Apparent Photorespiration Research Articles

Elevated efficiency of C3 photosynthesis in bamboo grasses: A possible consequence of enhanced refixation of photorespired CO2

AbstractBamboos are productive grasses that currently yield a high‐quality wood and potentially an abundance of lignocellulose for bioenergy. All are C3 grasses of warm habitats, where they are prone to significant photorespiratory inhibition and competitive suppression by C4 grasses. Here, we investigate whether three bamboo species from the Brazilian Cerrado (Dendrocalamus asper, Guadua angustifolia, and Guadua magna) exhibit unique adaptations that suppress photorespiratory costs and enhance photosynthetic efficiency. We evaluated photosynthetic efficiency of the bamboos and rice (Oryza sativa) by measuring C*, the CO2 compensation point in the absence of mitochondrial respiration. At 25℃, C* averaged 2.81 Pa in each of the bamboo species, which is closer to a C2 plant (2.71 Pa) than the C3 plant rice (3.31 Pa). Assuming a chloroplast CO2 concentration of 200 µmol mol−1, this represents an 18% lower cost of apparent photorespiration in bamboo than rice. Light and transmission electronic microscopy of the bamboo leaves exhibited few organelles in the bundle and mestome sheath cells, and mesophyll (M) cells are deeply lobed with 99% of the cell periphery adjacent to intercellular air space covered by chloroplast and stromules. The chloroplast layer in bamboo M cells is thick, with mitochondria adjacent to or engulfed by chloroplasts. This arrangement slows CO2 efflux and facilitates refixation of photorespired CO2, which could explain the low C* in the bamboos. The bamboos also had higher water use efficiency than rice, which may reflect efficient refixation of photorespired CO2.

Loss of the M-box from the glycine decarboxylase P-subunit promoter in C2 Moricandia species

C2 photosynthesis operates by shuttling photorespiratory glycine (C2) from mesophyll (M) to bundle sheath (BS) cells, followed by decarboxylation and release of CO2 around RubisCO. C2 plants are characterized by low apparent photorespiration and enhanced refixation of photorespiratory CO2 and the C2 pathway is thought to represent an intermediate step for the evolution from C3 to C4 photosynthesis. Restriction of glycine decarboxylation to the BS cells is considered to be a prerequisite for C2 photosynthesis. In the C3 plant species Arabidopsis thaliana, a cis-element required for expression of the P-subunit of glycine decarboxylase (GDC—P) in M cells (termed the M-box) was previously identified in the promoter of A. thaliana glycine decarboxylase P-subunit 1 (AtGldp1). Consequently, the loss of this element restricted Gldp1 expression to the BS cells. To investigate conservation, Gldp promoter sequences from another C3 and two additional C2 Moricandia species were isolated by genome walking. In comparison to AtGldp1, the M-box was conserved in the promoter of C3 Moricandia moricandioides, but was not found in the promoters of M. nitens, M. suffruticosa, and M. arvensis, indicating the loss of the M-box from several C2 Moricandia species. The AtGldp1 M-box was further analyzed in detail using promoter::GUS fusions. Results show that interaction between two promoter regions containing predicted CAAT and GATA elements are required for expression of the GUS reporter in M cells and these elements including their spacing are conserved in the promoters of different members of the Brassicaceae.

Fitting mitochondrial respiration rates under light by photosynthetic CO<inf>2</inf> response models

