CAM-cycling in the cycad Dioon edule Lindl. in its natural tropical deciduous forest habitat in central Veracruz, Mexico

The cycad Dioon edule Lindl. inhabits a seasonally-dry tropical forest along with associated CAM plants such as bromeliads and cacti. To test the hypothesis that D . edule might also be a CAM plant, diel total-acid fluctuation was measured through the dry to wet seasons of 4 consecutive years on adult D . edule plants in their natural forest habitat in Veracruz, Mexico. Correlations between acid fluctuation index and climatic data, and also soil water potential were determined over this period. Laboratory trials were followed up to estimate diel patterns of CO₂ exchange and estimation of δ¹³C value. A comparison of stomatal density cm⁻² with other C₃, CAM and CAM-facultative plants was made. The diel total titratable-acid fluctuation values, although variable, were found to be consistent and significant for the dry season. Carbon dioxide exchange was found to be typical of C₃ plants when hydrated but when the plant was stressed by withholding water, although the leaf remained healthy, there was no significant dark-period CO₂ output. Stomatal density was comparable to other CAM and CAM-facultative plants. It was concluded that D. edule is a C₃ plant that shows CAM-cycling metabolism when water stressed. © 2002 The Linnean Society of London, Botanical Journal of the Linnean Society , 138 , 155–161.     

Interaction of elevated CO2 and O3 on growth, photosynthesis and respiration of three perennial species grown in low and high nitrogen

Seedlings of three species native to central North America, a C₃ tree, Populus tremuloides Michx., a C₃ grass, Agropyron smithii Rybd., and a C₄ grass, Bouteloua curtipendula Michx., were grown in all eight combinations of two levels each of CO₂, O₃ and nitrogen (N) for 58 days in a controlled environment. Treatment levels consisted of 360 or 674 μmol mol^-1 CO₂, 3 or 92 nmol mol^-1 O₃, and 0.5 or 6.0 m M N. In situ photosynthesis and relative growth rate (RGR) and its determinants were obtained at each of three sequential harvests, and leaf dark respiration was measured at the second and third harvests. In all three species, plants grown in high N had significantly greater whole-plant mass, RGR and photosynthesis than plants grown in low N. Within a N treatment, elevated CO₂ did not significantly enhance any of these parameters nor did it affect leaf respiration. However, plants of all three species grown in elevated CO₂ had lower stomatal conductance compared to ambient CO₂-exposed plants. Seedlings of P. tremuloides (in both N treatments) and B. curtipendula (in high N) had significant ozone-induced reductions in whole-plant mass and RGR in ambient but not under elevated CO_2. This negative O₃ impact on RGR in ambient CO₂ was related to increased leaf dark respiration, decreased photosynthesis and/or decreased leaf area ratio, none of which were noted in high O₃ treatments in the elevated CO₂ environment. In contrast, A. smithii was marginally negatively affected by high O_3.     

Drought-induced shifts in daily CO2 uptake patterns for leafy cacti

For cacti with persistent, relatively large leaves, most shoot CO₂ uptake under well-watered conditions occurs by the leaves using the C₃ pathway. For three species in the primitive subfamily Pereskioideae, droughts of 7 or 14 days decreased leaf daytime net CO₂ uptake by an average of 49 and 88%, respectively; these species always had a net CO₂ release at night by the leaves and both at night and during the day by the stems. For three leafy species in subfamily Opuntioideae, 7 and 14 days of drought reduced leaf daytime net CO₂ uptake by 90 and 100%, respectively. Although drought reduced the total CO₂ uptake over 24 h, the average percentage occurring at night by the leaves of these species increased from 5% under wet conditions to 71% after 7 days of drought to 99% after 14 days of drought. For two of the three species of Opuntioideae, 7 days of drought caused the small net CO₂ uptake by the sterns to shift from the daytime to the nighttime, while for the third species drought caused a reduction of its stem nocturnal net CO₂ uptake. Thus, shifts from predominantly daytime to predominantly nighttime net CO₂ uptake can be induced by drought for the leaves and the stems of leafy cacti in subfamily Opuntioideae, indicating a high degree of biochemical versatility.     

