Malate Synthesis in Crassulacean Leaves. I. The Distribution of C in Malate of Leaves Exposed to CO(2) in the Dark

Diversity of CAM plant photosynthesis (crassulacean acid metabolism): a tribute to Barry Osmond

This special issue is a tribute to the Australian plant biologist Professor Charles Barry Osmond - Fellow of the Australian Academy of Sciences, the Royal Society of London, and Leopoldina, the German National Academy of Sciences - and his many contributions to our understanding of the biochemistry and physiological ecology of CAM (crassulacean acid metabolism) photosynthesis. This water-conserving photosynthetic pathway is characterised by nocturnal uptake of atmospheric CO2 and typically enables succulent plants to perform and survive in warm semiarid terrestrial and epiphytic habitats. The idea for this issue is to mark the occasion of Barry's 80th birthday in 2019. The foreword highlights some of his outstanding contributions and introduces the research papers of the special issue.

Metabolic Fate of the Carboxyl Groups of Malate and Pyruvate and their Influence on δ(13)C of Leaf-Respired CO2 during Light Enhanced Dark Respiration

The enhanced CO2 release of illuminated leaves transferred into darkness, termed "light enhanced dark respiration (LEDR)", is often associated with an increase in the carbon isotope ratio of the respired CO2 (δ(13)CLEDR). The latter has been hypothesized to result from different respiratory substrates and decarboxylation reactions in various metabolic pathways, which are poorly understood so far. To provide a better insight into the underlying metabolic processes of δ(13)CLEDR, we fed position-specific (13)C-labeled malate and pyruvate via the xylem stream to leaves of species with high and low δ(13)CLEDR values (Halimium halimifolium and Oxalis triangularis, respectively). During respective label application, we determined label-derived leaf (13)CO2 respiration using laser spectroscopy and the (13)C allocation to metabolic fractions during light-dark transitions. Our results clearly show that both carboxyl groups (C-1 and C-4 position) of malate similarly influence respiration and metabolic fractions in both species, indicating possible isotope randomization of the carboxyl groups of malate by the fumarase reaction. While C-2 position of pyruvate was only weakly respired, the species-specific difference in natural δ(13)CLEDR patterns were best reflected by the (13)CO2 respiration patterns of the C-1 position of pyruvate. Furthermore, (13)C label from malate and pyruvate were mainly allocated to amino and organic acid fractions in both species and only little to sugar and lipid fractions. In summary, our results suggest that respiration of both carboxyl groups of malate (via fumarase) by tricarboxylic acid cycle reactions or by NAD-malic enzyme influences δ(13)CLEDR. The latter supplies the pyruvate dehydrogenase reaction, which in turn determines natural δ(13)CLEDR pattern by releasing the C-1 position of pyruvate.

Crassulacean acid metabolism photosynthesis: ;working the night shift'

Crassulacean acid metabolism (CAM) can be traced from Roman times through persons who noted a morning acid taste of some common house plants. From India in 1815, Benjamin-Heyne described a 'daily acid taste cycle' with some succulent garden plants. Recent work has shown that the nocturnally formed acid is decarboxylated during the day to become the CO(2) for photosynthesis. Thus, CAM photosynthesis extends over a 24-hour day using several daily interlocking cycles. To understand CAM photosynthesis, several landmark discoveries were made at the following times: daily reciprocal acid and carbohydrate cycles were found during 1870 to 1887; their precise identification, as malic acid and starch, and accurate quantification occurred from 1940 to 1954; diffusive gas resistance methods were introduced in the early 1960s that led to understanding the powerful stomatal control of daily gas exchanges; C(4) photosynthesis in two different types of cells was discovered from 1965 to approximately 1974 and the resultant information was used to elucidate the day and night portions of CAM photosynthesis in one cell; and exceptionally high internal green tissue CO(2) levels, 0.2 to 2.5%, upon the daytime decarboxylation of malic acid, were discovered in 1979. These discoveries then were combined with related information from C(3) and C(4) photosynthesis, carbon biochemistry, cellular anatomy, and ecological physiology. Therefore by approximately 1980, CAM photosynthesis finally was rigorously outlined. In a nutshell, 24-hour CAM occurs by phosphoenol pyruvate (PEP) carboxylase fixing CO(2)(HCO(3) (-)) over the night to form malic acid that is stored in plant cell vacuoles. While stomata are tightly closed the following day, malic acid is decarboxylated releasing CO(2) for C(3) photosynthesis via ribulose bisphosphate carboxylase oxygenase (Rubisco). The CO(2) acceptor, PEP, is formed via glycolysis at night from starch or other stored carbohydrates and after decarboxylation the three carbons are restored each day. In mid to late afternoon the stomata can open and mostly C(3) photosynthesis occurs until darkness. CAM photo-synthesis can be both inducible and constitutive and is known in 33 families with an estimated 15 to 20 000 species. CAM plants express the most plastic and tenacious photosynthesis known in that they can switch photosynthesis pathways and they can live and conduct photosynthesis for years even in the virtual absence of external H(2)O and CO(2), i.e., CAM tenaciously protects its photosynthesis from both H(2)O and CO(2) stresses.

