Differential effects of drought and light levels on accumulation of citric and malic acids during CAM in Clusia

Night-time citrate accumulation has been proposed as a response to stress in CAM plants. To address this hypothesis, gas exchange patterns and nocturnal acid accumulation in three species of Clusia were investigated under controlled conditions with regard to water stress and responses to low and high photosynthetic photon flux density (PPFD). Under high PPFD, leaves of Clusia nocturnally accumulated large amounts of both malic and citric acids. Under low PPFD and well-watered conditions, substantial night-time citrate accumulation persisted, whereas malate accumulation was close to zero. Malate accumulation and night-time CO₂ uptake from the atmosphere declined in all three species during prolonged drought periods, whereas citrate accumulation remained similar or increased. Recycling of respiratory CO₂ was substantial for both well-watered and water-stressed plants. The suggestion that citrate accumulation is energetically more favourable than malate accumulation is not supported if the source of CO₂ for the formation of malate is respiratory CO₂. However, the breakdown of citric acid to pyruvate in the light period releases three molecules of CO₂, while the breakdown of malic acid releases only one CO₂ per pyruvate formed. Thus, citric acid should be more effective than malic acid as a mechanism to increase CO₂ concentration in the mesophyll and may help to prevent photoinhibition. Organic acid accumulation also affected the vacuolar pH, which reached values of 2·6–3·0 at dawn. At these pH values, the transport of 2H⁺/ATP is still feasible, suggesting that it is the divalent form of citrate which is being transported in the vacuoles. Since citrate is a well-known buffer, and Clusia spp. show the largest day-night changes in organic acid levels measured in any CAM plant, it is possible that citrate increases the buffer capacity of the vacuoles. Indeed, malate and titratable acidity levels are positively related to citrate levels. Moreover, Clusia species that show the highest nocturnal accumulation of organic acids are also the ones that show the greatest changes in citric acid levels.     

Is Sedum acre L. a CAM plant?

Summary Sedum acre L. collected from its natural stands south of Darmstadt (Germany) showed ¹³C values typical for C₃ plants. This suggests that in situ at the natural stand CO₂ was fixed mainly via the C₃ mode of photosynthesis rather than via the CAM mode. However, experimental water stress shifts the CO₂ exchange pattern from the C₃ type to CAM type. Simultaneously, a diurnal rhythm of malic acid oscillation, typical for CAM, and increase of PEP-carboxylase and malic enzyme activities developed. Hence, Sedum acre is obviously to be classified as a facultative CAM plant. Because of the temperature characteristics of CO₂ exchange in Sedum acre, in situ CO₂ should be harvested from the atmosphere mainly during the seasons where water stress situations capable of inducing CAM are unlikely to occur.     

One Morphotype, Three Physiotypes: Sympatric Species of Clusia with Obligate C3 Photosynthesis, Obligate CAM and C3-CAM Intermediate Behaviour

Abstract: Among about 150 species of the genus of Clusia, morphologically very similar neotropical shrubs and trees, 20 have been studied ecophysiologically and 39 original publications were screened here for key data on the potential performance of the various species. I have collected data on carbon isotope discrimination (8⁷³C), day-night oscilliations of malate (Amal), citrate (Acitr) and titratable protons (AH), maximum rates of net CO₂ exchange in the dark (Dark J_co2) and in the light (Light J_co2) apparent rate of photosynthetic electron transport at PS II (ETR), effective quantum yield of non-cyclic electron transport at PS II (AF/F'_m) at 1000 umol m-² s-¹ PPFD, and potential quantum yield of PS II of leaves adapted to short periods of darkness at midday (F_v/F_m midday) and after relaxation overnight (F_v/F_m predawn). The 20 species can be arranged from predominant C₃ photosynthesis to principally CAM. There may be only a few bona fide obligatory CAM species in the genus, but it is doubtful if there are obligatory C₃ species. The data were then used to provide comparisons of intrinsic capabilities for Clusia species occurring sympatrically at two sites each, in Brazil and Venezuela. No clear advantage of CAM emerges from these comparisons under the stress of high insolation and low water availability at these sites. Different Clusia species are successful in different ways and with different intrinsic ecophysiological capacities. The conventional expectations in CAM as a drought and light stress adaptation are confounded in Clusia to the extent that on some occasions C₃ photosynthesis seems to be the superior strategy. However, it appears, that C₃/CAM plasticity which is so widespread in the genus, with many species and potentially rapid speciation, allows a particularly wide ecological amplitude.     

