Mode Of Photosynthesis Research Articles

Sequencing complex plants on a budget: The development of Kalanchoë blossfeldiana as a C3, CAM comparative tool

Societal Impact StatementResearch efforts in plant biology have often been focused on sequenced and well‐studied ‘model’ organisms. Despite the advent of relatively inexpensive genome sequencing, most plant taxonomic groups are underrepresented, with few species that ‘represent’ the diversity of whole genera. This study describes an economical guide to sequencing a non‐model organism, which may be useful in reducing the cost of sequencing more species within genera and across plant life. This method was used to develop Kalanchoë blossfeldiana as a resource for comparing C3 and the water‐conserving mode of photosynthesis known as Crassulacean acid metabolism (CAM) within the same plant.Summary Despite the increasing number of well‐studied plant species with well‐annotated genomes across plant life, there are few densely sampled genera with more than a couple of genome sequences representing the diversity of whole genera. Here, we develop an economic approach to full‐genome sequencing that could be used to sequence many species within a genus. We made use of the Nanopore rapid sequencing kit to assist in plant genome assembly, dramatically reducing the cost. Here we applied this method to cost‐effectively develop genomic resources for Kalanchoë blossfeldiana, a commercially important ornamental, in which Crassulacean Acid Metabolism (CAM), a water‐conserving mode of photosynthesis can be induced. We present a physiological and biochemical characterisation of Kalanchoe blossfeldiana with its nuclear and chloroplastic genome and a comparative C3, CAM dusk transcriptome. We apply this approach to a complex tetraploid genome, making use of a relative species for chromosomal scaffolding to reduce assembly ploidy, we provide a resource for future gene expression studies. We highlight its limitations, e.g. the need for deeper sequencing to accurately resolve genome structure and haplotypes without using a relative species for scaffolding. The study demonstrates the merits of K. blossfeldiana as a comparative system for studying C3 and CAM within a plant and has identified substantial changes in the dusk transcriptome between young C3 and mature CAM K. blossfeldiana leaves in response to age‐induced CAM, and shows that in the absence of abiotic stress, CAM induction still involves the engagement of drought and abscisic acid (ABA) response pathways.

Evolution of Crassulacean acid metabolism in response to the environment: past, present, and future

Crassulacean acid metabolism (CAM) is a mode of photosynthesis that evolved in response to decreasing CO2 levels in the atmosphere some 20 million years ago. An elevated ratio of O2 relative to CO2 caused many plants to face increasing stress from photorespiration, a process exacerbated for plants living under high temperatures or in water-limited environments. Today, our climate is again rapidly changing and plants’ ability to cope with and adapt to these novel environments is critical for their success. This review focuses on CAM plant responses to abiotic stressors likely to dominate in our changing climate: increasing CO2 levels, increasing temperatures, and greater variability in drought. Empirical studies that have assessed CAM responses are reviewed, though notably these are concentrated in relatively few CAM lineages. Other aspects of CAM biology, including the effects of abiotic stress on the light reactions and the role of leaf succulence, are also considered in the context of climate change. Finally, more recent studies using genomic techniques are discussed to link physiological changes in CAM plants with the underlying molecular mechanism. Together, the body of work reviewed suggests that CAM plants will continue to thrive in certain environments under elevated CO2. However, how CO2 interacts with other environmental factors, how those interactions affect CAM plants, and whether all CAM plants will be equally affected remain outstanding questions regarding the evolution of CAM on a changing planet.

