Oxygen and carbon dioxide exchanges in crassulacean-acid-metabolism-plants

The 24 h O₂ uptake and release together with the CO₂ balance have been measured in two CAM plants, one a non-succulent Sempervivum grandifolium, the other a succulent Prenia sladeniana. The O₂ uptake was estimated by the use of ¹⁸O₂. It was found that the mean hourly O₂ uptake in the light was 7 times that in the dark for Sempervivum and 5 times that for Prenia, after correction for the lightdark temperature difference. It was estimated that oxygen uptake in the light was 2.4 times greater than oxygen release (=net photosynthesis) in Sempervivum and 1.4 times greater in Prenia. In both plants there was a positive carbon balance over the 24 h period under the experimental conditions. It was estimated that malate formed during the night could, if completely oxidized to CO₂ and water, account for 74% of the light phase O₂ uptake in Sempervivum. In Prenia the O₂ uptake was more than sufficient to account for a full oxidation of malate.Abbreviations CAM Crassulacean acid metabolism - PAR photosynthetically active radiation - PEP phosphoenolpyruvate - RrBP ribulose-1,5-bisphosphate - TCA tricarboxylic acid cycle     

Restriction analysis and sequencing of the ITS regions and 5.8S gene of rDNA of Ganoderma isolates from infected oil palm and coconut stumps in Malaysia

Use of Crassulacean acid metabolism (CAM) plants in México and worldwide has a long history, but the morphological and photosynthetic aspects of these plants have only been considered recently. Emphasis in this article is on the daily net CO₂ uptake ability by three species of agaves and three species of cacti that are currently extensively cultivated in México for beverages, food, fodder, and forage ‐ Agave mapisaga, A. salmiana, A. tequilana, Opuntia ficus‐indica, O. robusta and Stenocereus queretaroensis. Data under controlled conditions are used to help interpret seasonal net CO₂ uptake patterns observed in the field. These CAM plants have instantaneous and total daily net CO₂ uptake values similar to those for highly productive C₃ and C₄ crops. The future increase in the cultivated area of CAM plants will have both agronomical and ecological ramifications because of the ability of these plants to endure prolonged drought and to sequester carbon during extended dry periods when few C₃ and C₄ crops and non‐CAM native plants can fix atmospheric CO₂.     

Differentiation among populations of Sedum wrightii (Crassulaceae) in response to limited water availability: water relations,CO2 assimilation,growth and survivorship

Summary Sedum wrightii is one of only a few species in the Crassulaceae for which there is evidence for a high degree of variability in the ratio of daytime to nighttime CO₂ assimilation. There are both environmental and genetic components to this variability. S. wrightii grows over a wide altitudinal gradient. The purpose of this study was to compare low, intermediate, and high altitude populations with respect to the degree of CAM expression and the capability to tolerate limited water availability. We utilized clonallyreplicated genotypes of plants from each population in common environment greenhouse experiments. Genetic differences among the populations were found in long-term water use efficiency, in 24 hour CO₂ exchange patterns, in biomass ¹³C values, in carbon allocation, and in water status and ultimately survival during prolonged drought. The differences among the populations appear to be closely related to differences in the native habitats. The low altitude, desert plants had the greatest ability to grow and survive under conditions of limited water availability and appear to have the greatest shift to nighttime CO₂ uptake during periods without water, while the high altitude plants had the poorest performance under these conditions and appear to shut down net carbon uptake when severely water limited.     

Characterisation of CO2 and HCO3 − uptake in the cyanobacterium Synechocystis sp. PCC6803

