Diurnal Changes in Metabolite Levels and Crassulacean Acid Metabolism in Kalanchoë daigremontiana Leaves

Metabolic profiling of epidermal and mesophyll tissues under water-deficit stress in Opuntia ficus-indica reveals stress-adaptive metabolic responses

Cactus pear (Opuntia ficus-indica) is a high productivity species within the Cactaceae grown in many semiarid parts of the world for food, fodder, forage, and biofuels. O. ficus-indica utilises obligate crassulacean acid metabolism (CAM), an adaptation that greatly improves water-use efficiency (WUE) and reduces crop water usage. To better understand CAM-related metabolites and water-deficit stress responses of O. ficus-indica, comparative metabolic profiling was performed on mesophyll and epidermal tissues collected from well-watered and water-deficit stressed cladodes at 50% relative water content (RWC). Tissues were collected over a 24-h period to identify metabolite levels throughout the diel cycle and analysed using a combination of acidic/basic ultra-high-performance liquid chromatography/tandem mass spectrometry (UHPLC/MS/MS) and gas chromatography/mass spectrometry (GC/MS) platforms. A total of 382 metabolites, including 210 (55%) named and 172 (45%) unnamed compounds, were characterised across both tissues. Most tricarboxylic acid (TCA) cycle and glycolysis intermediates were depleted in plants undergoing water-deficit stress indicative of CAM idling or post-idling, while the raffinose family oligosaccharides (RFO) accumulated in both mesophyll and epidermal tissues as osmoprotectants. Levels of reduced glutathione and other metabolites of the ascorbate cycle as well as oxylipins, stress hormones such as traumatic acid, and nucleotide degradation products were increased under water-deficit stress conditions. Notably, tryptophan accumulation, an atypical response, was significantly (24-fold) higher during all time points in water-deficit stressed mesophyll tissue compared with well-watered controls. Many of the metabolite increases were indicative of a highly oxidising environment under water-deficit stress. A total of 34 unnamed metabolites also accumulated in response to water-deficit stress indicating that such compounds might play important roles in water-deficit stress tolerance.

Hierarchical clustering reveals unique features in the diel dynamics of metabolites in the CAM orchid Phalaenopsis

Crassulacean acid metabolism (CAM) is a major adaptation of photosynthesis that involves temporally separated phases of CO2 fixation and accumulation of organic acids at night, followed by decarboxylation and refixation of CO2 by the classical C3 pathway during the day. Transitory reserves such as soluble sugars or starch are degraded at night to provide the phosphoenolpyruvate (PEP) and energy needed for initial carboxylation by PEP carboxylase. The primary photosynthetic pathways in CAM species are well known, but their integration with other pathways of central C metabolism during different phases of the diel light-dark cycle is poorly understood. Gas exchange was measured in leaves of the CAM orchid Phalaenopsis 'Edessa' and leaves were sampled every 2 h during a complete 12-h light-12-h dark cycle for metabolite analysis. A hierarchical agglomerative clustering approach was employed to explore the diel dynamics and relationships of metabolites in this CAM species, and compare these with those in model C3 species. High levels of 3-phosphoglycerate (3PGA) in the light activated ADP-glucose pyrophosphorylase, thereby enhancing production of ADP-glucose, the substrate for starch synthesis. Trehalose 6-phosphate (T6P), a sugar signalling metabolite, was also correlated with ADP-glucose, 3PGA and PEP, but not sucrose, over the diel cycle. Whether or not this indicates a different function of T6P in CAM plants is discussed. T6P levels were low at night, suggesting that starch degradation is regulated primarily by circadian clock-dependent mechanisms. During the lag in starch degradation at dusk, carbon and energy could be supplied by rapid consumption of a large pool of aconitate that accumulates in the light. Our study showed similarities in the diel dynamics and relationships between many photosynthetic metabolites in CAM and C3 plants, but also revealed some major differences reflecting the specialized metabolic fluxes in CAM plants, especially during light-dark transitions and at night.

