Crassulacean Acid Metabolism in the Succulent C(4) Dicot, Portulaca oleracea L Under Natural Environmental Conditions

Crassulacean acid metabolism (CAM) was examined under natural environmental conditions in the succulent C₄ dicot Portulaca oleracea L. Two groups of plants were monitored; one was watered daily (well watered), while the other received water once every 3 to 4 weeks to produce a ψ of −8 bars (drought stressed). Gas exchange, transpiration rate, and titratable acidity were measured for 24-hour periods during the growing season. CAM activity was greatest in drought-stressed plants during late August which had 13 hour days and day/night temperatures of 35/15°C. Under these conditions net CO₂ uptake occurred slowly throughout the night. Diurnal fluctuations of titratable acidity took place in both leaves and stems with amplitudes of 17 and 47 microequivalents per gram fresh weight, respectively. Transpiration data indicated greater opening of stomata during the night than the day. CAM was less pronounced in drought-stressed P. oleracea plants in July and September; neither dark CO₂ uptake nor positive carbon balance occurred during the July measurements. In contrast, well-watered plants appeared to rely on C₄ photosynthesis throughout the season, although some acid fluctuations occurred in stems of these plants during September.     

Seasonal Shifts of Photosynthesis in Portulacaria afra (L.) Jacq

Portulacaria afra (L.) Jacq., a perennial facultative Crassulacean acid metabolism (CAM) species, was studied under natural photoperiods and temperatures in San Diego, California. The plants were irrigated every fourth day throughout the study period. Measurements of ¹⁴CO₂ uptake, stomatal resistance, and titratable acidity were made periodically from July 1981 through May 1982. P. afra maintained C₃ photosynthesis during the winter and the spring. Diurnal acid fluctuations were low and maximal ¹⁴CO₂ uptake occurred during the day. The day/night ratio of carbon uptake varied from 5 to 10 and indicated little nocturnal CO₂ uptake. CAM photosynthesis occurred during the summer and a mixture of both C₃ and CAM during the fall. Large acid fluctuations of 100 to 200 microequivalents per gram fresh weight were observed and maximal ¹⁴CO₂ uptake shifted to the late night and early morning hours. Daytime stomatal closure was evident. A reduction in the day/night ratio of carbon uptake to 2 indicated a significant contribution of nocturnal CO₂ uptake to the overall carbon gain of the plant. The seasonal shift from C₃ to CAM was facilitated by increasing daytime temperature and accompanied by reduced daytime CO₂ uptake despite irrigation.     

Recycling of CO2 during induction of CAM by drought in Talinum paniculatum (Portulacaceae)

To investigate the possible induction of Crassulacean acid metabolism (CAM) by drought in Talinum paniculatum ([Jacq.] Gaertn.), a deciduous herb with succulent leaves and lignified stems, nocturnal acid accumulation and CO₂-exchange were studied in watered and droughted greenhouse-grown plants. Watered plants had a typical C3 pattern of CO₂-exchange. When plants were subjected to drought, nocturnal acid accumulation increased significantly from 0.9 to 13.4 μmol H⁺ cm^?2 after 21 days. Water deficit provoked a rapid reduction of daytime CO₂ assimilation of as much as 92% and a slower increase in night-time fixation. A maximum of 24% of the diel carbon gain was contributed by dark fixation in droughted plants. After 34 days of drought, only CO₂ compensation and a small accumulation of acid (idling) was detected during the night. Relative recycling of respiratory CO₂ was approximately 100% for most of the water deficit treatment, the amount of CO₂ recycled showing a high positive correlation with nocturnal acid accumulation. A low rate of nocturnal loss of CO₂ in watered plants did not explain the amount recycled nightly in droughted plants, implying that respiration increased with drought. Leaf lamina area was reduced by 49% during drought due to rolling. Leaf biomass remained unchanged during the water-deficit treatment. Neither apparent quantum yield nor light-saturated photosynthetic rate differed significantly between control and 14-day water-stressed plants rewatered for 20 h. Chlorophyll content did not change with drought. These results confirm that CAM is induced by drought in T. paniculatum; the carbon acquired through this pathway only contributes to maintain, but not to increase, leaf biomass; also, CAM is responsible for a high recycling of respiratory CO₂ during the night. Recycling through CAM, plus the reduction of exposed leaf area during drought, may help explain the maintenance of chlorophyll, quantum yield and saturated photosynthetic rates in water-stressed plants of T. paniculatum.     