Aims The objective of this study was to compare the values of respiration under light derived by fitting a photosynthetic CO2 response model and measurements, in order to provide information for model optimization. Methods Using combined gas exchange measurements and a low O2 (2% O2) method, the responses of photosynthetic rate (Pn) to CO2 at different light intensities (2 000, 1 500, 1 000 and 500 μmol·m·s) in the flag leaves of wheat were measured. The measured data were fitted by a biochemical model, a rectangular hyperbola model and a modified rectangular hyperbola model of the photosynthetic response to intercellular CO2 concentration (A/Ci) and air CO2 concentration (A/Ca), aiming to approach the reasonability of the fitted results obtained from the models. Important finding The sequence of fitting effect of the three CO2 response models in descending order was as follows: modified rectangular hyperbola model > rectangular hyperbola model > biochemical model. Fitted values of A/Ca curve was more reasonable than A/Ci curve, because the photorespiration and mitochondrial respiration under light (Rd) estimated by the former better matched the measured values. However, there were significant differences in the whole between the fitted and measured values. The reason could be that the effect of CO2 concentrations on Rd and apparent photorespiration (Rpa) is neglected in all the current CO2 response models. Our results showed that CO2 concentration had a marked effect on Rpa and Rd. With increasing CO2 concentration, Rpa and Rd increased first, and then decreased sharply. Take 2 000 μmol·m·s for example, Rpa varied between 5.035 and 11.670 μmol CO2·m·s, and Rd varied between 0.491 and 2.987 μmol CO2·m·s. Regression analyses 康华靖等: 植物光合 CO2响应模型对光下(暗)呼吸速率拟合的探讨 1357 doi: 10.3724/SP.J.1258.2014.00130 indicated that Rpa and Rd were well related to CO2 concentrations at different light intensities.

Specific features of photorespiration in photosynthetically active organs of C3 plants

Photorespiration of photosynthetically active organs of C3 plants (leaf, ear, stem, and leaf sheath) and C4 plants (leaf, tassel, stem, leaf sheath, ear husk) grown under greenhouse and field conditions was studied. Photorespiration was measured using a PTM-48A high-technology monitor of photosynthesis (Bioinstruments S.R.L., Moldova). It is shown that photorespiration (CO2 ejection after light turning off — apparent photorespiration) in C3 plants is characteristic only for their leaves. In other photosynthesizing organs, photorespiration was absent, like in the photosynthesizing organs of C4 plants. The absence of such after-light CO2 outburst was observed for 31 genotypes: 18 cereal species belonging to four species (Triticum aestivum L., T. durum Desf., Secale cereale L., and Triticale); 6 grain legumes belonging to 2 species (Pisum sativum L. and Glycine max L.); 7 species of wild and rarely cultivated genotypes (T. boeoticum Boiss., T. dicoccoides Koern., T. dicoccum Schuebl., T. spelta L., T. compactum Host., T. monococcum L., and T. sphaerococcum Persiv.), and 2 genotypes of C4 plants (Zea mays L. and Sorgum vulgaris L.). In all tested photosynthetically active genotypes, except of the C3 plant leaves, apparent photorespiration was absent, but rather active glycolate cycle operated. The activity of this cycle was determined from the activity of the key enzyme of this cycle — glycolate oxidase. It was supposed that C3 plants have two mechanisms of CO2 assimilation: the first one — the mechanism of C3 type localized in the leaves and the second one localized in other photosynthesizing organs, similar or with some elements of C4 mechanism of CO2 assimilation, limiting after-light CO2 ejection during the metabolism of glycolate.

Crassulacean acid metabolism enhances underwater photosynthesis and diminishes photorespiration in the aquatic plant Isoetes australis

• Underwater photosynthesis by aquatic plants is often limited by low availability of CO(2), and photorespiration can be high. Some aquatic plants utilize crassulacean acid metabolism (CAM) photosynthesis. The benefits of CAM for increased underwater photosynthesis and suppression of photorespiration were evaluated for Isoetes australis, a submerged plant that inhabits shallow temporary rock pools. • Leaves high or low in malate were evaluated for underwater net photosynthesis and apparent photorespiration at a range of CO(2) and O(2) concentrations. • CAM activity was indicated by 9.7-fold higher leaf malate at dawn, compared with at dusk, and also by changes in the titratable acidity (μmol H(+) equivalents) of leaves. Leaves high in malate showed not only higher underwater net photosynthesis at low external CO(2) concentrations but also lower apparent photorespiration. Suppression by CAM of apparent photorespiration was evident at a range of O(2) concentrations, including values below air equilibrium. At a high O(2) concentration of 2.2-fold the atmospheric equilibrium concentration, net photosynthesis was reduced substantially and, although it remained positive in leaves containing high malate concentrations, it became negative in those low in malate. • CAM in aquatic plants enables higher rates of underwater net photosynthesis over large O(2) and CO(2) concentration ranges in floodwaters, via increased CO(2) fixation and suppression of photorespiration.