Continuous high and low temperature induced a decrease of photosynthetic activity and changes in the diurnal fluctuations of organic acids in Opuntia streptacantha

Opuntia plants grow naturally in areas where temperatures are extreme and highly variable in the day during the entire year. These plants survive through different adaptations to respond to adverse environmental conditions. Despite this capability, it is unknown how CAM photosynthetic activity and growth in Opuntia plantlets is affected by constant heat or cold. Therefore, the main objective of this research was to evaluate the short-term effect of high (40°C) and low (4°C) continuous temperatures on the photosynthetic efficiency, the organic acid content (malic acid) and the relative growth rate (RGR) in seven-month-old Opuntia streptacantha plantlets during 5, 10, and 15 days. Chlorophyll fluorescence analysis allowed us to determine that high temperatures negatively impact the photosynthetic efficiency of O. streptacantha plantlets, which exhibited the lowest values of maximum quantum efficiency of the photosystem II (Fv/Fm = 52%, Fv/F0 = 85%), operational quantum yield of PS (ΦPSII = 65%) and relative electron transport rate (rETR = 65%), as well as highest values of basal fluorescence (F0 = 226%) during 15 days of treatment. Similarly, low temperatures decreased Fv/Fm (16%), Fv/F0 (50%), ΦPSII and rETR (16%). High temperatures also decreased nocturnal acidification in approximately 34-50%, whereas low temperatures increased it by 30-36%. Additionally, both continuous temperatures affected drastically diurnal consumption of malic acid, which was related to a significant RGR inhibition, where the specific photosynthetic structure area component was the most affected. Our results allowed determining that, despite the high tolerance to extreme temperatures described for Opuntia plants, young individuals of O. streptacantha suffered photosynthetic impairment that led to the inhibition of their growth. Thus, the main findings reported in this study can help to predict the potential impact of climatic change on the establishment and survival of succulent species of arid and semiarid regions of Mexico.

A comparative study on diurnal changes in metabolite levels in the leaves of three crassulacean acid metabolism (CAM) species, Ananas comosus, Kalanchoë daigremontiana and K. pinnata

A comparative study on diurnal changes in metabolite levels associated with crassulacean acid metabolism (CAM) in the leaves of three CAM species, Ananas comosus (pineapple), a hexose-utilizing species, and Kalanchoë daigremontiana and K. pinnata, two starch-utilizing species, were made. All three CAM species showed a typical feature of CAM with nocturnal malate increase. In the two Kalanchoë species, isocitrate levels were higher than citrate levels; the reverse was the case in pineapple. In the two Kalanchoë species, a small nocturnal citrate increase was found and K. daigremontiana showed a small nocturnal isocitrate increase. Glucose 6-phosphate (G-6-P), fructose 6-phosphate (F-6-P) and glucose 1-phosphate (G-1-P) levels in the three CAM species rose rapidly during the first part of the dark period and decreased during the latter part of the dark period. The levels of the metabolites also decreased during the first 3 h of the light period, then, remained little changed through the rest of the light period. Absolute levels of G-6-P, F-6-P and G-1-P were higher in pineapple than in the two Kalanchoë species. Fructose 1,6-bisphosphate (F-1,6-P(2)) levels in the three CAM species increased during the dark period, then dramatically decreased during the first 3 h of the light period and remained unchanged through the rest of the light period. The extent of nocturnal F-1,6-P(2) increase was far greater in the two Kalanchoë species than in pineapple. Absolute levels of F-1,6-P(2) were higher in the two Kalanchoë species than in pineapple, especially during dark period. Diurnal changes in oxaloacetate (OAA), pyruvate (Pyr) and phosphoenolpyruvate (PEP) levels in the three CAM species were similar.