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.     

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 C₃ and C₄ 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 CO₂ and O₂ 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 CO₂. Green CAM tissues increased their specific weight at night in contrast to control C₃ and C₄ 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 CO₂ 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.

     

Influence of Photoperiod and Leaf Age on Crassulacean Acid Metabolism in Portulacaria afra (L.) Jacq

The possibility that Crassulacean acid metabolism (CAM) is subject to long day photoperiodic control in Portulacaria afra (L.) Jacq., a facultative CAM plant, was studied. Periodic measurements of ¹⁴CO₂ uptake, stomatal resistance, and titratable acidity were made on plants exposed to long and short day photoperiods. Results indicates that waterstressed P. afra had primarily nocturnal CO₂ uptake, daytime stomatal closure, and a large diurnal acid fluctuation in either photoperiod. Mature leaf tissue from nonstressed plants under long days exhibited a moderate diurnal acid fluctuation and midday stomatal closure. Under short days, there was a reduced diurnal acid fluctuation in mature leaf tissue. Young leaf tissue taken from nonstressed plants did not utilize the CAM pathway under either photoperiod as indicated by daytime CO₂ uptake, lack of diurnal acid fluctuation, and incomplete daytime stomatal closure.

The induction of CAM in P. afra appears to be related to the water status of the plant and the age of the leaf tissue. The photosynthetic metabolism of mature leaves may be partly under the control of water stress and of photoperiod, where CAM is favored under long days.

     

Developmental Control of CAM in Peperomia scandens

Experiments were conducted to examine the development of photosynthetic carbon metabolism in Peperomia scandens, a tropical epiphyte. Leaves were sampled during a 10-day period when they were between 30 to 165 days old. P. scandens exhibits a C₃ to CAM-cycling to CAM shift during maturation with the magnitude of CAM increasing with age. Initially, during both day and night, no significant CO₂ uptake or diurnal acid flux was evident. C₃ gas exchange was detected at 41 days of age with a gradual shift towards CAM gas exchange maximized thereafter. An acidity flux of 130 to 150 microequivalents per gram fresh weight was evident by 41 days. Between 40 and 90 days, the leaves shifted their CO₂ uptake pattern from a daytime to a nighttime peak. After 90 days, the leaves remained in CAM. The δ¹³C values became progressively less negative as the leaves matured. In the 30-day-old leaves, the δ¹³C value was −21.1% while in the 165-day-old leaves the δ¹³C value was −18.3%. The time-dependent shift from C₃ to CAM-cycling to CAM in P. scandens does not appear to result from changes in water, light, or temperature regimes since these variables were constant for all leaves sampled.     

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 CO₂ 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; C₄ photosynthesis in two different types of cells was discovered from 1965 to ∼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 CO₂ 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 C₃ and C₄ photosynthesis, carbon biochemistry, cellular anatomy, and ecological physiology. Therefore by ∼1980, CAM photosynthesis finally was rigorously outlined. In a nutshell, 24-hour CAM occurs by phosphoenol pyruvate (PEP) carboxylase fixing CO₂(HCO₃ ⁻) 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 CO₂ for C₃ photosynthesis via ribulose bisphosphate carboxylase oxygenase (Rubisco). The CO₂ 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 C₃ 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 H₂O and CO₂, i.e., CAM tenaciously protects its photosynthesis from both H₂O and CO₂ stresses. This revised version was published online in August 2006 with corrections to the Cover Date.     

