Relationships between Photosynthetically Active Radiation, Nocturnal Acid Accumulation, and CO(2) Uptake for a Crassulacean Acid Metabolism Plant, Opuntia ficus-indica

The influences of photosynthetically active radiation (PAR) and water status on nocturnal Crassulacean acid metabolism (CAM) were quantitatively examined for a widely cultivated cactus, Opuntia ficus-indica (L.) Miller. When the total daily PAR was maintained at 10 moles photons per square meter per day but the instantaneous PAR level varied, the rate of nocturnal H⁺ accumulation (tissue acidification) became 90% saturated near 700 micromoles per square meter per second, a PAR level typical for similar light saturation of C₃ photosynthesis. The total nocturnal H⁺ accumulation and CO₂ uptake reached 90% of maximum for a total daily PAR of about 22 moles per square meter per day. Light compensation occurred near 0 moles per square meter per day for nocturnal H⁺ accumulation and 4 moles per square meter per day for CO₂ uptake. Above a total daily PAR of 36 moles per square meter per day or for an instantaneous PAR of 1150 micromoles per square meter per second for more than 6 hours, the nocturnal H⁺ accumulation actually decreased. This inhibition, which occurred at PAR levels just above those occurring in the field, was accompanied by a substantial decrease in chlorophyll content over a 1-week period.

A minimum ratio of H⁺ accumulated to CO₂ taken up of 2.5 averaged over the night occurred for a total daily PAR of 31 moles per square meter per day under wet conditions. About 2 to 6 hours into the night under such conditions, a minimum H⁺-to-CO₂ ratio of 2.0 was observed. Under progressively drier conditions, both nocturnal H⁺ accumulation and CO₂ uptake decreased, but the H⁺-to-CO₂ ratio increased. A ratio of two H⁺ per CO₂ is consistent with the H⁺ production accompanying the conversion of starch to malic acid, and it apparently occurs for O. ficus-indica when CAM CO₂ uptake is strongly favored over respiratory activity.

     

ORIENTATION,PAR INTERCEPTION,AND NOCTURNAL ACIDITY INCREASES FOR TERMINAL CLADODES OF A WIDELY CULTIVATED CACTUS,OPUNTIA FICUS-INDICA

Terminal vertical cladodes (flattened stems) of Opuntia ficus-indica growing in widely separated locations were nonrandomly oriented. On plantations at 33°S latitude in Chile individual cladodes tended to orient in the same direction as the planted cladodes on which they developed. However, after 2 years unshaded new cladodes tended to face east-west. Terminal cladodes also tended to face east-west for irrigated O. ficus-indica in California (at 34°N) and in Israel (32 to 33°N), but cladodes developing in the winter tended to face north-south. Except for the residual effect of initial planting direction, the observed patterns tended to maximize the interception of photosynthetically active radiation (PAR). Specifically, east-west cladode orientation would maximize PAR interception, except for cladodes developing near the winter solstice at latitudes more than 27° from the equator. Nocturnal acidity increases and hence productivity would generally be light-limited, since the nocturnal increase in acidity was 90% saturated for a total daytime PAR of 24 mol m^–2 day^–1 and the PAR received on vertical surfaces is usually less than this. Topographical features can modify the orientation patterns, since at a site where PAR was considerably blocked by surrounding mountains the maximal nocturnal acidity increases and peak in cladode orientation occurred 20° from facing east–west. Laboratory studies showed that developing cladodes oriented toward a horizontal light and were rotated an average of 16° in a direction that increased PAR interception compared to the cladodes on which they developed. Such phototropic responses, the higher productivity of favorably oriented cladodes, and the tendency to orient similarly to the underlying cladode presumably accounts for the overall orientation patterns observed, where up to four times more cladodes may face in a particular direction than at right angles to it.     

Effects of Irradiance on Crassulacean Acid Metabolism in the Epiphyte Tillandsia usneoides L. (Bromeliaceae)

Spanish moss (Tillandsia usneoides L.) was collected in South Carolina, maintained in a greenhouse, then exposed to five levels of photosynthetic photon flux density (PPFD) for 3 weeks. Following this treatment, plants were sampled for chlorophyll concentrations, nocturnal acid accumulations, and photosynthetic responses to subsequent exposure at a range of PPFD. No acclimation to PPFD was observed; all plants exhibited similar patterns of nocturnal CO₂ uptake and acid accumulation regardless of initial PPFD treatment. These patterns revealed that at a PPFD level of approximately 200 micromoles per square meter per second (daytime integrated PPFD of 10 moles per square meter per day), CAM saturated or, in low-PPFD plants, was optimal. The results of this study indicate that adaptation to high PPFD is not necessarily a requirement of CAM.     

