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.     

Vertical distribution of methane metabolism in microbial mats of the Great Sippewissett Salt Marsh

Methane metabolism was investigated with respect to depth in intertidal microbial mats of the Great Sippewissett Salt Marsh, Massachusetts. Although sulfate-reducing organisms dominate anaerobic carbon consumption in marine microbial mats, methanogens persist and their activity varies vertically and temporally in the mat system. In the Sippewissett mats, potential methane production for all mat layers was higher in the spring (17.2 ± 4.5 nmol CH₄ cm⁻² day⁻¹) than in the fall (3.0 ± 1.1 nmol CH₄ cm⁻² day⁻¹) and maximal rates were consistently observed in proximity to the chemocline (5–10 mm depth). The methane flux from the mat surface did not vary appreciably over time due to the ability of methanotrophic activity to limit net methane production. Evidence indicates that both aerobic and anaerobic oxidation of methane occurs in this system. The importance of H₂ as a substrate for methanogenesis appeared to be the greatest at the mat surface (0–10 mm), and the proportion of methylotrophic methanogens generally increased with depth. These results suggest that both non-equilibrium H₂ dynamics and the use of non-competitive substrates permit coexistence of methanogens and sulfate-reducing organisms in the mat system.     

Metabolism of a desert stream

SUMMARY. Rates of photosynthesis and community respiration were determined for benthic assemblages in Sycamore Creek, a Sonoran Desert stream in Arizona. Benthos in this stream can be separated into (1) mats of Cladophora glomerata and associated epiphytes and (2) assemblages of epipelic diatoms and blue-green algae. Community respiration and net photosynthesis were measured for these assemblages using submerged light-dark chambers in situ . Multiple regression analysis was used to predict (1) gross photosynthesis as a function of photosynthetically active radiation, temperature and chlorophyll-α concentration; and (2) community respiration as a function of temperature and biomass.
Calculations suggest that Sycamore Creek is autotrophic during the summer ( P/R = 1.7) and that the rates of gross photosynthesis ( P =8.5 g O₂ m⁻² day⁻¹) and community respiration ( R = 5.1 g O₂ m⁻² day⁻¹) are high for a small stream. Considerable difference exists between the Cladophora mat assemblages, in which mean P is 12.5gO₂m⁻² day⁻¹and the P/R ratio is 2.3, and the epipelic assemblages in which mean P is 4.4 g O₂m⁻² day⁻¹ and P/R is 0.96. The high rate of gross photosynthesis, low litter inputs, high biomass of algae and the intermittent but severe floods that characterize Sycamore Creek indicate that this stream and other similar desert streams are net exporters of organic matter and are, thereby, truly autotrophic stream ecosystems.     

Detection of different quorum-sensing signal molecules in a virulent Edwardsiella tarda strain LTB-4

Aims:  The aim of this study was to elucidate the potential quorum-sensing (QS) signal molecules of an emerging pathogen ( Edwardsiella tarda strain LTB-4) of cultured turbot ( Scophthalmus maximus ).
Methods and Results:  A sensitive and rapid double-layer plate method using biosensor strain Agrobacterium tumefaciens KYC55 was developed to detect the N-acylhomoserine lactone (AHL)-related compounds in bacteria. LTB-4 was found to have two QS systems, one was based on the AHLs and the other was based on the autoinducer-2 (AI-2). The AI-2 activity produced by LTB-4 was growth phase dependent and topped at OD₆₀₀ of 1·0. The protocol to detect cholerae autoinducer 1 (CAI-1) activity in bacteria was modified, lowering the background luminescence of biosensor strain Vibrio harveyi JAF375. CAI-1 activity could not be detected in LTB-4.
Conclusion:  Edwardsiella tarda LTB-4 produced at least four kinds of AHLs during its whole growth phase. In comparison with the AHL-inducing QS, AI-2 may be the first predominant signal, functioning at early exponential phase. LTB-4 did not produce any CAI-1 activity.
Significance and Impact of the Study:  Different QS signal molecules of Edw. tarda LTB-4 were clarified by improved bioassays. In contrast to earlier studies detecting two types of AHLs, strain LTB-4 produced at least four kinds of AHLs, which seemed to be C₄-HSL, C₆-HSL, 3-oxo-C₆-HSL and an uncharacterized AHL molecule.     

