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     

Comparative ecophysiology of Encelia farinosa and Encelia frutescens

Summary Encelia farinosa and Encelia frutescens are drought-decidous shrubs whose distributions overlap throughout much of the Sonoran Desert. During hot and dry periods, leaves of E. farinosa utilize increased leaf reflectance to reduce leaf temperature, whereas leaves of E. frutescens have substantially higher leaf conductances and rely on increased transpirational cooling to reduce leaf temperature. E. farinosa is common on the dry slope microhabitats, whereas E. frutescens occurs only in wash microhabitats where greater soil moisture is available to provide the water necessary for transpirational cooling. E. farinosa tends not to persist in wash microhabitats because of its greater susceptibility to flashfloods. The consequences and significance of increased leaf reflectance versus increased transpirational cooling to leaf temperature regulation are discussed.     

Plasticity in the degree of CAM-cycling and its relationship to drought stress in five species of Talinum (Portulacaceae)

Summary The degree of CAM-cycling was examined in plants from 23 populations representing five morphologically similar species of Talinum to determine how CAM-cycling correlates with site aridity and drought stress. In the field, CAM-cycling, as indicated by the amount of malic acid accumulated in plant tissues overnight, and stable carbon isotope ratio (¹³C) were positively correlated with an index of site aridity. The relative levels of CAM-cycling and ¹³C values among populations in the field reversed when plants were grown under less arid conditions in a growth chamber such that populations with the highest average CAM-cycling and ¹³C in the field had the lowest averages in the growth chamber. In both cases, plants from all populations showed significantly higher levels of CAM-cycling under drought-stressed conditions relative to conditions known or predicted to be less arid. CAM-cycling was also positively correlated with tissue water content in both well-watered and drought-stressed plants, possibly reflecting greater water conservation associated with reductions in stomatal conductance. Biomass accumulation in plants exhibiting the greatest degree of CAM-cycling in the growth chamber was suppressed by drought stress to a similar degree as in the other plants, yet reproductive biomass of these plants was inhibited to a smaller degree. Thus, the importance of CAM-cycling increases in these species of Talinum during drought stress, due to increased malic acid accumulation overnight, potentially enhancing their water status, survival, and reproduction.     

Carbon isotope composition of canopy leaves in a tropical forest in Panama throughout a seasonal cycle

The seasonal variation in ¹³C values was measured in leaves from 17 upper canopy, five mid- canopy and in four gap tree species, as well as in five epiphyte and five vine species, in a seasonally dry lowland tropical forest at Parque Natural Metropolitano near Panama City, Republic of Panama. No seasonal variation was detected in the ¹³C values of mature exposed leaves from either the upper or mid- canopy. However, canopy position did influence the ¹³C value. The mean isotopic composition of leaves from the mid- canopy was more negative than that of the upper canopy throughout the year. The ¹³C value was also influenced by leaf development, with juvenile leaves on average 1.5 less negative than mature leaves. The five epiphyte species exhibited ¹³C values that were typical of crassulacean acid metabolism (CAM). Codonanthe uleana, with isotopic values of –19.9 to –22.1, is only the second species in the Gesneriaceae reported to express CAM, whereas values between –14.6 and –22.0 indicate that Peperomia macrostachya can exhibit different degrees of CAM. The isotopic composition of exposed mature leaves from the vines showed little interspecific variation and was similar to the upper-canopy leaves of the trees.     

Crassulacean acid metabolism in selected terrestrial succulents in southeastern Jamaica,including two species in the Commelinaceae

Summary Using determinations of overnigh changes in tissue titratable acidity and of tissue stable carbon isotope ratios, 10 species of terrestrial succulents were investigatedin situ in southeastern Jamaica for the presence of Crassulacean acid metabolism (CAM). Eight of the 10 species exhibited CAM (sensu lato), confirming past reports of CAM inClusia flava (Clusiaceae),Bryophyllum pinnatum (Crassulaceae),Euphorbia tirucalli (Euphorbiaceae), andPedilanthus tithymaloides (Euphorbiaceae) and extending the number of species with CAM in two genera previously known to contain CAM species (Agave sobolifera [Agavaceae] andSansevieria metalllica [Liliaceae]). Stems of bothE. tirucalli andP. tithymaloides exhibited CAM while the leaves of both species were intermediate with regard to photosynthetic pathway. The lack of CAM acid fluctuations inTalinum paniculatum (Portulacaceae) was surprising given past findings with all other species investigated in this genus. Shoots ofPilea microphylla (Urticaceae) were C₃ yet were remarkable in their extremely high pH. Both species require further investigation. Nocturnal acid accumulations indicative of CAM were found inTripogandra multiflora andCallisia fragrans, both members of the Commelinaceae. This represents the first report of CAM (probably “CAM-cycling”) in this family.     