Oxygen sensitivity of photosynthesis and photorespiration in different photosynthetic types in the genus Flaveria.

Two major indicators were used to access the degree of photorespiration in various photosynthetic types of Flaveria species (C3, C3-C4, C4-like, and C4): the O2 inhibition of photosynthesis measured above the O2 partial pressure which gives a maximum rate, and O2- and light-dependent whole-chain electron flow measured at the CO2 compensation point (Γ). The optimum level of O2 for maximum photosynthetic rates under atmospheric levels of CO2 (34 Pa) was lower in C3 and C3-C4 species (ca. 2 kPa) than in C4-like and C4 species (ca. 9 kPa). Increasing O2 partial pressures from the optimum for photosynthesis up to normal atmospheric levels (ca. 20 kPa) caused an inhibition of photosynthesis which was more severe under lower CO2. This inhibition was calculated as the O2 inhibition index (ΘA, the percentage inhibition of photosynthesis per kPa increase in O2). From measurements of 18 Flaveria species at atmospheric CO2, the ΘA values decreased from C3 (1.9-2.1) to C3-C4 (1.2-1.6), C4-like (0.6-0.8) and C4 species (0.3-0.4), indicating a progressive decrease in apparent photorespiration in this series. With increasing irradiance at Γ under atmospheric levels of O2, and increasing O2 partial pressure at 300 μmol quanta·m-2·s-1, there was a similar increase in the rate of O2 evolution associated with whole-chain electron flow (Jo2, calculated from chlorophyll fluorescence analysis) in the C3 and C3-C4 species compared to a much lower rate in the C4-like and C4 species. The results indicate that there is substantial O2-dependent electron flow in C3 and C3-C4 species, reflecting a high level of photorespiration compared to that in C4-like and C4 species. Consistent with these results, there was a significant decrease in Γ from C3 (6-6.2 Pa) to C3-C4 (1.0-3.0 Pa), to C4-like and C4 species (0.3-0.8 Pa), indicating a progressive decrease in apparent photorespiration. However, C3 and C3-C4 species examined had high intrinsic levels of photorespiration with the latter maintaining low apparent rates of photorespiration and lower Γ values, primarily by refixing photorespired CO2. The C4-like and C4 Flaveria species had low, but measurable, levels of photorespiration via selective localization of ribulose-1,5-bisphosphate carboxylase in bundle sheath cells and operation of a CO2 pump via the C4 pathway.

C4 Photosynthesis (The CO2-Concentrating Mechanism and Photorespiration).

Despite previous reports of no apparent photorespiration in C4 plants based on measurements of gas exchange under 2 versus 21% O2 at varying [CO2], photosynthesis in maize (Zea mays) shows a dual response to varying [O2]. The maximum rate of photosynthesis in maize is dependent on O2 (approximately 10%). This O2 dependence is not related to stomatal conductance, because measurements were made at constant intercellular CO2 concentration (Ci); it may be linked to respiration or pseudocyclic electron flow. At a given Ci, increasing [O2] above 10% inhibits both the rate of photosynthesis, measured under high light, and the maximum quantum yield, measured under limiting light ([phi]CO2). The dual effect of O2 is masked if measurements are made under only 2 versus 21% O2. The inhibition of both photosynthesis and [phi]CO2 by O2 (measured above 10% O2) with decreasing Ci increases in a very similar manner, characteristically of O2 inhibition due to photorespiration. There is a sharp increase in O2 inhibition when the Ci decreases below 50 [mu]bar of CO2. Also, increasing temperature, which favors photorespiration, causes a decrease in [phi]CO2 under limiting CO2 and 40% O2. By comparing the degree of inhibition of photosynthesis in maize with that in the C3 species wheat (Triticum aestivum) at varying Ci, the effectiveness of C4 photosynthesis in concentrating CO2 in the leaf was evaluated. Under high light, 30[deg]C, and atmospheric levels of CO2 (340 [mu]bar), where there is little inhibition of photosynthesis in maize by O2, the estimated level of CO2 around ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the bundle sheath compartment was 900 [mu]bar, which is about 3 times higher than the value around Rubisco in mesophyll cells of wheat. A high [CO2] is maintained in the bundle sheath compartment in maize until Ci decreases below approximately 100 [mu]bar. The results from these gas exchange measurements indicate that photorespiration occurs in maize but that the rate is low unless the intercellular [CO2] is severely limited by stress.