Young daughter cladodes affect CO2 uptake by mother cladodes of Opuntia ficus-indica

BACKGROUND AND AIMS: Drought damages cultivated C3, C4 and CAM plants in the semi-arid lands of central Mexico. Drought damage to Opuntia is common when mother cladodes, planted during the dry spring season, develop young daughter cladodes that behave like C3 plants, with daytime stomatal opening and water loss. In contrast, wild Opuntia are less affected because daughter cladodes do not develop on them under extreme drought conditions. The main objective of this work is to evaluate the effects of the number of daughter cladodes on gas exchange parameters of mother cladodes of Opuntia ficus-indica exposed to varying soil water contents. METHODS: Rates of net CO2 uptake, stomatal conductance, intercellular CO2 concentration, chlorophyll content and relative water content were measured in mature mother cladodes with a variable number of daughter cladodes growing in spring under dry and wet conditions. KEY RESULTS: Daily carbon gain by mother cladodes was reduced as the number of daughter cladodes increased to eight, especially during drought. This was accompanied by decreased mother cladode relative water content, suggesting movement of water from mother to daughter cladodes. CO2 assimilation was most affected in phase IV of CAM (late afternoon net CO2 uptake) by the combined effects of daughter cladodes and drought. Rainfall raised the soil water content, decreasing the effects of daughter cladodes on net CO2 uptake by mother cladodes. CONCLUSIONS: Daughter cladodes significantly hasten the effects of drought on mother cladodes by competition for the water supply and thus decrease daily carbon gain by mother cladodes, mainly by inhibiting phase IV of CAM.     

LEAF ORIENTATION,RADIATION INTERCEPTION,AND NOCTURNAL ACIDITY INCREASES BY THE CAM PLANT AGAVE DESERTI (AGAVACEAE)

Both field measurements and a computer model were used to study the interception of photosynthetically active radiation (PAR) by Agave deserti (Engelm.), a desert CAM plant with a basal rosette of massive opaque leaves. PAR interception was determined in the winter and the summer for upper and lower leaf surfaces on a plant with about 60 leaves. Total daily PAR on the leaf surfaces was approximately 10 mol m^-2 for a winter day and 20 mol m^-2 for a summer day. For a PAR of 15 mol m^-2, the nocturnal increase in acidity was about 0.6 mol m^-2 for both leaf surfaces and various leaf orientations, except for the oldest most horizontal leaves where the increase was less than half as large. The acidity increase measured in the field was 90% saturated at 25 mol m^-2. Thus, daytime PAR in the desert is often limiting for the nocturnal acidity increase, especially for the lower leaf surfaces. Simulated tilting of the plant by 55° so that the vertical axis pointed to the sun at solar noon on a winter day increased the PAR incident on the upper surfaces of the leaves, but did not affect the total nocturnal increase in acidity by the whole plant. Although simulated removal of alternate leaves increased the PAR per unit leaf area for the remaining leaves, it reduced the total increase in nocturnal acidity of the whole plant by 31%. PAR interception by plants on slopes facing steeply north, east, or west was substantially reduced compared to the horizontal. Thus, the model proved to be quite useful for quantifying the relation between leaf orientation, PAR interception, and nocturnal increases in acidity by A. deserti, and it indicated that the lower frequency of plants on north- compared to south-facing slopes was due to PAR limitations.     

Field productivity of a CAM plant, Agave salmiana, estimated using daily acidity changes under various environmental conditions

Nocturnal acidity increases of the Crassulaceae acid metabolism succulent Agave salmiana Otto ex Salm. ssp. crassispina (Trel.) Gentry were determined for various photosynthetically active radiation (PAR) levels in the field and various day/night temperatures and soil water potentials in the laboratory. When a particular factor was optimal for nocturnal acid accumulation, it was assigned an index value of unity. The product of the three indices was termed the environmental productivity index (EPI), which summarizes the effect of PAR, temperature, and water status on nocturnal acid accumulation. A monthly value for EPI was determined using microclimatic conditions at the field site near Salinas de Hidalgo, San Luis Potosf, Mexico. EPI was highly correlated with the number of leaves unfolding monthly from plants in the field (r²= 0.95), where monitoring of such leaf unfolding is a non-destructive technique for assessing plant productivity. By using the measured leaf area index for four groups of A. salmiana and a relationship between acid accumulation and net CO₂ up-take, the EPI acidity data were converted to dry weight gain per unit ground area. This measure of productivity closely agreed with the 1.05 kg m⁻² year⁻¹ determined by conventional harvesting techniques. The productivity is also in agreement with studies on other desert agaves, but about 10-fold higher than that generally considered for desert ecosystems.     