Barriers to movement and the response of herbivores to alternative cropping patterns

Summary Clusia rosea Jacq. is a hemiepiphyte having Crassulacean Acid Metabolism (CAM). In its natural habitat Clusia begins its life cycle as an epiphyte and eventually becomes a rooted tree. These two stages of the life cycle of Clusia represent markedly different water regimes. Our CO₂ exchange, stomatal conductance, titratable acidity, and stable carbon isotope ratio measurements indicate that Clusia has a flexible photosynthetic mode, where CO₂ is fixed mostly via CAM during its epiphytic stage, when water availability is low, and via both CAM and C₃ during its rooted stage.     

A Comparative Study by δ13C-Analysis of Crassulacean Acid Metabolism (CAM) in Kalanchoë (Crassulaceae) Species of Africa and Madagascar

The carbon isotope ratios (δ¹³C values) of samples of Kalanchoë species collected in Africa were compared with previous data obtained with species from Madagascar. In contrast to the Malagasy species which cover the whole range of δ¹³C values from ?10 to ? 30%_o, indicating high inter- and intraspecific diversity of CAM performance, in the African species nearly all δ¹³C values were less negative than ?18%₀. Thus, in the African species the CAM behaviour is characterized by CO₂ uptake proceeding mainly during the night. The distribution of δ¹³C values among the species clearly mirrors the taxonomic groups and the three sections of the genus Kalanchoë sensu lato. The Kitchingia section comprises only groups having CAM with a high proportion of carbon acquisition by the C₃-pathway of photosynthesis. The same holds true for the first three groups of the Bryophyllum section, whereas in the following groups of the section CAM with CO₂ proceeding mainly during the night is common. The latter CAM mode is typical also for the majority of groups and species in the section Eukalanchoë. The African Kalanchoë species belong to the Eukalanchoë section, whereas in Madagascar all three sections are abundant. The data support the view that the centre of adaptive radiation of the genus is located in Madagascar. They also suggest that high CAM variability is abundant in the more primitive taxa of the genus, whereas the phylogenetically more derived taxa show a stereotype CAM with CO₂ uptake taking place only during the night.     

Morphological and physiological variation in western redcedar (<Emphasis Type="Italic">Thuja plicata</Emphasis>) populations under contrasting soil water conditions

Adaptation to precipitation conditions may induce genetic diversity that changes morphological and physiological traits. This hypothesis was investigated in the seedlings of seven western redcedar (Thuja plicata Donn ex D. Don) populations, which were collected along a precipitation transect from the Pacific coast to the southern interior of British Columbia, Canada. The experimental seedlings were either well-watered or soil-droughted and measured for growth, gas exchange rates, transpiration efficiency, and carbon isotope discrimination during or at the end of the third growing season. Significant variation was found in most of these morphological and physiological traits among the populations. Much of this variation occurred under well-watered, but not so much under droughted conditions. Mean height increments and transpiration efficiency showed a significant linear relationship, but biomass increments exhibited a quadratic relationship with precipitation on the origin site of these populations. Measurements of water use efficiency obtained from instantaneous gas exchange measurements, carbon isotope discrimination, and transpiration efficiency were intercorrelated in the seedlings. However, neither did any of these measurements consistently rank the populations, nor were they indicative of adaptation to climatic precipitation conditions in these western redcedar populations.     