The availability of a complete genome database for the cyanobacterium Synechocystissp. PCC6803 (glucose-tolerant strain) has raised expectations that this organism would become a reference strain for work aimed at understanding the CO₂-concentrating mechanism (CCM) in cyanobacteria. However, the amount of physiological data available has been relatively limited. In this report we provide data on the relative contributions of net HCO₃ ⁻ uptake and CO₂ uptake under steady state photosynthetic conditions. Cells were compared after growth at high CO₂ (2% v/v in air) or limiting CO₂ conditions (20 ppm CO₂). Synechocystishas a very high dependence on net HCO₃ ⁻ uptake at low to medium concentrations of inorganic carbon (Ci). At high Ci concentrations net CO₂ uptake became more important but did not contribute more than 40% to the rate of photosynthetic O₂ evolution. The data also confirm that high Ci cells of Synechocystissp. PCC6803 possess a strong capacity for net HCO₃ ⁻ uptake under steady state photosynthetic conditions. Time course experiments show that induction of maximal Ci uptake capacity on a shift from high CO₂ to low CO₂ conditions was near completion by four hours. By contrast, relaxation of the induced state on return of cells to high CO₂, takes in excess of 230 h. Experiments were conducted to determine if Synechocystissp. PCC6803 is able to exhibit a `fast induction' response under severe Ci limitation and whether glucose was capable of causing a rapid inactivation in Ci uptake capacity. Clear evidence for either response was not found. This revised version was published online in August 2006 with corrections to the Cover Date.     

Remarks on mixotrophic and autotrophic carbon nutrition of Vitis plantlets cultured in vitro

Two Vitis species were cultured in vitro under photoautrophic (sucrose-free culture medium) and photomixotrophic (sucrose 15 g l^-1) conditions during the period following microcutting rooting (day 34 to day 120). Several parameters were measured at the end of the culture: growth, plant dry weight, carbohydrate uptake from the medium and rates of photosynthesis and dark respiration. The two species behaved very differently. Under photoautotrophic conditions, dark respiration, net photosynthesis and daily CO₂ fixation were higher in Vitis vinifera than in Vitis rupestris. Culture under mixotrophic conditions caused increase in growth, respiration and photosynthesis in Vitis rupestris. In contrast, photosynthesis decreased in Vitis vinifera under the same conditions.     

A method for the study of CO2 exchanges in in vitro cultured Vitis rupestris plantlets

A CO₂ assay circuit adapted to in vitro culture was designed to investigate CO₂ exchanges in test tube-grown Vitis rupestris plantlets. The CO₂ concentration of the air in culture tubes was measured by injection of samples in the open circuit. It was observed under the culture conditions used that the CO₂ content stabilized during the light phase at 3 times the CO₂ compensation point.Measurements of dark respiration under closed circuit conditions at every two-hour interval during the night did not reveal any limiting by lack of the substrate under mixotrophic culture conditions. A mathematical model of the influence of ambient CO₂ concentration on net CO₂ uptake rates under closed circuit conditions was devised and used to compare net photosynthesis at different lighting levels. Measurement of CO₂ evolution into CO₂-free air under open circuit conditions revealed a post-illumination burst characteristic of photorespiration which increased with the temperature.     

The effects of salinity,crassulacean acid metabolism and plant age on the carbon isotope composition of <Emphasis Type="Italic">Mesembryanthemum crystallinum</Emphasis> L., a halophytic C<Subscript>3</Subscript>-CAM species

The carbon isotope composition of the halophyte Mesembryanthemum crystallinum L. (Aizoaceae) changes when plants are exposed to environmental stress and when they shift from C₃ to crassulacean acid metabolism (CAM). We examined the coupling between carbon isotope composition and photosynthetic pathway by subjecting plants of different ages to salinity and humidity treatments. Whole shoot ¹³C values became less negative in plants that were exposed to 400 mM NaCl in the hydroponic solution. The isotopic change had two components: a direct NaCl effect that was greatest in plants still operating in the C₃ mode and decreased proportionally with increasing levels of dark fixation, and a second component related to the degree of CAM expression. Ignoring the presumably diffusion-related NaCl effect on carbon isotope ratios results in an overestimation of nocturnal CO₂ gain in comparison to an isotope versus nocturnal CO₂ gain calibration established previously for C₃ and CAM species grown under well-watered conditions. It is widely taken for granted that the shift to CAM in M. crystallinum is partially under developmental control and that CAM is inevitably expressed in mature plants. Plants, cultivated under non-saline conditions and high relative humidity (RH) for up to 63 days, maintained diel CO₂ gas-exchange patterns and ¹³C values typical of C₃ plants. However, a weak CAM gas-exchange pattern and an increase in ¹³C value were observed in non-salt-treated plants grown at reduced RH. These observations are consistent with environmental control rather than developmental control of the induction of CAM in mature M. crystallinum under non-saline conditions.     