Meta‐analysis of primary producer amino acid δ15N values and their influence on trophic position estimation

Abstract Compound‐specific stable isotope analysis of individual amino acids (CSIA‐AA) has emerged as a transformative approach to estimate consumer trophic positions (TPCSIA) that are internally indexed to primary producer nitrogen isotope baselines. Central to accurate TPCSIA estimation is an understanding of beta (β) values—the differences between trophic and source AA δ15N values in the primary producers at the base of a consumers’ food web. Growing evidence suggests higher taxonomic and tissue‐specific β value variability than typically appreciated. This meta‐analysis fulfils a pressing need to comprehensively evaluate relevant sources of β value variability and its contribution to TPCSIA uncertainty. We first synthesized all published primary producer AA δ15N data to investigate ecologically relevant sources of variability (e.g. taxonomy, tissue type, habitat type, mode of photosynthesis). We then reviewed the biogeochemical mechanisms underpinning AA δ15N and β value variability. Lastly, we evaluated the sensitivity of TPCSIA estimates to uncertainty in mean βGlx‐Phe values and Glx‐Phe trophic discrimination factors (TDFGlx‐Phe). We show that variation in βGlx‐Phe values is two times greater than previously considered, with degree of vascularization, not habitat type (terrestrial vs. aquatic), providing the greatest source of variability (vascular autotroph = −6.6 ± 3.4‰; non‐vascular autotroph = +3.3 ± 1.8‰). Within vascular plants, tissue type secondarily contributed to βGlx‐Phe value variability, but we found no clear distinction among C3, C4 and CAM plant βGlx‐Phe values. Notably, we found that vascular plant βGlx‐Lys values (+2.5 ± 1.6‰) are considerably less variable than βGlx‐Phe values, making Lys a useful AA tracer of primary production sources in terrestrial systems. Our multi‐trophic level sensitivity analyses demonstrate that TPCSIA estimates are highly sensitive to changes in both βGlx‐Phe and TDFGlx‐Phe values but that the relative influence of β values dissipates at higher trophic levels. Our results highlight that primary producer β values are integral to accurate trophic position estimation. We outline four key recommendations for identifying, constraining and accounting for β value variability to improve TPCSIA estimation accuracy and precision moving forward. We must ultimately expand libraries of primary producer AA δ15N values to better understand the mechanistic drivers of β value variation.

Possible roles for phytohormones in controlling the stomatal behavior of Mesembryanthemum crystallinum during the salt-induced transition from C3 to crassulacean acid metabolism

The halophyte ice plant (Mesembryanthemum crystallinum) converts its mode of photosynthesis from C3 to crassulacean acid metabolism (CAM) during severe water stress. During the transition to CAM, the plant induces CAM-related genes and changes its diurnal stomatal behavior to take up CO2 efficiently at night. However, limited information concerning this signaling exists. Here, we investigated the changes in the diurnal stomatal behavior of M. crystallinum during its shift in photosynthesis using a detached epidermis. M. crystallinum plants grown under C3 conditions opened their stomata during the day and closed them at night. However, CAM-induced plants closed their stomata during the day and opened them at night. Quantitative analysis of endogenous phytohormones revealed that trans-zeatin levels were high in CAM-induced plants. In contrast, the levels of jasmonic acid (JA) and JA–isoleucine were severely reduced in CAM-induced plants, specifically at night. CAM induction did not alter the levels of abscisic acid; however, inhibitors of abscisic acid synthesis suppressed CAM-induced stomatal closure. These results indicate that M. crystallinum regulates the diurnal balance of cytokinin and JA during CAM transition to alter stomatal behavior.

Carbon isotopic analysis of reactive organic matter using a new pyrolysis-cryotrapping-isotope ratio mass spectrometry method: The isotope variation of organic matter within the S1 and S2 peaks of Rock-Eval