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

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

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

     

Polyamine levels during the onset of “CAM” inOpuntia F. indica (Miller)

Summary Polyamines have been related to the Crassulacean Acid Metabolism (CAM) in higher plants. Such relationship was however observed in plants where CAM activity is inducible by external factors. Results presented here indicate that, inOpuntia F. indica, cladodes where onset of CAM is dependent on internal conditions, i.e. leaf age, the concentration of putrescine increases in parallel to the acidity of the cytoplasm. The parallel increase of putrescine concentration and acidity (malic acid concentration) can be best evaluated during the onset of CAM (young cladodes), while such correlation is not observed in mature cladodes where CAM is already in it's full function. Spermidine and spermine show no correlation with CAM activity neither during the onset of CAM nor during it's full function. However, spermidine levels correlate negatively to CAM activity when cladodes attain > 30 days of age. The results suggest that putrescine in free form could possibly counteract the increase of cellular acidity during onset of CAM inOpuntia F. indica; the possible roles of spermidine are discussed.Abbreviations PA polyamines - put putrescine - spd spermidine - spm spermine - HPLC high performance liquid chromatography This work was supported by grants from Italian Ministry of University and Scientific Research (60%).     

Photosynthetic pathways in a midwestern rock outcrop succulent,Sedum nuttallianum Raf. (Crassulaceae)

Shoots of Sedum nuttallianum exhibited CAM^* acid fluctuations in the field. These nocturnal acid accumulations persisted in the laboratory under well-watered and water-stressed conditions. Simultaneous measurements of transpiration, however, indicated daytime stomatal opening and nocturnal stomatal closure. Measurements of CO₂ and H₂O vapor exchange continuously for six days after watering substantiated these results in part: the majority of CO₂ uptake occurred during the day early in the experiment; however, after several days without water, nighttime CO₂ uptake was stimulated and eventually was greater than the drastically reduced daytime CO₂ uptake. This nighttime uptake was never quite sufficient to account for all estimated increases in tissue acidity. Thus, a combination of CAM and CAM-cycling occurred early in the desiccation experiment. Evidence for CAM and a form of CAM-idling was found later in the experiment. Though nighttime CO₂ uptake occurred and persisted after only one day without water, rates were too low to alter the tissue ¹³C/¹²C value from a C₃-like number (–30). Thus, although CAM and CAM-idling may have survival value during extended droughts, shoots of S. nuttallianum apparently utilize the C₃ pathway to obtain most of their carbon.Abbreviations C₃ pathway - CO₂ fixation pathway in which an intermediate containing 3 carbon atoms is formed - CAM Crassulacean acid metabolism - Chl Chlorophyll - c_i internal CO₂ concentration - DW Dry weight - g_c mean conductance to CO₂ - FW Fresh weight - PAR Photosynthetically active radiation - SD Standard deviation - vpd Vapor pressure deficit - WUE Water use efficiency     

Drought-stress-induced up-regulation of CAM in seedlings of a tropical cactus, Opuntia elatior, operating predominantly in the C3 mode

Immediately after unfolding, cotyledons of the tropical platyopuntoid cactus, Opuntia elatior Mill., exhibited a C(3)-type diel CO(2) exchange pattern characterized by net CO(2) uptake in the light. Significant nocturnal increases in titratable acidity typical of crassulacean acid metabolism (CAM) were not detected at this early developmental stage. As cotyledons matured and the first cladode (flattened stem) developed, features of CAM were observed and the magnitude of CAM increased. Nonetheless, in well-watered seedlings up to 10 cm tall, C(3) photosynthetic CO(2) fixation in the light remained the major pathway of carbon fixation. Reduced soil water availability led to an up-regulation of net dark CO(2) fixation and greater nocturnal increases in tissue acidity, consistent with facultative CAM. These observations demonstrate that C(3) photosynthesis, drought-stress-related facultative CAM, and developmentally controlled constitutive CAM can all contribute to the early growth of O. elatior. The strong C(3) component and facultative CAM features expressed in young O. elatior contrast with mature plants in which obligate CAM is the major pathway of carbon acquisition.     

