Crassulacean acid metabolism in the epiphytic ferns Drymoglossum piloselloides and Pyrrosia longifolia: studies on responses to environmental signals

Abstract The paper reports the results of the comprehensive study of crassulacean acid metabolism in two epiphytic tropical ferns, Drymoglossum piloselloides and Pyrrosia longifolia. The plants were investigated under different light, temperature and water status. It was found that both species are obligate CAM plants. The diurnal acidity rhythm is due to the fluctuation in malic acid concentration, which accounts for the change in titratable acidity. Besides malic acid, shikimate and oxalate are found to be present, but not contributing to the CAM acid rhythm. The diurnal rhythm of malic acid content results in a corresponding rhythm in leaf water relations. Both Φ_Φ and Φ_total, were lowest at the end of the night, i.e. when the level of malic acid was highest. The effects of temperature on CO₂ exchange were inverse to those observed in other CAM plants. In both ferns studied, dark CO₂ fixation increased when the night temperature was increased. Increase in day temperature reduced CO₂ uptake during phase IV and during the following night. The observed responses of the ferns to temperature changes suggest that the in situ environmental conditions are optimal for their CAM performance. In weak light, the plants showed net CO₂ output during the midday deacidification period. Increases in light intensity reduced such CO₂ output. Under drought conditions, the CO₂ exchange in the ferns was reduced to zero within 5–6 d, indicating that the ferns studied are more susceptible to water deficiency than other CAM plants. This could be due to a higher cuticular conductance for water. The results are discussed, in particular, in relation to CAM performance of epiphytes growing in the wet tropics.     

The Effect of Temperature and Light on Gas Exchange and Acid Accumulation in the C3-CAM Plant Clusia minor L

Gas exchange and organic acid accumulation of the C₃-CAM intermediateClusia minor L. were investigated in response to various day/nighttemperatures and two light regimes (low and high PAR). For bothlight levels equal day/night temperatures between 20°C and30°C caused a typical C₃ gas exchange pattern with all CO₂uptake occurring during daylight hours. A day/ night temperatureof 15°C caused a negative CO₂ balance over a 24 h periodfor low-PAR-grown plants while high-PAR-grown plants showeda CAM gas exchange pattern with most CO₂ uptake taking placeduring the dark period. However, there was always a considerablenight-time accumulation of malic acid which increased when thenight-time temperature was lowered and had its maximum (54 mmolm^–2) at day/night temperature of 30/15°C. A significantamount of malic acid accumulation (23 mmol m^–2) in low-PAR-grownplants was observed only at 30/15°C. Recycling of respiratoryCO₂ in terms of malic acid accumulation reached between 2·0and 21·5 mmol m_–2 for high-PAR-grown plants whilethere was no significant recycling for low-PAR-grown plants.Both low and high-PAR-grown plants showed considerable night-timeaccumulation of citric acid. Indeed under several temperatureregimes low-PAR-grown plants showed day/night changes in citricacid levels whereas malic acid levels remained approximatelyconstant or slightly decreased. It is hypothesized that lowand high-PAR-grown plants have different requirements for citrate.In high-PAR-grown plants, the breakdown of citrate preventsphotoinhibition by increasing internal CO₂ levels, whereas inlow-PAR-grown plants the night-time accumulation of citric acidmay function as an energy and carbon saving mechanism. Key words: C. minor, C₃, CAM, citric acid, light intensity     

Correlation between CAM-Cycling and Photosynthetic Gas Exchange in Five Species of Talinum (Portulacaceae)

