CAM variations in the leaf-succulent Delosperma tradescantioides (Mesembryanthemaceae), native to southern Africa

Drought responses of diurnal gas exchange, malic acid accumulation and water status were examined in Delosperma tradescantioides , a succulent that grows in drought-prone microenvironments in summer rainfall and all-year rainfall regions of southern Africa. When well-watered, this species exhibited Crassulacean acid metabolism (CAM)-cycling, but its carbon fixation pattern changed during the development of drought, shifting to either low-level CAM or to CAM-idling. The rate and pattern of this change depended on environmental conditions, duration of water stress and leaf age. At the onset of drought, diurnal malate fluctuation increased, but was strongly depressed (by ca 70%) as drought continued, and when leaf water content and water potential were low (ca 35 and 50% of the initial levels, respectively). When rewatered, rates of growth and photosynthesis, gas exchange and water status recovered fully to pre-stressed values within two days. Whole-shoot carbon uptake rates suggested that leaf growth had continued unabated during a short-term (≅ one week) drought. This emphasises that CAM-idling allows the maintenance of active metabolism with negligible gas exchange when soil water is limiting. It is possible that old or senescent leaves may provide water for the expansion of developing leaves during initial periods of drought. Regardless of the water regime and environmental conditions, leaf nocturnal malate accumulation and water content were positively correlated and increased with leaf age. Thus the gradual loss of water from older mature leaves may induce CAM-idling, which reduces water loss. An important ecological consequence of this combination of CAM modes is the potential to switch rapidly between fast growth via C₃ gas exchanges when well-watered to water-conserving CAM-idling during drought.     

Shifts in the Carbon Metabolism of Xerosicyos danguyi H. Humb. (Cucurbitaceae) Brought About by Water Stress : II. Enzymology

Xerosicyos danguyi Humbert (Cucurbitaceae) is a leaf succulent endemic to Madagascar. Under well-watered conditions, the plant exhibited Crassulacean acid metabolism (CAM) but shifted to a dampened form of CAM, CAM-idling, when subjected to water stress. The purpose of this investigation was to examine the effects of a shift in carbon metabolism on phosphoenolpyruvate carboxylase and on NADP-malic enzyme in X. danguyi. Experiments were conducted to determine the diurnal patterns of enzyme activity and pH optima of both enzymes, as well as the approximate molecular mass, kinetic patterns, malate inhibition, and glucose-6-phosphate stimulation of phosphoenolpyruvate carboxylase. The two enzymes extracted from well-watered and water-stressed plants were similar in most parameters investigated; thus, CAM-idling appeared to be only a dampened form of CAM photosynthesis.     

CAM-idling in Hoya carnosa (Asclepiadaceae)

In the leaf sueculent Asclepial Hoya carnosa (L.) R. Br., CAM photosynthesis occurred under well-watered conditions, as characterized by diurnal gas exchange and changes in titratable acidity. Following 10–12 days of severe water stress, the plants shifted from CAM to a modified CAM-idling mode of metabolism. CAM-idling was characterized by complete or almost complete stomatal closure accompanied by CAM-like diurnal changes in titratable acidity. H. carnosa plants maintained this CAM-idling mode of photosynthesis for at least 8 weeks. Upon reirrigation, the plants returned to the original CAM mode within 1 week. These results suggested that CAM-idling is a reversible, intermediate form of sustained metabolism which enables plant survival under conditions of extended drought.This work was supported in part by NSF Grant PCM 8200366 and in part by the Science and Education Administration of the United States Department of Agriculture under Competitive Grant 5901-0420-8-0018-0.     

CAM-idling in Hoya carnosa (Asclepiadaceae)

In the leaf succulent Asclepiad Hoya carnosa (L.) R. Br., CAM photosynthesis occurred under well-watered conditions, as characterized by diurnal gas exchange and changes in titratable acidity. Following 10–12 days of severe water stress, the plants shifted from CAM to a modified CAM-idling mode of metabolism. CAM-idling was characterized by complete or almost complete stomatal closure accompanied by CAM-like diurnal changes in titratable acidity. H. carnosa plants maintained this CAM-idling mode of photosynthesis for at least 8 weeks. Upon reirrigation, the plants returned to the original CAM mode within 1 week. These results suggested that CAM-idling is a reversible, intermediate form of sustained metabolism which enables plant survival under conditions of extended drought.This work was supported in part by NSF Grant PCM 8200366 and in part by the Science and Education Administration of the United States Department of Agriculture under Competitive Grant 5901-0420-8-0018-0.     