Physiological studies on acid metabolism in green plants IX. The distribution of 14 C in malate of darkened Kalanchoё leaf fragments after infiltration with labelled pyruvate

When carboxyl-, carbonyl- or methyl-labelled pyruvates are supplied to darkened fragments of Kalanchoё crenata Haw. leaves, malate usually becomes the most heavily labelled metabolite. To help in elucidating the processes involved in the labelling, the distribution of 14 C within the malate molecule has been determined. Investigations were made for leaves in which dark acidification led to a net synthesis of malate and for leaves in which dark deacidification led to a net consumption of malate. Some experiments were carried out in the presence of aerobic atmospheres containing 20 % CO 2 . The results are consistent with the oxidation of the pyruvate by pyruvic oxidase and enzymes associated with the conventional TCA cycle. In addition to such slight net synthesis of malate as may have occurred by this sequence there appeared to be two other distinct synthetic processes involving β -carboxylations. In one of these, 14 CO 2 that arose from pyruvate or a labelled product thereof was fixed into malate by the enzymic system responsible for the massive CO 2 fixation and resulting acidification in darkened Kalanchoё leaves. In this reaction, apparently, the carbon skeleton of pyruvate was not directly incorporated into the malate. In the other carboxylative synthesis the carbon skeleton of labelled pyruvate appeared to be incorporated as a whole into malic acid. Only a very small proportion of the total fixation of 14 CO 2 by the leaves was attributable to this synthesis. It is suggested that the first of these carboxylations involves phosphoenolpyruvic carboxylase acting in conjunction with other enzymes Support is provided for the concept that most of the malate synthesized when CO 2 is fixed during dark acidification is transferred to a storage pool. The present analyses suggest that malate synthesized from the labelled pyruvate by action of malic enzymeor in oxidative reactions of the TCA cycle is relatively rapidly metabolized. The implications of the possible occurrence of the acids in Kalanchoё leaves partly in storage pools, where they are not subject to metabolic transformations, and partly in active pools, where they are rapidly transformed in metabolic reactions, are discussed briefly with respect to the interpretation of results observed when labelled metabolites are supplied to plant tissues.

Malate Metabolism in the Dark After CO(2) Fixation in the Crassulacean Plant Kalanchoë tubiflora

The metabolism of [(13)C]malate was studied in the Crassulacean plant Kalanchoë tubiflora following exposure to (13)CO(2) for 2 hour intervals during a 16 hour dark cycle. Nuclear magnetic resonance spectroscopy of [(13)C]malate extracted from labeled tissue revealed that the transient flux of malate to the mitochondria, estimated by the randomization of [4-(13)C]malate to [1- (13)C]malate by fumarase, varied substantially during the dark period. At both 15 and 25 degrees C, the extent of malate label randomization in the mitochondria was greatest during the early and late parts of the dark period and was least during the middle of the night, when the rate of (13)CO(2) uptake was highest. Randomization of labeled malate continued for many hours after malate synthesis had initially occurred. Internally respired (12)CO(2) also served as a source of carbon for malate formation. At 15 degrees C, 15% of the total malate was formed from respired (12)CO(2), while at 25 degrees C, 49% of the accumulated malate was derived from respired (12)CO(2). Some of the malate synthesized from external (13)CO(2) was also respired during the night. The proportion of the total [(13)C]malate respired during the dark period was similar at 15 and 25 degrees C, and respiration of newly formed [(13)C]malate increased as the night period progressed. These data are discussed with regard to the relative fluxes of malate to the mitochondria and the vacuole during dark CO(2) fixation.