Enhancement of the Stomatal Response to Blue Light by Red Light, Reduced Intercellular Concentrations of CO(2), and Low Vapor Pressure Differences

The effects of environmental parameters on the blue light response of stomata were studied by quantifying transient increases in stomatal conductance in Commelina communis following 15 seconds by 0.100 millimole per square meter per second pulses of blue light. Because conductance increases were not observed following red light pulses of the same or greater (30 seconds by 0.200 millimole per square meter per second) fluences, the responses observed could be reliably attributed to the specific blue light response of the guard cells, rather than to guard cell chlorophyll. In both Paphiopedilum harrisianum, which lacks guard cell chloroplasts, and Commelina, the blue light response was enhanced by 0.263 millimole per square meter per second continuous background red light. Thus, the blue light response and its enhancement do not require energy derived from red-light-driven photophosphorylation by the guard cell chloroplasts. In Commelina, reduction of the intercellular concentration of CO₂ by manipulation of ambient CO₂ concentrations resulted in an enhanced blue light response. In both Commelina and Paphiopedilum, the blue light response was decreased by an increased vapor pressure difference. The magnitude of blue-light-specific stomatal opening thus appears to be sensitive to environmental conditions that affect the carbon and water status of the plant.     

Crassulacean Acid Metabolism and Photochemical Efficiency of Photosystem II in the Adaxial and Abaxial Parts of the Succulent Leaves of Kalanchoë daigremontiana Grown at Four Photon Flux Densities

Kalanchoë daigremontiana, a species possessing crassulacean acid metabolism, was grown at four photon flux densities (1300, 400, 60, and 25 micromole photons per square meter per second). In leaves which had developed at 1300 and 400 micromole photons per square meter per second, CO₂ was mainly incorporated through the lower, shaded leaf surfaces, and the chlorenchyma adjacent to the lower surfaces showed a higher degree of nocturnal acid synthesis than the chlorenchyma adjacent to the upper surfaces. In leaves acclimated to 60 and 25 micromole photons per square meter per second, the gradient in CAM activity was reversed, i.e. more CO₂ was taken up through the upper than through the lower surfaces and nocturnal acidification was higher in the tissue next to the upper surfaces. Total net carbon gain and total nocturnal acid synthesis were highest in leaves which had developed at 400 micromole photons per square meter per second. Chlorophyll content was markedly reduced in leaves which had developed at 1300 micromole photons per square meter per second, especially in the exposed adaxial parts. There was also a sustained reduction in photosystem II photochemical efficiency as indicated by measurements of the ratio of variable over maximum chlorophyll a fluorescence. These findings suggest that, at high growth photon flux densities, the reduced activity of the exposed portions of these succulent leaves is caused by (a) the adverse effects of excess light, (b) together with a genotypic component which favors CO₂ uptake and acid synthesis in the abaxial (lower) leaf parts even when light is not or only marginally excessive. This latter component is predominant at medium photon flux densities, e.g. at 400 micromole photons per square meter per second. It becomes overridden, however, under conditions of deep shade when strongly reduced light levels in the abaxial parts of the leaf chlorenchyma severely limit photosynthesis.     

Exploring the Limits of Crop Productivity : I. Photosynthetic Efficiency of Wheat in High Irradiance Environments

The long-term vegetative and reproductive growth rates of a wheat crop (Triticum aestivum L.) were determined in three separate studies (24, 45, and 79 days) in response to a wide range of photosynthetic photon fluxes (PPF, 400-2080 micromoles per square meter per second; 22-150 moles per square meter per day; 16-20-hour photoperiod) in a near-optimum, controlled-environment. The CO₂ concentration was elevated to 1200 micromoles per mole, and water and nutrients were supplied by liquid hydroponic culture. An unusually high plant density (2000 plants per square meter) was used to obtain high yields. Crop growth rate and grain yield reached 138 and 60 grams per square meter per day, respectively; both continued to increase up to the highest integrated daily PPF level, which was three times greater than a typical daily flux in the field. The conversion efficiency of photosynthesis (energy in biomass/energy in photosynthetic photons) was over 10% at low PPF but decreased to 7% as PPF increased. Harvest index increased from 41 to 44% as PPF increased. Yield components for primary, secondary, and tertiary culms were analyzed separately. Tillering produced up to 7000 heads per square meter at the highest PPF level. Primary and secondary culms were 10% more efficient (higher harvest index) than tertiary culms; hence cultural, environmental, or genetic changes that increase the percentage of primary and secondary culms might increase harvest index and thus grain yield. Wheat is physiologically and genetically capable of much higher productivity and photosynthetic efficiency than has been recorded in a field environment.     