Daily Changes in CO(2) and Water Vapor Exchange, Chlorophyll Fluorescence, and Leaf Water Relations in the Halophyte Mesembryanthemum crystallinum during the Induction of Crassulacean Acid Metabolism in Response to High NaCl Salinity

Simultaneous measurements of net CO₂ exchange, water vapor exchange, and leaf water relations were performed in Mesembryanthemum crystallinum during the development of crassulacean acid metabolism (CAM) in response to high NaCl salinity in the rooting medium. Determinations of chlorophyll a fluorescence were used to estimate relative changes in electron transport rate. Alterations in leaf mass per unit area, which—on a short-term basis—largely reflect changes in water content, were recorded continuously with a beta-gauge. Turgor pressure of mesophyll cells was determined with a pressure probe. As reported previously (K Winter, DJ von Willert [1972] Z Pflanzenphysiol 67: 166-170), recently expanded leaves of plants grown under nonsaline conditions showed gas-exchange characteristics of a C₃ plant. Although these plants were not exposed to any particular stress treatment, water content and turgor pressure regularly decreased toward the end of the 12 hour light periods and recovered during the following 12 hours of darkness. When the NaCl concentration of the rooting medium was raised to 400 millimolar, in increments of 100 millimolar given at the onset of the photoperiods for 4 consecutive days, leaf water content and turgor pressure decreased by as much as 30 and 60%, respectively, during the course of the photoperiods. These transient decreases probably triggered the induction of the biochemical machinery which is required for CAM to operate. After several days at 400 millimolar NaCl, when leaves showed features typical of CAM, overall turgor pressure and leaf mass per unit area had increased above the levels before onset of the salt treatment, and diurnal alterations in leaf water content were reduced. Net carbon gain during photoperiods and average intercellular CO₂ partial pressures at which net CO₂ uptake occurred, progressively decreased upon salinization. Reversible diurnal depressions in leaf conductance and net CO₂ uptake, with minima recorded in the middle of the photoperiods, preceded the occurrence of nocturnal net CO₂ uptake. During these reductions, intercellular CO₂ partial pressure and rates of photosynthetic electron transport decreased. With advancing age, leaves of plants grown under nonsaline conditions exhibited progressively greater diurnal reductions in turgor pressure and developed a low degree of CAM activity.     

Deciphering the mechanisms involved in Portulaca oleracea (C4) response to drought: metabolic changes including crassulacean acid‐like metabolism induction and reversal upon re‐watering

Portulaca oleracea is a C₄ plant; however, under drought it can change its carbon fixation metabolism into a crassulacean acid metabolism (CAM)‐like one. While the C₃‐CAM shift is well known, the C₄‐CAM transition has only been described in Portulaca. Here, a CAM‐like metabolism was induced in P. oleracea by drought and then reversed by re‐watering. Physiological and biochemical approaches were undertaken to evaluate the drought and recovery responses. In CAM‐like plants, chlorophyll fluorescence parameters were transitory affected and non‐radiative energy dissipation mechanisms were induced. Induction of flavonoids, betalains and antioxidant machinery may be involved in photosynthetic machinery protection. Metabolic analysis highlights a clear metabolic shift, when a CAM‐like metabolism is induced and then reversed. Increases in nitrogenous compounds like free amino acids and urea, and of pinitol could contribute to withstand drought. Reciprocal variations in arginase and urease in drought‐stressed and in re‐watered plants suggest urea synthesis is strictly regulated. Recovery of C₄ metabolism was accounted by CO₂ assimilation pattern and malate levels. Increases in glycerol and in polyamines would be of importance of re‐watered plants. Collectively, in P. oleracea multiple strategies, from induction of several metabolites to the transitory development of a CAM‐like metabolism, participate to enhance its adaptation to drought.     

水分胁迫下露花光合型的转变

CAM植物按其对环境的反应可分为两种类型:即专一CAM植物和兼性CAM植物。前者不易受外界环境的变化而改变其CAM性质;后者的光合型可随季节和水分胁迫等而发生变化,也可因人工诱导作用而使其由C_3型转变为CAM型。     

Some Effects of Abscisic Acid and Water Stress on Stomata of Vicia faba L.