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

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

Vibrio vulnificus produces quorum sensing signals of the AHL-class

Vibrio vulnificus is an aquatic pathogenic bacterium that can cause vibriosis in humans and fish. The species is subdivided into three biotypes with the fish-virulent strains belonging to biotype 2. The quorum sensing (QS) phenomenon mediated by furanosyl borate diester or autoinducer 2 (AI-2) has been described in human strains of biotype 1, and here we show that the luxS gene which encodes AI-2 is present in all strains of V. vulnificus regardless of origin, biotype or serovar. In this study, we also demonstrate that V. vulnificus produces QS signals of the acylated homoserine lactone (AHL) class (AI-1). AHLs were detected in strains of biotype 1 and 2 from water, fish and human wound infections but not in strains isolated from human septicaemic cases. The AHL compound was identified as N -butanoyl-homoserine-lactone (C₄-HL) by both reporter strains and by HPLC-high-resolution MS. C₄-HL was detected when AHL-positive strains were grown in low-nutrient medium [modified sea water yeast extract (MSWYE)] but not in rich media (tryptic soy broth or brain–heart infusion) and its production was enhanced when blood factors were added to MSWYE. C₄-HL was detected in vivo , in eels infected with AHL-positive biotype 2 strains. No known AHL-related gene was detected by PCR or Southern blot suggesting that AHL-related genes in V. vulnificus are different from those found in other Gram-negative bacteria.     

C2H2 and Ar induce rapid declines in nitrogenase activity and CO2 evolution in nodules of Datisca glomerata

Preliminary studies have indicated that after addition of C₂H₂ there is a rapid decline in nitrogenase activity in the nodules of Datisca glomerata . The present work was undertaken to determine whether (1) there is also a decline in respiration and (2) the decline is associated with the cessation of ammonia production. The rates of C₂H₄ and CO₂ evolution by nodulated root systems of Datisca were measured as a function of time after exposure to C₂H₂. The peak rate of C₂H₄ evolution occurred at 30 s after C₂H₂ exposure, while the rate of CO₂ evolution started to decline at 60 s after exposure to C₂H₂. Incubation of nodules in a gas mixture containing Ar also caused a decline in CO₂ evolution. Further, pretreatment with Ar eliminated most of the C₂H₂-induced decline in nitrogenase activity and CO₂ evolution. These C₂H₂- and Ar-induced declines in Datisca nodules are more rapid than those reported in any other nodules. They are evidence that continued ammonia formation is essential for maintenance of normal nitrogenase activity in Datisca nodules.     

Diurnal and seasonal variation in the carbon isotope composition of leaf dark-respired CO2 in velvet mesquite (Prosopis velutina)

We evaluated diurnal and seasonal patterns of carbon isotope composition of leaf dark-respired CO₂ ( δ ¹³C_l) in the C₃ perennial shrub velvet mesquite ( Prosopis velutina ) across flood plain and upland savanna ecosystems in the south-western USA. δ ¹³C_l of darkened leaves increased to maximum values late during daytime periods and declined gradually over night-time periods to minimum values at pre-dawn. The magnitude of the diurnal shift in δ ¹³C_l was strongly influenced by seasonal and habitat-related differences in soil water availability and leaf surface vapour pressure deficit. δ ¹³C_l and the cumulative flux-weighted δ ¹³C value of photosynthates were positively correlated, suggesting that progressive ¹³C enrichment of the CO₂ evolved by darkened leaves during the daytime mainly resulted from short-term changes in photosynthetic ¹³C discrimination and associated shifts in the δ ¹³C signature of primary respiratory substrates. The ¹³C enrichment of dark-respired CO₂ relative to photosynthates across habitats and seasons was 4 to 6‰ at the end of the daytime period (1800 h), but progressively declined to 0‰ by pre-dawn (0300 h). The origin of night-time and daytime variations in δ ¹³C_l is discussed in terms of the carbon source(s) feeding respiration and the drought-induced changes in carbon metabolism.     