Novel characteristics of cassava, Manihot esculenta Crantz, a reputed C3-C 4 intermediate photosynthesis species

The cassava plant, Manihot esculenta, grows exceptionally well in low fertility and drought prone environments, but the mechanisms that allow this growth are unknown. Earlier, and sometimes contradictory, work speculated about the presence of a C4-type photosynthesis in cassava leaves. In the present work we found no evidence for a C4 metabolism in mature attached cassava leaves as indicated i) by the low, 2 to 8%, incorporation of (14)CO2 into C4 organic acids in short time periods, 10 s, and the lack of (14)C transfer from C4 acids to other compounds in (12)CO2, ii) by the lack of C4 enzyme activity changes during leaf development and the inability to detect C4 acid decarboxylases, and iii) by leaf CO2 compensation values between 49 and 65 μl of CO2 1(-1) and by other infrared gas exchange photosynthetic measurements. It is concluded that the leaf biochemistry of cassava follows the C3 pathway of photosynthesis with no indication of a C3-C4 mechanism.However, cassava leaves exhibit several novel characteristics. Attached leaves have the ability to effectively partition carbon into sucrose with nearly 45% of the label in sucrose in about one min of (14)CO2 photosynthesis, contrasting with 34% in soybean (C3) and 25% in pigweed (C4). Cassava leaves displayed a strong preference for the synthesis of sucrose versus starch. Field grown cassava leaves exhibited high rates of photosynthesis and curvilinear responses to increasing sunlight irradiances with a tendency to saturate only at high irradiances, above 1500 μmol m(-2) s(-1). Morphologically, the cassava leaf has papillose epidermal cells on its lower mesophyll surface that form 'fence-like' arrangements encircling guard cells. It is proposed that the active synthesis of sugars has osmotic functions in the cassava plant and that the papillose epidermal cells function to maintain a healthy leaf water status in various environments.

The Effect of Adenine Nucleotides on Purified Phosphoenolpyruvate Carboxylase from the CAM Plant Crassula argentea

The effects of adenine nucleotides on phosphoenolypyruvate carboxylase were investigated using purified enzyme from the CAM plant, Crassula argentea. At 1 millimolar total concentration and with limiting phosphoenolpyruvate, AMP had a stimulatory effect, lowering the K(m) for phosphoenolpyruvate, ADP caused less stimulation, and ATP decreased the activity by increasing the K(m) for phosphoenolpyruvate. Activation by AMP was not additive to the stimulation by glucose 6-phosphate. Furthermore, AMP increased the K(a) for glucose 6-phosphate. Inhibition by ATP was competitive with phosphoenolpyruvate. In support of the kinetic data, fluorescence binding studies indicated that ATP had a stronger effect than AMP on phosphoenolpyruvate binding, while AMP was more efficient in reducing glucose 6-phosphate binding. As free Mg(2+) was held constant and saturating, these effects cannot be ascribed to Mg(2+) chelation. Accordingly, the enzyme response to the adenylate energy charge was basically of the "R" type (involving enzymes of ATP regenerating sequences) according to D. E. Atkinson's (1968 Biochemistry 7: 4030-4034) concept of energy charge regulation. The effect of energy charge was abolished by 1 millimolar glucose 6-phosphate. Levels of glucose 6-phosphate and of other putative regulatory compounds of phosphoenolpyruvate carboxylase were determined in total leaf extracts during a day-night cycle. The level of glucose 6-phosphate rose at night and dropped sharply during the day. Such a decrease in glucose 6-phosphate concentration could permit an increased control of phosphoenolpyruvate carboxylase by energy charge during the day.