The source material, the mode of photosynthesis, the organic matter (OM) type, the paleoenvironmental conditions and thermal degradation all determine the isotopic composition of OM. This study presents a novel analytical technique and demonstrates its potential to enhance the understanding of the preservation and maturation-related isotopic compositional changes in OM from different paleoenvironments. A new Rock-Eval type pyrolysis coupled to isotope ratio mass spectrometry (py-IRMS) method has been developed to monitor the isotopic changes during Rock-Eval type pyrolysis while simultaneously recording pyrograms to obtain S1 and S2 peaks compatible with standard Rock-Eval pyrolysis. An automated cryotrapping method allows the sequential sampling of product from defined pyrolysis portions. Each subsample is subsequently transferred to the isotopic ratio mass spectrometer for carbon isotopic measurements. In addition, the cryotrap serves to focus sample peaks, enabling robust and reproducible results even at standard Rock-Eval heating rates. In combination with a standard elemental analyser coupled to an isotopic ratio mass spectrometer (EA–IRMS) it was possible to deconvolute the carbon isotope composition of the reactive components, bitumen and inert components of OM. This allowed us to observe depositional control as well as changes imposed by maturation. Our initial results show the potential of the proposed isotopic screening method to deduce a variety of organic geochemical properties of different OM types based on differences in their carbon isotope compositions. Our results suggest that the ability to produce significant isotopic fractionation during the proposed pyrolysis method is highly dependent on the preservation state of the OM and the thermal maturity.

Enhancement of Algal Biofeedstocks in a Mixotrophic Batch Culture Supplemented with Exogenous Glycerol

Chlorella vulgaris is a fast growing, unicellular microalgae. With its limited nutritional requirements and ability to grow in adverse environments, it is a promising biodiesel feedstock that will not compete with agricultural resources.1 Published literature has shown C. vulgaris can produce up to 42% lipids per dry mass, the most desired of which are triacylglycerols (TAGs).2 TAGs consist of three fatty acids esterified to a glycerol backbone and are the most reduced, energy‐dense biomolecules. To generate drop‐in ready biodiesel, TAGs are transesterified to yield methyl‐ester alkyls and a glycerol byproduct.To realistically produce fuel on an industrial scale, microalgal TAG production must be increased beyond current levels. Previous work published by the Kolling Lab increased lipid accumulation in C. vulgaris by using an exogenous carbon source to form a heteromixotrophic culture, in which the cells are energetically supplemented by both exogenous carbon as well as photosynthetic light reactions.3 This project seeks to further increase TAG production by supplying cells with exogenous glycerol, thus providing a use for the transesterification byproduct while reducing the cellular energy investment for TAG production.While published studies have found 10 g/L glycerol to be optimal for lipid production in C. vulgaris, this was completed heterotrophically.4 While growing mixotrophically, we have increased lipid accumulation beyond previously published glycerol studies and have increased overall lipid productivity in glycerol concentrations of 31.5 and 63 g/L. We also monitored photosynthetic performance before and after glycerol addition and observed an increased reliance on cyclic electron transport (a mode of photosynthesis that does not result in the production of NADPH), indicating that the ATP need is greater than that of NADPH for TAG formation under photomixotrophic conditions. Finally, using isotopically labeled glycerol, we have used GC‐MS to determine the rate of glycerol uptake and that exogenously provided glycerol is directly used for the TAG backbone and not metabolized for the generation of fatty acids.Support or Funding InformationThis material is based upon work supported by the National Science Foundation under Cooperative Agreement No. OIA‐1458952.Amanda Smythers acknowledges a fellowship from the NASA West Virginia Space Grant Consortium, Grant #NNX15AK74A and Training Grant # NNX15AI01H.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Nitrogen metabolism-related enzymes in Mesembryanthemum crystallinum after Botrytis cinerea infection