Seasonal differences in photosynthesis between the C3 and C4 subspecies of Alloteropsis semialata are offset by frost and drought

The regional abundance of C₄ grasses is strongly controlled by temperature, however, the role of precipitation is less clear. Progress in elucidating the direct effects of photosynthetic pathway on these climate relationships is hindered by the significant genetic divergence between major C₃ and C₄ grass lineages. We addressed this problem by examining seasonal climate responses of photosynthesis in Alloteropsis semialata , a unique grass species with both C₃ and C₄ subspecies. Experimental manipulation of rainfall in a common garden in South Africa tested the hypotheses that: (1) photosynthesis is greater in the C₄ than C₃ subspecies under high summer temperatures, but this pattern is reversed at low winter temperatures; and (2) the photosynthetic advantage of C₄ plants is enhanced during drought events. Measurements of leaf gas exchange over 2 years showed a significant photosynthetic advantage for the C₄ subspecies under irrigated conditions from spring through autumn. However, the C₄ leaves were killed by winter frost, while photosynthesis continued in the C₃ plants. Unexpectedly, the C₄ subspecies also lost its photosynthetic advantage during natural drought events, despite greater water-use efficiency under irrigated conditions. This study highlights previously unrecognized roles for climatic extremes in determining the ecological success of C₃ and C₄ grasses.     

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.     

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 .     

Seasonal patterns of water relations and enzyme activity of the facultative CAM plant Portulacaria afra (L.) Jacq.

Abstract. Portulacaria afra (L.) Jacq. is a perennial facultative CAM species showing a seasonal shift from C₃ to CAM photosynthesis. The shift to CAM during the summer occurs despite continued irrigation of the plants. The authors examined the hypothesis that the seasonal shift to CAM occurred because of low transient water potentials. They measured changes in whole leaf water, osmotic and pressure potentials over the course of the shift. They also studied changes in enzyme activity to ascertain if PEP carboxylase and PEP carboxykinase were induced during the seasonal shift to CAM. Water potentials were high, from -0.1 to -0.5 MPa, predawn and midday, when the C₃ pathway of photosynthesis was utilized. Osmotic potentials were constant, from -0.7 to - 0.8 MPa, indicating very little change in turgor. P. afra shifted to CAM indicated by large diurnal acid fluctuations (300 400 meq m⁻²) despite C₃-like predawn water potentials. Midday water potentials usually decreased 0.2-0.7 MPa, while the osmotic potential remained unchanged or decreased slightly. Thus, a midday loss of turgor was associated with the use of the CAM pathway. The results support the hypothesis that the induction of CAM occurred due to low transient water potentials and may be partially mediated through the loss of turgor. The shift to CAM is only a partial induction with PEP carboxykinase showing high activity all year round while PEP carboxylase increases three-to five-fold over C₃ levels. Relatively high levels of CAM enzyme activity enables the utilization of the CAM pathway in the winter and spring in response to high daytime temperatures and increased evaporative demand. These results would lead to an increase in water use efficiency during such periods when compared to other inducible CAM species.     

Identification of the Sources of Energy for Nitrogen Fixation and Physiological Characterization of Nitrogen-Fixing Members of a Marine Microbial Mat Community

Experimental manipulations of a microbial mat community were performed to determine sources of energy and reductant used for nitrogen fixation and to physiologically characterize the responsible diazotrophs. The dominant photolithotrophic members of this community were nonheterocystous cyanobacteria, but other potential nitrogen-fixing microorganisms were also present. Pronounced diel variability in rates of acetylene reduction was observed, with nighttime rates a factor of three to four higher than daytime rates. Acetylene reduction measured at night was dependent upon the occurrence of oxygenic photosynthesis the preceding day; mats incubated in the dark during the daytime reduced acetylene at rates comparable to those of light-incubated mats but were not able to reduce acetylene at the normally high rates the following night. The addition of various exogenous carbon compounds to these dark-incubated mats did not elicit nighttime acetylene reduction. Nighttime acetylene reduction apparently proceeds under anoxic conditions in these mats; the highest rates of acetylene reduction occur late at night. Additions of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (an inhibitor of oxygenic photosynthesis) to mats resulted in a pronounced stimulation of acetylene reduction during the day, but acetylene reduction the next night proceeded at greatly reduced rates (relative to untreated mats). This daytime stimulation, under the 3-(3,4-dichlorophenyl)-1,1-dimethylurea-induced anoxic conditions in the experimentally treated mats, was light dependent. These results suggest that nitrogen fixation in these mats may be attributed to the activities of nonheterocystous cyanobacteria utilizing storage products of oxygenic photosynthesis under anoxic conditions at night.     