Induction of crassulacean acid metabolism in Mesembryanthemum crystallinum increases reproductive success under conditions of drought and salinity stress

Summary Mesembryanthemum crystallinum L., an inducible crassulacean acid metabolism (CAM) plant, was grown for approximately 5 weeks following germination in well-watered, non-saline soil in a controlled-environment chamber. During this time, plants were characterized by C₃ photosynthetic carbon metabolism. After the initial 5 weeks, CAM was induced by a combination of high soil salinity and reduced soil water content. One group of plants was allowed to engage in CAM by being continuously exposed to normal CO₂-containing air (about 350–400 ppm). A second group of plants was deprived of ambient CO₂ each night (12 h dark period) until completion of their life cycle, thereby minimizing potential carbon gain via dark CO₂ fixation. The capacity to express CAM under conditions of drought and salinity stress markedly improved reproductive success: plants kept in normal CO₂-containing air produced about 10 times more seeds than plants kept in CO₂-free air during dark periods. Seeds from plants deprived of ambient CO₂ overnight had more negative ¹³C values than seeds from plants kept in normal air.     

Diurnal Pattern of Transpiration,Water Uptake and Water Budget of Succulents with Different CO2 Fixation Pathways

The water fluxes and the CO₂ exchange of three leaf succulents, Othonna opima, Cotyledon orbiculata and Senecio medley-woodii, with different leaf anatomy, growth form and CO₂ fixation pathways (C₃, CAM) were monitored with a gas exchange cuvette which was combined with a potometric system to quantify water uptake. Measurements, which are primarily valid for plants with a sufficient water supply, were made during 6 to 10 consecutive days under constant experimental conditions. Water uptake for 24 h exceeded water loss by transpiration only for a S, medley-woodii plant with 10 expanding but only 7 mature leaves. In this case the gained water evidently is put into leaf expansion. All other plants showed balanced transpiration and water uptake rates. O. opima and C. orbiculata have a similar life form, similar water storage volumes and the same natural habitat but their diurnal water uptake patterns differ significantly. In the C₃ plant O. opima water uptake increased when the transpiration increased or transpiration rates were higher than uptake rates and vice versa. On the contrary the CAM plant C. orbiculata transpired during the dark period at constant or decreasing rates but showed steadily increasing uptake rates. Senecio medley-woodii- and C. orbiculata are CAM plants with similar diurnal water uptake patterns with its maximum in uptake during or towards the end of the CO₂ dark fixation period. Water uptake of C. orbiculata was at its minimum at the end of the light period despite transpiration being maximal. The results were discussed considering the different CO₂ fixation pathways. In the investigated CAM succulents, C. orbiculata and S. medley-woodii, the CAM influenced water uptake throughout the whole day and not only during the CO₂ dark fixation period.     

Phosotynthesis in hemiepiphytic species of Clusia and Ficus

Summary Hemiepiphytic species in the genera Clusia and Ficus were investigated to study their mode of photosynthetic metabolism when growing under natural conditions. Despite growing sympatrically in many areas and having the same growth habit, some Clusia species show Crassulacean acid metabolism (CAM) whereas all species of Ficus investigated are C₃. This conclusion is based on diurnal CO₂ fixation patterns, diurnal stomatal conductances, diurnal titratable acidity fluctuations, and ¹³C isotope ratios. Clusia minor, growing in the savannas adjacent to Barinas, Venezuela, shows all aspects of Crassulacean acid metabolism (CAM) on the basis of nocturnal gas exchange, stomatal conductance, total titratable acidity, and carbon isotope composition when measured during the dry season (February 1986). During the wet season (June 1986), the plants shifted to C₃-type gas exchange with all CO₂ uptake occurring during the daylight hours. The carbon isotope composition of new growth was-28 to-29 typical of C₃ plants.     