We compared C3 and CAM (crassulacean acid metabolism) states in Mesembryanthemum crystallinum, a facultative CAM species, with respect to the involvement of phosphoenolpyruvate carboxylase (PEPC) and nitrogen metabolismrelated enzymes in plant response to Botrytis cinerea infection. The enzyme activities were monitored both in pathogeninoculated 2nd leaf pair and non-inoculated 3rd leaf pair. The control activities of most studied enzymes were dependent on the mode of photosynthesis. Compared to C3 plants, those performing CAM exhibited higher PEPC, nitrate reductase (NR), and deaminating glutamate dehydrogenase (NAD-GDH) activities but lower glutamine synthetase (GS) and alanine aminotransferase (ALT) activities. Regardless of the mode of photosynthetic carbon assimilation, the plants responded to infection with enhancement of PEPC and inhibition of NR activities in the inoculated leaves. Whereas the activity of GS remained unaffected, those of all glutamate-yielding enzymes, namely ferredoxin-dependent glutamate synthase (Fd-GOGAT), aspartate aminotransferase (AST), ALT, and aminating glutamate dehydrogenase (NADHGDH) were altered after infection. However, the time-course and extent of the observed changes differed in C3 and CAM plants. In general, CAM plants responded to infection with an earlier increase in PEPC and Fd-GOGAT activities as well as later inhibition of NR activity. Contrary to C3 plants, in those performing CAM the activities of PEPC, Fd-GOGAT, NADH-GDH, and AST in the non-inoculated 3rd leaf pair were similarly influenced by infection as in leaves directly inoculated with the pathogen. This implies that the local infection induced an alteration of carbon/nitrogen status in healthy upper leaves. This reprogramming resulting from changes in PEPC and nitrogen metabolism-related enzymes was C3- and CAM-specific.

Exotic grassland species have stronger priority effects than natives regardless of whether they are cultivated or wild genotypes.

During community assembly, early arriving exotic species might suppress other species to a greater extent than do native species. Because most exotics were intentionally introduced, we hypothesize there was human selection on regeneration traits during introduction. This could have occurred at the across- or within-species level (e.g. during cultivar development). We tested these predictions by seeding a single species that was either native, exotic 'wild-type' (from their native range), or exotic 'cultivated' using 28 grassland species in a glasshouse experiment. Priority effects were assessed by measuring species' effect on establishment of species from a seed mix added 21 d later. Exotic species had higher germination and earlier emergence dates than native species, and differences were found in both 'wild' and 'cultivated' exotics. Exotic species reduced biomass and species diversity of later arriving species much more than native species, regardless of seed source. Results indicate that in situations in which priority effects are likely to be strong, effects will be greater when an exotic species arrives first than when a native species arrives first; and this difference is not merely a result of exotic species cultivation, but might be a general native-exotic difference that deserves further study.

Pollen morphological studies in subfamily Suaedoideae (Chenopodiaceae)

The pollen morphology of two Bienertia species and 28 Suaeda species from the Old World has been studied with light and scanning electron microscopy. Pollen morphological characters used in this study include pollen diameter, pore number, pore diameter, operculum diameter, chord distance, exine thickness, plus number and characteristics of exine spinules and operculum spinules. Using these characters, the pollen of Bienertia is clearly distinguished from that of Suaeda. In addition, pollen data support recent molecular studies which merge the genus Borszczowia into Suaeda. The pollen grains of Suaeda show notable variation; in some cases this allows morphological distinctions to be made between species. Taxonomically, pore number is observed to be a more useful character than pollen diameter. Principal components analysis (PCA) shows only a weak correlation between pollen characters and habit (life form), habitat, mode of photosynthesis or phylogenetic relationships. Interestingly, however, the annual C3 and C4 species of section Schoberia are usually characterised by a higher pore number than the perennial C4 species.

Carbon isotope composition and mode of photosynthesis in Clusia species from Mexico

The neotropical genus Clusia comprises arborescent species exhibiting Crassulacean Acid Metabolism (CAM) as was first reported for a Mexican species, Clusia lundellii. Here, the occurrence of CAM photosynthesis was studied in 20 species of Clusia, 18 from Mexico, and 2 from Guatemala, using leaf carbon isotopic composition. In most species, samples from individuals collected in different locations were analyzed. CAM was present in at least 11 species, eight of which contained specimens with δ13C values less negative than -20.0 ‰, indicating strong CAM (C. chanekiana, C. flava, C. lundellii, C. mexicana, C. quadrangula, C. rosea, C. suborbicularis, and C. tetra-trianthera). δ13C was highly variable in some species, but CAM expression was not correlated to life form (epiphytic, hemiepiphytic, terrestrial) or habitat. CAM specimens were not collected at altitudes above 1 700 m a.s.l.