Photosynthetic gas exchange and malic acid fluctuations were monitored in 69 well-watered plants from five morphologically similar species of Talinum in an investigation of the ecophysiological significance of the Crassulacean acid metabolism (CAM)-cycling mode of photosynthesis. Unlike CAM, atmospheric CO₂ uptake in CAM-cycling occurs exclusively during the day; at night, the stomata are closed and respiratory CO₂ is recaptured to form malic acid. All species showed similar patterns of day-night gas exchange and overnight malic acid accumulation, confirming the presence of CAM-cycling. Species averages for gas exchange parameters and malic acid fluctuation were significantly different such that the species with the highest daytime gas exchange had the lowest malic acid accumulation and vice versa. Also, daytime CO₂ exchange and transpiration were negatively correlated with overnight malic acid fluctuation for all individuals examined together, as well as within one species. This suggests that malic acid may effect reductions in both atmospheric CO₂ uptake and transpiration during the day. No significant correlation between malic acid fluctuation and water-use efficiency was found, although a nonsignificant trend of increasing water-use efficiency with increasing malic acid fluctuation was observed among species averages. This study provides evidence that CO₂ recycling via malic acid is negatively correlated with daytime transpirational water losses in well-watered plants. Thus, CAM-cycling could be important for survival in the thin, frequently desiccated soils of rock outcrops on which these plants occur.     

The Regulation of Citric Acid Accumulation and Carbon Recycling During CAM in Ananas comosus

The carbon balance of shade-grown Ananas comosus was investigatedwith regard to nitrogen supply and responses to high PAR. Netdark CO₂ uptake was reduced from 61.2 to 38.5 mmol CO₂ m^–2in N limited (–N) plants grown under low PAR (60 µmolm^–2 s^–1) and apparent photon yield declined from0.066 to 0.034 (mol 0².mol^–1 photon), although photosyntheticcapacities (measured under 5% CO₂) were similar. Following transferfor 7 d to high PAR (600. µmol m^–2 s^–1), netCO² uptake at night increased by 14% in +N plants, and daytimephotosynthetic capacity was higher, with a maximum value of7.8 µmol m^–2 s^–1. The magnitude of dark CO₂ fixation during CAM was measured asdawn—dusk variations in leaf-sap titratable acidity (H+)and as the proportion of malic and citric acids. The contributionfrom re-fixation of respiratory CO₂ recycling (measured as thedifference between net CO₂ uptake and malic acid accumulation)varied with growth conditions, although it was generally lower(30%) than reported for other bromeliads. Assuming a stoichiometryof 2H+: malate and 3H+: citrate, there was a good agreementbetween titratable protons and enzymatically determined organicacids. The accumulation of citric acid was related to nitrogensupply and PAR regime, increasing from 7.0 mol m–3 (+Nplants) to 18 mol m^–3 (–N plants) when plants weretransferred to high PAR; malate: citrate ratios decreased from13.1 to 2.5 under these conditions. Under the low PAR regime, leaf-sap osmotic pressure increasedat night in proportion to malic acid accumulation. However,following the transfer to high PAR for 7 d, there was a muchgreater depletion of soluble sugars at night which correspondedto a decrease in leaf-sap osmotic pressure. Although a rolefor citric acid in CAM has not been properly defined, it appearsthat the accepted stoichiometry for CAM in terms of gas exchange,titratable acidity, malic acid and osmotic pressure may nothold for plants which accumulate citric acid. Key words: Ananas comosus, CAM, citric acid accumulation, carbon recycling     

Influence of Drought on Mitochondrial Activity, Photosynthesis, Nocturnal Acid Accumulation and Water Relations in the CAM Plants Prenia sladeniana(ME-type) and Crassula lycopodioides(PEPCK-type)

The activity of photosynthesis and mitochondrial respiration,nocturnal organic acid accumulation and water relations wereinvestigated in Prenia sladeniana L. Bol. [malic enzyme (ME)-type]andCrassula lycopodioides Lam. [phosphoenolpyruvate carboxykinase(PEPCK)-type] to compare the physiological responses to waterdeficit in crassulacean acid metabolism (CAM) plants differingin their decarboxylating enzyme systems. Withholding water inhibiteddaytime gas exchange within 2 d while night time CO₂gain andmalic acid accumulation remained relatively unchanged in bothspecies. In P. sladeniana, maximum photochemical efficiency(F_v/F_m) and photosynthetic electron transport declined to nearlythe same degree as CO₂supply was restricted during drought.Despite limited CO₂availability, photosynthetic activity waslargely unaffected in C. lycopodioides, as were mitochondrialproperties. There is no indication of a drought-induced increasein the capability to totally oxidize malate, yielding 4 CO₂, in either species. Nevertheless, the enhanced ratio of malateto glycine oxidation may have increased the in vivo capabilityfor malate oxidation in P. sladeniana. Although pressure potentialwas maintained throughout the experiment in both species, activeosmotic adaptation occurred only inP. sladeniana. The observeddecrease in photosynthetic and mitochondrial activity may haveresulted from the large increase in osmotic concentration inthis species. Copyright 2000 Annals of Botany Company Chlorophyll fluorescence analysis, Crassula lycopodioides Lam., crassulacean acid metabolism, citric acid, gas exchange, malic acid, mitochondria, photosynthetic electron transport, Prenia sladeniana L. Bol., water relations     