Seasonal response to drought and rewatering in Portulacaria afra (L.) Jacq.

Summary Gas exchange characteristics of droughted and rewatered Portulacaria afra were studied during the seasonal shift from CAM to C₃ photosynthesis. ¹⁴CO₂ uptake, stomatal conductance, and total titratable acidity were determined for both irrigated and 2, 4, and 7.5 month waterstressed plants from summer 1984 to summer 1985. Irrigated P. afra plants were utilizing the CAM pathway throughout the summer and shifted to C₃ during the winter and spring. Beginning in September, P. afra plants shifted from CAM to CAM-idling after 2 months of water-stress. When water-stress was initiated later in the fall, exogenous CO₂ uptake was still measurable after 4 months of drought. After 7.5 months of stress, exogenous CO₂ uptake was absent. The shift from CAM to CAM-idling or C₃ in the fall and winter was related to when water stress was initiated and not to the duration of the stress. Gas exchange resumed within 24 h of rewatering regardless of the duration of the drought. In the winter and spring, rewatering resulted in a full resumption of daytime CO₂ uptake. Whereas during the summer, rewatering quickly resulted in early morning CO₂ uptake, but nocturnal CO₂ uptake through the CAM pathway was observed after 7 days. Gas exchange measurements, rewatering characteristics, and transpirational water loss support the hypothesis that the C₃ pathway was favored during the winter and spring. The CAM pathway was functional during the summer when potential for water loss was greater. Our investigations indicate that P. afra has a flexible photosynthetic system that can withstand long-term drought and has a rapid response to rewatering.     

Enhanced root system C-sink activity,water relations and aspects of nutrient acquisition in mycotrophic Bouteloua gracilis subjected to CO2 enrichment

In order to better elucidate fixed-C partitioning, nutrient acquisition and water relations of prairie grasses under elevated [CO₂], we grew the C₄ grass Bouteloua gracilis (H.B.K.) lag ex Steud. from seed in soil-packed, column-lysimeters in two growth chambers maintained at current ambient [CO₂] (350 μL L⁻¹) and twice enriched [CO₂] (700 μL L⁻¹). Once established, plants were deficit irrigated; growth chamber conditions were maintained at day/night temperatures of 25/16°C, relative humidities of 35%/90% and a 14-hour photoperiod to simulate summer conditions on the shortgrass steppe in eastern Colorado. After 11 weeks of growth, plants grown under CO₂ enrichment had produced 35% and 65% greater total and root biomass, respectively, and had twice the level of vesicular-arbuscular mycorrhizal (VAM) infection (19.8% versus 10.8%) as plants grown under current ambient [CO₂]. The CO₂-enriched plants also exhibited greater leaf water potentials and higher plant water use efficiencies. Plant N uptake was reduced by CO₂ enrichment, while P uptake appeared little influenced by CO₂ regime. Under the conditions of the experiment, CO₂ enrichment increased root biomass and VAM infection via stimulated growth and adjustments in C partitioning below-ground. The U.S. Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged. The U.S. Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

Phosotynthesis in hemiepiphytic species of Clusia and Ficus

Summary Hemiepiphytic species in the genera Clusia and Ficus were investigated to study their mode of photosynthetic metabolism when growing under natural conditions. Despite growing sympatrically in many areas and having the same growth habit, some Clusia species show Crassulacean acid metabolism (CAM) whereas all species of Ficus investigated are C₃. This conclusion is based on diurnal CO₂ fixation patterns, diurnal stomatal conductances, diurnal titratable acidity fluctuations, and ¹³C isotope ratios. Clusia minor, growing in the savannas adjacent to Barinas, Venezuela, shows all aspects of Crassulacean acid metabolism (CAM) on the basis of nocturnal gas exchange, stomatal conductance, total titratable acidity, and carbon isotope composition when measured during the dry season (February 1986). During the wet season (June 1986), the plants shifted to C₃-type gas exchange with all CO₂ uptake occurring during the daylight hours. The carbon isotope composition of new growth was-28 to-29 typical of C₃ plants.     