Does Bienertia cycloptera with the single-cell system of C4 photosynthesis exhibit a seasonal pattern of δ13C values in nature similar to co-existing C4 Chenopodiaceae having the dual-cell (Kranz) system?

Family Chenopodiaceae is an intriguing lineage, having the largest number of C(4) species among dicots, including a number of anatomical variants of Kranz anatomy and three single-cell C(4) functioning species. In some previous studies, during the culture of Bienertia cycloptera Bunge ex Boiss., carbon isotope values (delta(13)C values) of leaves deviated from C(4) to C(3)-C(4) intermediate type, raising questions as to its mode of photosynthesis during growth in natural environments. This species usually co-occurs with several Kranz type C(4) annuals. The development of B. cycloptera morphologically and delta(13)C values derived from plant samples (cotyledons, leaves, bracts, shoots) were analyzed over a complete growing season in a salt flat in north central Iran, along with eight Kranz type C(4) species and one C(3) species. For a number of species, plants were greenhouse-grown from seeds collected from the site, in order to examine leaf anatomy and C(4) biochemical subtype. Among the nine C(4) species, the cotyledons of B. cycloptera, and of the Suaeda spp. have the same respective forms of C(4) anatomy occurring in leaves, while cotyledons of members of tribe Caroxyloneae lack Kranz anatomy, which is reflected in the delta(13)C values found in plants grown in the natural habitat. The nine C(4) species had average seasonal delta(13)C values of -13.9 per thousand (with a range between species from -11.3 to -15.9 per thousand). The measurements of delta(13)C values over a complete growing season show that B. cycloptera performs C(4) photosynthesis during its life cycle in nature, similar to Kranz type species, with a seasonal average delta(13)C value of -15.2 per thousand.

Effects of salinity, high irradiance, ozone, and ethylene on mode of photosynthesis, oxidative stress and oxidative damage in the C3/CAM intermediate plant Mesembryanthemum crystallinum L.

ABSTRACTThe occurrence of oxidative stress and oxidative damage in the halophyte Mesembryanthemum crystallinum during C3/crassulacean acid metabolism (CAM) transition evoked by salinity and high irradiance was investigated. The hypothesis tested was that CAM induction will effectively protect M. crystallinum against oxidative damage even under additionally applied oxidative stress via ozone (150 p.p.b.) exposure. Salinity (0.4 m NaCl), high irradiance and ozone induced the cytosolic CuZn‐superoxide dismutase (SOD; EC 1.15.1.1) transcript abundance and activity indicating oxidative stress. However, ozone did not induce CAM. Contrary to the hypothesis, that ethylene may be involved in CAM induction, gassing with ethylene (200 p.p.b.) did not lead to CAM, but revealed that in M. crystallinum ethylene biosynthesis is induced by autostimulation. A putative fragment of the hitherto unknown nu‐cleotide sequence of 1‐aminocyclopropane‐1‐carboxylate oxidase (ACC; EC 4.4.1.14) was isolated and used as a specific probe against the ACC oxidase transcript to monitor oxidative damage. Ozone exposure increased the ACC oxidase expression only in plants with C3‐photosynthesis which also showed severe necrosis and reduced photosynthetic activity, whereas plants in CAM mode (salt induced) remained unaffected. Natural stressors such as salinity and high irradiance induce CAM and up‐regulate the ARS component CuZn‐SOD very effectively to protect M. crystallinum from oxidative damage even when additional oxidative stress such as ozone is present.

Mode of photosynthesis during different life stages of hemiepiphytic Clusia species.