Photosynthetic and Photorespiratory Characteristics of Flaveria Species

The genus Flaveria shows evidence of evolution in the mechanism of photosynthesis as its 21 species include C(3), C(3)-C(4), C(4)-like, and C(4) plants. In this study, several physiological and biochemical parameters of photosynthesis and photorespiration were measured in 18 Flaveria species representing all the photosynthetic types. The 10 species classified as C(3)-C(4) intermediates showed an inverse continuum in level of photorespiration and development of the C(4) syndrome. This ranges from F. sonorensis with relatively high apparent photorespiration and lacking C(4) photosynthesis to F. Among the intermediates, the photosynthetic CO(2) compensation points at 30 degrees C and 1150 micromoles quanta per square meter per second varied from 9 to 29 microbars. The values for the three C(4)-like species varied from 3 to 6 microbars, similar to those measured for the C(4) species. The activities of the photorespiratory enzymes glycolate oxidase, hydroxypyruvate reductase, and serine hydroxymethyltransferase decreased progressively from C(3) to C(3)-C(4) to C(4)-like and C(4) species. On the other hand, most intermediates had higher levels of phosphenolpyruvate carboxylase and NADP-malic enzyme than C(3) species, but generally lower activities compared to C(4)-like and C(4) species. The levels of these C(4) enzymes are correlated with the degree of C(4) photosynthesis, based on the initial products of photosynthesis. Another indication of development of the C(4) syndrome in C(3)-C(4)Flaveria species was their intermediate chlorophyll a/b ratios. The chlorophyll a/b ratios of the various Flaveria species are highly correlated with the degree of C(4) photosynthesis suggesting that the photochemical machinery is progressively altered during evolution in order to meet the specific energy requirements for operating the C(4) pathway. In the progression from C(3) to C(4) species in Flaveria, the CO(2) compensation point decreased more rapidly than did the decrease in O(2) inhibition of photosynthesis or the increase in the degree of C(4) photosynthesis. These results suggest that the reduction in photorespiration during evolution occurred initially by refixation of photorespired CO(2) and prior to substantive reduction in O(2) inhibition and development of the C(4) syndrome. However, further reduction in O(2) inhibition in some intermediates and C(4)-like species is considered primarily due to the development of the C(4) syndrome. Thus, the evolution of C(3)-C(4) intermediate photosynthesis likely occurred in response to environmental conditions which limit the intercellular CO(2) concentration first via refixation of photorespired CO(2), followed by development of the C(4) syndrome.

Leaf Anatomy of C3-C4 Species as Related to Evolution of C4 Photosynthesis

This study was undertaken to examine the degree of Kranz anatomy development in the species intermediate to C(3) and C(4) types (C(3)-C(4)) in Panicum, Neurachne, Flaveria, and Moricandia. In each genus, C(3) and/or C(4) species were used for comparison. Leaf transections from each species were examined by light and transmission electron microscopy. The percentages of leaf photosynthetic cell profiles partitioned to bundle sheaths were higher in C(4) than in C(3) species, while C(3)-C(4) species tended to be in between. However, percentages for C(3)-C(4) species in Moricandia and some C(3)-C(4)Flaveria species were not greater than C(3). When expressed on a cell profile area basis, C(3)-C(4) species partitioned more photosynthetic tissue to bundle sheaths than C(3) species in Moricandia, but not in Flaveria. Neurachne minor S. T. Blake (C(3)-C(4)) partitioned a very small portion of cell profile area to the inner bundle sheaths (5%) compared to Neurachne munroi F. Muell (C(4)) (21%). The percentage of organelles partitioned to bundle sheaths was much greater in C(3)-C(4) than in C(3) species. The average C(3) percentages for mitochondria plus peroxisomes were 19, 8, and 19.5% for Neurachne, Flaveria, and Moricandia, respectively, compared to 41, 29, and 46.5% for the C(3)-C(4) species. The CO(2) compensation concentration was negatively related to the partitioning of tissue to bundle sheaths and to the percentage of organelles in bundle sheaths. It is concluded that all of the C(3)-C(4) species examined have developed some degree of Kranz anatomy and that this altered anatomy is involved in their reduced apparent photorespiration.