Comparative measurements of chlorophyll a fluorescence, acid accumulation and gas exchange in exposed and shaded plants of Clusia minor L. and Clusia multiflora H. B. K. in the field

 Changes in chlorophyll a fluorescence during the day and diurnal-changes of net CO₂-exchange and organic acid contents were determined in two species of the genus Clusia during the dry season in Venezuela. The investigations included plants of the C₃/CAM intermediate species Clusia minor and the C₃ species C. multiflora growing at exposed and shaded sites. Both species showed a C₃ pattern of net CO₂-exchange at the exposed site. In the shade under extreme drought stress C. minor showed a weak expression of CAM without CO₂-uptake during the afternoon (phase IV of CAM). C. multiflora growing in the shade exhibited a C₃-pattern of net CO₂-exchange and a small but significant nocturnal accumulation of citrate. Shaded plants of C. minor were able to double their light utilisation for electron transport and to reduce non-photochemical quenching during phase III compared to phase II of CAM. Furthermore, increase of electron transport rate through photosystem II in phase III of CAM is correlated to decarboxylation of malate. At the exposed site C. multiflora was less negatively affected by high PPFD than C. minor. This was shown by a lower reduction of potential electron quantum yield (F_v/F_m) and higher light utilisation of electron transport of C. multiflora compared to C. minor. At the exposed site C. minor did not make use of the CAM option to increase light utilisation of electron transport and to reduce non-photochemical quenching as did the plants growing in the shade. Received: 20 March 1996 / Accepted: 24 June 1996     

Seasonal response to drought and rewatering in Portulacaria afra (L.) Jacq.

Summary Gas exchange characteristics of droughted and rewatered Portulacaria afra were studied during the seasonal shift from CAM to C₃ photosynthesis. ¹⁴CO₂ uptake, stomatal conductance, and total titratable acidity were determined for both irrigated and 2, 4, and 7.5 month waterstressed plants from summer 1984 to summer 1985. Irrigated P. afra plants were utilizing the CAM pathway throughout the summer and shifted to C₃ during the winter and spring. Beginning in September, P. afra plants shifted from CAM to CAM-idling after 2 months of water-stress. When water-stress was initiated later in the fall, exogenous CO₂ uptake was still measurable after 4 months of drought. After 7.5 months of stress, exogenous CO₂ uptake was absent. The shift from CAM to CAM-idling or C₃ in the fall and winter was related to when water stress was initiated and not to the duration of the stress. Gas exchange resumed within 24 h of rewatering regardless of the duration of the drought. In the winter and spring, rewatering resulted in a full resumption of daytime CO₂ uptake. Whereas during the summer, rewatering quickly resulted in early morning CO₂ uptake, but nocturnal CO₂ uptake through the CAM pathway was observed after 7 days. Gas exchange measurements, rewatering characteristics, and transpirational water loss support the hypothesis that the C₃ pathway was favored during the winter and spring. The CAM pathway was functional during the summer when potential for water loss was greater. Our investigations indicate that P. afra has a flexible photosynthetic system that can withstand long-term drought and has a rapid response to rewatering.     

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.     

Studies on carbon flow in Crassulacean acid metabolism during the initial light period

In the Crassulacean acid metabolism (CAM) plants Kalanchoë tubiflora and Sedum morganianum a shift in the pathways occurs by which external CO₂ enters the metabolism during the initial light period (phase II of the diurnal CAM cycle). At the beginning of phase II, CO₂ is fixed mainly by the C₄ pathway; during late phase II, however, it is fixed mainly via the C₃ pathway. The C₃ pathway contributes to the phosphoenolpyruvate-carboxylase-mediated CO₂ fixation by the provision of three-carbon skeletons. Since the shift in the carbon-flow pathway is delayed after a CO₂-free night when malic-acid accumulation in the vacuoles is prevented, it is very likely that the amount of malic acid in the vacuole is integrated in the mechanism which controls CAM during the initial light period. A light-on signal at the beginning of phase II is not required to bring about the shifts in the carbon-flow pathways, as is shown by the reaction of plants to a prolonged dark period. A model of carbon flow during phase II is proposed.Abbreviations CAM Crassulacean acid metabolism - PEP-Case phosphoenolpyruvate carboxylase     