Photosynthetic acclimation and water-use efficiency of three species of understory herbaceous bamboo (Gramineae) in Panama

Summary To assess the role of photosynthetic acclimation in the response of tropical understory herbs to treefall light gaps, photosynthetic response curves were determined for three species of herbaceous bamboo growing in treatments of sun and shade at Barro Calorado Island, Panama. Increased maximum photosynthetic capacity did not always accompany higher ramet production in the sun treatment. Pharus latifolius reproduced abundantly in both treatments, and produced more ramets and developed higher maximum photosynthetic capacity under higher irradiance. Streptochaeta spicata also produced a high percentage of reproductive ramets in both treatments and produced more ramets in the sun, did not show any significant differences in photosynthetic parameters between treatments. Streptochaeta sodiroana did not change maximum photosynthetic capacity in the sun, and had higher photosynthetic efficiency and lower mortality in the shade. Stable carbon isotope composition of leaves indicated that all three species developed higher water-use efficiency under higher irradiance. Photosynthetic flexibility may contribute to the ability of P. latifolius to reproduce in treefall gaps, whereas S. spicata and S. sodiroana may maintain the ability to fix carbon efficiently in low irradiance even when growing or persisting in gaps.     

Carbon isotope composition in relation to leaf gas exchange and environmental conditions in Hawaiian Metrosideros polymorpha populations

Summary Carbon isotope composition, photosynthetic gas exchange, and nitrogen content were measured in leaves of three varieties of Metrosideros polymorpha growing in sites presenting a variety of precipitation, temperature and edaphic regimes. The eight populations studied could be divided into two groups on the basis of their mean foliar ¹³C values, one group consisting of three populations with mean ¹³C values ca.-26 and another group with ¹³C values ca.-28. Less negative ¹³C values appeared to be associated with reduced physiological availability of soil moisture resulting from hypoxic conditions at a poorly drained high elevation bog site and from low precipitation at a welldrained, low elevation leeward site. Gas exchange measurements indicated that foliar ¹³C and intrinsic wateruse efficiency were positively correlated. Maximum photosynthetic rates were nearly constant while maximum stomatal conductance varied substantially in individuals with foliar ¹³C ranging from-29 to-24. In contrast with the patterns of ¹³C observed, leaf nitrogen content appeared to be genetically determined and independent of site characteristics. Photosynthetic nitrogenuse efficiency was nearly constant over the range of ¹³C observed, suggesting that a compromise between intrinsic water- and N-use efficiency did not occur. In one population variations in foliar ¹³C and gas exchange with leaf cohort age, caused the ratio of intercellular to atmospheric partial pressure of CO₂ predicted from gas exchange and that calculated from ¹³C to be in close agreement only in the two youngest cohorts of fully expanded leaves. The results indicated that with suitable precautions concerning measurement protocol, foliar ¹³C and gas exchange measurements were reliable indicators of potential resource use efficiency by M. polymorpha along environmental gradients.     

Gas exchange and water balance of a mistletoe species and its mangrove hosts

Summary The gas exchange and water relations of the hemiparasite Pthirusa maritima and two its mangrove host species, Conocarpus erectus and Coccoloba uvifera, were studied in an intertidal zone of the Venezuelan coast. Carbon uptake and transpiration, leaf osmotic and total water potential, as well as nutrient content in the xylem sap and leaves of mistletoes and hosts were followed through the dry and wet season. In addition, carbon isotope ratios of leaf tissue were measured to further evaluate water use efficiency. Under similar light and humidity conditions, mistletoes had higher transpiration rates, lower leaf water potentials, and lower water use efficiencies than their hosts. Potassium content was much higher in mistletoes than in host leaves, but mineral nutrient content in the xylem sap of mistletoes was relatively low. The resistance of the liquid pathway from the soil to the leaf surface of mistletoes was larger than the total liquid flow resistance of host plants. Differences in the daily cycles of osmotic potential of the xylem sap also indicate the existence of a high resistance pathway along the vascular connection between the parasite pathway along the vascular connection between the parasite and its host. P. maritima mistletoes adjust to the different physiological characteristics of the host species which it parasitizes, thus ensuring an adequate water and carbon balance.     