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.

Immunogold localization of primary carboxylases in leaves of aquatic and A C 3-C 4 intermediate species

Using protein A-gold labeled antibodies, attempts were made to determine the cellular location of ribulose bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) in two aquatic macrophytes and a terrestrial C 3-C 4 intermediate plant. Hydrilla verticillata and Myriophyllum spicatum are submersed angiosperms which exhibit variable photorespiratory states. Leaf sections indicated that neither possessed Kranz anatomy. In Myriophyllum, the chloroplasts were confined to a single epidermal layer. The Hydrilla leaf was composed of two morphologically distinct chloroplastic cell layers. Immunocytochemical studies showed that Rubisco was distributed in the chloroplasts of all the cells of both layers, and that no difference in Rubisco labeling between cells was apparent, even though low-photorespiration Hydrilla has many biochemical and physiological characteristics of C 4-photosynthesis. In Hydrilla the location of PEPC was largely cytosolic and was in all leaf cells. These data support the postulate that an intracellular separation of C 4 and C 3 fixation events may account for the low photorespiration state in Hydrilla. PEPC labeling was undectectable in Myriophyllum, which correlates with its lack of C 4-like photosynthetic biochemistry. The terrestrial C 3-C 4 intermediate species, Moricandia arvensis, has leaves with photosynthetic cells differentiated into a chloroplast-containing bundle sheath layer surrounded by mesophyll tissue. Rubisco labeling was found in the chloroplasts of both cell types. Thus, the reduced apparent photorespiration in Moricandia is not achieved through a separation of CO 2 fixation events into different cells, as occurs in C 4 plants with Kranz anatomy, but is more likely due to efficient refixation of photorespiratory CO 2 by Rubisco.

Photosynthesis and transpiration of healthy and diseased spruce trees in the course of three vegetation periods

CO2- and H2O-gas exchange of 20- to 25-year-old spruce trees from a plantation in the Hunsruck mountains were investigated over a period of 3 years. All measurements were made as pair comparisons, i.e., in each case the gas exchange of a damaged tree and of a relatively healthy tree in its immediate vicinity was measured simultaneously. A second plantation in the Westerwald mountains consisted of 18-year-old apparently healthy spruce trees. Pair comparison at this location meant comparison of two healthylooking trees. The investigations at both locations included diurnal course measurements of photosynthesis and transpiration, and light saturation curves and CO2-saturation curves of photosynthesis. The reduced photosynthesis parameters of the phenotypically damaged trees at the Hunsruck location indicates massive damage to the photosynthetic apparatus. Measurements of H2O-gas exchange showed that there are disturbances in stomatal regulation of the needles of damaged trees. As a result, the water use efficiency of these needles proved to be significantly lower. In addition, apparent photorespiration of the damaged trees was decreased, whereas their light- and CO2-compensation points and their dark respiration were increased. In contrast to the Hunsruck plantation, no such effects were detectable when the healthy-looking Westerwald trees were subjected to pair comparison of gas exchange. Reduced photosynthetic capacity and disturbances of the stomatal regulation of the phenotypically damaged Hunsruck trees may be due to damage in the cellular membranes. Furthermore, a comparison of three growing seasons led to the conclusion that the gas exchange of spruce trees in their natural habitat is markedly influenced by climatic conditions.

PHOTOSYNTHETIC CARBON METABOLISM IN ENTEROMORPHA COMPRESSA (CHLOROPHYTA)1

ABSTRACTThe intertidal macroalga Enteromorpha compressa showed the ability to use HCO3− as an exogenous inorganic carbon (Ci) source for photosynthesis. However, although the natural seawater concentration of this carbon form was saturating, additional CO2 above ambient Ci levels doubled net photosynthetic rates. Therefore, the productivity of this alga, when submerged, is likely to be limited by Ci. When plants were exposed to air, photosynthetic rates saturated at air‐levels of CO2 during mild desiccation.Based on carbon fixing enzyme activities and Ci pulsechase incorporation patterns, it was found that Enteromorpha is a C3 plant. However, this alga did not show O2 inhibited photosynthetic rates at natural seawater Ci conditions. It is suggested that such a C4‐ like gas exchange response is due to the HCO3− utilization system concentrating CO2 intracellularly, thus alleviating apparent photorespiration.