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     

Environmental Influences on Carbon Recycling in a Terrestrial CAM Bromeliad, Bromelia humilis Jacq.

The effects of night-time temperature, leaf-to-air vapour pressuredeficit (VPD) and water stress on CO₂ recycling in Bromeliahumilis Jacq. grown under two light and nitrogen regimes wereinvestigated. At night-time temperatures above 30°C, integratednet dark CO₂ uptake was severely reduced and CO₂ for malatesynthesis was mainly derived from dark respiration. At 35°C,up to 84% of the CO₂ liberated by dark respiration was refixedinto malic acid. Below 30 °C only nitrogen deficient plantsshowed significant recycling. No significant differences wereobserved between high and low light grown plants in CO₂ recycling.A doubling of leaf-to-air VPD from 7-46 Pa kPa^–1 to 15.49Pa kPa^–1 resulted in a 2- to 20-fold decrease in leafconductance and about 50 to 65% reduction in integrated darkCO₂ uptake. However, about twice as much CO₂ was recycled atthe higher VPD as in the lower. Ten days of water stress resultedin 80 to 100% recycling of respiratory CO₂. Under high VPD andwater stress treatments, the amount of water potentially savedthrough recycling of CO₂ reached 2- to 6-fold of the actualtranspiration. In general, nitrogen deficient plants had higherper cent recycling of respiratory CO₂ in response to high night-timetemperature, increased VPD or water stress. The results emphasizethe ecological relevance of carbon recycling in CAM plants. Key words: Bromelia humilis, CAM, PPFD, dark respiration, temperature, VPD, water stress     

Induction and Repression of CAM in Sedum telephium L. in Response to Photoperiod and Water Stress

Lee, H. S. J. and Griffiths, H. 1987. Induction and repressionof CAM in Sedurn relephluni L. in response to photopcnod andwater stress.—J. exp. Bot. 38: 834–841. The introduction and repression of CAM in Sedurn telephiunmL, a temperate succulent, was investigated in watered, progressivelydrouglited and rewatered plants in growth chambers. Measurementswere made of water vapour and CO₂ exchange, titratable acidity(TA) and xylem sap tension. Effects of photoperiod were alsostudied. CAM was induced by drought under long or short days,although when watered no CAM activity was expressed. C₃-CAM intermediate plants were used for the investigation ofwater supply. Those which had received water and those drought-stressedboth displayed a similar nocturnal increase in TA, with a day-nightmaximum (H+) of 69 µmol g^–1 fr. wt. The wateredplants took up CO₂ at a maximum rate of 2?2 µmol m^–2s^–1 only in the light period, while the droughted plantsshowed a maximum nocturnal CO₂ uptake rate of 0?69 µmolm^–2 s^–1. Subsequently, as CAM was repressed, thewatered S. telephiwn displayed little variation in TA, withconstant levels at 42 µmol g^–1 fr. wt. (day 10).After 10 d of drought stress, the CAM characteristics of S.telephiurn were aLso affected, with reduced net CO₂ uptake andH+. The transition between C₃ and CAM in S. telephium can be describedas a progression in terms of the proportion of respiratory CO₂which is recycled and refixed at night as malic acid, in comparisonwith net CO₂ uptake. Recycling increased from 20% (day 1) to44% (day 10) as a result of the drought stress and was highin both the CAM-C₃ stage (no net CO2 uptake at night) and alsoin the drought-stressed CAM stage (reduced net CO₂ uptake atnight). The complete C₃-CAM transition occurred in less than8 d, and the stages could be characterized by xylem sap tensionmeasurements: CAM = 0?50 MPa C₃-CAM = 0?36 MPa C₃ = 0?29 MPa. Key words: CAM, Sedum telephium L., recycling     