Rapid-cycling CAM; an hypothetical variant of photosynthetic metabolism

The currently recognized forms of CAM photosynthesis do not represent all possible variants on the theme. It is predicted here that there may exist an undiscovered variant in which the CO₂-acquiring and CO₂-reducing phases of CAM alternate over time periods shorter (possibly much shorter) than the diel cycle. The process would occur entirely in the photoperiod and may be likened to a form of C₄ photosynthesis in which the same photosynthetic cell alternates between performing the functions of a C₄ mesophyll and C₄ bundle sheath cell. Rapid-cycling CAM, which would be unlikely to be detected by methods commonly used to measure photosynthesis, could provide a CO₂-concentrating mechanism in both unicellular and multicellular plants.     

The appearance of a new molecular species of phosphoenolpyruvate carboxylase (PEPC) and the rapid induction of CAM in Sedum telephium L.

Abstract. When detached leaves of Sedum telephium are incubated in the absence of water, a rapid switch from C₃ photosynthesis to CAM (as indicated by the onset of day-to-night fluctuations in titratable acidity. ΔH⁺) occurs within the first dark period. The C₃-CAM switch in intact plants occurs within 3 5d. Extractable activity of phospho enol pyruvate carboxylase (PEPC) increases five-fold in intact plants during CAM induction; however, during rapid CAM induction in detached leaves, there is only a very small increase in PEPC activity. Fractionation by anion exchange chromatography of crude extracts from leaves of intact plants subjected to water deficit shows that CAM induction is associated with the appearance of a molecular species of PEPC termed PEPC I. PEPC I is barely detectable in well-watered plants which are not performing CAM. The major form in these plants is termed PEPC II. In leaves from intact plants, there is a significant positive correlation between PEPC I activity and ΔH⁺ during a period of increasing water deficit. PEPC I exhibits day to night fluctuations in malate sensitivity, being less sensitive during the dark period. In contrast, PEPC II is more sensitive to inhibition by malate and has no day to night fluctuation in sensitivity. In detached leaves deprived of water, a small increase in PEPC I capacity is detected at the end of the first dark period (20 h after the start of treatment). The results suggest that PEPC I is required for attainment of maximum nocturnal malic acid synthesis. There is a significant correlation between leaf water status (relative water content), ΔH⁺, total PEPC and PEPC I activity suggesting that the internal water status of the plant may be a trigger for CAM induction. Abscisic acid applied to detached leaves does not cause nocturnal acidification.     

Mechanistic basis of differences in water-use efficiency between a CAM and a C3 species of Peperomia (Piperaceae)

Under ecologically realistic environmental conditions, the water-use efficiency (WUE) of Peperomia scandens , a CAM plant, was higher than that of the C₃ congener P. obtusifolia . This difference has been attributed to differences in stomatal activity between C₃ and CAM plants, coupled with differences in the evaporative demand of the atmosphere during which the stomata are open. This explanation has apparently not, however, been experimentally tested. Thus, WUEs were compared in these species in two experiments in which the atmospheric evaporative demand was identical (or nearly so) during the period of stomatal opening (i.e. during the night for the CAM plant and during the day for the C₃ species). In both experiments, the WUE of the CAM species was higher than that of the C₃ species. These results suggest that factors other than differences in atmospheric environmental conditions must also be responsible for the observed differences in WUE. Because CO₂ uptake rates of the CAM species were substantially lower than those of the C₃ species, the lower WUE in the CAM species resulted primarily from lower transpiration rates. Lower rates of water loss in P. scandens , relative to rates in P. obtusifolia , were ascribed, in part, to lower stomatal densities. Thus, leaf morphological differences, in addition to differences in atmospheric evaporative demand, help to explain the high WUE typically measured in CAM plants.     

