Crassulacean Acid Metabolism and Survival of Asexual Propagules of Sedum wrightii

In Sedum wrightii grown in a growth chamber, detached leaves could survive for at least 120 d with a high rate of success for propagule formation. The pattern of gas exchange, associated with CAM, may be important in extending the period during which the detached leaf remains physiologically active. The added benefit for the developing propagule, still attached to the parent leaf, is an additional source of water and saccharide reserves over an extended period necessary for rooting. Drought survival of propagules may be determined by the amount of water-storing tissue in the detached leaf.     

Photosynthesis in Isolated Chloroplasts of the Crassulacean Acid Metabolism Plant Sedum praealtum

Intact chloroplasts were isolated from protoplasts of the Crassulacean acid metabolism plant Sedum praealtum D.C. Typical rates of CO₂ fixation or CO₂-dependent O₂ evolution ranged from 20 to 30 micromoles per milligram chlorophyll per hour and could be stimulated 30 to 50% by several Calvin cycle intermediates. The pH optimum for CO₂ fixation was 7.0 to 7.6 with considerable activity as low as pH 6.4. Low concentrations of orthophosphate (Pi) (optimum 0.4 millimolar) stimulated photosynthesis while high concentrations (5 millimolar) caused some inhibition. Both CO₂ fixation and CO₂-dependent O₂ evolution exhibited a relatively long lag phase (4 to 6 minutes) which remained constant between 0.4 to 5 millimolar Pi. The lag phase could be decreased by addition of dihydroxyacetone-phosphate or ribose 5-phosphate. Further results are presented which suggest these chloroplasts have a functional phosphate translocator.     

Crassulacean Acid Metabolism and Crassulacean Acid Metabolism Modifications in Peperomia camptotricha

Peperomia camptotricha, a tropical epiphyte from Mexico, shows variable forms of Crassulacean acid metabolism (CAM). Young leaves exhibit CAM-cycling, while mature leaves show an intermediate type of metabolism, between CAM and CAM-cycling, having approximately the same amount of nighttime gas exchange as daytime. Metabolism of young leaves appears independent of daylength, but mature leaves have a tendency toward more CAM-like metabolism under short days (8 hours). Large differences in the physical appearance of plants were found between those grown under short daylengths and those grown under long daylengths (14 hours). Some anatomical differences were also detected in the leaves. Water stress caused a switch to CAM in young and mature leaves, and as water stress increased, they shifted to CAM-idling.     

Intracellular Localization of Some Key Enzymes of Crassulacean Acid Metabolism in Sedum praealtum

The intracellular locations of six key enzymes of Crassulacean acid metabolism were determined using enzymically isolated mesophyll protoplasts of Sedum praealtum D.C. Data from isopycnic sucrose density gradient centrifugation established the chloroplastic location of pyruvate Pi dikinase, the mitochondrial location of NAD-linked malic enzyme, and exclusively nonparticulate (not associated with chloroplasts, peroxisomes, or mitochondria) locations of phosphoenolpyruvate carboxylase, NADP-linked malic enzyme, enolase, and phosphoglycerate mutase. The consequences of this enzyme distribution with respect to compartmentalization of the pathway and the transport of metabolites in Crassulacean acid metabolism are discussed.     

Quantum Requirement for Photosynthesis in Sedum praealtum during Two Phases of Crassulacean Acid Metabolism

The quantum requirement (QR) for photosynthesis in Sedum praealtum, a Crassulacean acid metabolism plant, was compared with that of wheat, a C₃ plant, and maize, a C₄ plant, at 30 C. During the deacidification phase in S. praealtum, approximately 16 moles quanta were absorbed per mole malate consumed. This is equivalent to 16 moles quanta per mole CO₂ fixed, assuming 1 mole CO₂ is assimilated per mole malate decarboxylated. This QR for Crassulacean acid metabolism is similar to that of the C₃ or C₄ plant under atmospheric conditions, even though there are considerable differences in the biochemistry of photosynthesis. During late-afternoon C₃-like fixation of atmospheric CO₂ in S. praealtum, the QR was relatively high with values of 41 under 21% O₂ and 19 under 2% O₂. During the deacidification phase in S. praealtum, the relatively low QR can be accounted for by the repression of photorespiration and saturation of photosynthesis from the elevated CO₂ concentration in the leaves during malate decarboxylation.     