Alkane composition diversity among populations of dwarf forms of Juniperus communis L.: comparison between western Europe and northern American populations

The cuticular wax composition of leaves has been analysed in three western European populations (Corsica, central Pyrenees, northern Alps) of Juniperus communis var. saxatilis Pall. (=  J. nana Willd., nom illeg.) and in one population of J. communis L. var. depressa Pursh. from North America (Sierra Nevada). Gas chromatography shows the presence of 13 alkanes in all samples ranging from C₂₃ to C₃₅ with important intraspecific polymorphism in alkane content. The dominant alkanes range from C₃₃ to C₃₅. Alkanes C₂₁ and C₂₂ were found only in Corsica and Sierra Nevada populations. Canonical discriminant analysis separated the J. communis L. var. depressa Pursh. of the population of Sierra Nevada from other populations of J. communis var. saxatilis Pall. on the basis of their higher C₃₁ content and the constant presence of C₂₁ and C₂₂ alkanes. J. communis var. saxatilis Pall. populations from the Pyrenees are close to northern Alps populations characterized by high concentrations of C₃₃, C₃₄ and C₃₅ alkanes. This paper confirms the existence of Juniperus var. saxatilis Pall. in the Pyrenees (France).  © The Linnean Society of London, Botanical Journal of the Linnean Society , 2002, 140 , 165–168.     

Uptake and utilization of n-octacosane and n-nonacosane by Arthrobacter nicotianae KCC B35

Arthrobacter nicotianae KCC B35 isolated from blue-green mats densely covering oil sediments along the Arabian Gulf coast grew well on C₁₀ to C₄₀ n -alkanes as sole sources of carbon and energy. Growth on C₂₀ to C₄₀ alkanes was even better than on C₁₀ to C₁₈ alkanes. Biomass samples incubated for 6 h with n -octacosane (C₂₈) or n -nonacosane (C₂₉) accumulated these compounds as the predominant constituent alkanes of the cell hydrocarbon fractions. The even chain hexadecane C₁₆ and the odd chain pentadecane C₁₅ were the second dominant constituent alkanes in C₂₈ and C₂₉ incubated cells, respectively. n -Hexadecane-incubated cells accumulated in their lipids higher proportions of C₁₆-fatty acids than control cells not incubated with hydrocarbons. On the other hand, C₂₈ and C₂₉-incubated cells did not contain any fatty acids with the equivalent chain lengths, but the fatty acid patterns of the cell lipids suggest that there should have been mid-chain oxidation of these very long chain alkanes. This activity qualifies A. nicotianae KCC B35 to be used in cocktails for bioremediating environments polluted with heavy oil sediments.     

Crop colonisation, feeding, and reproduction by the predatory beetle, Hippodamia convergens, as indicated by stable carbon isotope analysis

Abstract.  1. Habitat management to enhance natural enemy populations in agricultural systems may help regulate levels of crop pests, but little research addresses the behaviour of immigrating beneficial insects.
2. Stable carbon isotopes were used in complementary laboratory and field studies to examine colonisation behaviour of an ephemeral agricultural habitat by the lady beetle, Hippodamia convergens Guérin-Méneville.
3. Under laboratory conditions, H. convergens carbon isotope ratios, δ¹³C, changed after its food supply was shifted from a C₄- to a C₃-based diet of aphids produced on grain sorghum or cotton respectively. Final isotope ratios of adult H. convergens were closer to that of the new C₃-based diet, with most change in δ¹³C occurring within 3 days after the diet shift.
4. The carbon isotope ratios of lady beetle adults collected in cotton fields suggested that grain sorghum was a continuous source for H. convergens until many nearby sorghum fields matured and senesced.
5. When cotton aphid ( Aphis gossypii Glover) prey were absent, carbon isotope ratios of beetle populations did not change over time and virtually no egg production by H. convergens was detected. This indicates that beetles were feeding little on non-aphid resources originating in cotton.
6. With cotton aphids present, beetle isotope ratios decreased towards the carbon isotope ratio of cotton, indicating adult feeding in cotton. As a result, egg masses produced had carbon isotope ratios in the C₃ range of values.
7. The results suggest that some predator species may be retained in habitats without large prey populations, a quality essential in controlling pests in agricultural systems.     