Vicia faba seedlings grown under a plastic tent in the laboratorywere either watered well throughout their growth period or weresubjected to a water stress treatment for several days priorto an experimental treatment. The effects of a further waterstress treatment or an application of an aqueous solution ofabscisic acid (ABA) on the stomata of these plants were determined.Stomata of previously water-stressed plants proved to be moresensitive than stomata of well watered plants to ABA appliedthrough the petiole via the transpiration stream and sprayedonto leaf surfaces. Stomata of previously water-stressed plantsclosed more rapidly and to a greater degree than stomata ofwell watered plants. The hormone had only a small effect whenapplied directly to epidermal fragments removed from both groupsof plants. Stomata of plants which had received a water stresspretreatment were less sensitive to a subsequent period of waterstress than were stomata of previously well watered plants.It is proposed that stomatal adaptation to water stress maybe related to changes in the hormonal balance of the plant.     

Effects of Various Levels of CO(2) on the Induction of Crassulacean Acid Metabolism in Portulacaria afra (L.) Jacq

In response to water stress, Portulacaria afra (L.) Jacq. (Portulacaceae) shifts its photosynthetic carbon metabolism from the Calvin-Benson cycle for CO₂ fixation (C₃) photosynthesis or Crassulacean acid metabolism (CAM)-cycling, during which organic acids fluctuate with a C₃-type of gas exchange, to CAM. During the CAM induction, various attributes of CAM appear, such as stomatal closure during the day, increase in diurnal fluctuation of organic acids, and an increase in phosphoenolpyruvate carboxylase activity. It was hypothesized that stomatal closure due to water stress may induce changes in internal CO₂ concentration and that these changes in CO₂ could be a factor in CAM induction. Experiments were conducted to test this hypothesis. Well-watered plants and plants from which water was withheld starting at the beginning of the experiment were subjected to low (40 ppm), normal (ca. 330 ppm), and high (950 ppm) CO₂ during the day with normal concentrations of CO₂ during the night for 16 days. In water-stressed and in well-watered plants, CAM induction as ascertained by fluctuation of total titratable acidity, fluctuation of malic acid, stomatal conductance, CO₂ uptake, and phosphoenolpyruvate carboxylase activity, remained unaffected by low, normal, or high CO₂ treatments. In well-watered plants, however, both low and high ambient concentrations of CO₂ tended to reduce organic acid concentrations, low concentrations of CO₂ reducing the organic acids more than high CO₂. It was concluded that exposing the plants to the CO₂ concentrations mentioned had no effect on inducing or reducing the induction of CAM and that the effect of water stress on CAM induction is probably mediated by its effects on biochemical components of leaf metabolism.     

Characterization and Partial Purification of Aldose-6-phosphate Reductase (Alditol-6-Phosphate:NADP 1-Oxidoreductase) from Apple Leaves

The Pereskia are morphologically primitive, leafed members of the Cactaceae. Gas exchange characteristics using a dual isotope porometer to monitor ¹⁴CO₂ and tritiated water uptake, diurnal malic acid fluctuations, phosphoenolpyruvate carboxylase, and malate dehydrogenase activities were examined in two species of the genus Pereskia, Pereskia grandifolia and Pereskia aculeata. Investigations were done on well watered (control) and water-stressed plants. Nonstressed plants showed a CO₂ uptake pattern indicating C₃ carbon metabolism. However, diurnal fluctuations in titratable acidity were observed similar to Crassulacean acid metabolism. Plants exposed to 10 days of water stress exhibited stomatal opening only during an early morning period. Titratable acidity, phosphoenolpyruvate carboxylase activity, and malate dehydrogenase activity fluctuations were magnified in the stressed plants, but showed the same diurnal pattern as controls. Water stress causes these cacti to shift to an internal CO₂ recycling (“idling”) that has all attributes of Crassulacean acid metabolism except nocturnal stomata opening and CO₂ uptake. The consequences of this shift, which has been observed in other succulents, are unknown, and some possibilities are suggested.     