Chlorophyll Fluorescence and Organic Acid Oscillations during Transition from CAM to C3-photosynthesis inClusia minor L. (Clusiaceae)

In species of Clusia, switching from C₃-photosynthesis (C₃-PS)to crassulacean acid metabolism (CAM) may be a means of optimizingwater use, plant carbon balance and photon utilization duringperiods of stress. We ask whether, in perennial species of Clusia,the switch from CAM back to C₃-PS is also of ecophysiologicalsignificance. Our objective was to investigate the performanceof C. minor L. during a short-term shift from CAM to C₃-PS.During the transition from CAM to C₃-PS, nocturnal malate andcitrate accumulation decreased whereas CO₂uptake increased duringthe daytime. However, after 7 d, marked nocturnal accumulationof citrate and 24 h CO₂uptake occurred. In contrast to C₃-likephotosynthesis, a pronounced reduction in the effective quantumyield of photosystem II,     

Lateral diffusion of CO<Subscript>2</Subscript> in leaves of the crassulacean acid metabolism plant<Emphasis Type="Italic"> Kalanchoë daigremontiana</Emphasis> Hamet et Perrier

Dynamic patchiness of photosystem II (PSII) activity in leaves of the crassulacean acid metabolism (CAM) plant Kalanchoë daigremontiana Hamet et Perrier, which was independent of stomatal control and was observed during both the day/night cycle and circadian endogenous oscillations of CAM, was previously explained by lateral CO₂ diffusion and CO₂ signalling in the leaves [Rascher et al. (2001) Proc Natl Acad Sci USA 98:11801–11805; Rascher and Lüttge (2002) Plant Biol 4:671–681]. The aim here was to actually demonstrate the importance of lateral CO₂ diffusion and its effects on localized PSII activity. Covering small sections of entire leaves with silicone grease was used for local exclusion of a contribution of atmospheric CO₂ to internal CO₂ via transport through stomata. A setup for combined measurement of gas exchange and chlorophyll fluorescence imaging was used for recording photosynthetic activity with a spatiotemporal resolution. When remobilization of malic acid from vacuolar storage and its decarboxylation in the CAM cycle caused increasing internal CO₂ concentrations sustaining high PSII activity behind closed stomata, PSII activity was also increased in adjacent leaf sections where vacuolar malic acid accumulation was minimal as a result of preventing external CO₂ supply due to leaf-surface greasing, and where therefore CO₂ could only be supplied by diffusion from the neighbouring malic acid-remobilizing leaf tissue. This demonstrates lateral CO₂ diffusion and its effect on local photosynthetic activity.     

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.     

Crassulacean acid metabolism (CAM) in leaves of Aloe arborescens mill

In the succulent leaves of Aloe arborescens Mill diurnal oscillations of the malic acid content, being indicative of Crassulacean Acid Metabolism (CAM), were exhibited only by the green mesophyll. In contrast, the malic acid level of the central chloroplast-free water-storing tissue remained constant throughout the day-night cycle. Apart from malate, the green tissue contained high amounts of isocitrat which was lacking in the water tissue. There was no significant transfer from the green mesophyll to the water tissue of ¹⁴C fixed originally via dark ¹⁴CO₂ fixation in the mesophyll. Both isolated mesophyll and water tissue were capable of dark CO₂ fixation yielding mainly malate as the first stable product. Both tissues have phosphoenolpyruvate carboxylase. However, the enzymes derived from the both sources could be distinguished by their molecular weights and by their kinetic properties, suggesting different phosphoenolpyruvate carboxylase proteins. The conclusion drawn from the experiments is that in a. arborescens the CAM cycle proceeds exclusively in the green mesophyll and that the water tissue, though capable of malate synthesis via -carboxylation of phosphoenolpyruvate, behaves as an independent metabolic system where CAM is lacking. This view is supported by the finding that the cell walls bordering the green mesophyll from the water tissue lack plasmodesmata, hence conveniant pathways of metabolite transport.Abbreviations CAM Crassulacean acid metabolism - PEP phosphoenolpyruvate - PEP-C phosphoenolpyruvate carboxylase     