Regulation of malic-acid metabolism in Crassulacean-acid-metabolism plants in the dark and light: In-vivo evidence from (13)C-labeling patterns after (13)CO 2 fixation

The labeling patterns in malic acid from dark (13)CO2 fixation in seven species of succulent plants with Crassulacean acid metabolism were analysed by gas chromatography-mass spectrometry and (13)C-nuclear magnetic resonance spectrometry. Only singly labeled malic-acid molecules were detected and on the average, after 12-14 h dark (13)CO2 fixation the ratio of [4-(13)C] to [1-(13)C] label was 2:1. However the 4-C carboxyl contained from 72 to 50% of the label depending on species and temperature. The (13)C enrichment of malate and fumarate was similar. These data confirm those of W. Cockburn and A. McAuley (1975, Plant Physiol. 55, 87-89) and indicate fumarase randomization is responsible for movement of label to 1-C malic acid following carboxylation of phosphoenolpyruvate. The extent of randomization may depend on time and on the balance of malic-acid fluxes between mitochondria and vacuoles. The ratio of labeling in 4-C to 1-C of malic acid which accumulated following (13)CO2 fixation in the dark did not change during deacidification in the light and no doubly-labeled molecules of malic acid were detected. These results indicate that further fumarase randomization does not occur in the light, and futile cycling of decarboxylation products of [(13)C] malic acid ((13)CO2 or [1-(13)C]pyruvate) through phosphoenolpyruvate carboxylase does not occur, presumably because malic acid inhibits this enzyme in the light in vivo. Short-term exposure to (13)CO2 in the light after deacidification leads to the synthesis of singly and multiply labeled malic acid in these species, as observed by E.W. Ritz et al. (1986, Planta 167, 284-291). In the shortest times, only singly-labeled [4-(13)C]malate was detected but this may be a consequence of the higher intensity and better detection statistics of this ion cluster during mass spectrometry. We conclude that both phosphoenolpyruvate carboxylase (EC 4.1.1.32) and ribulose-1,5-biphosphate carboxylase (EC 4.1.1.39) are active at this time.

Characterization of carbon metabolism in Opuntia ficus-indica Mill. exhibiting the idling mode of Crassulacean acid metabolism

Upon transfer from well-watered conditions to total drought, long-day-grown cladodes of Opuntia ficus-indica Mill. shift from full Crassulacean acid metabolism (CAM) to CAM-idling. Experiments using (14)C-tracers were conducted in order to characterize the carbon-flow pattern in cladodes under both physiological situations. Tracer was applied by (14)CO2 fumigations and NaH(14)CO3 injections during the day-night cycle. The results showed that behind the closed stomata, mesophyll cells of CAM-idling plants retained their full capacity to metabolize CO2 in light and in darkness. Upon the induction of CAM-idling the level of the capacity of phosphoenolpyruvate carboxylase (EC 4.1.1.31) was maintained. By contrast, malate pools decreased, displaying finally only a small or no day-night oscillation. The capacity of NADP-malic enzyme (EC 1.1.1.40) decreased in parallel with the reduction in malate pools. Differences in the labelling patterns, as influenced by the mode of tracer application, are discussed.

Mass-spectrometric evidence for the double-carboxylation pathway of malate synthesis by Crassulacean acid metabolism plants in light

Phyllodia of the Crassulacean acid metabolism (CAM) plant Kalanchoë tubiflora were allowed to fix (13)CO2 in light and darkness during phase IV of the diurnal CAM cycle, and during prolongation of the regular light period. After (13)CO2 fixation in darkness, only singly labelled [(13)C]malate molecules were found. Fixation of (13)CO2 under illumination, however, produced singly labelled malate as well as malate molecules which carried label in two, three or four carbon atoms. When the irradiance during (13)CO2 fixation was increased, the proportion of singly labelled malate decreased in favour of plurally labelled malate. The irradiance, however, did not change either the ratio of labelled to unlabelled malate molecules found in the tissue after the (13)CO2 application, or the magnitude of malate accumulation during the treatment with label. The ability of the tissue to store malate and the labelling pattern changed throughout the duration of the prolonged light period. The results indicate that malate synthesis by CAM plants in light can proceed via a pathway containing two carboxylation steps, namely ribulose-1,5-bisphosphate-carboxylase/oxygenase (EC 4.1.1.39) and phosphoenolpyruvate carboxylase (EC 4.1.1.31) which operate in series and share common intermediates. It can be concluded that, in light, phosphoenolpyruvate carboxylase can also synthesize malate independently of the proceeding carboxylation step by ribulose-1,5-bisphosphate carboxylase/oxygenase.