Effects of water stress on gas exchange and water relations of a succulent epiphyte,Kalanchoë uniflora

Summary Measurements of gas exchange, xylem tension and nocturnal malate synthesis were conducted with well-watered and droughted plants of Kalanchoë uniflora. Corresponding results were obtained with plants grown in 9 h and 12 h photoperiods. In well-watered plants, 50 to 90% of total CO₂-uptake occurred during the light period. Nocturnal CO₂-uptake and malate synthesis were higher and respiration rate was lower in old leaves (leaf pairs 6 to 10) compared to young leaves (leaf pairs 1 to 5). Within four days of drought distinct physiological changes occurred. Gas exchange during the light period decreased and CO₂-uptake during the dark period increased. Nocturnal malate synthesis significantly increased in young leaves.Respiration rate decreased during periods of drought, this decrease being more pronounced in young leaves compared to old leaves. Restriction of gas exchange during the light period resulted in a decrease of transpiration ratio from more than 100 to about 20. The difference between osmotic pressure and xylem tension decreased in young leaves, indicating a reduction in bulk leaf turgor-pressure.We conclude that both the CAM-enhancement in young leaves and the decrease of respiration rate are responsible for the increase of nocturnal CO₂-uptake during water stress. During short drought periods, which frequently occur in humid habitats, the observed physiological changes result in a marked reduction of water loss while net CO₂-uptake is maintained. This might be relevant for plant growth in the natural habitat.Abbreviations LP light period - DP dark period - CAM crassulacean acid metabolism     

Diel leaf growth cycles in Clusia spp. are related to changes between C3 photosynthesis and crassulacean acid metabolism during development and during water stress

This study reports evidence that the timing of leaf growth responds to developmental and environmental constraints in Clusia spp. We monitored diel patterns of leaf growth in the facultative C₃-crassulacean acid metabolism (CAM) species Clusia minor and in the supposedly obligate CAM species Clusia alata using imaging methods and followed diel patterns of CO₂ exchange and acidification. Developing leaves of well-watered C. minor showed a C₃-like diel pattern of gas exchange and growth, with maximum relative growth rate (RGR) in the early night period. Growth slowed when water was withheld, accompanied by nocturnal CO₂ exchange and the diel acid change characteristic of CAM. Maximum leaf RGR shifted from early night to early in the day when water was withheld. In well-watered C. alata , similar changes in the diel pattern of leaf growth occurred with the development of CAM during leaf ontogeny. We hypothesize that the shift in leaf growth cycle that accompanies the switch from C₃ photosynthesis to CAM is mainly caused by the primary demand of CAM for substrates for nocturnal CO₂ fixation and acid synthesis, thus reducing the availability of carbohydrates for leaf growth at night. Although the shift to leaf growth early in the light is presumably associated with the availability of carbohydrates, source–sink relationships and sustained diurnal acid levels in young leaves of Clusia spp. need further evaluation in relation to growth processes.     

Availability of water controls Crassulacean acid metabolism in succulents of the Richtersveld (Namib desert,South Africa)

Features of Crassulacean acid metabolism (CAM) were studied in a variety of different succulents in response to climatic conditions between March 1977 and October 1983 in the southern Namib desert (Richtersveld). A screening in 1977 and 1978 revealed that nearly all investigated succulents performed a CAM, but overnight accumulation of malate declined gradually with decreasing soil water potential, tissue osmotic potential, and leaf water content. This was further substantiated by an extended period of insufficient rainfall in 1979 and 1980 which damaged the evergreen CAM succulents between 80 and 100%. In most of the species still living, neither CO₂-gas exchange nor diurnal acid fluctuation, indicative of CAM, could be detected unless an abundant rainfall restored both CAM features. Plants persisted in a stage of latent life.Water supply is one necessary prerequisite for CAM in the Richtersveld. But even well-watered plants with CAM were sensitive to short-term water stress caused by high water-vapour partialpressure deficit (VPD) in the night, which reduced or prevented CO₂ uptake and resulted in a linear relation between overnight accumulated malate and VPD. The results do not support the opinion that, for the Namib succulents, CAM is an adaptive mechanism to water stress since long-term and short-term water stress stopped nocturnal malate synthesis, but instead lead to the conclusion that nocuturnal CO₂ fixation is only performed when the water status of the plant can be improved simultaneously.Abbreviations CAM Crassulacean acid metabolism - VPD water vapour pressure deficit Dedicated to Professor H. Ziegler on the occasion of his 60th birthday     

