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Characterizing pedestal turbulence in the tokamak I-mode is a crucial step in understanding how particle and heat transport decouple during I-mode operation. This work models an ASDEX Upgrade I-mode discharge for the first time via linear and nonlinear gyrokinetic simulations with the GENE code. L-mode and I-mode regimes at two different pedestal locations are investigated. A microtearing mode that is not apparent in initial value linear L-mode simulations is found to dominate in I-mode simulations at both radial positions, and ion-scale instabilities are characterized for all four scenarios linearly. Computed nonlinear heat flux values approach experimental measurements with nominal input parameters in three of the four cases, and heat transport is found to be dominated by ion-scale electrostatic turbulence. Electrostatic potential oscillation frequencies, as well as potential-temperature and potential-density crossphases, are compared linearly and nonlinearly, and agreement is found at wavenumber ranges corresponding to peaks in the simulated heat flux spectra at one radial position for L-mode and I-mode. [ABSTRACT FROM AUTHOR]

The results described represent the first detailed measurements of gas exchange of epiphytic plants with crassulacean acid metabolism (CAM) in the humid tropics. A portable steady-state CO2 and H2O porometer was used to measure net exchange rates of CO2 and H2O vapour (Jco2, JH2O), leaf temperature (TI), air temperature (TA), air relative humidity (RH) and photosynthetically active radiation (RH) and photosynthetically active radiation (PAR) for bromeliads in the field during the dry season in February and March 1983 on the tropical island of Trinidad. Different lengths of tubing (up to 25 m) were used so that the gas exchange could be measured of bromeliads in situ in their epiphytic habitats. Derived parameters such as leaf-air water-vapour-concentration difference (Δw), water-vapour conductance of leaves (g) and internal CO2 partial pressure (pi,CO2) could be calculated. The particular problems of making such measurements in the humid tropics due to high relative humidities and high dew-point temperatures are discussed. The long and often broad, strap-like leaves of bromeliads are well suited for measurements with the steady-state porometer. It is shown that CAM activity varies along the length of individual leaves, and variability between different leaves is also demonstrated. The major phases of CAM, i.e. nocturnal stomatal opening, CO2 uptake and dark fixation as malic acid (Phase I), daytime stomatal closure and light-dependent assimilation CO2 derived from decarboxylation of the malic acid (Phase III), and late-afternoon stomatal opening with direct light-dependent assimilation of atmospheric CO2 (Phase IV) were all clearly shown by CAM bromeliads in situ. Their expression and magnitude depended on the environmental conditions. An early-morning peak of CO2 uptake as is characteristic of Phase II of CAM was not detected during the night-day transition. A bromeliad intermediate between C3 and CAM, Guzmania monostachia, showed substantial net CO2 uptake in the early morning but no net uptake integrated over the whole of the night. [ABSTRACT FROM AUTHOR]

summary: The perennial halophytes, Batis maritima L., Sesuvium portulacastrum L., and Portulaca rubricaulis H.B.K. which inhabit the extreme environment on parts of the salt plain and at the edges of the vegetation islands of Ciénega el Ostional were examined to determine their strategies for growth and survival in the rainy and dry seasons. All of the three species are leaf‐succulents. High leaf‐sap osmotic pressures, xylem tensions and Na+ and Cl− levels indicated that S. portulacastrum and B. maritima are salt‐accumulating halophytes. Succulence, leaf‐sap osmolalities and Na+ and Cl− levels increased in the dry season in B. maritima by a factor of 1.5‐2.0 and in S. Portulacastrum by a factor of 1.9–2.7. B. maritima also accumulated sulphate with a two‐fold increase of concentrations in the dry season. In S. portulacastrum Na+ accumulation much exceeded Cl− accumulation and oxalate synthesis was found to serve charge balance. In this species the compatible solutes, proline and pinitol, were clearly detectable in both seasons; their levels increased by a factor of about 6 in the dry season. Exchange of water vapour and CO2 was measured with a portable steady‐state porometer. Photosynthesis in B. maritima showed little response to the transition from the rainy to the dry season while S. portulacastrum was severely impaired in the dry season, showing pronounced midday depressions of gas exchange and about 40% inhibition of light‐saturated rates of CO2 uptake. P. rubricaulis shed its leaves in the dry season. According to carbon isotope ratios (δ13C), B. maritima (δ13C =−26.4‰) and S. portulacastrum (δ13C =−25.8 ‰) are C3 plants while P. rubricaulis (δ13C =−12.3‰) performed C4 photosynthesis. [ABSTRACT FROM AUTHOR]