Effects of irradiance and temperature on photosynthesis in C3, C4 and C3/C4 Panicum species

Species in the Laxa and Grandia groups of the genus Panicum are adapted to low, wet areas of tropical and subtropical America. Panicum milioides is a species with C3 photosynthesis and low apparent photorespiration and has been classified as a C3/C4 intermediate. Other species in the Laxa group are C3 with normal photorespiration. Panicum prionitis is a C4 species in the Grandia group. Since P. milioides has some leaf characteristics intermediate to C3 and C4 species, its photosynthetic response to irradiance and temperature was compared to the closely related C3 species, P. laxum and P. boliviense and to P. prionitis. The response of apparent photosynthesis to irradiance and temperature was similar to that of P. laxum and P. boliviense, with saturation at a photosynthetic photo flux density of about 1 mmol m(-2) s(-1) at 30°C and temperature optimum near 30°C. In contrast, P. prionitis showed no light saturation up to 2 mmol m(-2) s(-1) and an optimum temperature near 40°C. P. milioides exhibited low CO2 loss into CO2-free air in the light and this loss was nearly insensitive to temperature. Loss of CO2 in the light in the C3 species, P. laxum and P. boliviense, was several-fold higher than in P. milioides and increased 2- to 5-fold with increases in temperature from 10 to 40°C. The level of dark respiration and its response to temperature were similar in all four Panicum species examined. It is concluded that the low apparent photorespiration in P. milioides does not influence its response of apparent photosynthesis to irradiance and temperature in comparison to closely related C3 Panicum species.

Reduced Apparent Photorespiration by the C3-C4 Intermediate Species, Moricandia arvensis and Panicum milioides

The CO(2)/O(2) specificity factor of sucrose gradient purified ribulose 1,5-bisphosphate carboxylase/oxygenase from the C(3)-C(4) intermediate plants Moricandia arvensis (79 +/- 1) and Panicum milioides (89 +/- 2) was similar to the respective values of the enzyme from the closely related C(3) species, Moricandia foetida (80 +/- 5) and Panicum laxum (86 +/- 2). Thus, the kinetic properties of this bifunctional enzyme do not explain the reduced rates of photorespiration exhibited by either of these intermediate species.Dark/light ratios for aminoacetonitrile-sensitive (14)CO(2) evolution during decarboxylation of exogenous [1-(14)C]glycine by leaf discs had values of 9.0 with M. arvensis and 11.8 with P. milioides. Equivalent ratios with M. foetida and P. laxum were 2.5 and 3.2, respectively. Similar results were obtained using [1-(14)C]glycolate as the exogenous photorespiratory substrate, with dark/light (14)CO(2) evolution ratios for the C(3)-C(4) and C(3) leaf discs averaging 6.6 and 2.0, respectively. Stimulating photosynthetic CO(2) fixation by progressively increasing photon flux density from 0 to 1900 micromoles per square meter per second caused a concomitant reduction in (14)CO(2) evolution from leaf discs of M. arvensis and P. milioides supplied with [1-(14)C]glycine. Conversely, inhibition of photosynthesis by DCMU or the Calvin cycle inhibitor dl-glyceraldehyde increased (14)CO(2) evolution in the light to rates comparable to those in the dark. The data suggest that P. milioides and M. arvensis are capable of a more efficient internal recycling of photorespiratory CO(2) via ribulose bisphosphate carboxylase/oxygenase than closely related C(3) plants, and that this may partially account for the reduced rates of apparent photorespiration by these intermediate species.