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

Upon transfer from well-watered conditions to total drought, long-day-grown cladodes of Opuntia ficus-indica Mill. shift from full Crassulacean acid metabolism (CAM) to CAM-idling. Experiments using ¹⁴C-tracers were conducted in order to characterize the carbon-flow pattern in cladodes under both physiological situations. Tracer was applied by ¹⁴CO₂ fumigations and NaH¹⁴CO₃ injections during the day-night cycle. The results showed that behind the closed stomata, mesophyll cells of CAM-idling plants retained their full capacity to metabolize CO₂ in light and in darkness. Upon the induction of CAM-idling the level of the capacity of phosphoenolpyruvate carboxylase (EC 4.1.1.31) was maintained. By contrast, malate pools decreased, displaying finally only a small or no day-night oscillation. The capacity of NADP-malic enzyme (EC 1.1.1.40) decreased in parallel with the reduction in malate pools. Differences in the labelling patterns, as influenced by the mode of tracer application, are discussed.Abbreviations CAM Crassulacean acid metabolism - PEP-Case phosphoenolpyruvate carboxylase     

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.     

Photosynthetic flexibility in Pedilanthus tithymaloides poit,a CAM plant

The induction of CAM in Pedilanthus tithymaloides (Euphorbiaceae) under water-limited conditions was evaluated by following diurnal oscillations of CO2 fixation, titratable acidity and malic acid content in the leaf extracts. CAM induction was assessed by measuring the activities of phosphoenolpyruvate carboxylase (PEPC), NADH-malate dehydrogenase (MDH) and phosphoenolpyruvate caroxykinase (PEPCK) in the leaves as well. Drought resulted in large increases in the nocturnal acid accumulation and rates of CO2 uptake in the leaves of P. tithymaloides. The drought-induced CAM activity tended to be reversible after re-watering. Nevertheless, under well-watered conditions, plants of P. tithymaloides showed day time CO2 uptake patterns with less pronounced diurnal oscillations of organic acids. Our data indicate that although P. tithymaloides is a CAM plant, environmental variables like drought induce photosynthetic flexibility in this species. This type of plasticity in CAM and metabolic versatility in P. tithymaloides should be an adaptation for prolonged survival under natural adverse edaphic and microclimate situations.     

Interactive effects of photon fluence rates and drought on CAM‐cycling in Delosperma tradescantioides (Mesembryanthemaceae)

The ability of photosynthesis and CAM to acclimate to low (220 µmol m^?2 s^?1; LL) and relatively high (550 µmol m^?2 s^?1; HL) photosynthetic photon flux densities (PPFD) was investigated in the CAM-cycling species Delosperma tradescantioides by means of CO₂ gas exchange and chlorophyll fluorescence analysis. Furthermore, the influence of short-term drought on malic acid accumulation and the activity of photosystem II (PSII) was studied to assess the possible interactions between drought and the prevailing PPFD in this species. HL plants showed features of sun versus shade acclimation relative to LL plants. Nocturnal malic acid accumulation (Δ-malate) and leaf water content also tended to be higher in HL plants. Irrespective of the PPFD during growth, the weak Δ-malate doubled within 3 days of drought. Despite largely restricted CO₂ uptake, photosynthetic activity as estimated from fluorescence analysis declined only ca 5%. After 7 days of drought, when plants showed CAM-idling and Δ-malate had decreased again, potential carbon assimilation was still ca 84% of that in well-watered plants and remained relatively constant throughout the day. Decarboxylation of malic acid accounted for ca 23% of potential assimilation assuming total oxidation of a maximum portion of this organic acid. Drought did not affect predawn maximum photochemical efficiency (F_v/F_m). Nonphotochemical quenching (qN) increased (24%) in response to desiccation and resulted in a more or less constant reduction state of PSII. This increase in qN resulted mainly from the change in its fast-relaxing component (qNF), while the slow component (qNS) was significant only at or above saturating PPFD in both HL and LL plants. The photon response characteristics of PSII, which differed between LL and HL plants, were unaffected by short-term drought. Photon harvesting and photon use were always adjusted to guarantee a low reduction state of PSII. Results suggest that in both LL and HL plants CAM-cycling may help to stabilize photosynthesis but to a large extent by other means than simply providing internally derived CO₂.     