Fructose 2,6-bisphosphate, carbohydrate partitioning, and crassulacean Acid metabolism

Fructose 2,6-bisphosphate (F 2,6-P(2)) was detected in the CAM species, Ananas comosus and Bryophyllum tubiflorum, and in C(3)- and CAM-Mesembryanthemum crystallinum. In both Mesembryanthemum tissues, F 2,6-P(2) was located outside the chloroplast. The levels of F 2,6-P(2), malate, starch, or soluble sugars were measured during various periods during the day-night cycle in the leaves of Ananas, a species which stores carbohydrate in an extrachloroplastic compartment, and in Bryophyllum, a species which stores carbon as starch in the chloroplast. In both species, the levels of F 2,6-P(2) were correlated with the stages of the day-night CAM cycle. Immediately following the dark-light transition the F 2,6-P(2) levels exhibited a rapid transient increase followed by a decrease. F 2,6-P(2) reached a daily minimum soon after the onset of deacidification and remained low until the malic acid pools approached their daily minima; the levels of F 2,6-P(2) then began a slow increase which accelerated during the period of afternoon CO(2) uptake. Immediately following the light-dark transition F 2,6-P(2) levels fluctuated. The levels were usually low after the fluctuations had ceased. The pools then increased as the rate of malate synthesis increased, remained at relatively constant high levels when the rates of malate synthesis were constant, and decreased as malate synthesis decreased towards the end of the dark period. The absolute levels of F 2,6-P(2) were always higher in Ananas than in Bryophyllum. The ratios of the activity of pyrophosphate fructose-6-phosphate l-phosphotransferase to cytoplasmic fructose 1,6-bisphosphatase and to phosphofructokinase were also far higher in Ananas than in Bryophyllum or in C(3)- or CAM-Mesembryanthemum.

Persistent circadian rhythms in the phosphorylation state of phosphoenolpyruvate carboxylase from Bryophyllum fedtschenkoi leaves and in its sensitivity to inhibition by malate

Phosphoenolpyruvate carboxylase (EC 4.1.1.31; PEPCase) from Bryophyllum fedtschenkoi leaves has previously been shown to exist in two forms in vivo. During the night the enzyme is phosphorylated and relatively insensitive to feedback inhibition by malate whereas during the day the enzyme is dephosphorylated and more sensitive to inhibition by malate. These properties of PEPCase have now been investigated in leaves maintained under constant conditions of temperature and lighting. When leaves were maintained in continuous darkness and CO2-free air at 15°C, PEPCase exhibited a persistent circadian rhythm of interconversion between the two forms. There was a good correlation between periods during which the leaves were fixing respiratory CO2 and periods during which PEPCase was in the form normally observed at night. When leaves were maintained in continuous light and normal air at 15°C, starting at the end of a night or the end of a day, a circadian rhythm of net uptake of CO2 was observed. Only when these constant conditions were applied at the end of a day was a circadian rhythm of interconversions between the two forms of PEPCase observed and the rhythms of enzyme interconversion and CO2 uptake did not correlate in phase or period.

Purification and characterization of phosphoenolpyruvate carboxylase from a cyanobacterium

Phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) was purified 100-fold from the cyanobacterium Coccochloris peniocystis with a yield of 10%. A single isozyme was found at all stages of purification, and activity of other beta-carboxylase enzymes was not detected. The apparent molecular weight of the native enzyme was 560,000. Optimal activity was observed at pH 8.0 and 40 degrees C, yielding a Vmax of 8.84 mumol/mg of protein per min. The enzyme was not protected from heat inactivation by aspartate, malate, or oxalacetate. Michaelis-Menten reaction kinetics were observed for various concentrations of PEP, Mg2+, and HCO3-, yielding Km values of 0.6, 0.27, and 0.8 mM, respectively. Enzyme activity was inhibited by aspartate and tricarboxylic acid cycle intermediates and noncompetitively inhibited by oxalacetate, while activation by any compound was not observed. However, the enzyme was sensitive to metabolic control at subsaturating substrate concentrations at neutral pH. These data indicate that cyanobacterial PEP carboxylase resembles the enzyme isolated from C3 plants (plants which initially incorporate CO2 into C3 sugars) and suggest that PEP carboxylase functions anapleurotically in cyanobacteria.

