxygen. However, under natural conditions L:D ratios were below unity most of the time, and CO2 loss in the light was never >4% of photosynthate fixed per hour. Experimental enhancement of photorespiration with oxygen was never as great as the inhibition of photosynthesis at comparable oxygen concentrations. Although plants exhibited a low CO2 compensation point and high light and temperature optima, photosynthesis enzymology and fixation products indicated that C4 photosynthesis was not a significant factor in maintenance of the low photorespiration rates.