Diurnal Rhythm and Characteristics of Photosynthesis and Respiration in the Leaf and Root of a Phalaenopsis Plant

A hybrid Phalaenopsis plant was grown hydroponically on a nutrientsolution of Hyponex in the greenhouse. Typical daily patternsof CO₂ fixation, production of malic acid and citric acid andpH of the leaf and root were determined. The rate of primaryfixation of CO₂ in the leaf increased markedly in the evening,remained high until well into the afternoon, then decreasedsharply. The pattern of production of organic acids resultedfrom the fact that the rate of uptake of CO₂ gas was highestat night and fell during the day. A high correlation betweenpH value and level of malic acid in the leaves was observed,and an exponential relationship appeared to exist between theseparameters. These rhythmic sequences were not observed in theroot. The results suggest that metabolism in the leaf had characteristicsof CAM (Crassulacean Acid Metabolism), while the root did not. (Received May 6, 1988; Accepted October 25, 1988)     

Photosynthetic Changes in the Inducible CAM Plant Sedum telephium L. Following the Imposition of Water Stress. 1. General Characteristics

The effects of water stress (drought) on the pattern of photosynthesisin Sedum telephium have been determined. Well-watered plantsexhibit a weak-CAM pattern, with substantial CO₂ fixation inthe day, a low level of CO₂ fixation at night, high daytimestomatal conductance with a lower conductance at night, andno diurnal fluctuation in acid content. Imposition of water-stress causes a switch from weak-CAM toa full-CAM mode of photosynthesis, as indicated by cessationof daytime CO₂ fixation, a marked increase in night-time CO₂fixation, very low daytime stomatal conductance, increased night-timeconductance and significant diurnal fluctuations in acid content. Sedum telephium, CAM, CO₂ fixation, drought, malate, photosynthesis, water stress     

The Response of the C3-CAM Tree, Clusia rosea, to Light and Water Stress: I. GAS EXCHANGE CHARACTERISTICS

Gas exchange measurements were undertaken on 2-year-old plantsof Clusia rosea. The plants were shown to have the ability toswitch from C₃-photosynthesis to CAM and vice versa regardlessof leaf age and, under some conditions, CO₂ was taken up continuously,throughout the day and night. The light response was saturatedby 120 µmol m^–2 s^–1 typical of a shade plant. Gas exchange patterns in response to light, water and VPD wereexamined. All combinations of daytime and night-time CO₂ uptakewere observed, with rates of CO₂ uptake ranging from 2 to 11µmol m^–2 s^–1 depending upon water status andlight. Categorization of this plant asC₃, CAM or an intermediateis impossible. Differing VPD affected the magnitude of changesfrom CAM to C₃-photosynthesis (0 to 0.5 and 0 to 6.0 µmolm^–2 s^–1 CO₂, respectively) when plants were watered.Under well-watered conditions, but not under water stress, highPPFD elicited changes from CAM to C₃ gas exchange. This is unusualnot only for a shade plant but also for a plant with CAM. Itis of ecological importance for C. rosea, which may spend theearly years of its life as an epiphyte or in the forest understorey,to be able to maximize photosynthesis with minimal water loss. Key words: Clusia rosea, CAM, C₃, stress     

The Influence of Nitrogen, Light and Water Stress on CO2 Exchange and Organic Acid Accumulation in the Tropical C3-CAM Tree, Clusia minor