Properties of phosphoenol pyruvate carboxylase in Sedum species in relation to Crassulacean Acid Metabolism (CAM)

Abstract Parameters of Crassulacean Acid Metabolism (carbon isotope composition [δ¹³C values], diurnal acid cycles) were studied together with properties of phosphoenolpyruvate carboxylases (PEP-C) in eight species of the genus Sedum. The δ¹³C values indicate a considerable variability of CAM capacity among the species investigated. The spectra of organic acids were similar in all species. Malate, citrate, and isocitrate were the main components of the acid fraction. Quinic acid occurred only during the light period. Molecular weights of PEP-C were in the range of 160,000 and showed no correlation to CAM as indicated by the δ¹³C values. However, there were differences in the kinetic data of PEP-C. Sedum species with less negative δ¹³C (i.e. with high CAM capacity) have PEP-C with higher V_max, higher Km (PEP) and higher sensitivity for malate inhibition.     

Drought Adaptation in Opuntia basilaris: Significance of Recycling Carbon through Crassulacean Acid Metabolism

Contrasting metabolic regimes operate in Opuntia basilaris Engelm. and Bigelov, before and after precipitation. During periods of drought, atmospheric CO₂ exchange and transpiration are greatly reduced throughout the day/night cycle by stomatal closure and a highly impervious cuticle. The hypothesis is that endogenously produced CO₂ is retained and recycled through dark CO₂ fixation, organic acid transformations, photosynthesis, and respiration. Immediately following precipitation, nighttime stomatal opening is initiated, permitting increased atmospheric CO₂ assimilation and organic acid synthesis.     

Molecular Genetics of Crassulacean Acid Metabolism

Most higher plants assimilate atmospheric CO2 through the C3 pathway of photosynthesis using ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). However, when CO2 availability is reduced by environmental stress conditions, the incomplete discrimination of CO2 over O2 by Rubisco leads to increased photorespiration, a process that reduces the efficiency of C3 photosynthesis. To overcome the wasteful process of photorespiration, approximately 10% of higher plant species have evolved two alternate strategies for photosynthetic CO2 assimilation, C4 photosynthesis and Crassulacean acid metabolism. Both of these biochemical pathways employ a "CO2 pump" to elevate intracellular CO2 concentrations in the vicinity of Rubisco, suppressing photorespiration and therefore improving the competitiveness of these plants under conditions of high light intensity, high temperature, or low water availability. This CO2 pump consists of a primary carboxylating enzyme, phosphoenolpyruvate carboxylase. In C4 plants, this CO2-concentrating mechanism is achieved by the coordination of two carboxylating reactions that are spatially separated into mesophyll and bundle-sheath cell types (for review, see R.T. Furbank, W.C. Taylor [1995] Plant Cell 7: 797-807;M.S.B. Ku, Y. Kano-Murakami, M. Matsuoka [1996] Plant Physiol 111: 949-957). In contrast, Crassulacean acid metabolism plants perform both carboxylation reactions within one cell type, but the two reactions are separated in time. Both pathways involve cell-specific changes in the expression of many genes that are not present in C3 plants.     

Changes in Mulberry Leaf Metabolism in Response to Water Stress

A series of experiments were conducted to characterize the water stress-induced changes in the activities of RuBP carboxylase (RuBPCO) and sucrose phosphate synthase (SPS), photosystem 2 activity, and contents of chlorophylls, carotenoids, starch, sucrose, amino acids, free proline, proteins and nucleic acids in mulberry (Morus alba L. cv. K-2) leaves. Water stress progressively reduced the activities of RuBPCO and SPS in the leaf extracts, the chlorophyll content, and PS2 activity in isolated chloroplasts. Plants exposed to drought showed lower content of starch and sucrose but higher total sugar content than control plants. While the soluble protein content decreased under water stress, the amino acid content increased. Proline accumulation (2.5-fold) was noticed in stressed leaves. A reduction in the contents of DNA and RNA was observed. Reduced nitrogen content was associated with the reduction in nitrate reductase activity. SDS-PAGE protein profile showed few additional proteins (78 and 92 kDa) in the water stressed plants compared to control plants.     