This paper originates from a presentation at the IIIrd International Congress on Crassulacean Acid Metabolism, Cape Tribulation, Queensland, Australia, August 2001. Carbon isotope fractionation and nocturnal acid accumulation in Clusia osaensis Hammel-ined., C. peninsulae Hammel-ined. and C. valerii Standl. were investigated during the seedling, epiphytic and hemiepiphytic phases in a lowland tropical forest in Costa Ricato study photosynthetic adaptations of different plant growth stages to their habitat. Foliar δ 13C values around -24 to -32‰ indicate predominant C3 fixation of CO2 and low crassulacean acid metabolism (CAM) activity in all three Clusia species. Only terrestrially rooted plants of C. osaensis showed increased CAM expression. However, all developmental stages exhibited significant CAM cycling as shown by significant day-night fluctuations of titratable protons and of malic and citric acid. In C. valerii and C. peninsulae, an increase in CAM expression with plant development was not observed, and CAM cycling in hemiepiphytic-stage plants was completely repressed during the high rainfall season. The expression of CAM in the three Clusia species is therefore not developmentally controlled but triggered by environmental factors such as water availability and light intensity. These factors remain relatively stable in this ecosystem and CAM is therefore not fully expressed. However, CAM cycling may be of ecophysiological significance in all life stages as it serves as a mechanism to improve carbon economy by reducing respiratory CO2 losses.

Influence of light intensity and salt-treatment on mode of photosynthesis and enzymes of the antioxidative response system of Mesembryanthemum crystallinum.

The metabolic switch from C3-photosynthesis to crassulacean acid metabolism (CAM),and the antioxidative response of Mesembryanthemum crystallinum L. plants cultured under severe salt stress and high light intensities, and a combination of both stress conditions, were studied. High light conditions led to a more rapid CAM induction than salinity. The induction time was still shortened when both stress factors were combined. A main pattern observed in CAM plants was a decrease in mitochondrial Mn-superoxide dismutase (SOD) activity during the day. The activities of the chloroplastic Fe-SOD and cytosolic CuZn-SOD were increased due to salt treatment after a lag phase, while catalase activity was decreased. Combination of salt and light stress did not lead to a higher SOD activity as found after application of one stress factor alone, indicating that there is a threshold level of the oxidative stress response. The fact that salt-stressed plants grown under high light conditions showed permanent photoinhibition and lost the ability for nocturnal malate storage after 9 d of treatment indicate serious malfunction of metabolism, leading to accelerated senescence. Comparison of CuZn-SOD activity with CuZn-SOD protein amount, which was determined immunologically, indicates that the activity of the enzyme is at least partially post-translationally regulated.

Expression of C3 and C4 photosynthetic characteristics in the amphibious plant Eleocharis vivipara: structure and analysis of the expression of isogenes for pyruvate, orthophosphate dikinase.

Eleocharis vivipara, a unique leafless amphibious sedge, adopts the C4 mode of photosynthesis under terrestrial conditions and the C3 mode under submerged aquatic conditions. To analyze the molecular basis of these responses to the contrasting environments, we isolated and characterized two full-length cDNAs for a key C4 enzyme, pyruvate, orthophosphate dikinase (PPDK; EC 2.7.9.1). The isogenes for PPDK, designated ppdk1 and ppdk2, were highly homologous to one another but not identical. The PPDK1 protein, deduced from the nucleotide sequence of the cDNA, contained an extra domain at the amino terminus which, presumably, serves as a chloroplast transit peptide, while PPDK2 lacked this extra domain. It seems likely, therefore, that the ppdk1 and ppdk2 genes encode a chloroplastic and a cytosolic PPDK, respectively. Genomic Southern blot analysis revealed the existence of a small family of genes for PPDK in the genome of E. vivipara. Northern blot analysis indicate that both chloroplastic and cytosolic genes for PPDK are expressed simultaneously in the culms, a photosynthetic organ, of E. vivipara and that the pattern of expression of these genes differs between the growth forms.