A phylogenetic view of low-level CAM in Pelargonium (Geraniaceae)

Crassulacean acid metabolism (CAM) is common in several plant families and is often associated with succulence. Few studies have examined the occurrence of CAM from a phylogenetic perspective. The genus Pelargonium is promising for such a study because members are characterized by dramatic variation in growth form (including geophytes, shrubs, and stem succulents) and because growth form diversity is expressed to the greatest extent in a monophyletic group comprising 80% of Pelargonium species. This clade, predominantly from the winter rainfall region of southern Africa, likely proliferated in response to Miocene or Pliocene aridification. We present a survey for CAM across Pelargonium, emphasizing the winter rainfall clade. Dawn/dusk fluctuations in titratable acidity were examined in 41 species, with detailed measurements of carbon uptake and stomatal conductance under progressive water stress in four species. No species exhibited obligate CAM. When well-watered, most species exhibited stomatal conductances and acid fluctuations characteristic of C(3) photosynthesis, though some exhibited more pronounced increases in nocturnal acidity, suggesting CAM cycling. In four species examined during dry-down, water stress led to increased nighttime acid levels and decreased daytime stomatal conductance. Ultimately, stomata closed and external carbon uptake ceased, consistent with CAM idling. These results are discussed from the perspective of the evolution of CAM flexibility.     

Stomatal Opening Mechanism of CAM Plants

Stomata usually open when leaves are transferred from darkness to light. However, reverse-phase stomatal opening in succulent plants has been known. CAM plants such as cacti and Opuntia ficus–indica achieve their high water use efficiency by opening their stomata during the cool, desert nights and closing them during the hot, dry days. Signal transduction pathway for stomatal opening by blue light photoreceptors including phototropins and the carotenoid pigment zeaxanthin has been suggested. Blue light regulated signal transduction pathway on stomatal opening could not be applied to CAM plants, but the most possible theory for a nocturnal response of stomata in CAM plants is photoperiodic circadian rhythm.     

Rapid and Specific Modulation of Stomatal Conductance by Blue Light in Ivy (Hedera helix) : An Approach to Assess the Stomatal Limitation of Carbon Assimilation

Low intensity (0.015 millimole per square meter per second) blue light applied to leaves of Hedera helix under a high intensity red light background (0.50 millimole per square meter per second red light) induced a specific stomatal opening response, with rapid kinetics comparable to those previously reported for stomata with `grass type' morphology. The response of stomatal conductance to blue light showed a transient `overshoot' behavior at high vapor pressure difference (2.25 ± 0.15 kiloPascals), but not at low vapor pressure difference (VPD) (0.90 ± 0.10 kilo-Pascal). The blue light-induced conductance increase was accompanied by an increase in net photosynthetic carbon assimilation, mediated by an increase in the intercellular concentration of carbon dioxide. Values of assimilation once the blue light-stimulated conductance increase reached steady state were less than those at the peak of the overshoot, but the ratios of assimilation to transpiration (A/E) and blue light-stimulated ΔA/ΔE were greater during the steady-state response than during the overshoot. These results indicate that significant stomatal limitation of assimilation can occur, but that this limitation may improve water use efficiency under high VPD conditions. Under high intensity red light, the decline in A/E associated with an increase in VPD was minimized when conductance was stimulated by additional low intensity blue light. This effect indicates that the blue light response of stomata may be important in H. helix for the optimization of water use efficiency under natural conditions of high irradiance and VPD.     