Le vie fotosintetiche C3, C4 e CAM nel contesto ecologico

Abstract

Ecological aspects of C₃, C₄ and CAM photosynthetic pathways. - Three different photosynthetic CO₂ fixation pathways are known to occur in higher plants. However all three pathways ultimately depend on the Calvin-Benson cycle for carbon reduction. The oxygenase activity of RuBP carboxilase is responsible for photorespiratory CO₂ release. Both C₄ and CAM pathways behave as a CO₂ concentrating mechanism which prevent photorespiration. The CO₂-concentrating mechanism in C₄ plants is based on intracellular symplastic transport of C₄ dicarboxylic acids from mesophyll-cells to the adjacent bundle-sheath cells. On the contrary in CAM plants the CO₂-concentrating mechanism is based on the intracellular transport of malic acid into and out of the vacuole.

The C₄ photosynthetic pathway as compared to the C₃ pathway permits higher rates of CO₂ fixation in high light and high temperature environments at low costs in terms of water loss, given the stability of the photosynthetic apparatus under such conditions.

CAM is interpreted as an adaptation to arid environments because it enables carbon assimilation to take place at very low water costs during the night when the evaporative demand is low. Nevertheless many aquatic species of Isoetes and some relatives are CAM, suggesting the adaptive role of CAM to environments which become depleted in CO₂.

The photosynthetic carbon fixation pathway certainly contributes to the ecological success of plants in different environments. However the distribution of plants may also reflect their biological history. On the other hand plants with different photosynthetic pathways coexist in many communities and tend to share resources in time. In any case some generalizations are possible: C₄ plants enjoy an ecological advantage in hot, moist, high light regions while the majority of species in desert environments are C₃; CAM plants are more frequent in semiarid regions with seasonal rainfall, coastal fog deserts, and in epiphytic habitats in tropical rain forests.     

Light and dark CO2 fixation in Clusia uvitana and the effects of plant water status and CO2 availability

Summary In well-watered plants of Clusia uvitana, a species capable of carbon fixation by crassulacean acid metabolism (CAM), recently expanded leaves gained 5 to 13-fold more carbon during 12 h light than during 12 h dark periods. When water was withheld from the plants, daytime net CO₂ uptake strongly decreased over a period of several days, whereas there was a marked increase in nocturnal carbon gain. Photosynthetic rates in the chloroplasts were hardly affected by the water stress treatment, as demonstrated by measurements of chlorophyll a fluorescence of intact leaves, indicating efficient decarboxylation of organic acids and refixation of carbon in the light. Within a few days after rewatering, plants reverted to the original gas exchange pattern with net CO₂ uptake predominantly occurring during daytime. The reversible increase in dark CO₂ fixation was paralleled by a reversible increase in the content of phosphoenolpyruvate (PEP) carboxylase protein. In wellwatered plants, short-term changes in the degree of dark CO₂ fixation were induced by alterations in CO₂ partial pressure during light periods: a decrease from 350 to 170 bar CO₂ caused nocturnal carbon gain, measured in normal air (350 bar), to increase, whereas an increase to 700 bar CO₂, during the day, caused net dark CO₂ fixation to cease. The increased CAM activity in response to water shortage may, at least to some extent, be directly related to the reduced carbon gain during daytime.     