Interaction between SO 2 and NO 2 and their effects on photosynthetic properties of soybean Glycine max

Soybean Glycine max plants were fumigated for a 2-h period with either SO 2 or NO 2 alone, or in combination, at concentrations from 0 to 0·6 μl litre −1. Gas exchange was measured before, during, 1 h after and 24 h after fumigation. Post-fumigation determinations were made for dark respiration, apparent quantum yield, photorespiration and for leaf conductance at both saturating light intensity and saturating CO 2 concentration. Leaf resistance to CO 2 was partitioned into its various components. A stepwise multiple regression analysis of variance was used to identify statistically significant relationships between plant response and the concentration of pollutant during treatment. Summary of results 1. (1) Photosynthesis 1.1. (a) reduced by SO 2 but not by NO 2 alone. 1.2. (b) reduced significantly more by an interaction between SO 2 and NO 2 than by either pollutant alone. 1.3. (c) decreased to lower values 1 h after the end of fumigation than during the fourth half-hour of fumigation with combinations of SO 2 and NO 2 over 0·4 μl litre −1 for plants continuously exposed to bright light. 1.4. (d) showed signs of recovery within 3 h for plants that were put in the dark and re-illuminated. 1.5. (e) recovered to pre-fumigation values 24 h after fumigation. 2. (2) Stomatal conductance was reduced significantly by SO 2 alone, NO 2 alone, and by an interaction between SO 2 and NO 2. 3. (3) Residual conductance 3.1. (a) reduced by SO 2 alone but not by NO 2 alone. 3.2. (b) reduced more by an interaction between SO 2 and NO 2 than by SO 2 alone. 4. (4) Apparent quantum yield was reduced by SO 2 and NO 2 in an additive fashion. 5. (5) Dark respiration and apparent photorespiration were reduced by NO 2 but not by SO 2, and there was no interactive effect of SO 2 and NO 2. 6. (6) Resistance to CO 2 transport across mesophyll cell walls accounted for nearly all of the substomatal resistance to CO 2, and was sensitive to the combination of SO 2 and NO 2 but not the individually applied pollutants. 7. (7) Carboxylation resistance was not sensitive to SO 2 or NO 2.

A comparison of photosynthesis in two thalloid liverworts.

Standard infra-red gas analysis techniques were used to compare the photosynthesis of the liverworts Marchantia foliacea Mitt. and Monoclea forsteri Hook. Parameters measured include net photosynthetic rates, light response curves, quantum efficiencies, diffusive resistances to CO2 and water, apparent photorespiration and chlorophyll content. A series of morphological measurements were also made to determine resistance of pores and the 'mesophyll' to dorsal surface ratio, A mes/A. Marchantia has a cuticularised thallus with the photosynthetic tissues arranged in air chambers giving an A mes/A of 9 whilst Monoclea has a solid thallus, A mes/A of 1. Both species are shade adapted and it was found that whilst the air chambers were advantageous for water relations they increased maximum photosynthesis only slightly. Calculations showed that the solid thallus would be photosynthetically superior in very moist environments. The results are discussed with reference to existing ideas on the evolution of the structure of land plants.

Carbon Dioxide Exchange in Lichens: Apparent Photorespiration and Possible Role of CO2Refixation in Some Members of the Stictaceae (Lichenes)

Carbon Dioxide Exchange in Lichens: Apparent Photorespiration and Possible Role of CO₂Refixation in Some Members of the Stictaceae (Lichenes)