Diurnal regulation of phosphoenolpyruvate carboxylase from crassula

Phosphoenolpyruvate carboxylase appears to be located in or associated with the chloroplasts of Crassula. As has been found with this enzyme in other CAM plants, a crude extract of leaves gathered during darkness and rapidly assayed for phosphoenolpyruvate carboxylase (PEPc) activity is relatively insensitive to inhibition by malate. After illumination begins, the PEPc activity becomes progressively more sensitive to malate. This enzyme also shows a diurnal change in activation by glucose-6-phosphate, with the enzyme from dark leaves more strongly activated than that from leaves in the light.When the enzyme is partially purified in the presence of malate, the characteristic sensitivity of the day leaf enzyme is largely retained. Partial purification of the enzyme from dark leaves results in a small increase in sensitivity to malate inhibition.Partially purified enzyme is found by polyacrylamide gel electrophoresis analysis to have two bands of PEPc activity. In enzymes from dark leaves, the slower moving band predominates, but in the light, the faster moving band is preponderant. Both of these bands are shown by sodium dodecyl sulfate-polyacrylamide gel electrophoresis to be composed of the same subunit of 103,000 daltons.The enzyme partially purified from night leaves has a pH optimum of 5.6, and is relatively insensitive to malate inhibition over the range from pH 4.5 to 8. The enzyme from day leaves has a pH optimum of 6.6 and is strongly inhibited by malate at pH values below 7, but becomes insensitive at higher pH values.Gel filtration of partially purified PEPc showed two activity peaks, one corresponding approximately to a dimer of the single subunit, and the other twice as large. The larger protein was relatively insensitive to malate inhibition, the smaller was strongly inhibited by malate.Kinetic studies showed that malate is a mixed type inhibitor of the sensitive, day, enzyme, increasing K(m) for phosphoenolpyruvate and reducing V(max). With the insensitive, night, enzyme, malate is a K type inhibitor, reducing the K(m) for phosphoenolpyruvate, but having little effect on V(max). The inhibition of the insensitive enzyme by malate appears to be hysteretic, taking several minutes to be expressed during assay, probably indicating a change in the conformation or aggregation state of the enzyme.Activation by glucose-6-phosphate is of the mixed type for the day form of the enzyme, causing both a decreased K(m) for phosphoenolpyruvate and an increased V(max), but the night, or insensitive, form shows only an increase in V(max) in response to glucose-6-phosphate.

Day-night changes in leaf water relations associated with the rhythm of crassulacean acid metabolism in Kalanchoë daigremontiana

A study was made of the day-night changes under controlled environmental conditions in the bulk-leaf water relations of Kalanchoë daigremontiana, a plant showing Crassulacean acid metabolism. In addition to nocturnal stomatal opening and net CO2 uptake, the leaves of well-watered plants showed high rates of gas exchange during the whole of the second part of the light period. Measurements with the pressure chamber showed that xylem tension increased during the night and then decreased towards a minimum at about midday; a significant increase in xylem tension was also seen in the late afternoon. Cell-sap osmotic pressure paralleled leaf malate content and was maximum at dawn and minimum at dusk. The relationship between these two variables indicated that the nocturnally synthesized malate was apparently behaving as an ideal osmoticum. To estimate bulk-leaf turgor pressure, values for water potential were derived by correcting the pressurechamber readings for the osmotic pressure of the xylem sap. This itself was found to depend on the malate content of the leaves. Bulk-leaf turgor pressure changed rhythmically during the day-night cycle; turgor was low during the late afternoon and for most of the night, but increased quickly to a maximum of 0.20 MPa around midday. In water-stressed plants, where net CO2 uptake was restricted to the dark period, there was also an increase in bulk-leaf turgor pressure at the start of the light period, but of reduced magnitude. Such changes in turgor pressure are likely to be of considerable ecological importance for the water economy of crassulacean-acid-metabolism plants growing in their natural habitats.