CAM induction in Clusia minor L. during the transition from wet to dry season in Trinidad: the role of organic acid speciation and decarboxylation

The interrelationships between the induction of CAM and the turnover of malate and citrate in the dicotyledenous tree Clusia minor were compared with seasonal changes in rainfall, leaf water status, PFD and photoinhibitory responses during the transition from wet to dry season in Trinidad. Over a period of 8 weeks, as rainfall declined from a maximum observed around week 3, leaf xylem tensions measured at dusk and dawn reflected the concurrent reduction in day-time carbon gain and an increase in the diel turnover of malate (exposed leaves) and citrate (shaded leaves). Clear seasonal trends were observed in the turnover of malate and citrate during the transition from wet to dry season. In contrast to the declining back-ground concentrations of citrate during the wet-dry season transition, malate accumulation was markedly enhanced and the ratio of malalc:citrate accumulated overnight increased as the dry season advanced. Photo-inhibitory responses, assessed by chlorophyll fluorescence, indicated that photochemistry was largely determined by the diurnal course of PFD incident on leaves, regardless of the magnitude of internal CO₂ release from malate and citrate decarboxylation. In the long term, photochemical efficiency in both shaded and exposed leaves appeared to decline as the dry season progressed. Although there was a clear linear relationship between integrated PFD and overnight accumulation of malate, no such correlation was found for citrate. However, citrate breakdown during the day showed a much closer correlation with PFD as compared to malate, with levels of citrate measured at dusk declining in response to higher daily light intensities. Moreover, enhanced citrate decarboxylation during the day was strongly correlated with increased CAM and overnight accumulation of both malate and citrate.     

Leaf development in Ricinus communis during drought stress: dynamics of growth processes, of cellular structure and of sink-source transition

Dicot leaf growth is characterized by partly transient tip-to-base gradients of growth processes, structure and function. These gradients develop dynamically and interact with dynamically developing stress conditions like drought. In Ricinus communis plants growing under well-watered and drought conditions growth rates peaked during the late night and minimal values occurred in the late afternoon. During this diurnal course the leaf base always showed much higher rates than the leaf tip. The amplitude of this diurnal course decreased when leaves approached maturity and during drought stress without any significant alteration of the diurnal pattern and it increased during the first days after rewatering. Unique relationships between leaf size and cytological structure were observed. This provided the framework for the analysis of changes in assimilation, transpiration and dark respiration, chlorophyll, protein, carbohydrate, and amino acid concentrations, and of activities of sink-source-related enzymes at the leaf tip and base during leaf development in well-watered and drought-stressed plants. Gas exchange was dominated by physiological rather than by anatomical properties (stomatal density). Tip-to-base gradients in carbohydrate concentrations per dry weight and sink-source-related enzymes were absent, whereas significant gradients were found in amino acid concentrations per dry weight. During drought stress, growing leaves developed source function at smaller leaf size, before specific physiological adaptations to drought occurred. The relevance of the developmental status of individual leaves for the drought-stress response and of the structural changes for the biochemical composition changes is discussed.     

Daily malate fluctuations in Sedum dasyphyllum L.

Abstract

Sedum dasyphyllum L. leaves of well-watered plants kept outdoors and under controlled conditions show diurnal malic acid fluctuations. In well-watered plants growing outdoors the malate accumulation undergoes seasonal variations and seems to be inhibited by short photoperiod and/or by low temperature. The seasonal variations of CAM activity correspond to seasonal variations of mesophyll succulence. Water stress markedly depressed CAM activity. In fact, plants of S. dasyphyllum show, under controlled conditions, a decrease of malate accumulation as relative water content decreases. Recovery from water stress is fairly slow. Water potential quickly increases during rewatering and exceeds the original value after few days, suggesting a consumption of osmotic compounds during the water stress period.     

Shifts in the Carbon Metabolism of Xerosicyos danguyi H. Humb. (Cucurbitaceae) Brought About by Water Stress : I. General Characteristics

Xerosicyos danguyi H. Humb. (Cucurbitaceae) is an unusual leaf succulent endemic to Madagascar. Under well-watered conditions the plant exhibited Crassulacean acid metabolism (CAM), as characterized by large diurnal changes in titratable acidity, predominantly nighttime stomatal opening and CO₂ uptake, and high δ¹³C values. When plants were exposed to water stress for a minimum of a month, they shifted to a mode of carbon metabolism previously labeled CAM-idling. Under this mode of metabolism, the plants exhibited reduced stomatal opening, reduced CO₂ uptake, dampened diurnal fluctuations in titratable acidity, and no apparent changes in the δ¹³C values. Additionally, investigations showed that the stress hormones 1-aminocyclopropane-1-carboxylic acid (an ethylene precursor) and abscisic acid increased as much as 6-fold in the water-stressed plants. The results are discussed in relation to physiological significance and evolution of the CAM-idling mode of metabolism.     