Gas exchange and metabolite fluctuations in green and yellow bands of variegated leaves of the monocotyledonous CAM species Agave americana

The variegated leaves of the Crassulacean acid metabolism (CAM) species Agave americana have a large central longitudinal green band with narrow yellow bands on either side. The yellow bands had 97% less pigment content, 84% lower ribulose‐1,5‐bisphosphate carboxylase/oxygenase activity, but only 20% lower phosphoenolpyruvate carboxylase activity than the green band. The green bands exhibited gas exchange typical of CAM plants, with most CO₂ uptake occurring at night, leading to a daily net CO₂ uptake of 127 mmol m⁻² day⁻¹. The yellow bands had some nighttime net CO₂ uptake but a larger loss during the daytime, indicating that they were sink tissues. Nocturnal citrate and malate accumulations for the yellow bands were 65 and 75%, respectively, of those of the green bands; sucrose supported 64‐83% of their nocturnal acid accumulation. This is the first evidence that agaves, which are malic‐enzyme‐type CAM plants, use sucrose as the carbon source for nocturnal acid accumulation. About 44% of the carbon demand of the yellow bands can be supplied by sucrose diffusing via the symplast from the adjacent green band, about 25% from fructose and glucose diffusion, and some via the apoplast.     

The Genus Clusia L.: Molecular Evidence for Independent Evolution of Photosynthetic Flexibility

Abstract: Members of the Clusiaceae genus Clusia (tropical trees and shrubs) belong to the small group of dicotyledonous trees which are able to perform crassulacean acid metabolism (CAM). Most of the species are able to switch between C₃ and CAM modes of photosynthesis and only a few are restricted to either C₃ or CAM. In order to discover possible phylogenetic relationships with regard to the mode of photosynthesis, we investigated 17 species of the genus Clusia, and one species each of the Clusiaceae genera Oedematopus and Hypericum on the basis of internal transcribed spacer (ITS) sequences between the 18S and 26S coding regions of nuclear ribosomal DNA. Little length variation was detected in the ITS region of Clusia species. ITS1 sequences ranged from 255 to 260 bp and ITS2 sequences from 208 to 210 bp. Neighbour-joining and parsimony analyses of these sequences resulted in considerable differences in cluster formation when compared to a classification based on morphological characteristics. The molecular data also give no indication of a group-specific evolution of modes of photosynthesis, i.e., C₃ and CAM. We thus conclude that CAM has evolved independently several times within the genus Clusia.     

Expression of Modes of Photosynthesis (C3, CAM) in Clusia criuva Camb. in a CerradolGallery Forest Transect

Abstract: The large majority of the Ca . 150 species of the neotropical shrub and tree genus Clusia have the potential to perform Crassulacean acid metabolism (CAM). They are either obligate CAM plants or C₃/CAM intermediate plants. Only a very small percentage of the plants studied so far are apparently obligate C₃ species. Among these was C. criuva , until recent laboratory studies showed that it may also have a certain CAM capacity under artificial stress conditions. Measurements of stomatal conductance (porometry) and chlorophyll fluorescence variables of C. criuva occurring along a transect from deep shade inside a gallery forest across the semi-shaded ecotone towards a cerrado and into the exposed cerrado itself in central Brazil now show that it can adapt its photosynthetic apparatus to effective performance of C₃ photosynthesis under highly different photosynthetic photon flux densities. In addition, however, it does have a certain potential for CAM and cannot be considered as a strictly obligate C₃ plant. Should a basic capacity for performing CAM be a general property of the genus, the quest for CAM traits in other remaining putatively obligate C₃ species of the genus ought to be pursued.     

Stomatal responses to light in the facultative Crassulacean acid metabolism species, Portulacaria afra

Guard cell responses to light are mediated by guard cell chlorophyll and by a specific blue light photoreceptor. Gas exchange and epidermal peel techniques were employed to investigate these responses in the facultative Crassulacean acid metabolism (CAM) species, Portulacaria afra (L.) Jacq. In P. afra individuals performing C₃ metabolism, red light stimulated an increase in leaf conductance in intact leaves and stomatal opening in isolated epidermal peels, indicating the presence in guard cells of the chlorophyll-mediated response to light. Under a background of continuous red illumination, conductance exhibited transient increases following pulses of blue but not red light, indicating that the specific stomatal response to blue light was also operative. In contrast, in CAM individuals, conductance in gas exchange experiments and stomatal opening in epidermal peel experiments were not stimulated by red light. In CAM plants, conductance did not increase following blue light pulses administered over a range of temperatures, vapor pressure differences (VPD), ambient CO₂ concentrations and background red light intensities. These results indicate that P. afra does possess typical guard cell responses to light when performing C₃ metabolism. The metabolic pathways mediating these responses are either lost or inhibited when CAM is induced.     