Circadian rhythms inKalanchoë: the pathway of(14)CO 2 fixation during prolonged light

(14)CO2 was applied repeatedly at 3- to 6-h intervals toKalanchoë daigremontiana leaves during continuous light of differing irradiances. The circadian rhythm in net CO2 uptake in gasexchange measurements and its disappearance at high irradiances was confirmed by oscillating rates of(14)CO2 incorporation. At 10-30 W m(-2) a markedly circadian oscillation in the(14)CO2-uptake rate was measured; with increasing energy fluence rate the oscillation levelled off at a constant high uptake rate. The labelling patterns obtained during the 10 min of(14)CO2 fixation indicated that the rhythm of CO2 exchange is the consequence of a rhythmic behaviour in the C4 pathway of CO2 fixation. During the mininum of(14)CO2 uptake no C4 products were labelled; however, substantial amounts of label were transferred to C4 products during the peaks of(14)CO2 uptake. Metabolism of C3 and C4 products was also studied in pulsechase experiments at different points of the circadian cycle. In bright light (100 W m(-2)), when the(14)CO2 uptake was constantly high, the transfer of label into C4 products (malic acid) was high in spite of the fact that the malate pool is known to be reduced to a permanently low level under these conditions. This led us to the conclusion that it is not the capacity of the phosphoenolpyruvatecarboxylase-mediated CO2 fixation but rather the storage of malic acid in the vacuole that is disturbed under bright-light conditions when the circadian oscillation levelled off.

Carbon Dioxide and Water Vapor Exchange in the Crassulacean Acid Metabolism Plant Kalanchoë pinnáta during a Prolonged Light Period: METABOLIC AND STOMATAL CONTROL OF CARBON METABOLISM

Net CO(2) and water vapor exchange were studied in the Crassulacean acid metabolism plant Kalanchoë pinnáta during a normal 12-hour light/12-hour dark cycle and during a prolonged light period. Leaf temperature and leaf-air vapor pressure difference were kept constant at 20 C and 9 to 10 millibar. There was a 25% increase in the rate of CO(2) fixation during the first 6 hours prolonged light without change in stomatal conductance. This was associated with a decrease in the intracellular partial pressure of CO(2), a decrease in the stimulation of net CO(2) uptake by 2% O(2), and a decrease in the CO(2) compensation point from 45 to 0 microbar. In the normal light period after deacidification, leaves showed a normal light dependence of CO(2) uptake but, in prolonged light, CO(2) uptake was scarcely light-dependent. The increase in titratable acidity in prolonged light was similar to that in the dark.The results suggest a change from C(3) photosynthetic CO(2) fixation in the second part of the 12-hour light period to a mixed metabolism in prolonged light with both ribulose bisphosphate carboxylase and phosphoenolpyruvate carboxylase as primary carboxylating enzymes.

Dark Carbon Dioxide Fixation under Aerobic and Anaerobic Conditions in Maize Leaves after Preillumination in the Absence of Oxygen: Ribulose 1,5-Bisphosphate Can Serve as a Primary Acceptor of Carbon Dioxide

When dark (14)CO(2) fixation in maize leaves was carried out under anaerobic conditions after preillumination in the absence of O(2), the (14)C incorporation in aspartic acid was transient; its maximum level was very low compared with that of malic acid. The addition of 5% O(2) during the dark fixation period increased the total uptake of (14)CO(2) and the (14)C incorporation into aspartic acid.A study of the intramolecular distribution of radioactivity showed that 71 to 76% of the (14)C was located in the C(4) (beta-carboxyl) of malate and aspartate and the remainder in the C(1). This intramolecular labeling pattern did not change during the 5- to 60-second dark (14)CO(2) fixation period and was scarcely altered by the presence of O(2). Three degradation techniques led to similar data.The significance of these results is discussed taking into account the known possible carboxylation pathways. It is concluded that ribulose 1,5-bisphosphate can be a primary acceptor of CO(2) when maize leaves are preilluminated in the absence of O(2).