Recycling of CO2 via Crassulacean acid metabolism in the rock outcrop succulent Sedum pulchellum Michx. (Crassulaceae)

Under well-watered conditions in the laboratory, Sedum pulchellum assimilated CO₂ only during the day, yet exhibited small nocturnal increases in tissue acid content followed by deacidification in the light (CAM-cycling). When drought-stressed, little CO₂ was fixed in the day and none at night, yet even greater acid fluctuations were observed (CAM-idling). Calculations indicate that water savings associated with CAM-cycling when water is available are small. Water saving is more likely to be significant during CAM-idling when water supply is limited and stomata are closed day and night. Thus, in this species, CAM-idling may be of greater benefit to the plant, relative to CAM-cycling, in surviving habitats prone to frequent drought stress.Abbreviations A CO₂ exchange rate - CAM Crassulacean acid metabolism - c_i shoot internal CO₂ concentration - g_c shoot conductance to CO₂ - PPFD photosynthetic photon flux density - WUE water-use efficiency Supported by National Science Foundation Grant No. DMB 8506093.     

Carbon isotope ratios and the variation in the diurnal pattern of malate accumulation in aerial roots of CAM species of <Emphasis Type="Italic">Phalaenopsis</Emphasis> (Orchidaceae)

We investigated the carbon isotope ratios and the diurnal pattern of malate accumulation in leaves and aerial roots of eight species of Phalaenopsis grown in greenhouses. The leaves of all the species showed carbon isotope ratios and the diurnal patterns of malate content typical of CAM plants. However, the aerial roots exhibited a large variation in the diurnal pattern of malate content among species and even among plants within the same species, although carbon isotope ratios were always CAM-like values. Some aerial roots showed the typical diurnal pattern of CAM, but others maintained high or low malate contents during a day without fluctuation. In order to characterize more strictly the nature of the malate variation in the aerial roots, we further investigated a possible variation of the diurnal pattern of malate among different aerial roots within an individual for Phalaenopsis amabilis and P. cornu-cervi. The diurnal pattern of malate content was varied even among different aerial roots within the same plant. Thus the photosynthetic carbon metabolism in aerial roots of orchids is fairly complex.     

Differential usage of storage carbohydrates in the CAM bromeliad Aechmea 'Maya' during acclimation to drought and recovery from dehydration

CAM requires a substantial investment of resources into storage carbohydrates to account for nocturnal CO₂ uptake, thereby restricting carbohydrate partitioning to other metabolic activities, including dark respiration, growth and acclimation to abiotic stress. Flexible modulation of carbon flow to the different competing sinks under changing environmental conditions is considered a key determinant for the growth, productivity and ecological success of the CAM pathway. The aim of the present study was to examine how shifts in carbohydrate partitioning could assure maintenance of photosynthetic integrity and a positive carbon balance under conditions of increasing water deprivation in CAM species. Measurements of gas exchange, leaf water relations, malate, starch and soluble sugar (glucose, fructose and sucrose) contents were made in leaves of the CAM bromeliad Aechmea ‘Maya’ over a 6‐month period of drought and subsequently over a 2‐month period of recovery from drought. Results indicated that short‐term influences of water stress were minimized by elevating the level of respiratory recycling, and carbohydrate pools were maintained at the expense of export for growth while providing a comparable nocturnal carbon gain to that in well‐watered control plants. Longer term drought resulted in a disproportionate depletion of key carbohydrate reserves. Sucrose, which was of minor importance for providing substrate for the dark reactions under well‐watered conditions, became the major source of carbohydrate for nocturnal carboxylation as drought progressed. Flexibility in terms of the major carbohydrate source used to sustain dark CO₂ uptake is therefore considered a crucial factor in meeting the carbon and energy demands under limiting environmental conditions. Recovery from CAM‐idling was found to be dependent on the restoration of the starch pool, which was used predominantly for provision of substrate for nocturnal carboxylation, while net carbon export was limited. The conservation of starch for the nocturnal reactions might be adaptive with regard to responding efficiently to a return of water stress.     

Water economy and photosynthesis of the CAM plant Senecio medley-woodii during increasing drought

Abstract CO₂ gas exchange, transpiration and water uptake of the succulent Senecio medley-woodii were monitored simultaneously during a 10 day period of increasing drought. The measurements were performed with a combination of a CO₂ gas exchange chamber and a potometer system. Further, leaf water relations and CO₂ gas exchange of a branched potted plant were measured during 15 days of water shortage. The enhancement of CO₂ dark fixation at the beginning of drought modifies the leaf water relations according to the increased malate accumulation during the dark period. The enhancement of water uptake from dusk to dawn corresponds to the increase of Ψ_leaf during the same period. Therefore at the beginning of drought a short time improvement of plant water status through the increased CO₂ dark fixation and malate accumulation can be maintained.