Elevated CO2 enhances stomatal responses to osmotic stress and abscisic acid in Arabidopsis thaliana

A , carbon assimilation rate
ABA, abscisic acid
Ci , intercellular space CO₂ concentration
g , leaf conductance
WUE, water use efficiency

Carbon dioxide and abscisic acid (ABA) are two major signals triggering stomatal closure. Their putative interaction in stomatal regulation was investigated in well-watered air-grown or double CO₂-grown Arabidopsis thaliana plants, using gas exchange and epidermal strip experiments. With plants grown in normal air, a doubling of the CO₂ concentration resulted in a rapid and transient drop in leaf conductance followed by recovery to the pre-treatment level after about two photoperiods. Despite the fact that plants placed in air or in double CO₂ for 2 d exhibited similar levels of leaf conductance, their stomatal responses to an osmotic stress (0·16–0·24 MPa) were different. The decrease in leaf conductance in response to the osmotic stress was strongly enhanced at elevated CO₂. Similarly, the drop in leaf conductance triggered by 1 μ M ABA applied at the root level was stronger at double CO₂. Identical experiments were performed with plants fully grown at double CO₂. Levels of leaf conductance and carbon assimilation rate measured at double CO₂ were similar for air-grown and elevated CO₂-grown plants. An enhanced response to ABA was still observed at high CO₂ in pre-conditioned plants. It is concluded that: (i) in the absence of stress, elevated CO₂ slightly affects leaf conductance in A. thaliana ; (ii) there is a strong interaction in stomatal responses to CO₂ and ABA which is not modified by growth at elevated CO₂.     

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.     

Interaction of photoperiod and gibberellin on growth and photosynthesis of high-latitude Poa pratensis

The relative growth rate of pot-grown plants of Poa pratensis L. cv. Holt, origin 69s°N, was increased by 20–40% by photoperiod extension with low intensity incandescent light from 8 to 24 h at 9–21°C. The main increase occurred over the 14 to 18 h photoperiod range. The true photoperiodic nature of the response was demonstrated by the effectiveness of night interruption in stimulating growth. Fortnightly sprayings with gibberellic acid (GA₃) (3 × 10^-6 to 3 × 10^-5 M ) mimicked all the effects of long days, whereas (2-chloroethyl)-trimethylammonium chloride (CCC) counteracted the effects of long days. Both growth substances exhibited pronounced interactions with photoperiod, GA₃ being most effective in short days and CCC in long days. The growth stimulation, whether caused by long days or GA₃, was exerted mainly through increases in individual and total leaf area. This was associated with a reduction in CO₂, exchange rate and a parallel fall in specific leaf weight. Proportionally, however, the increase in leaf area was greater than the fall in CO₂ exchange rate, resulting in a 38 to 118% increase in photosynthesis per leaf. No evidence was found of any direct and promotive effect of transition to long days on the CO₂ exchange rate of already expanded leaves.     