Canopy CO2 concentrations and Crassulacean acid metabolism in Hoya carnosa in a subtropical rain forest in Taiwan: consideration of CO2 availability and the evolution of CAM in epiphytes

The potential importance of CO₂ derived from host tree respiration at night as a substrate for night time CO₂ uptake during CAM was investigated in the subtropical and tropical epiphytic vine Hoya carnosa in a subtropical rainforest in north-eastern Taiwan. Individuals were examined within the canopies of host trees in open, exposed situations, as well as in dense forests. Although night time CO₂ concentrations were higher near the epiphytic vines at night, relative to those measured during the day, presumably the result of CO₂ added to the canopy air by the host tree, no evidence for substantial use of this CO₂ was found. In particular, stable carbon isotope ratios of H. carnosa were not substantially lower than those of many other CAM plants, as would be expected if host-respired CO₂ were an important source of CO₂ for these CAM epiphytes. Furthermore, laboratory measurements of diel CO₂ exchange revealed a substantial contribution of daytime CO₂ uptake in these vines, which should also result in lower carbon isotope values than those characteristic of a CAM plant lacking daytime CO₂ uptake. Overall, we found that host-respired CO₂ does not contribute substantially to the carbon budget of this epiphytic CAM plant. This finding does not support the hypothesis that CAM may have evolved in tropical epiphytes in response to diel changes in the CO₂ concentrations within the host tree canopy.     

Photosynthetic characteristics of Cymbidium plantlet in vitro

The photosynthetic characteristics of the Cymbidium plantlet in vitro cultured on Hyponex-agar medium with 2% sucrose were determined based on the measurements of CO₂ concentration inside and outside of the culture vessels. The CO₂ measurements were made with a gas chromatograph at a PPF (photosynthetic photon flux) of 35, 102 and 226 mol m^-2 s^-1, a chamber air temperature of 15, 25 and 35°C and a CO₂ concentration outside the vessel of approximately 350, 1100 and 3000 ppm. The net photosynthetic rates were determined on individual plantlets and were expressed on a dry weight basis. The steady-state CO₂ concentration during the photoperiod was lower inside the vessel than outside the vessel at any PPF greater than 35 mol m^-2s^-1 and at any chamber air temperature. The photosynthetic response curves relating the net photosynthetic rate, PPF, and CO₂ concentration in the vessel and chamber air temperature were similar to those for Cymbidium plants grown outside and other C₃ plants grown outside under shade. The results indicate that CO₂ enrichment for the plantlets in vitro at a relatively high PPF would promote photosynthesis and hence the growth of chlorophyllous shoots/plantlets in vitro and that the plantlets in vitro would make photoautotrophic growth under environmental conditions favorable for photosynthesis.Abbreviations C_in CO₂ concentration in the culture vessel - C_out CO₂ concentration outside the vessel (in the culture room) - PPF photosynthetic photon flux     

Elevated CO2 alters anatomy,physiology, growth,and reproduction of red mangrove (Rhizophora mangle L.)

Mangroves, woody halophytes restricted to protected tropical coasts, form some of the most productive ecosystems in the world, but their capacity to act as a carbon source or sink under climate change is unknown. Their ability to adjust growth or to function as potential carbon sinks under conditions of rising atmospheric CO₂ during global change may affect global carbon cycling, but as yet has not been investigated experimentally. Halophyte responses to CO₂ doubling may be constrained by the need to use carbon conservatively under water-limited conditions, but data are lacking to issue general predictions. We describe the growth, architecture, biomass allocation, anatomy, and photosynthetic physiology of the predominant neotropical mangrove tree, Rhizophora mangle L., grown solitarily in ambient (350 ll^–1) and double-ambient (700 ll^–1) CO₂ concentrations for over 1 year. Mangrove seedlings exhibited significantly increased biomass, total stem length, branching activity, and total leaf area in elevated CO₂. Enhanced total plant biomass under high CO₂ was associated with higher root:shoot ratios, relative growth rates, and net assimilation rates, but few allometric shifts were attributable to CO₂ treatment independent of plant size. Maximal photosynthetic rates were enhanced among high-CO₂ plants while stomatal conductances were lower, but the magnitude of the treatment difference declined over time, and high-CO₂ seedlings showed a lower P_max at 700 ll^–1 CO₂ than low-CO₂ plants transferred to 700 ll^–1 CO₂: possible evidence of downregulation. The relative thicknesses of leaf cell layers were not affected by treatment. Stomatal density decreased as epidermal cells enlarged in elevated CO₂. Foliar chlorophyll, nitrogen, and sodium concentrations were lower in high CO₂. Mangroves grown in high CO₂ were reproductive after only 1 year of growth (fully 2 years before they typically reproduce in the field), produced aerial roots, and showed extensive lignification of the main stem; hence, elevated CO₂ appeared to accelerate maturation as well as growth. Data from this long-term study suggest that certain mangrove growth characters will change flexibly as atmospheric CO₂ increases, and accord with responses previously shown in Rhizophora apiculata. Such results must be integrated with data from sea-level rise studies to yield predictions of mangrove performance under changing climate.     