Relationships between carbon and hydrogen isotope ratios and nitrogen levels in leaves ofClusia species and two other Clusiaceae genera at various sites and different altitudes in Venezuela

Samples of the Clusiaceae generaClusia, Oedematopus andDystovomita were collected at various sites and different altitudes in northern and south-western Venezuela. Analyses of stable isotopes of carbon and hydrogen and of leaf-nitrogen levels were performed on the dried samples. Correlations among these variables, i.e. carbon isotope discrimination (), hydrogen isotope ratio (D) and N-levels, and with altitude were assessed. In the samples, where values of above 15 indicate predominant performance of C₃ photosynthesis, there were slight tendencies of increasing , D and N-levels with increasing altitude and of increasing with increasing N. Although these correlations taken separately were not statistically significant, they support each other and indicate increasing transpiration and increased leaf-nutrient supply at increasing altitude. Performance of crassulacean acid metabolism (CAM) in species ofClusia appears to be restricted to altitudes below 1500 m a.s.l. There was a significant negative correlation of with altitude in the samples, where values of below 10 indicated predominant performance of CAM. This suggests that phases II and IV of CAM are progressively suppressed towards the upper altitudinal limit of CAM inClusia in northern Venezuela. It is concluded that among the large number of environmental factors and combinations thereof, which determine the expression of CAM inClusia and trigger C₃-CAM transitions in C₃/CAM intermediate species, low availability of water is the most important.     

Comparative studies on immunological and molecular properties of phosphoenolpyruvate carboxylase in species of Sedum and Kalanchoë performing crassulacean acid metabolism (CAM)

Abstract. PEP-carboxylase was extracted and partially purified from nine species of the genus Sedum and three species of the genus Kalanchoe, all performing CAM. Immunological and molecular properties of these enzymes were compared. Molecular weight estimation with gradient slab gels showed identical molecular weights of about 232,000 for all PEP-carboxylases. Ouchlerlony double-diffusion analysis, immunotitralion and SDS polyacrylamide clectrophoresis indicated the presence of PEP-c dimers consisting of monomers of MW 105,000 and 115,000. A model of PEP-c substructure is proposed. The results are discussed in the context of CAM performance in the genus Sedum     

Ecophysiological studies on orchids of Madagascar: incidence and plasticity of crassulacean acid metabolism in species of the genus Angraecum Bory

The present study is an investigation on photosynthetic options in an orchid taxon and deals with the mainly epiphytes comprising genus Angraecum Bory. The incidence of crassulacean acid metabolism (CAM) in Angraecum species collected at various habitats in Madagascar was surveyed by analysis of stable carbon isotope composition (¹³C values). The values showed both inter- and intraspecific variability and suggest that in situ about 50% of the analysed species perform C3 photosynthesis, 20% moderate CAM (fixation of external CO₂ during day-and night-time) and 30% pronounced CAM (CO₂ uptake entirely during the night). The photosynthetic behaviour of the species indicated by the ¹³C values was clearly related to the habitat from where the samples derived. In A. eburneum, A. sororium and particularly in A. sesquipedale the stable carbon isotope analysis was complemented by measurements of CAM performance under controlled conditions. The experiments with A. sesquipedale revealed that drought and temperature are important factors modulating CAM, whereas variation of the leaf-to-air water vapor pressure difference was less effective. Altogether, the results of the study support the view that the high biological adaptability and thus the ecological success of the genus Angraecum is largely based on genotypic diversity and intraspecific plasticity of the photosynthetic behaviour.     

Short-term changes in carbon-isotope discrimination identify transitions between C3 and C4 carboxylation during Crassulacean acid metabolism

Short-term measurements of instantaneous carbon-isotope discrimination have been determined from mass-spectrometric analyses of CO₂ collected online during gas exchange for the epiphytic bromeliad Tillandsia utriculata L. Using this technique, the isotopic signature of CO₂ exchange for each phase of Crassulacean acid metabolism (CAM) has been characterised. During night-time fixation of CO₂ (Phase I), discrimination () ranged from 4.4 to 6.6, equivalent to an effective carbon-isotope ratio (¹³C) of –12.3 to –14.5 versus Pee Dee Belemnite (PDB). These values reflected the gross photosynthetic balance between net CO₂ uptake and refixation of respiratory CO₂, characteristic of CAM in the Bromeliaceae. When for the relative proportion of external (p _a ) and internal (p _i) CO₂ is taken into account, calculated p _i/p _a decreased during the later part of the dark period from 0.68 to 0.48. Measurements of during Phase II, early in the light period, showed the transition between C₄ and C₃ pathways, with carboxylation being increasingly dominated by ribulose bisphosphate carboxylase (Rubisco) as increased from 10.5 to 21.2 During decarboxylation in the light period (Phase III), CO₂ leaked out of the leaf and the inherent discrimination of Rubisco was expressed. The value of calculated from on-line measurements (64.4) showed that the CO₂ lost was considerably enriched in ¹³C, and this was confirmed by direct analysis of the CO₂ diffusing out into a CO₂-free atmosphere ( ¹³C = + 51.6 versus PDB). Instantaneous discrimination was characteristic of the C₃ pathway during Phase IV (late in the light period), but a reduction in showed an increasing contribution from phosphoenolpyruvate carboxylase. The results from this non-invasive technique confirm the observations that double carboxylation involving both phosphoenolpyruvate carboxylase and Rubisco occurs during the transient phases of CAM (II and IV) in the light period.Abbreviations and Symbols CAM Crassulacean acid metabolism - H⁺ (dawn-dusk) variation in titratable acidity - ¹³C carbonisotope ratio of plant organic material, relative to Pee Dee Belemnite (vs. PDB) - discrimination against ¹³CO₂, - p _i, p _a internal, external partial pressures of CO₂ - Rubisco ribulose1,5-bisphosphate carboxylase - PAR photosynthetically active radiation - PEPCase phosphoenolpyruvate carboxylase We are grateful for financial support in respect of research grants (GR3/5360, GR3/6419) and a studentship awarded by the Natural Environment Research Council, UK.     