Short-Term Regulation of Crassulacean Acid Metabolism Activity in a Tropical Hemiepiphyte,Clusia uvitana

Diel courses of net CO2 exchange of leaves were studied in Clusia uvitana (Clusiaceae), a tropical Crassulacean acid metabolism (CAM) hemiepiphyte, growing in the crown of a 47-m tall kapok tree on Barro Colorado Island, Panama. Measurements on days without precipitation showed that net uptake of atmospheric CO2 occurred at night, a feature of CAM, as well as in the early morning and late afternoon. During 36 h of almost continuous rainfall, nocturnal net CO2 uptake was abolished and the diel pattern of net CO2 exchange became similar to that of a C3 plant. Exposing well-watered, potted plants of Clusia in the laboratory to temperatures and photosynthetic photon flux densities similar to those during the tropical rainstorm also abolished nocturnal net CO2 uptake. In contrast, Kalanchoe pinnata (Crassulaceae), an obligate CAM plant, still showed net CO2 dark fixation following the same low-light and moderate-temperature conditions, albeit at decreased rates. During these 12-h photoperiods, titratable acidity in Clusia increased slightly above its high level measured at the end of the previous dark period, whereas in Kalanchoe, the acid content decreased by about 40%. A survey among outer canopy leaves of Clusia on Barro Colorado Island showed that leaves that exhibited little or no nocturnal acidification maintained high levels of H+ at dawn and dusk. Progressively lower levels of H+ at dusk were accompanied by progressively higher nocturnal increases in H+. The data suggest that in C. uvitana the rapid switching between CAM- and C3-type carbon fixation that may occur within 24 h in response to environmental changes is controlled by the acidity status of the leaves in the light. Nocturnal CO2 fixation is enhanced by conditions that decrease the organic acid content during the light period.     

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.     

Are legume‐feeding herbivores buffered against direct effects of elevated carbon dioxide on host plants? A test with the sulfur butterfly,Colias philodice

When grown under elevated atmospheric carbon dioxide (CO₂), leaf nitrogen content decreases less for legumes than for nonlegume C₃ plants. Given that elevated CO₂ adversely affects insect herbivores primarily through dilution of plant nitrogen, it is reasonable to expect that legume-feeding herbivores will be relatively buffered against CO₂-induced reduction in performance. However, despite their ecological and economic importance, very few studies have addressed the effects of elevated CO₂ on legume-feeding herbivores. Unlike the responses of the vast majority of nonlegume C₃ plants, when the legumes Trifolium pratense and Melilotus alba were grown under elevated (742 ppm) CO₂, leaf nitrogen and carbon contents and C : N ratios did not change. For Colias philodice larvae fed T. pratense , elevated CO₂ had little or no effect on consumption, digestion, or conversion of whole food or nitrogen and, consequently, no effect on growth rate, instar duration, or pupal weight. For larvae fed M. alba , elevated CO₂ had little or no effect on consumption of whole food or nitrogen, increased digestion but decreased conversion of both and, consequently, had no effect on growth rate, instar duration or pupal weight. These results suggest that, relative to herbivores of nonlegume C₃ plants, legume-feeding herbivores will be less affected as atmospheric CO₂ continues to rise.     

δ 13C of CO2 respired in the dark in relation to δ13C of leaf carbohydrates in Phaseolus vulgaris L. under progressive drought

The variations in δ ¹³C in both leaf carbohydrates (starch and sucrose) and CO₂ respired in the dark from the cotyledonary leaves of Phaseolus vulgaris L. were investigated during a progressive drought. As expected, sucrose and starch became heavier (enriched in ¹³C) with decreasing stomatal conductance and decreasing p _i/ p _a during the first half (15 d) of the dehydration cycle. Thereafter, when stomata remained closed and leaf net photosynthesis was near zero, the tendency was reversed: the carbohydrates became lighter (depleted in ¹³C). This may be explained by increased p _i/ p _a but other possible explanations are also discussed. Interestingly, the variations in δ ¹³C of CO₂ respired in the dark were correlated with those of sucrose for both well-watered and dehydrated plants. A linear relationship was obtained between δ ¹³C of CO₂ respired in the dark and sucrose, respired CO₂ always being enriched in ¹³C compared with sucrose by ≈ 6‰. The whole leaf organic matter was depleted in ¹³C compared with leaf carbohydrates by at least 1‰. These results suggest that: (i) a discrimination by ≈ 6‰ occurs during dark respiration processes releasing ¹³C-enriched CO₂; and that (ii) this leads to ¹³C depletion in the remaining leaf material.     