CO2 exchange characteristics during dark-light transitions in wild-type and mutant Chlamydomonas reinhardii cells

A burst of net CO₂ uptake was observed during the first 3–4 min after the onset of illumination in both wild-type Chlamydomonas reinhardii in which carbonic anhydrase was chemically inhibited with ethoxyzolamide and in a mutant of C. reinhardii (ca-1-12-1C) deficient in carbonic anhydrase activity. The burst was followed by a rapid decrease in the CO₂ uptake rate so that net evolution often occurred. After a 2–3 min period of CO₂ evolution, net CO₂ uptake again increased and ultimately reached a steady-state, positive rate. From [¹⁴CO₂]-tracer studies it was determined that CO₂ fixation proceeded at a nearly linear rate throughout the period of illumination. Thus, prior to reaching a steady state, there was a rapid accumulation of inorganic carbon inside the cells which apparently reached a supercritical concentration and the excess was excreted, causing a subsequent efflux of CO₂. A post illumination burst of net CO₂ efflux was also observed in ethoxyzolamide-inhibited wild type and ca-1 mutant cells, but not in the unihibited wild type. [¹⁴CO₂]-tracer experiments revealed that this burst was the result of a collapse of a large internal inorganic carbon pool at the onset of darkness rather than a photorespiratory post-illumination burst. These results indicate that upon illumination, chemical or genetic inhibition of carbonic anhydrase initially causes an accumulation of excess inroganic carbon in C. reinhardii cells, and that unknown regulatory mechanisms correct for this imbalance by first excreting the excess inorganic carbon and then, after several dampened oscillations, achieving an equilibrium between bicarbonate uptake, bicarbonate dehydration, and CO₂ fixation.     

High photosynthetic photon flux and high CO2 concentration under increased number of air exchanges promote growth and photosynthesis of four kinds of orchid plantlets in vitro

Summary In vitro plantlets of Phalaenopsis ‘Happy Valentine’, Neofinetia falcate Hu, Cymbidium kanran Makino, and Cymbidium goeringii Reichb. f. were grown under photoautotrophic [high photosynthetic photon flux (PPF), high CO₂ concentration, and increased number of air exchanges] and heterotrophic (low PPF, low CO₂ concentration, no air exchanges) culture conditions. After 40 d of culture, a significant difference in plantlet growth was observed between the two cultures. Total fresh and dry mass were on average 1.5 times greater in photoautotrophic culture than in heterotrophic culture. Higher net photosynthetic rates were also observed for Phalaenopsis in photoautotrophic culture. In photoautotrophic culture, little difference was observed in air temperature between the inside and outside of the culture vessel, whereas in heterotrophic culture, air temperature inside the culture vessel was 1–2°C higher than that outside the culture vessel. Relative humidity inside the culture vessel was remarkably different between the two cultures: 83–85% in photoautotrophic culture and 97–99% in heterotrophic culture. These results indicated that growth and net photosynthetic rate of in vitro orchid plantlets were susceptible to the culture environments such as PPF, CO₂ concentration, relative humidity (RH), and the number of air exchanges, which would allow a more efficient micropropagation system for these orchid plants.     