Light and Temperature Regulation of Early Morning Crassulacean Acid Metabolism in Opuntia erinacea var Columbiana (Griffiths) L. Benson

During the early morning period, light and temperature exert distinctively different influences on the gas exchange patterns of the Crassulacean acid metabolism plant Opuntia erinacea through their effects on acid metabolism. An initial decrease in CO(2) uptake was triggered by illumination and was apparently due to a decreased CO(2) diffusion gradient through light-mediated decarboxylation of malate. In contrast, the morning burst of CO(2) uptake occurred at high temperature presumably in response to increases in both stomatal conductance and the CO(2) diffusion gradient, resulting from the temperature-regulated fixation of endogenous CO(2), primarily into malate. Subsequent stomatal closure, apparently due to elevated levels of internal CO(2) through rapid decarboxylation of malate at high temperature, was primarily responsible for the final termination of early morning Crassulacean acid metabolism.

Maintenance carbon cycle in crassulacean Acid metabolism plant leaves : source and compartmentation of carbon for nocturnal malate synthesis

The reciprocal relationship between diurnal changes in organic acid and storage carbohydrate was examined in the leaves of three Crassulacean acid metabolism plants. It was found that depletion of leaf hexoses at night was sufficient to account quantitatively for increase in malate in Ananas comosus but not in Sedum telephium or Kalanchoë daigremontiana. Fructose and to a lesser extent glucose underwent the largest changes. Glucose levels in S. telephium leaves oscillated diurnally but were not reciprocally related to malate fluctuations.Analysis of isolated protoplasts and vacuoles from leaves of A. comosus and S. telephium revealed that vacuoles contain a large percentage (>50%) of the protoplast glucose, fructose and malate, citrate, isocitrate, ascorbate and succinate. Sucrose, a major constituent of intact leaves, was not detectable or was at extremely low levels in protoplasts and vacuoles from both plants.In isolated vacuoles from both A. comosus and S. telephium, hexose levels decreased at night at the same time malate increased. Only in A. comosus, however, could hexose metabolism account for a significant amount of the nocturnal increase in malate. We conclude that, in A. comosus, soluble sugars are part of the daily maintenance carbon cycle and that the vacuole plays a dynamic role in the diurnal carbon assimilation cycle of this Crassulacean acid metabolism plant.

Photosynthetic/Photorespiratory Carbon Metabolism in the C(3)-C(4) Intermediate Species, Moricandia arvensis and Panicum milioides

The distribution of (14)C in photosynthetic metabolites of two naturally occurring higher plants with reduced photorespiration, Moricandia arvensis and Panicum milioides, in pulse and pulse-chase (14)CO(2) incorporation experiments was similar to that for the C(3) species, M. foetida and Glycine max. After 6 seconds of (14)CO(2) incorporation, only about 6% of the total (14)C fixed was in malate and aspartate in both M. arvensis and P. milioides. The apparent turnover of the C(4) acids was very slow, and malate accumulated during the day in M. arvensis. Thus, C(4) acid metabolism by M. arvensis and P. milioides had no significant role in photosynthetic carbon assimilation under the conditions of our experiments (310 microliters CO(2) per liter, 21% O(2), 1100 or 1900 micromoles photon per square meter per second, 27 degrees C).After a 36-second chase period in air containing 270 microliters CO(2) per liter, about 20% of the total (14)C fixed was in glycine with M. arvensis, as compared to 15% with M. foetida, 14% with P. milioides, and 9% with G. max. After a 36-second chase period in 100 microliters CO(2) per liter, the percentage in glycine was about twice that at 270 microliters CO(2) per liter in the C(3) species and P. milioides, but only 20% more (14)C was in glycine in M. arvensis. These data suggest that either the photorespiratory glycine pool in M. arvensis is larger than in the other species examined or the apparent turnover rate of glycine and the flow of carbon into glycine during photorespiration are less in M. arvensis. An unusual glycine metabolism in M. arvensis may be linked to the mechanism of photorespiratory reduction in this crucifer.