Plant Responses to Elevated Atmospheric CO 2 among Terrestrial Biomes

Although many researchers have stressed the importance of among-species variation in plant response to elevated CO 2 , none have quantitatively tested whether variation exists among biomes. I compiled data from the literature and found that, although C 3 plants did respond more than C 4 plants (as predicted), biome origin was a better predictor (accounted for more of the variation) of plant response to elevated CO 2 than mode of photosynthesis. Variation in plant response among biomes was found both between and within latitudinal zones, with plant species from tropical and temperate biomes responding more than plant species from a polar biome. Within the temperate zone, species from forested biomes responded more than species from a grassland biome, and this provides further evidence that forests are acting as major sinks for increasing levels of atmospheric CO 2 . A more than 4-fold difference was found among ecosystems in coefficients of variation (calculated across mean species response within each ecosystem). Based on this difference, I suggest that ecosystems may vary in the amount of change in species composition in response to elevated CO 2 .

Enzymic Activities Associated with the Ability of Aerial and Submerged Forms ofLittorella uniflora(L.) Aschers to perform CAM

Groenhof, A. C., Smirnoff, N. and Bryant, J. A. 1988. Enzymic activities associated with the ability of aerial and submerged forms of Littorella uniflora (L.) Aschers to perform CAM.—J. exp. Bot. 39: 353-361. The submerged form of Littorella uniflora shows a full CAM mode of photosynthesis as shown by diel acid fluctuations and elevated activities (in comparison to non-submerged leaves) of the enzymes phosphoeno/pyruvate carboxylase (PEPC) and NADP-malic enzyme. Non-submerged plants exhibit no diel fluctuations of acidity and no changes in activity of NADP-malic enzyme or PEPC. PEPC activity is low and NADP-malic enzyme is not detectable. Further characterization of PEPC extracted from submerged plants during the light and dark periods of a diel cycle shows that the enzyme extracted in the dark is more active. In addition, the enzyme shows a decrease in Km (PEP) and an increase in Kmax in the presence of glucose-6-phosphate, whilst in the presence of malate K„ (PEP) is increased and Vmax decreased; this response to malate is only observed in the light and at pH 7-2. Molecular weight determinations using a Sephacryl S-300 column show that the enzyme extracted from plants during the dark period has an apparent mol. wt. of 375 KDa and the enzyme extracted from plants during the light period has an apparent mol. wt. of 307 KDa. Key words—Littorella uniflora (shoreweed), Crassulacean acid metabolism, PEP carboxylase, malic enzyme. Correspondence to: Department of Biological Sciences, University of Exeter, Washington Singer Laboratories, Perry Road, Exeter EX4 4QG, U.K.

Limited photosynthetic plasticity in Sedum pulchellum Michx.

Carbon dioxide exchange pattern, carboxylating enzyme specific activity and titratable acidity were measureed for field collected individuals of Sedum pulchellum Michx., a winter annual of xeric cedar glade communities in central Tennessee. Evidence of a weak-CAM mode of photosynthesis was found on 4 of 27 sampling dates during a three year period. Three of these dates occurred during 1978, the driest study season, and nocturnal CO2 uptake contributed a maximum of 10% of the plants daily CO2 uptake. Weak CAM in the field was associated with elevated phosphoenol-pyruvate (PEP) carboxylase activity and, to a lesser degree, with decreased ribulose bisphosphate (RuBP) carboxylase activity. In contrast, carboxylating enzyme specific activities of the sympatric, obligate CAM species Opuntia compressa, a dominant perennial of the cedar glades, were highly stable.Weak CAM was also induced in S. pulchellum upon water stress in the laboratory. Again, weak CAM was associated with elevated PEP carboxylase and decreased RuBP carboxylase activities and represented a relatively minor CO2 uptake contribution. The potential ecological significance of this limited photosynthetic flexibility in S. puchellum is not obvious but it may play an important role in successful yearly reproduction in this annual species at a time when water stress is frequent.

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.

Occurrence of two different modes of photosynthesis in Dendrobium cultivars

Photosynthetic modes of 58 cultivars of Dendrobium were examined. All the phalaenopsis-cane type cultivars were recognized as CAM (Crassulacean Acid Metabolism) plants, whereas nobile types were not CAM plants. Mode of photosynthesis might be a characteristic of taxonomic interest in the genus Dendrobium.