Effects of CO2 enrichment and supporting materialin vitro on photoautotrophic growth ofEucalyptus plantletsin vitro andex vitro

Summary Eucalyptus camaldulensis shoots were cultured photoautotrophicallyin vitro for 6 wk with four different types of supporting materials (agar matrix, Gelrite matrix, plastic net, or vermiculite) under CO₂-nonenriched or CO₂-enriched conditions. Plantlets from each treatmentin vitro were then grownex vitro in a greenhouse for 4 wk. The growth and net photosynthetic rate of plantletsin vitro, as well as subsequent growth, survival percentage, transpiration rate, and net photosynthetic rate of plantletsex vitro were evaluated. CO₂ enrichment significantly increased growth (total dry weight and number of primary roots) and net photosynthetic rate of plantletsin vitro, as well as the growth and survival percentage of plantletsex vitro regardless of the type of supporting materials. The growthin vitro was greatest in the vermiculite, followed by the plastic net, Gelrite matrix, and agar matrix (in descending order) under either the CO₂-nonenriched or CO₂-enriched conditions. The growth and survival percentage of plantletsex vitro were highest in the vermicultie under the CO₂-enriched condition. The extensive root system producedin vitro was necessary for growth and survival of plantletsex vitro.     

Leaf and Stem CO(2) Uptake in the Three Subfamilies of the Cactaceae

Net CO₂ uptake over 24-hour periods was examined for the leaves and for the stems of 11 species of cacti representing all three subfamilies. For Pereskia aculeata, Pereskia grandifolia, and Maihuenia poeppigii (subfamily Pereskioideae), all the net shoot CO₂ uptake was by the leaves and during the daytime. In contrast, for the leafless species Carnegiea gigantea, Ferocactus acanthodes, Coryphantha vivipara, and Mammillaria dioica (subfamily Cactoideae), all the shoot net CO₂ uptake was by the stems and at night. Similarly, for leafless Opuntia ficus-indica (subfamily Opuntioideae), all net CO₂ uptake occurred at night. For leafy members of the Opuntioideae (Pereskiopsis porteri, Quiabentia chacoensis, Austrocylindropuntia subulata), at least 88% of the shoot CO₂ uptake over 24 hours was by the leaves and some CO₂ uptake occurred at night. Leaves responded to the instantaneous level of photosynthetically active radiation (PAR) during the daytime, as occurs for C₃ plants, whereas nocturnal CO₂ uptake by stems of O. ficus-indica and F. acanthodes responded to the total daily PAR, as occurs for Crassulacean acid metabolism (CAM) plants. Thus, under the well-watered conditions employed, the Pereskioideae behaved as C₃ plants, the Cactoideae behaved as CAM plants, and the Opuntioideae exhibited characteristics of both pathways.     

Effect of O2 and CO2 on net CO2 exchange in a high-CO2-requiring mutant of Chlamydomonas reinhardtii during dark-light-dark transitions

Net CO₂ exchange was monitored through a dark-light-dark transition, under 2% and 21% O₂ in the presence and absence of CO₂, in Chlamydomonas reinhardtii wild type and the high-CO₂-requiring mutant ca-1-12-1C. Upon illumination at 350 l/l CO₂, ca-1-12-1C cell exhibited a large decrease in net CO₂ uptake following an initial surge of CO₂ uptake. Net CO₂ uptake subsequently attained a steady-state rate substantially lower than the maximum. A large, O₂-enchanced post-illumination burst of CO₂ efflux was observed after a 10-min illumination period, corresponding to a minimum in the net CO₂ uptake rate. A smaller, but O₂-insensitive post-illumination burst was observed following a 30-min illumination period, when net CO₂ uptake was at a steady-state rate. These post-illumination bursts appeared to reflect the release of an intracellular pool of inorganic carbon, which was much larger following the initial surge of net CO₂ uptake than during the subsequent steady-state CO₂ uptake period.With the mutant in CO₂-free gas, O₂-stimulated, net CO₂ efflux was observed in the light, and a small, O₂-dependent post-illumination burst was observed. With wild-type cells no CO₂ efflux was observed in the light in CO₂-free gas under either 2% or 21% O₂, but a small, O₂-dependent post-illumination burst was observed. These results were interpreted as indicating that photorespiratory rates were similar in the mutant and wild-type cells in the absence of CO₂, but that the wild-type cells were better able to scavenge the photorespiratory CO₂.