Alternative Crassulacean Acid Metabolism Modes Provide Environment-Specific Water-Saving Benefits in a Leaf Metabolic Model

Crassulacean acid metabolism (CAM) evolved in arid environments as a water-saving alternative to C₃ photosynthesis. There is great interest in engineering more drought-resistant crops by introducing CAM into C₃ plants. However, it is unknown whether full CAM or alternative water-saving modes would be more productive in the environments typically experienced by C₃ crops. To study the effect of temperature and relative humidity on plant metabolism in the context of water saving, we coupled a time-resolved diel (based on a 24-h day-night cycle) model of leaf metabolism to an environment-dependent gas-exchange model. This combined model allowed us to study the emergence of CAM as a trade-off between leaf productivity and water saving. We show that vacuolar storage capacity in the leaf is a major determinant of the extent of CAM. Moreover, our model identified an alternative CAM cycle involving mitochondrial isocitrate dehydrogenase as a potential contributor to initial carbon fixation at night. Simulations across a range of environmental conditions show that the water-saving potential of CAM strongly depends on the daytime weather conditions and that the additional water-saving effect of carbon fixation by isocitrate dehydrogenase can reach 11% total water saving for the conditions tested.     

玉米根、叶质膜透性和叶片水分对土壤干旱胁迫的反应

利用大型活动式防雨旱棚 ,人工控制不同土壤含水量 ,全生育期系统研究了轻度及严重土壤干旱胁迫对夏玉米根系活力、叶片相对含水量、离体叶片保水力和根、叶质膜透性的影响 .结果表明 :土壤干旱胁迫下 ,玉米叶片相对含水量下降、离体叶片保水力降低 ;叶片及根系质膜透性上升 ,并且根的质膜透性比叶片上升快 ,根系活力下降 ;在干旱胁迫下 ,根系、叶片质膜透性与叶片相对含水量呈负相关 ,而根系、叶片质膜透性与离体叶片保水力呈显著正相关 ,根系膜透性与叶片膜透性也呈显著正相关 ,维持根系活力与保持较高的叶片含水量有密切关系 .另外 ,由于严重水分胁迫处理的上述特性和充分供水处理差异显著 ,而轻度胁迫和充分供水不显著 ,因此可以认为轻度水分胁迫 ,即土壤含水量为田间持水量的 6 0± 5 %为夏玉米正常生长发育的下限指标 ,可作为制定节水栽培措施的理论依据 .     

Leaf Water Potential Response to Transpiration by Citrus

This paper reports on further studies of a model for interpreting leaf water potential data for Citrus. Experimental data confirmed the assumption that the ratio of vapor pressure deficit to leaf diffusion resistance adequately estimates transpiration when leaf-to-air temperature differences are small. Data collected diurnally indicated that the relationship between leaf water potential and transpiration followed a sequence of steady states without hysteresis. No difference in water transport characteristics was found for Valencia orange on three rootstocks in well-watered soil, but the two rootstocks Cleopatra mandarin and Rangpur gave slightly greater leaf water stress in Valencia orange leaves than‘Troyer’ citrange rootstock at high transpiration rates under mild soil water deficits. In laboratory studies, previously unstressed seedlings had higher leaf water potentials than field trees at equivalent transpiration rates. After several drying cycles, however, leaf water potentials were similar to those observed in the field.     

Diel Patterns of Water Potential Components for the Crassulacean Acid Metabolism Plant Opuntia ficus-indica when Well-Watered or Droughted

Under well-watered conditions, chlorenchyma acidity in cladodes of Opuntia ficus-indica increased substantially at night, fully accounting for the 0.26-megapascal nocturnal increase in osmotic pressure in the outer 2 millimeters. Osmotic pressure in the inner part of the chlorenchyma and in the water-storage parenchyma did not change significantly over 24-hour periods. Three months of drought decreased nocturnal acid accumulation by 73% and essentially abolished transpiration; also, 27% of the chlorenchyma water and 61% of the parenchyma water was lost during such drought, but the average tissue osmotic pressure was little affected. Turgor pressure was maintained in the chlorenchyma after 3 months of drought, although it decreased sevenfold in the water-storage parenchyma compared with the well-watered condition. Moreover, the nocturnal increases in turgor pressure of about 0.08 megapascal in the outer part of the chlorenchyma was also unchanged by such drought. The water potential magnitudes favored water movement from the parenchyma to the chlorenchyma at the end of the night and in the reverse direction during the late afternoon. Experiments with tritiated water support this pattern of water movement, which is also in agreement with predictions based on electric-circuit analog models for Crassulacean acid metabolism plants.     