Malate synthesis by dark carbon dioxide fixation in leaves

The rates of dark CO(2) fixation and the label distribution in malate following dark (14)CO(2) fixation in a C-4 plant (maize), a C-3 plant (sunflower), and two Crassulacean acid metabolism plants (Bryophyllum calycinum and Kalanchoë diagremontianum leaves and plantlets) are compared. Within the first 30 minutes of dark (14)CO(2) fixation, leaves of maize, B. calycinum, and sunflower, and K. diagremontianum plantlets fix CO(2) at rates of 1.4, 3.4, 0.23, and 1.0 mumoles of CO(2)/mg of chlorophyll. hour, respectively. Net CO(2) fixation stops within 3 hours in maize and sunflower, but Crassulaceans continue fixing CO(2) for the duration of the 23-hour experiment.A bacterial procedure using Lactobacillus plantarum ATCC No. 8014 and one using malic enzyme to remove the beta-carboxyl (C(4)) from malate are compared. It is reported that highly purified malic enzyme and the bacterial method provide equivalent results. Less purified malic enzyme may overestimate the label in C(4) as much as 15 to 20%.The contribution of carbon atom 1 of malate is between 18 and 21% of the total carboxyl label after 1 minute of dark CO(2) fixation. Isotopic labeling in the two carboxyls approached unity with time. The rate of increase is greatest in sunflower leaves and Kalanchoë plantlets. In addition, Kalanchoë leaves fix (14)CO(2) more rapidly than Kalanchoë plantlets and the equilibration of the malate carboxyls occurs more slowly. The rates of fixation and the randomization are tissue-specific. The rate of fixation does not correlate with the rate of randomization of isotope in the malate carboxyls.

Changes in Metabolite Levels in Kalanchoë daigremontiana and the Regulation of Malic Acid Accumulation in Crassulacean Acid Metabolism

Changes in glucose-6-P, fructose-6-P, fructose-1,6-diP, 6-phospho-gluconate, phosphoenolpyruvate, 3-phosphoglycerate, and pyruvate levels in the leaves of the Crassulacean plant Kalanchoë daigremontiana Hammet et Perrier were measured enzymically during transitions from CO(2)-free air to air, air to CO(2)-free air, and throughout the course of acid accumulation in darkness. The data are discussed in terms of the involvement of phosphoenolpyruvate carboxylase in malic acid synthesis and in terms of the regulation of the commencement of malic acid synthesis and accumulation through the effects of CO(2) on storage carbohydrate mobilization and its termination through the effects of malic acid on phosphoenolpyruvate carboxylase activity.

Equilibration of Label in Malate during Dark Fixation of CO(2) in Kalanchoë fedtschenkoi

In vitro studies of dark (14)CO(2) fixation with isolated cell aggregates of Kalanchoë fedtschenkoi showed that malate synthesized after 20 sec is predominantly (85 to 92%) labeled at carbon 4, while after 20 min only 65 to 69% of the radioactivity was located in this position. The intramolecular labeling pattern of malate could not be changed by supplementing the cells with carboxylation reaction substrates such as ribulose diphosphate or phosphoenolpyruvate. The kinetic decline of label at carbon 4 of malate occurs independently of CO(2) fixation, since 4-(14)C-labeled aspartate fed to the cells gave rise to malate labeled 62% at carbon 4 after 20 min. Furthermore, the cells were capable of converting fed malate to fumarate. It is concluded that synthesis of malate during dark CO(2) fixation is accomplished by a single carboxylation step via phosphoenolpyruvate carboxylase and labeling patterns observed in malate are a consequence of the action of fumarase.