Comparative Study of the C3/CAM Intermediate Species Clusia parviflora Saldanha et Engi. and the Obligate CAM Species Clusia hilariana Schlecht. Growing Sympatrically Exposed and Shaded in the Coastal Restinga of Brazil

Abstract: The C₃/CAM intermediate species, C/usia parviflora Saldanha et EngI., and the obligate CAM species Clusia hilariana Schlecht., occur sympatrically in the coastal sand dune vegetation of the Restinga of Brazil. Their photosynthetic activity at an exposed and at a shaded site was compared by measuring gas exchange (porometry), chlorophyll a fluorescence parameters, organic acid levels (malic and citric) and carbon isotope ratios. At the shaded site, low photosynthetic photon flux densities (PPFD) strongly restricted photosynthetic activity. However, C parviflora could readily make use of light flecks. At the exposed site, C. parviflora was much less affected by photoinhibition than C. hilariana . The CAM species showed higher apparent rates of linear photosynthetic electron transport (ETR) and higher effective quantum yield of PSII (ΔF/F'_m) than did C. parviflora during high insolation in the middle of the day, i.e., the time of Phase Ill of CAM. Nevertheless, it suffered much more severe acute photoinhibition that was not reversible after 20 min of darkening during this time, and even some chronic photoinhibition not reversible overnight. Comparative studies of sympatric physiotypes with different modes of photosynthesis of a given leaf morphotype, as available in the genus Cksia , challenge some CAM dogmas, e.g., CAM may not always be superior at exposed sites and may not always provide better photoprotection at high PPFD. However, the idea that C₃/CAM plasticity allows occupation of a wider range of habitats is supported.     

Cladogram of Panamanian Clusia Based on Nuclear DNA: Implications for the Origins of Crassulacean Acid Metabolism

Abstract: The internal transcribed spacer (ITS) region of 18-26S nuclear ribosomal DNA repeat was sequenced for 31 of the approximately 40 species of Clusia known to occur in Panama. Several species from other genera of the Clusiaceae were used as outgroups in the phylogenetic calculation. High sequence alignment and minimal length variation among ITS-1, 5.8S and ITS-2 sequences facilitated determination of positional homology of nucleotide sizes. Sequence alignment was evaluated with character state (Maximum Parsimony) and distance methods (Neighbour Joining). Phylogenetic trees obtained with the two methods were largely concordant and revealed three main groups that roughly correspond to previous arrangements of species into three large morphological groups, the C. flava group, the C. minor group and the C. multiflora group. Because species of Clusia are either regular C₃ plants or exhibit crassulacean acid metabolism (CAM) involving varying proportions of CO₂ fixation in the dark versus the light, we mapped photosynthetic pathways onto the cladograms. Photosynthetic pathway classification was based on measurements of ¹³C/¹²C ratios of plant carbon and also on information available from the literature. Both the C. flava and C. minor group contained species exhibiting CAM, distributed on distinct branches of the cladograms, whereas the third group ( C. multiflora group) was composed of species which are not known to use CAM.     

CARBONIC ANHYDRASE ACTIVITY IN THE BLOOM-FORMING DINOFLAGELLATE PERIDINIUM GATUNENSE

The response of carbonic anhydrase (CA) activity in Peridinium gatunense Nygaard, the natural bloom-forming dinoflagellate in Lake Kinneret, to diel and seasonal variations in environmental conditions was characterized under controlled laboratory experiments. Simulated diel cycles demonstrated large changes in the ambient concentration of dissolved CO₂ and parallel changes in CA activity. The CA activity depended on the total concentrations of inorganic carbon (C₁) and in particular on the dissolved CO₂. Lowering the C₁ concentrations resulted in a large increase in CA activity within several hours. Light and photosynthesis were both required for the induction of CA activity. Under CO₂ -limited conditions, the dependence of the photosynthetic rate on CA (estimated from the ratio of photosynthetic rates in the presence or absence of CA inhibitors) was greater in P. gatunense than in other eukaryotic microalgae. This points to the ecological significance of CA in photosynthetic carbon uptake mechanisms of a large, dominant alga in a natural ecosystem .