Changes in levels of phosphoenolpyruvate carboxylase with induction of Crassulacean acid metabolism (CAM)-like behavior in the C4 plant Portulaca oleracea

Changes in levels of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31, orthophosphate: oxaloacetate carboxy-lyase, phosphorylating) were followed in leaves and stems of CAM-expressing and non-expressing Portulaca oleracea L. plants. CAM expression was induced by growing plants under an 8-h photoperiod and water stress conditions (SD-WS). Leaves and stems of these plants (designated CAM) expressed nocturnal acidification with an oscillation pattern and an amplitude characteristic of CAM plants. Generally, PEPC activity increased by ca 3-fold during the period of CAM induction. Over the day/night cycle. PEPC activity oscillated in a pattern typical of CAM plants. Treatment of the other plant group (designated as non-CAM) by growth under a 16-h photoperiod and well-watered conditions (LD-WW) did not induce expression of the tested criteria of CAM in plants. In these plants, nocturnal acidification as well as changes in the magnitude of PEPC, activity and fluctuation pattern were undetectable. SDS-PAGE of leaf extracts of the CAM-expressing plants and the corresponding densitometric scans show progressive increase in the amount of PEPC subunit protein (ca 95 kDa) during the period of CAM induction. These results show that induction of CAM-like characteristics in the C₄ plant Portulaca oleracea is also accompanied by increased PEPC activity, which may be partly due to an increase in enzyme synthesis.     

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.     

Photosynthetic light-use by three bromeliads originating from shaded sites (<Emphasis Type="Italic">Ananas ananassoides,Ananas comosus</Emphasis> cv. Panare) and exposed sites (<Emphasis Type="Italic">Pitcairnia pruinosa</Emphasis>) in the medium Orinoco basin,Venezuela

Three Bromeliaceae species of the medium Orinoco basin, Venezuela, were compared in their light-use characteristics. The bromeliads studied were two species of pineapple, i.e. the wild species Ananas ananassoides originating from the floor of covered moist forest, and the primitive cultivar Panare of Ananas comosus mostly cultivated in semi-shaded palm swamps, and Pitcairnia pruinosa, a species abundant in highly sun exposed sites on rock outcrops. Ananas species are Crassulacean acid metabolism (CAM) plants, P. pruinosa is C₃ plant. Plants were grown at low daily irradiance (LL = 1.3 mol m^–2 d^–1 corresponding to an incident irradiance of 30 mol m^–2 s^–1) and at high irradiance (HL = 14.7 mol m^–2 d^–1 or 340 mol m^–2 s^–1), and CO₂ and H₂O-vapour gas exchange and photochemical (q_P) and non-photochemical quenching (q_NP) of chlorophyll a fluorescence of photosystem 2 (PS2) were measured after transfer to LL, medium irradiance (ML = 4.1 mol m^–2 d^–1 or 95 mol m^–2 s^–1) and HL. All plants showed flexible light-use, and q_P was kept high under all conditions. LL-grown plants of Ananas showed particularly high rates of CAM-photosynthesis when transferred to HL and were not photoinhibited.     

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.