The carbon balance and changes in leaf structure in Clusia minorL., were investigated in controlled conditions with regardto nitrogen supply and responses to low and high photosyntheticallyactive radiation (PAR). Nitrogen deficiency and high PAR ledto the production of smaller leaves with higher specific leafdry weight (SLDW) and higher leaf water content, but with lowerchlorophyll content. Nitrogen and PAR levels at growth alsoaffected CO₂ exchange and leaf area. In – N conditions,total daily net CO₂ uptake and leaf area accumulation were slightlyless for high-PAR-grown plants. In contrast, high-PAR-grownplants supplied with nitrogen showed about a 4-fold higher totaldaily CO₂ uptake and about twice the total leaf area of low-PAR-grownplants. Although total daily net CO₂ uptake of +N plants wasonly slightly higher than –N plants under the low PARlevel, –N plants produced almost three times more leafarea but with lower SLDW. Under well-watered conditions, low-PAR-grownplants showed only CO₂ evolution during the night and malicacid levels decreased. However, there was considerable night-timeaccumulation of titratable protons due to day/night changesin citric acid levels. High-PAR-grown plants showed net CO₂uptake, malate and citrate accumulation during the dark period.However, most of the CO₂ fixed at night probably came from respiratoryCO₂. Positive night-time CO₂ exchange was readily observed forlow-PAR-grown plants when they were transferred to high PARconditions or when they were submitted to water stress. In plantsgrown in high and low PAR, CAM leads to a substantial increasein daily water use efficiency for water-stressed plants, althoughtotal net CO₂ uptake decreased.     

Occurrence of Inducible Crassulacean Acid Metabolism in Leaves of Talimum triangulare (Portulacaceae)

In this paper we report for the first time the occurrence ofan inducible weak CAM in leaves of Talinwn triangulare (Jacq.)Willd. This plant is a terrestrial perennial deciduous herbwith woody stems and succulent leaves which grows under fullexposure and in the shade in northern Venezuela. Plants grownin a greenhouse (‘sun’ plants) and a growth cabinet(‘shade’ plants) with daily irrigation showed CO₂uptake only during the daytime (maximum rate, 4?0 µmolm^–2 s^–1) and a small acid accumulation during thenight (6?0 µmol H⁺g^–1 FW). Twenty-four hours aftercessation of irrigation, no CO₂ exchange was observed duringpart of the night. Dark fixation reached a maximum (1?0 µmolCO₂ m^–2 s^–1, 100 µmol H⁺ g^–1 FW) onday 9 of drought. By day 30 almost no gas exchange was observed,while acid accumulation was still 10 µmol H⁺ g^–1FW. Rewatering reverted the pattern of CO₂ exchange to thatof a C₃ plant within 24 h. Daytime and night-time phosphoenolpyruvatecarboxylase activity increased up to 100% (shade) and 62% (sun)of control values after 10 and 15 d of drought, respectively.Light compensation point and saturating irradiance were similarin well-watered sun and shade plants, values being characteristicof sun plants. CAM seems to be important for the tolerance ofplants of this species to moderately prolonged (up to 2 months)periods of drought in conditions of full exposure as well asshade, and also for regaining high photosynthetic rates shortlyafter irrigation. Key words: Talinum triwigulare, inducible CAM, PEP-C activity, recycling     

Photorespiratory Enzyme Activities in C3 and CAM Forms of the Facultative CAM Plant, Mesembryanthemum crystallimum L.

The extent to which photorespiration occurs in CrassulaceanAcid Metabolism (CAM) plants has received limited attention.No comparative studies of C₃ and CAM development have been made.To address this problem, activities of several photorespiratoryenzymes were measured in a facultative CAM plant—Mesembryanthemumcrystallinum L.—during induction of CAM by water stress(NaCl-treatment). Salt-treatment over a 22 d period produceda progressive change in metabolism from C₃ to CAM. This wasconfirmed by (I) changes in gas exchange from C₃ fixation tothe characteristic CAM pattern of nocturnal CO2 uptake; (2)increases in did acid fluctuation and (3) a 30-fold increasein phosphoenol pyruvate (PEP) carboxylase activity. In contrast,no significant changes were observed in the activities of glycollateoxidase (GO), NAD-dependent-hydroxypyruvate reductase (HPR),glutamine synthetase (GS) or glutamate dehydrogenase (GDH) whenplants were induced into CAM. Ion exchange chromatography onDEAE Sephacel detected only one GS isoenzyme (GS2, chloroplastic)at all stages of CAM induction, Western blot analysis, however,detected an additional, although minor, band of GS1 (cytosolic),in C₃ plants, which disappeared following CAM induction. Ourresults show that, after development of CAM, these plants stillretain the capacity to photorespire. This may be an essentialrequirement of CAM plants growing in habitats with variablerainfall. When water availability is high, stomata may openin the light allowing ribulose bisphosphate carboxylase oxygenase(Rubisco) activity and photorespiration to occur. The inherentcapacity to photorespire would allow plants to survive variableperiods of rainfall. Key words: Mesembryanthemum, C₃, CAM, photorespiration     