Restoration of Organic Acid Accumulation in Sectioned Leaves of Bryophyllum tubiflorum Harv

When leaves of Bryophyllum tubiflorum were cut into transverse sections, and held at 20 C in the dark, the capacity to accumulate organic acid decreased with decreasing section thickness. In addition, the rate of respiration increased with decreasing section thickness and was unaffected by changes in O(2) concentration above 5% or by the presence (1%) of CO(2). It was concluded that O(2) ventilation is not a controlling factor in respiration. Malonate (0.1 m) and fluoroacetate (0.01 m) restored the capacity of sectioned leaves to accumulate acid to normal levels and depressed respiration in 1-millimeter sections. Acid accumulation in 8-millimeter sections remained essentially constant at 20, 15, and 10 C, and was equal to that in unsectioned leaves, but accumulation in 2-millimeter sections rose to normal levels as the temperature fell to 10 C. Twenty-three additional metabolic inhibitors (none specific to the tricarboxylic acid cycle) were screened, and none promoted acid accumulation in sectioned leaves at 20 C. The results suggest that sectioning stimulates a respiratory sequence which includes the tricarboxylic acid cycle. This sequence in turn competes with the synthesis or accumulation of malic acid.

The pathway of carbon dioxide fixation in crassulacean plants

Combined gas chromatography-mass spectrometry of malic acid derivatives has been used to show unequivocally that malic acid, synthesized during active acid accumulation in the dark by Kalanchoë daigremontiana Hammet et Perrier in the presence of (13)CO(2) is produced by a pathway involving a single carboxylation. The significance of the finding that crassulacean malate synthesized in the dark and in the presence of (14)CO(2) often contains 66% of the total carboxyl label in carbon atom 4, which has previously been taken to indicate the operation of a double carboxylation pathway or has been dismissed as an artefact, is discussed.

Relationship between leaf development and primary photosynthetic products in the C4 plant Portulaca oleracea L

The photosynthetic products of Portulaca oleracea differ greatly depending on leaf age and length of exposure to (14)CO2. Mature leaves of P. oleracea fix (14)CO2 primarily into organic and amino acids during a 10-s exposure period. Less than 2% of the (14)CO2 fixed appears in phosphorylated compounds. In contrast, incorporation into amino acids can account for over 60% of the total (14)CO2 fixed by young leaves in an equal time period, and incorporation into alanine alone can account for up to one half of this amount. Senescent leaves display a quantitative shift of primary products toward phosphorylated compounds with a concomitant reduction of the label residing in malate and asparate. About 8 times more phosphoglyceric acid is produced in senescent leaves than in mature leaves. The aspartate/ malate ratio is not constant and depends on the length of time the leaves are exposed to (14)CO2 and the age of the leaves under study. It appears as if the stage of leaf development is one of the most important factors determining the operation of a particular enzyme system in C4 plants.

Isolation of Mesophyll Cells from Sedum telephium Leaves

A technique is described for mechanically isolating metabolically active individual spongy mesophyll cells from the Crassulacean acid metabolism plant, Sedum telephium. Mature leaves are selected at about 2 PM when acidity is low, and three different media are used to reduce the problem of leaf acidity and to maintain isotonic conditions. The upper and lower epidermis is peeled from chilled leaves and the leaves are suspended in a buffered "soaking medium," then gently ground with a mortar and pestle. Cells and debris are separated using a "washing medium," with cells being filtered through a 136 micron net and collected on an 80 micron net. Cells then are suspended in a "cell suspension medium" and concentrated by centrifugation. Approximately 2 hours are required for the isolation procedure, and activity in CO(2) fixation is constant for up to 4 hours after isolation. Microscopic examination shows about 65% of the isolated cells appear intact and unplasmolyzed and are similar to leaf msophyll cells. The yield of cells on a leaf chlorophyll basis is about 1%. A light micrograph of the isolated cells is given.The isolated cells upon addition of phosphoenolpyruvate, 2-phosphoglycerate, and ribulose-1, 5-diphosphate fix CO(2) as rapidly as intact leaves; however, without exogenous substrate the cells only fix CO(2) between 10 and 20% of intact leaves. The temperature and pH optima for cellular CO(2) fixation in the presence of phosphoenolpyruvate is 35 to 45 C and 7.5 to 9.0, respectively. The light and dark portions of CO(2) fixation with the isolated cells are considered in relation to a scheme for net CO(2) fixation by Crassulacean acid metabolism plants.