Stomata of the CAM plant Tillandsia recurvata respond directly to humidity

Summary Under controlled conditions, CO₂ exchange of Tillandsia recurvata showed all characteristics of CAM. During the phase of nocturnal CO₂ fixation stomata of the plant responded sensitively to changes in ambient air humidity. Dry air resulted in an increase, moist air in a decrease of diffusion resistance. The evaporative demand of the air affected the level of stomatal resistance during the entire night period. Due to stomatal closure, the total nocturnal water loss of T. recurvata was less at low than at high humidity. It is concluded that stomata respond directly to humidity and not via bulk tissue water conditions of the leaves. Such control of transpiration may optimize water use efficiency for this almost rootless, extreme epiphyte.     

CARBON UPTAKE CHARACTERISTICS IN TWO HIGH ELEVATION POPULATIONS OF THE AQUATIC CAM PLANT ISOETES BOLANDERI (ISOETACAE)

In the aquatic CAM species Isoetes bolanderi, a resident of high elevation lakes in which ambient carbon levels are low and fairly constant throughout the day, CO₂ uptake by the leaves generally parallels changes in photosynthetic photon flux density as opposed to changes in ambient CO₂ levels. Overnight CO₂ uptake by the leaves of I. bolanderi contributed up to 33% of the total daily carbon assimilation from the water column which is typical of aquatic CAM species but low in comparison to the 30–50% seen in the seasonal pool congener Isoetes howellii. Two additional sources of carbon may supplement the 24-hour period for carbon assimilation conferred by CAM in I. bolanderi: the refixation of respiratory CO₂ and carbon acquisition from the interstitial sediment water via roots. CAM appears to play a significant role in enhancing carbon gain in I. bolanderi which dominates the littoral flora of Siesta and Ellery Lakes despite higher carbon uptake rates from the water column found in one associated macrophyte Fontinalis antipyretica.     

Patterns of gas exchange and organic acid oscillations in tropical trees of the genus Clusia

Summary Gas exchange patterns and nocturnal acid accumulation were examined in four species of Clusia under simulated field conditions in the laboratory. Clusia alata and C. major had midday stomatal closure, substantial net CO₂ exchange ( ) during the night, and the highest water use efficiency (WUE). C. venosa showed a pattern similar to a C₃ plant, with nighttime stomatal closure, while C. minor maintained positive continuously throughout a 24-h period. However, large changes in titratable acidity, which closely matched changes in citrate and malate levels, indicated that Crassulacean acid metabolism (CAM) is active in all four species. C. venosa showed dawn-dusk oscillations in titratable acidity that were higher than the values reported for other C₃-CAM intermediates, while the nighttime acid accumulation of 998 mol m^–3 observed in C. major is unsurpassed by any other CAM plant. Moreover, the dawn-dusk changes in citrate levels of over 65 mol m^–3 in C. alata and C. minor, and over 120 mol m^–3 in C. major, are 3–6 times higher than values reported for other CAM plants. Although these oscillations in citrate levels were quite large, and the nighttime dark respiration rates were high, the O₂ budget analysis suggestes that only part of the reducing power generated by the synthesis of citric acid enters the respiratory chain. Dawn-dusk changes in malate levels were just over 50 mol m^–3 for C. venosa but over 300 mol m^–3 for C. major. Between 28% (C. major) and 89% (C. venosa) of the malate accumulated during the night was derived from recycled respiratory CO₂. These daily changes in malate and citrate levels also contributed significantly to changes in leaf sap osmolality. This variability in CO₂ uptake patterns, the recycling of nighttime respiratory CO₂, and the high WUE may have contributed to the successful invasion of Clusia into a wide range of habitats in the tropics.     