Root Water Uptake, Leaf Water Storage and Gas Exchange of a Desert Succulent: Implications for Root System Redundancy

A technique used for hydroponics was adapted to measure instantaneousroot water uptake from the soil for a leaf succulent CAM species,Agave deserti. Comparisons were made to previously modelledwater fluxes for A. deserti and to Encelia farinosa, a non-succulentC₃species. Net CO₂uptake and transpiration forA. deserti underwell-watered conditions occurred primarily at night whereasroot water uptake was relatively constant over 24 h. Leaf thicknessdecreased when transpiration commenced and then increased whenrecharge from the stem and soil occurred, consistent with previousmodels. A drought of 90 d eliminated net CO₂uptake and transpirationand reduced the water content of leaves by 62%. Rewetting theentire root system for 7 d led to a full recovery of leaf waterstorage but only 56% of maximal net CO₂uptake. Root water uptakewas maximal immediately after rewetting, which replenished rootwater content, and decreased to a steady rate by 14 d. Whenonly the distal 50% of the root system was rewetted, the timefor net CO₂uptake and leaf water storage to recover increased,but by 30 d gas exchange and leaf water storage were similarto 100% rewetting. Rewetting 10 or 20% of the root system resultedin much less water uptake; these plants did not recover leafwater storage or gas exchange by 30 d after rewetting. A redundancyin the root system of A. deserti apparently exists for dailywater uptake requirements under wet conditions but the entireroot system is required for rapid recovery from drought.Copyright1999 Annals of Botany Company Agave deserti Engelm., desert, drought, gas exchange, rewetting, roots, succulent, water uptake.     

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.     

Nocturnal Accumulation of Acid in Leaves of Wall Pennywort (Umbilicus rupestris) Following Exposure to Water Stress

Physiological responses to water stress (drought) have beeninvestigated in Umbilicus rupestris (wall pennywort) by comparingcontrol (well-watered) and draughted plants with respect to(i) diurnal fluctuations in the acid content of the leaves,(ii) CO₂ exchange patterns and (iii) stomatal conductance. Controlplants show no diurnal fluctuations in acid content, whereasafter 6 d of drought a clear CAM-type pattern (nocturnal acidificationfollowed by deacidification in the light) is observed. In controlplants, the CO₂ exchange pattern over a 24 h period is of atypical C-3 ‘square-wave’ type, with extensive CO₂uptake in the light and CO₂ output in the dark. In droughtedplants the day-time CO₂ uptake is confined to a morning ‘burst’,whilst night-time CO₂ output is markedly reduced. There is howeverno net noctural uptake of CO₂. In control plants, stomatal conductanceis high during the day (especially in the first half of theday) falling to a low level at the onset of darkness, and thenrising slowly through the remainder of the night. In droughtedplants, stomatal conductance is very low, except that thereis morning ‘burst’ of high conductance and a periodduring the night when conductance is higher than in controlplants. These results are discussed in relation to the response of U.rupestris to drought both in laboratory and in field conditions. Umbilicus rupestris, wall pennywort, CO₂ exchange, Crassulacean acid metabolism, drought, stomatal conductance, water stress     

Long-term effects of a doubled atmospheric CO2 concentration on the CAM species Agave deserti