Effects of age, supra-ambient oxygen and repeated assays on acetylene reduction and root respiration in pea

An open flow-through gas system was used to investigate the effect of plant age on nitrogenase activity in relation to root respiration (measured as CO₂ release) and supra-ambient O₂ levels in 24- to 51-day-old, nodulated Pisum sativum L. cv. Bodil. The effect of assaying plants repeatedly was also studied. The respiratory efficiency of nitrogenase [mol CO₂ (mol C₂H₄)⁻¹] and the relative decline in nitrogenase (EC 1.7.99.2) activity in response to introduction of C₂H₂ in the gas stream were unaffected by plant age. In contrast, the nitrogenase-linked respiration as a proportion of total root respiration increased with time. Accordingly, the specific respiration linked-to growth and maintenace of the noduled root system decreased with time. C₂H₂ reduction and root respiration were increased by supra-ambient O₂ levels, but the tolerance to high O₂ concentrations seemed to decrease with plant age. Repeated C₂H₂ assays on the same plants decreased their rate of growth and N accumulation: in addition, nitrogenase activity and root respiration were somewhat negatively affected. The results indicate that results from experiments with plants of different ages cannot always be directly compared, and that repeated C₂H₂ assays on the same plants should be applied with caution in physiological work.     

Methylmercury enters an aquatic food web through acidophilic microbial mats in Yellowstone National Park, Wyoming

Microbial mats are a visible and abundant life form inhabiting the extreme environments in Yellowstone National Park (YNP), WY, USA. Little is known of their role in food webs that exist in the Park's geothermal habitats. Eukaryotic green algae associated with a phototrophic green/purple Zygogonium microbial mat community that inhabits low-temperature regions of acidic (pH ∼ 3.0) thermal springs were found to serve as a food source for stratiomyid (Diptera: Stratiomyidae) larvae. Mercury in spring source water was taken up and concentrated by the mat biomass. Monomethylmercury compounds (MeHg⁺), while undetectable or near the detection limit (0.025 ng l⁻¹) in the source water of the springs, was present at concentrations of 4–7 ng g⁻¹ dry weight of mat biomass. Detection of MeHg⁺ in tracheal tissue of larvae grazing the mat suggests that MeHg⁺ enters this geothermal food web through the phototrophic microbial mat community. The concentration of MeHg⁺ was two to five times higher in larval tissue than mat biomass indicating MeHg⁺ biomagnification occurred between primary producer and primary consumer trophic levels. The Zygogonium mat community and stratiomyid larvae may also play a role in the transfer of MeHg⁺ to species in the food web whose range extends beyond a particular geothermal feature of YNP.     

Developmental changes of enzymes of malate metabolism in relation to respiration, photosynthesis and nitrate assimilation in peach leaves

The developmental profile of the activities of some enzymes involved in malate metabolism, namely phosphoenolpyruvate carboxylase (PEPC; EC 4. 1. 1. 31), NAD⁺-linked (EC 1. 1. 1. 37) and NADP⁺-linked (EC 1. 1. 1. 82) malate dehydrosenase (MDH), NAD⁺linked (EC 1. 1. 1. 39) and NADP⁺-linked (EC 1. 1. 1. 40) malic enzyme (ME), has been determined in leaves of peach [ Prunus persica (L.) Batsch cv. Maycrest], a woody C₃ species. In order to study the role of these enzymes, their activities were related to developmental changes of photosynthesis, respiration, and capacity for N assimilation. Activities of PEPC, NAD(P)⁺-MDH and NADP⁺-ME were high in young expanding leaves and decreased 2- to 3-fold in mature ones, suggesting that such enzymes play some role during the early stages of leaf expansion. In leaves of peach, such a role did not seem to be linked to C₃ photosynthesis or nitrate assimilation, in that photosynthetic O₂ evolution and activities of nitrate reductase (EC 1. 6. 6. 1) and glutamine synthetase (EC 6. 3. 1. 2) increased during leaf development. In contrast, leaf respiration strongly decreased with increasing leaf age. We suggest that in expanding leaves of this woody species the enzymes associated with malate metabolism have anaplerotic functions, and that PEPC may also contribute to the recapture of respiratory CO₂.     

The evolutionary relationship between stomatal mechanism, crassulacean acid metabolism and C4 photosynthesis

Abstract. All of the features of crassulacean acid metabolism (CAM) and most characteristics of C₄ photosynthesis are exhibited by stomatal guard cells. It is proposed that CAM and possibly also C₄ photosynthesis result from the expression in photosynthetic cells of genetic information which is expressed only in guard cells of C₃ plants.