Changes in stomatal movements in isolated epidermal strips of a CAM plant, Notonia grandiflora DC

The diuranl rhythm of the stomatal aperture size was investigatedusing isolated epidermal strips from a CAM plant, Notonia grandiflora.Maximal stomatal opening occurred at pH 6.5. The nocturnal stomatalopening was stimulated by KCl which produced stomatal closurein light. The stimulatory effect of KCl on stomatal openingin drakness was suppressed by ABA. (Received January 24, 1978; )     

Interception of photosynthetically active radiation by cacti of different morphology

Summary Ribbing, orientation of cladodes, and variations in stem height were examined theoretically and in the field to see whether the morphology of various cacti could be interpreted as adaptations for intercepting photosynthetically active radiation (PAR). The total daytime PAR incident on different parts of the stem was related to the nocturnal increase in tissue acidity of these Crassulacean acid metabolism plants. The acidity change was 90% saturated for a total daytime PAR of about 22 mol m^-2, which indicates that the vertical stems of cacti are often light-limited in the desert.PAR interception by a cactus with ribs of various depths was simulated. Ribbing led to more surface area but a lower PAR per unit area, so net carbon gain was little influenced.Although the cladodes of Opuntia basilaris Engelm. and Bigel. var. basilaris were randomly oriented, those of another platyopuntia, O. chlorotica Engelm. and Bigel, preferentially faced northsouth. Compared to facing east-west, cladodes facing N-S would receive 52% more PAR at the winter solstice, a time when water is generally available and overheating is not a problem in the Mojave desert habitat of O. chlorotica.The maximum height of Stenocereus gummosus (Engelm.) Gibs. & Horak varied from 0.7 m in northern Baja California (31°52N) to 4.1 m at 23°47N. Stem height was positively correlated (r ²=0.93) with the height of the surrounding vegetation and negatively correlated (r ²=0.91) with the PAR 1 m above the ground. The great plasticity in stem height permits S. gummosus to obtain sufficient PAR in the presence of subtropical trees in the southern part of its range and yet expend less biomass to reach unobstructed PAR in the northern part of its range, where the surrounding vegetation is much shorter.     

Environmental Influences on Open Stomates of a Crassulacean Acid Metabolism Plant, Agave deserti

The major short term stomatal response of Agave deserti was to temperature; increases in leaf temperature led to decreases in water vapor conductance for stomatal opening during the daytime (C₃ mode) as well as at night (Crassulacean acid metabolism or CAM mode). Hourly changes in the water vapor concentration drop from leaf to air had no significant stomatal effect in either mode. Stomatal responses to external CO₂ levels up to 800 microliters per liter were not significant after 15 minutes and only moderate after a few hours, suggesting that CO₂ effects on open stomates of this succulent were indirect in both CAM and C₃ modes.     

Age-specific changes of acidity, phosphoenolpyruvate carboxylase, ribulose-1,5-bisphosphate carboxylase/oxygenase, abscisic acid and leaf water potential in Mesembryanthemum nodiflorum

Age-induced changes in 1) nocturnal and diurnal acidity fluctuations that coincide with the ongoing environmental conditions, 2) the build up of abscisic acid (ABA) in plant roots and leaves during sunrise, midday, and sunset in all growing stages, 3) the changes in phosphoenolpyruvate carboxylase (PEPC) and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) activities as key enzymes of the photosynthetic pathways of C3 and CAM, 4) leaf water potential (ψ1), and 5) Km and Vmax for PEPC to express its activity and affinity, were studied in Mesembryanthemum nodiflorum during transition from C3 to CAM mode of CO2 fixation. The acidity during sunset in mature stage was higher than in earlier stages and reflected the impact of environmental conditions on physiological and metabolic changes. Moreover, the higher acidity during sunrise and sunset was observed during the senescence than the mature stage; this might be due to CO2 release and oxygen intake during senescence induced ethylene formation that lead to increased malic acid formation. The ABA concentration was high in M. nodiflorum leaves, but stomatal closure was insensitive to elevated ABA concentrations recorded. Vmax of PEPC, Km, and the affinity of PEPC during later stages indicated the ability of PEPC to fix CO2 taking up at night in CAM cycle of M. nodiflorum. Less affinity during sunrise indicated inhibitory effect of malate on PEPC during the release of CO2. The second peak of PEPC activity before sunset caused CO2 fixation. The RuBPCO was inactive at night. Slight increase in ABA during sunset, and night drop in air temperature and increase in relative humidity reduced markedly transpiration rate without decreasing ψ1. This revised version was published online in July 2006 with corrections to the Cover Date.