Responses of succulents to plant water stress

Experiments were performed to test the hypothesis that succulents “shift” their method of photosynthetic metabolism in response to environmental change. Our data showed that there were at least three different responses of succulents to plant water status. When plant water status of Portulacaria afra (L.) Jacq. was lowered either by withholding water or by irrigating with 2% NaCl, a change from C₃-photosynthesis to Crassulacean acid metabolism (CAM) occurred. Fluctuation of titratable acidity and nocturnal CO₂ uptake was induced in the stressed plants. Stressed Peperomia obtusifolia A. Dietr. plants showed a change from C₃-photosynthesis to internal cycling of CO₂. Acid fluctuation commenced in response to stress but exogenous CO₂ uptake did not occur. Zygocactus truncatus Haworth plants showed a pattern of acid fluctuation and nocturnal CO₂ uptake typical of CAM even when well irrigated. The cacti converted from CAM to an internal CO₂ cycle similar to Peperomia when plants were water-stressed. Reverse phase gas exchange in succulents results in low water loss to carbon gain. Water is conserved and low levels of metabolic activity are maintained during drought periods by complete stomatal closure and continual fluctuation of organic acids.     

Crassulacean acid metabolism and fitness under water deficit stress: if not for carbon gain,what is facultative CAM good for?

Background

In obligate Crassulacean acid metabolism (CAM), up to 99 % of CO₂ assimilation occurs during the night, therefore supporting the hypothesis that CAM is adaptive because it allows CO₂ fixation during the part of the day with lower evaporative demand, making life in water-limited environments possible. By comparison, in facultative CAM (inducible CAM, C₃-CAM) and CAM-cycling plants drought-induced dark CO₂ fixation may only be, with few exceptions, a small proportion of C₃ CO₂ assimilation in watered plants and occur during a few days. From the viewpoint of survival the adaptive advantages, i.e. increased fitness, of facultative CAM and CAM-cycling are not obvious. Therefore, it is hypothesized that, if it is to increase fitness, CAM must aid in reproduction.

Scope

An examination of published reports of 23 facultative CAM and CAM-cycling species finds that, in 19 species, drought-induced dark CO₂ fixation represents on average 11 % of C₃ CO₂ assimilation of watered plants. Evidence is discussed on the impact of the operation of CAM in facultative and CAM-cycling plants on their survival – carbon balance, water conservation, water absorption, photo-protection of the photosynthetic apparatus – and reproductive effort. It is concluded that in some species, but not all, facultative and cycling CAM contribute, rather than to increase carbon balance, to increase water-use efficiency, water absorption, prevention of photoinhibition and reproductive output.Key words: Facultative CAM, CAM-cycling, water, crassulacean acid metabolism, deficit     

Crassulacean acid metabolism (CAM) in Kalanchoë: Changes in intercellular CO2 concentration during a normal CAM cycle and during cycles in continuous light or darkness

In the crassulacean acid metabolism (CAM) plant Kalanchoë daigremontiana, the internal CO₂ concentrations were measured throughout CAM cycles by gas chromatography. Under normal dark-light cycles, the internal CO₂ concentration was near that of the ambient air and increased up to 0.5% during the phase of maximum malate decarboxylation. A sharp increase in internal CO₂ concentration occurring after the first 12 h of the cycle was exhibited by the plants both when there was a normal day-night cycle and when the night was replaced by illumination, and also when the light period was replaced by darkness. Thus, the increase in internal CO₂ in the morning does not appear to be primarily determined by a light-on signal or by alterations of temperature rather than by inherent factors of the leaves. This view is supported further by a steep increase in ¹⁴CO₂ production from labeted malate occurring during extended darkness at a time when the light period would normally begin. The results are discussed in particular in relation to of how CAM can control stomata movement.Abbreviation CAM Crassulacean acid metabolism