Diurnal variations in leaf fluorescence induction kinetics: variable fluorescence in crassulacean Acid metabolism plants

The variable fluorescence of leaves from Kalanchoë daigremontiana and pineapple, Ananas comosus, both CAM plants, was found to change over a 24-hour cycle and to exhibit high temperature-dependent maxima during the night period. The time course of the induced fluorescence was correlated with malic acid accumulation but not with other aspects of CAM such as with the nature of the decarboxylation pathway or with stomatal movements. The variable fluorescences of sunflower (Helianthus annuus L.) and corn (Zea mays L.) leaves were compared with the CAM plants diurnally; both plants also exhibit high fluorescence maxima during the night period. We conclude that the assembly of the photosystems in the light is a primary process in photosynthesis induction and may be influenced by other cellular metabolic processes, specifically in the case of CAM leaves by malic acid accumulation.

Diurnal changes in volume and specific tissue weight of crassulacean Acid metabolism plants

The diurnal variations in volume and in specific weight were determined for green stems and leaves of Crassulacen acid metabolism (CAM) plants. Volume changes were measured by a water displacement method. Diurnal variations occurred in the volume of green CAM tissues. Their volume increased early in the light period reaching a maximum about mid-day, then the volume decreased to a minimum near midnight. The maximum volume increase each day was about 2.7% of the total volume. Control leaves of C(3) and C(4) plants exhibited reverse diurnal volume changes of 0.2 to 0.4%. The hypothesis is presented and supported that green CAM tissues should exhibit a diurnal increase in volume due to the increase of internal gas pressure from CO(2) and O(2) when their stomata are closed. Conversely, the volume should decrease when the gas pressure is decreased.The second hypothesis presented and supported was that the specific weight (milligrams of dry weight per square centimeter of green surface area) of green CAM tissues should increase at night due to the net fixation of CO(2). Green CAM tissues increased their specific weight at night in contrast to control C(3) and C(4) leaves which decreased their specific weight at night. With Kalanchoë daigremontiana leaves, the calculated increase in specific leaf weight at night based on estimates of carbohydrate available for net CO(2) fixation was near 6% and the measured increase in specific leaf weight was 6%.Diurnal measurements of CAM tissue water content were neither coincident nor reciprocal with their diurnal patterns of either volume or specific weight changes.

Photosynthetic characteristics of chloroplasts isolated fromMesembryanthemum crystallinum L., a halophilic plant capable of Crassulacean acid metabolism

Photosynthetically highly active chloroplasts were routinely obtained by rupture of leaf protoplasts from the halophyteMesembryanthemum crystallinum which exhibited the photosynthetic characteristics of either a C3 plant when grown with 20 mmol l(-1) NaCl in the rooting medium, or a Crassulacean-acid-metabolism (CAM) plant when grown with 400 mmol l(-1) NaCl. Photosynthesis rates of C3 and CAM chloroplasts were 150-250 and 90-150 μmol mg(-1) chlorophyll h(-1), respectively. Because of osmotic adjustment, CAM chloroplasts required higher sorbitol concentrations (0.7-0.8 mol l(-1)) in the assay medium than C3 chloroplasts (0.3-0.4 mol l(-1)) for optimum activity. Substitution of sorbitol by NaCl as the osmoticum strongly reduced photosynthesis of CAM chloroplasts. Rates of electron transport (ferricyanide reduction, uncoupled) remained unaffected over a range of sorbitol concentrations (0 to 1 mol l(-1)). Sensitivity of electron transport to increasing levels of NaCl was less pronounced than the NaCl-sensitivity of CO2 fixation by intact chloroplasts. The CAM chloroplasts showed a broad pH optimum of photosynthesis between pH 7.0 and 8.2; photosynthesis of C3 chloroplasts dropped markedly below pH 7.6. The CAM chloroplasts maintained a higher transenvelope proton gradient than C3 chloroplasts both in the light and dark. External pyruvate (5 mmol l(-1)) inhibited photosynthesis of CAM chloroplasts, but not of C3 chloroplasts. Inhibition was reduced by increased external concentrations of orthophosphate.