Photosynthesis of Grass Species Differing in Carbon Dioxide Fixation Pathways

The effects of temperature and photosynthetically active radiation levels on photorespiration were investigated in Panicum milioides Nees ex Trin. and Panicum schenckii Hack., species known to have low photorespiration rates and other characteristics intermediate between C(3) and C(4) species. Comparisons were made with the C(3) grass species tall fescue (Festuca arundinacea Schreb.). An increase in temperature from 20 to 35 C raised photorespiration from 7.3 to 10.2 milligrams per square decimeter per hour in tall fescue, but the increase in P. schenckii was less than 1 milligram per square decimeter per hour. Increases in temperature caused much less change in CO(2) compensation concentration in P. milioides and P. schenckii than in tall fescue, values of 160 microliters per liter being obtained in tall fescue at 40 C compared to about 40 microliters per liter for P. milioides and P. schenckii. Photorespiration in P. schenckii increased by only about 1 milligram CO(2) per square decimeter per hour as the photosynthetically active radiation level was raised from 100 to 2,000 microEinsteins per square meter per second. Loss of CO(2) into CO(2)-free air actually decreased from 2.2 to 1.0 milligrams per square decimeter per hour as the radiation level was raised from 100 to 1,100 microEinsteins per square meter per second but tended to rise again at 2,000 microEinsteins per square meter per second. In contrast, photorespiration in tall fescue tripled with radiation level over the same range, reaching a maximum value of 7.2 milligrams per square decimeter per hour as determined by extrapolation of the CO(2) response curves to zero CO(2). The CO(2) compensation concentration in tall fescue was nearly insensitive to photosynthetically active radiation above 140 microEinsteins per square meter per second but, in P. milioides and P. schenckii, it decreased from values of 69 and 62 microliters per liter, respectively, to values of 21 and 16 as the radiation level was increased from 50 to 1075 microEinsteins per square meter per second. Interpolation of CO(2)-response curves showed that an increase in photosynthetically active radiation level from 100 to 2,000 microEinsteins per square meter per second reduced the CO(2) compensation value of P. schenckii from 38 to 19 microliters per liter. Data from these experiments indicate reduced photorespiration or a CO(2)-recycling mechanism in P. milioides and P. schenckii which causes apparent photorespiration to be nearly insensitive to temperature in the 20 to 35 C range and to decrease at high radiation intensities.

Fractionation of the Stable Isotopes of Inorganic Carbon by Seagrasses

The delta(13)C values for seagrasses collected along the Texas Gulf Coast range from -10.9 to -11.4 per thousand. These values are similar to the delta(13)C values of terrestrial C(4) plants, but seagrasses lack bundle sheath cells which are important in determining the delta(13)C values of C(4) plants. This work attempts to explain the reason the delta(13)C values of seagrasses resemble the delta(13)C values of C(4) plants.Investigations on the photosynthetic characteristics of seagrasses show that dissolved CO(2) is the species of inorganic carbon absorbed or accumulated by Thalassia testudinum. The rate of photosynthetic CO(2) fixation varies from 9.6 to 129.0 micromoles CO(2) per milligram chlorophyll per hour in the presence of 0.042 to 1.9 millimolar dissolved CO(2) due to the high resistances of Thalassia leaves to CO(2) diffusion. Phosphoglyceric acid is the first stable product of photosynthetic CO(2) fixation in Thalassia which is a Calvin cycle plant. The light/dark ratios of (14)CO(2) release from submerged Thalassia leaf sections at 1, 21, and 100% O(2) indicate a small apparent photorespiration. Dark respiration continues in the light and is stimulated by 21 and 100% O(2). The low apparent photorespiration may be due to membrane and H(2)O resistances to CO(2) diffusion with subsequent refixation of the photorespired CO(2). The internal pool of CO(2) is not in equilibrium with the external pool of CO(2) which results in a closed system in the seagrasses.The delta(13)C value of CO(2) in sea H(2)O in isotopic equilibrium with HCO(3) (-) is -10.3 per thousand and the delta(13)C value of hexoses isolated from the leaves of Thalassia is -11.5 per thousand. In the closed system of the seagrasses there is a -1.2 per thousand fractionation of CO(2) by ribulose-1,5-bisphosphate carboxylase and the Calvin cycle. This contrasts to a fractionation of about -17 to -27 per thousand of the stable carbon isotopes of CO(2) by the Calvin cycle in the open system of terrestrial C(3) plants where the internal pool of CO(2) is in equilibrium with atmospheric CO(2).Among C(3) plants the seagrasses are very unusual in fixing CO(2) by the Calvin cycle in a closed system. This closed system metabolism is analogous to the fixation of CO(2) by the Calvin cycle in the bundle sheath cells of C(4) plants where all of the (12)CO(2) and (13)CO(2) is fixed by ribulose-1,5-bisphosphate carboxylase. Therefore, the reasons the delta(13)C values of seagrasses and C(4) plants are similar are: (a) the delta(13)C value of dissolved CO(2) in seawater resembles the delta(13)C value of the C(4) acids and the delta(13)C value of CO(2) in the bundle sheath cells, and (b) there is no fractionation of the stable carbon isotopes of CO(2) in the closed systems of the seagrasses or the bundle sheath cells of C(4) plants.