CO2 exchange of CAM exhibiting suceulents in the southern Namib desert in relation to microclimate and water stress

The responses of CO₂ exchange and overnight malate accumulation of leaf and stem succulent CAM-plants to water stress and the particular climatic conditiens of fog and föhn in the southern Namib desert have been investigated. In most of the investigated CAM plants a long term water stress gradually attenuated any uptake of external CO₂ and led to CO₂ release throughout day and night. No CAM-idling was observed. Rainfall or irrigation immediately restored daytime CO₂ uptake while the recovery of the noctural CO₂ uptake was delayed. Dawn peak of photosynthesis was only found in well watered plants but was markedly reduced by the short term water stress of a föhn-storm. Morning fog with its higher diffuse light intensity compared with clear days increased photosynthetic CO₂ uptake considerably. Even in well watered plants noctural CO₂ uptake and malate accumulation were strongly affected by föhn indicating that the water vapour pressure deficit during the night determines the degree of acidification.     

Proserpinaca: photoperiodic and chemical differentiation of leaf development and flowering

Proserpinaca palustris L. produced juvenile leaves on 8-hour photoperiods, adult leaves on 12-hour photoperiods, and adult leaves and flowers on 14-hour photoperiods. Treatment of plants growing on 8- and 14-hour photoperiods with gibberellic acid caused stem elongation and inhibited flowering. The treated plants on 8-hour photoperiods produced adult-like leaves.     

Crassulacean acid metabolism in the Basellaceae (Caryophyllales)

• C₄ and crassulacean acid metabolism (CAM) have evolved in the order Caryophyllales many times but neither C₄ nor CAM have been recorded for the Basellaceae, a small family in the CAM‐rich sub‐order Portulacineae.
• 24 h gas exchange and day–night changes in titratable acidity were measured in leaves of Anredera baselloides exposed to wet–dry–wet cycles.
• While net CO₂ uptake was restricted to the light period in well‐watered plants, net CO₂ fixation in the dark, accompanied by significant nocturnal increases in leaf acidity, developed in droughted plants. Plants reverted to solely C₃ photosynthesis upon rewatering.
• The reversible induction of nocturnal net CO₂ uptake by drought stress indicates that this species is able to exhibit CAM in a facultative manner. This is the first report of CAM in a member of the Basellaceae.

     

Effects of abscisic acid on CAM in Portulacaria afra

Water stress induces Crassulacean acid metabolism (CAM) in Portulacaria afra as manifested by day stomatal closure, organic acid fluctuation, and night CO₂ uptake. We now have evidence that abscisic acid treatment of leaves causes partial stomatal closure that is accompanied by the induction of CAM in a manner similar to water stress. There appears to be an inverse relationship between exogenous CO₂ uptake and decarboxylation of organic acids in that organic acids remain high during the day providing stomata are open. When stomata close, there is consumption of organic acids by decarboxylation. The hypothesis is that stomatal opening controls CAM in this species.This material is based upon work supported by the Science and Education Administration of the USDA under Competitive Grant No. 5901-0410-8-0018-0.     

CO2 exchange of CAM exhibiting succulents in the southern Namib desert in relation to microclimate and water stress

The responses of CO₂ exchange and overnight malate accumulation of leaf and stem succulent CAM-plants to water stress and the particular climatic conditions of fog and föhn in the southern Namib desert have been investigated. In most of the investigated CAM plants a long term water stress gradually attenuated any uptake of external CO₂ and led to CO₂ release throughout day and night. No CAM-idling was observed. Rainfall or irrigation immediately restored daytime CO₂ uptake while the recovery of the nocturnal CO₂ uptake was delayed. Dawn peak of photosynthesis was only found in well watered plants but was markedly reduced by the short term water stress of a föhn-storm. Morning fog with its higher diffuse light intensity compared with clear days increased photosynthetic CO₂ uptake considerably. Even in well watered plants nocturnal CO₂ uptake and malate accumulation were strongly affected by föhn indicating that the water vapour pressure deficit during the night determines the degree of acidification.     

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.