Effect of Severe Water Stress on Aspects of Crassulacean Acid Metabolism in Xerosicyos

Xerosicyos danguyi H.Humb. (Cucurbitaceae) is a Crassulacean acid metabolism (CAM) species native to Madagascar. Previously, it was shown that when grown under good water conditions, it is a typical CAM plant, but when water stressed, it shifts to a dampened form of CAM, termed CAM-idling, in which stomata are closed day and night but with a continued, low diurnal organic acid fluctuation. We have now studied the kinetics of some metabolic features of the shift from CAM to CAM-idling under severe water stress and the recovery upon rewatering. When water is withheld, there is a steady decrease in relative water content (RWC), reaching about 50%, at which point the water potential decreases precipitously from about -2 or -3 bars to -12 bars. Abscisic acid (ABA) increases sharply at about 75% RWC. Stomata close, which limits CO2 uptake, and there is a dampened diurnal organic acid fluctuation typical of CAM-idling. Throughout an extended stress period to 50% RWC, there is no change in chlorophyll, protein, and ribulose bisphosphate carboxylase activity compared with the well-watered plants. Despite the fact that the tissue was already in CAM, the stress is accompanied by an increase in phosphoenolpyruvate carboxylase (PEPc) mRNA, extractable PEPc activity, and PEPc protein (such that the specific activity remained approximately constant) and a decrease in the apparent Km(PEP). It is not known if the changes in Km(PEP) in response to drought are related to or are separate from the increases in PEPc protein and mRNA. The changes in Km(PEP) could be in response to the decreased endogenous levels of organic acids, but evidently are not an assay artifact. The increases in PEPc protein and mRNA appear to be related to the water-stress treatment and may result from the increased concentration of ABA or the decreased levels of endogenous organic acids. When rewatered, the metabolism quickly returns to the well-watered control typical of CAM.     

The Relation between Photosynthesis, Respiration, and Crassulacean Acid Metabolism in Leaf Slices of Aloe arborescens Mill

Leaves and leaf slices from Aloe arborescens Mill. were used to study the interrelations between Crassulacean acid metabolism, photosynthesis, and respiration. Oxygen exchange of leaf slices was measured polarographically. It was found that the photosynthetic utilization of stored malic acid resulted in a net evolution of oxygen. This oxygen production, and the decrease in acid content of the leaf tissue, were completely inhibited by amytal, although the rate of respiratory oxygen uptake was hardly affected by the presence of this inhibitor of mitochondrial electron transport. Other poisons of respiration (cyanide) and of the tricarboxylic acid cycle (trifluoroacetate, 2-diethyl malonate) also were effective in preventing acid-dependent oxygen evolution. It is concluded that the mobilization of stored acids during light-dependent deacidification of the leaves depends on the operation of the tricarboxylic acid cycle and of the electron transport of the mitochondria.     

Crassulacean Acid Metabolism in Three Species of Commelinaceae

Diurnal patterns of CO₂ exchange and fluctuations of tissuemalic acid concentrations were investigated in three speciesof Commelinaceae: Callisia fragrans and Tripogandra multiflorafrom Jamaica, and Tradescantia brevifolia from southern Texas.Very low levels of CAM gas exchange were induced by droughtstress in C. fragrans and T. multiflora. In addition, past indicationsof CAM-cycling in the two Jamaican species were confirmed indrought-stressed plants; however, only C. fragrans exhibitedCAM-cycling under well-watered conditions. CAM-cycling underdrought stress was also found in T. brevifolia. This constitutesthe first report of CAM (sensu lato) in the genus Tradescantia.The importance of low-level CAM in these three species is discussedas a potential adaptation to drought.Copyright 1994, 1999 AcademicPress Callisia fragrans, Tradescantia brevifolia, Tripogandra multiflora, Commelinaceae, CO₂ exchange, Crassulacean acid metabolism, CAM-cycling, CAM-idling, drought stress, malic acid fluctuations     

Properties of Leaf NAD-Malic Enzyme from the Inducible Crassulacean Acid Metabolism Species Mesembryanthemum crystallinum