To examine the effects of a doubled atmospheric CO₂ concentrationand other aspects of global climate change on a common CAM speciesnative to the Sonoran Desert, Agave deserti was grown under370 and 750 µmol CO₂ mol^–1 air and gas exchangewas measured under various environmental conditions. Doublingthe CO₂ concentration increased daily net CO₂ uptake by 49%throughout the 17 months and decreased daily transpiration by24%, leading to a 110% increase in water-use efficiency. Underthe doubled CO₂ concentration, the activity of ribulose-1,5-bisphosphatecarboxylase/oxygenase (Rubisco) was 11% lower, phosphoenolpyruvatecarboxylase was 34% lower, and the activated:total ratio forRubisco was 25% greater than under the current CO₂ concentration.Less leaf epicuticular wax occurred on plants under the doubledCO₂ concentration, which decreased the reflectance of photosyntheticphoton flux (PPF); the chlorophyll content per unit leaf areawas also less. The enhancement of daily net CO₂ uptake by doublingthe CO₂ concentration increased when the PPF was decreased below25 mol m^–2 d^–1 when water was withheld, and whenday/night temperatures were below 17/12 C. More leaves, eachwith a greater surface area, were produced per plant under thedoubled CO₂ concentration. The combination of increased totalleaf surface area and increased daily net CO₂ uptake led toan 88% stimulation of dry mass accumulation under the doubledCO₂ concentration. A rising atmospheric CO₂ concentration, togetherwith accompanying changes in temperature, precipitation, andPPF, should increase growth and productivity of native populationsof A. deserti. Key words: Crassulacean acid metabolism, gas exchange, global climate change, Sonoran Desert     

Changes in Crassulacean Acid Metabolism of Kalanchoe blossfeldiana by Different Nitrogen Sources

Kalanchoe blossfeldiana Poelln. cv. Hikan (a Crassulacean acidmetabolism (CAM) plant) was grown in pots containing soil for6 months and then cultured in nutrient solution containing 10mM nitrate or ammonium as a sole nitrogen source for 2 or 3months, under a long-day (16 h) condition. Plant growth was better in the nitrate medium. Leaves of thenitrate-grown plants showed greater diurnal fluctuations intitratable acidity and malate content than those of the ammonium-grownplants. The diurnal patterns in CO₂ exchange of nitrate-grownplants were basically similar for both groups, but the amountof net CO₂ uptake at night was twice as large in the nitrate-grownplants. The leaves of the nitrate-grown plants had 1.3 to 2.5times higher activities of phosphoenolpyruvate carboxylase (PEPC),phosphofructokinase (PFK) and NAD glycelaldehyde-3-phosphatedehydrogenase (G3PDH). These results indicate that K. blossfeldianagrown in nitrate medium showed more CAM activity than thosein ammonium medium. (Received August 13, 1987; Accepted February 22, 1988)     

Comparative ecophysiology of CAM and C3 bromeliads. II. Field measurements of gas exchange of CAM bromeliads in the humid tropics

Abstract The results described represent the first detailed measurements of gas exchange of epiphytic plants with crassulacean acid metabolism (CAM) in the humid tropics. A portable steady-state CO₂ and H₂O porometer was used to measure net exchange rates of CO₂ and H₂O vapour (J_CO2, J_H2O), leaf temperature (T₁), air temperature (T_A), air relative humidity (RH) and photosynthetically active radiation (PAR) for bromeliads in the field during the dry season in February and March 1983 on the tropical island of Trinidad. Different lengths of tubing (up to 25 m) were used so that the gas exchange could be measured of bromeliads in situ in their epiphytic habitats. Derived parameters such as leaf-air water-vapour-concentration difference (Δ_w), water-vapour conductance of leaves (g) and internal CO₂ partial pressure (pⁱ_CO2) could be calculated. The particular problems of making such measurements in the humid tropics due to high relative humidities and high dew-point temperatures are discussed. The long and often broad, strap-like leaves of bromeliads are well suited for measurements with the steady-state porometer. It is shown that CAM activity varies along the length of individual leaves, and variability between different leaves is also demonstrated. The major phases of CAM, i.e. nocturnal stomalal opening, CO₂ uptake and dark fixation as malic acid (Phase I), daytime stomatal closure and light-dependent assimilation of CO₂ derived from decarboxylation of the malic acid (Phase III), and late-afternoon stomatal opening with direct light-dependent assimilation of atmospheric CO₂ (Phase IV) were all clearly shown by CAM bromeliads in situ. Their expression and magnitude depended on the environmental conditions. An early-morning peak of CO₂ uptake as is characteristic of Phase II of CAM was not detected during the night-day transition. A bromeliad intermediate between C₃ and CAM, Guzmania monostachia, showed substantial net CO₂ uptake in the early morning but no net uptake integrated over the whole of the night.