Dark Fixation of CO(2) by Crassulacean Plants: Evidence for a Single Carboxylation Step

Malic acid isolated from Bryophyllum pinnatum (Lamk.) Oken (B. calycinum Salisb.), Bryophyllum tubiflorum Harv., Kalanchoë diagremontiana Hamet et Perrier and Sedum guatamalense Hemsl. after dark (14)CO(2) fixation was degraded by an in vitro NADP-malic enzyme technique. In the short term (5 to 30 seconds) the malic acid was almost exclusively labeled in the C-4 carboxyl carbon (greater than 90%). The percentage of (14)C in the C-4 carboxyl of malic acid declined slowly with time, reaching 70% in B. tubiflorum and 54% in B. pinnatum after 14 hours of exposure to (14)CO(2). It was found that malic acid-adapted Lactobacillus arabinosus may seriously underestimate the C-4 carboxyl component of label in malic acid-(14)C. The amount of substrate which the bacteria can completely metabolize was easily exceeded; there was a significant level of randomization of label even when beta-decarboxylation proceeded to completion, and in extended incubation periods, more than 25% of label was removed from malic acid-U-(14)C. The significance of these findings in relation to pathways of carbohydrate metabolism and malic acid synthesis in Crassulacean acid metabolism is discussed.

[On the analysis of CO2-exchange in bryophyllum : II. Inhibition of starch loss during the night in an atmosphere free from CO2]

Starch consumption during the dark period in detached phyllodia of Bryophyllum tubiflorum is inhibited, when the phyllodia are held in an atmosphere free from carbon dioxide during the night. This is true also in other succulent plants with Crassulacean acid metabolism=CAM (examined were Bryophyllum calycinum and Sedum morganianum). This effect seems to indicate that the role of starch in CAM is production of CO2 acceptors rather than production of carbon dioxide by respiration. If the CO2 acceptors are not used, starch consumption comes to an end.This hypothesis could also explain results of experiments in which phyllodia were held at different temperatures during the dark period, and net CO2 fixation, starch loss and malate gain were determined. At 10° CO2 uptake was at a maximum (the necessary supply of CO2 acceptors must have therefore been at a maximum, too). Under these conditions there was the greatest amount of starch consumption. At 23° C, CO2 uptake was clearly lowered, and this was also true for starch consumption. At 35° C net CO2 uptake was balanced by net CO2, output (no CO2 acceptors were needed in CO2 dark fixation). At this temperature no starch loss could be measured.

Fixation of carbon dioxide in the dark by the malic enzyme of bean and oat stem rust uredospores

Malic enzyme was found in both bean rust and cat stem rust uredospores. In bean rust uredospores it was shown to catalyze the formation of pyruvic acid from l-malic acid and to synthesize malic acid from pyruvic acid and CO(2). The malic enzyme from bean rust uredospores was specific for NADP and dependent on manganous ions for activity. The specific activity of the bean rust malic enzyme in crude extracts of ungerminated uredospores was approximately 6 times greater than that found in crude extracts obtained from germinated uredospores. The malic enzyme was also found in extracts obtained from healthy and rust-infected bean leaves. The specific activity of the enzyme was approximately 2 to 5 times greater in partially purified extracts obtained from the infected bean tissue at 6 days after inoculation. The specific activity of the malic enzyme in crude extracts obtained from oat stem rust uredospores was 2 times greater than the specific activity of this enzyme in crude extracts obtained from bean rust uredospores. Phosphoenolpyruvate carboxylase activity could not be demonstrated in crude extracts obtained from the ungerminated uredospores of the bean rust fungus.

Temperature features of enzymes affecting crassulacean Acid metabolism

Enzymes involved in malic acid production via a pathway with 2 carboxylation reactions and in malic acid conversion via total oxidation have been demonstrated in mitochondria of Bryophyllum tubiflorum Harv. Activation of the mitochondria by Tween 40 was necessary to reveal part of the enzyme activities. The temperature behavior of the enzymes has been investigated, revealing optimal activity of acid-producing enzymes at 35 degrees . Even at 53 degrees the optimum for acid-converting enzymes was not yet reached. From the simultaneous action of acid-producing and acid-converting enzyme systems the overall result at different temperatures was established. Up to 15 degrees the net result was a malic acid production. Moderate temperatures brought about a decrease in this accumulation, which was partly accompanied by a shift to isocitrate production, while at higher temperatures total oxidation of the acids exceeded the production.

Dark Fixation of CO(2) by Tobacco Leaves

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