NAD-malic enzyme (NAD-ME) functions to decarboxylate malatein the light in leaves of certain species displaying Crassulaceanacid metabolism (CAM). The properties of NAD-ME in desaltedextracts from the inducible CAM species, Mesembryanthemum crystallinumwere examined. The shapes of the malate saturation curve andthe activity versus pH curve at 10 mM malate were dependenton the presence of the activator CoA. The malate saturationcurve was sigmoidal in the absence of an activator and hyperbolicin the presence of CoA. The pH optimum with 10mM malate andMn²⁺ as cofactor was as low as 6.5 without an activator, andincreased to 7.2 in the presence of CoA. Fumarate activationwas synergistic with CoA above pH 7.2. The enzyme displayedhysteretic behavior under suboptimal assay conditions. Rapid extraction and desalting of the enzyme (<1.5 mim) followedimmediately by assay did not reveal any difference in the propertiesof the enzyme on a day/night basis. It is proposed that diurnalregulation of the enzyme in vivo is mediated by pH and malatelevel without a change in the oligomeric form of the enzyme.The molecular weight of the enzyme was approximately 350,000at pH 6.5 or 7.8. The enzyme obtained from M. crystallinum inthe C₃ mode was very similar to the CAM enzyme except that itdisplayed a lower V_max. ³ Current address: MSU-DOE Plant Research Lab, Michigan StateUniversity, E. Lansing, Michigan, U.S.A. 48824. (Received October 2, 1984; Accepted December 20, 1984)     

Isolation and Oxidative Properties of Intact Mitochondria from the Leaves of Sedum praealtum: A Crassulacean Acid Metabolism Plant

A procedure is described for preparing intact mitochondria from leaves of Sedum praealtum D.C., a plant showing Crassulacean acid metabolism. These mitochondria oxidized malate, pyruvate, α-ketoglutarate, succinate, NADH, NADPH, and isocitrate with good respiratory control and ADP/O ratios better than those observed in mitochondria from other photosynthetic tissues.     

Organic Acid Metabolism of Sedum praealtum

1. The organic acids present are citric, isocitric, and l-malic,with a small residue of unidentified acids.
2. The diurnal variationin acidity is due chiefly to changes,in malic acid, with aparallel fluctuation shown by citric acid.Under these conditionsisocitric acid shows little change.
3. The importance of carbondioxide during acidification is confirmed,and it is shown thatat room temperatures or higher the CO₂produced in respirationis sufficient to produce maximum acidification.At lower temperaturesthe supply of CO₂ limits acid production.
4. In the absence ofoxygen no acidification occurs, but even smallquantities (approx.1 per cent.) are sufficient to cause someacid production.
5. Completebalance-sheets are presented for acids, carbohydrates,CO₂ andoxygen for leaves maintained in the dark at high andlow temperatures.As acids are produced there is a correspondingloss of carbohydrate(chiefly starch). A scheme of reactionsis suggested to explainthe experimental results.

     

Crassulacean Acid Metabolism and Epiphytism Linked to Adaptive Radiations in the Orchidaceae

Species of the large family Orchidaceae display a spectacular array of adaptations and rapid speciations that are linked to several innovative features, including specialized pollination syndromes, colonization of epiphytic habitats, and the presence of Crassulacean acid metabolism (CAM), a water-conserving photosynthetic pathway. To better understand the role of CAM and epiphytism in the evolutionary expansion of tropical orchids, we sampled leaf carbon isotopic composition of 1,103 species native to Panama and Costa Rica, performed character state reconstruction and phylogenetic trait analysis of CAM and epiphytism, and related strong CAM, present in 10% of species surveyed, to climatic variables and the evolution of epiphytism in tropical regions. Altitude was the most important predictor of photosynthetic pathway when all environmental variables were taken into account, with CAM being most prevalent at low altitudes. By creating integrated orchid trees to reconstruct ancestral character states, we found that C₃ photosynthesis is the ancestral state and that CAM has evolved at least 10 independent times with several reversals. A large CAM radiation event within the Epidendroideae, the most species-rich epiphytic clade of any known plant group, is linked to a Tertiary species radiation that originated 65 million years ago. Our study shows that parallel evolution of CAM is present among subfamilies of orchids, and correlated divergence between photosynthetic pathways and epiphytism can be explained by the prevalence of CAM in low-elevation epiphytes and rapid speciation of high-elevation epiphytes in the Neotropics, contributing to the astounding diversity in the Orchidaceae.