summary: The terrestrial CAM plant Bromelia humilis was examined in the salinas of the Ciénega el Ostional, on the north coast of Venezuela, in the rainy and dry seasons. Three colour forms were distinguished; yellow (in full sun), green exposed (also in sun) and green shaded (beneath woodland). Plant size decreased with increasing irradiance. An examination was made of the three phenotypes in terms of CO2 exchange (Jco2), dawn‐dusk changes in titratable acidity (ΔH+) and malate and citrate levels, osmotic pressure, xylem tension, sugar and amino acids levels, nitrogen and ion concentrations and ambient temperature fluorescence. All phenotypes exhibited lowered Jco2 and ΔH+ in the dry as compared to the rainy season. Citrate, as well as malate, showed dawn‐dusk fluctuations. Soluble sugars were the major source of carbon skeletons for nocturnal organic acid production. The dawn‐dusk changes in osmotic pressure were negligible. Yellow plants performed poorly in contrast to shaded plants in both seasons. The former showed higher dawn‐dusk changes of citrate levels and contained much less nitrogen than shaded plants. Nocturnal recycling of respiratory CO2 was more important in yellow plants and, in the dry season, reached 87%. These differences were reflected in the overall productivity, shaded plants showing increases in size whereas yellow plants utilized energy mainly for leaf replacement. Water availability and nitrogen supply appear to be the overriding factors determining higher productivity and CO2 assimilation in partially shaded plants as compared with plants in full sun. [ABSTRACT FROM AUTHOR]

summary: Seasonal effects on leaf gas exchange and water relations were compared for Avicennia germinans, a true mangrove, and Conocarpus erectus, a mangrove associate, at coastal sites in northern Venezuela. On the Ciénega el Ostional at Chichiriviche, A. germinans was most abundant around lagoons on the seaward side of the vegetation‐free alluvial sand plain. C. erectus was the dominant shrub in inland communities, but the two species co‐occurred on vegetation islands at the landward edge of the alluvial plain. On the vegetation islands of the Ciénega el Ostional, gas exchange in A. germinans (a species with foliar salt glands) was less severely curtailed in the dry season compared with the rainy season than was gas exchange in C. erectus (a species lacking salt glands). Average rates of photosynthesis at near‐saturating light intensities and total diurnal CO2 uptake were reduced in the dry season to 69 and 61%, respectively, of their values in the rainy for A. germinans, but to 48 and 30%, respectively, of their rainy‐season values for C. erectus. Similarly, stomatal conductance and transpirational water loss were more reduced in the dry season for C. erectus than for A. germinans, with the result that C. erectus showed a 3.4‐fold increase in water‐use efficiency in the dry season compared with the rainy season. The importance of the soil environment in determining plant gas‐exchange Patterns was evidenced by large seasonal shifts in dawn xylem tension for the two species (which increased from 1.34 MPa in the rainy season to 5.50 MPa in the dry season for A. germinans, and from 0.40 to 5.78 MPa for C. erectus). These values reflected changes in the soil environment caused by inundation of the upper soil layers by fresh water in the rainy season and a progressive increase in salt concentrations (to almost twice those in sea water) by evaporation from the soil in the dry season. Large changes in xylem tension were observed for both species during individual day–night cycles, reaching a maximum of 2.36 MPa for A. germinans. For C. erectus, the magnitude of these day‐night changes was greatly reduced in the dry season, consistent with its very low transpiration rates at this time of year. Leaf‐cell osmotic pressures also tended to be higher in A. germinans than C. erectus (attaining a maximum of 8.3 MPa for A. germinans in the dry season), and were related to the more seaward distribution of the true mangrove on the alluvial plain. Whereas leaves of A. germinans did not show any changes in succulence, leaf succulence in C. erectus increased with leafage and was slightly higher in the dry season than the rainy season. The more succulent leaves also had higher cell‐sap osmotic pressures and NaCl concentrations. The most succulent leaves of C. erectus were observed for exposed shrubs growing on the shoreline. During the dry season, these shoreline plants showed high rates of gas exchange and low values for dawn xylem tension (0.89 MPa), indicating that they had access to relatively non‐saline water from the shallow water table. On individual plants, exposed shoots had more succulent leaves and higher osmotic pressure and NaCl concentrations than sheltered leaves, demonstrating the importance of foliar absorption of salt borne in sea spray for the ionic relations of C. erectus. Thus, although the distribution of C. erectus is centred on brackish‐water zones, this species can apparently extend from habitats with permanent access to a shallow water table through to areas where it is seasonally exposed to low soil water potentials and high salt concentrations in the substratum. [ABSTRACT FROM AUTHOR]

summary: A comparison of the performance of two epiphytes with crassulacean acid metabolism (CAM) was made during the rainy season and dry season at the Ciénega el Ostional, Chichiriviche in northern Venezuela. The epiphytic bromeliad, Tillandsia flexuosa has water‐retaining tanks and leaf trichomes, and propagates mainly vegetatively to produce large populations in the shrubby island vegetation. The epiphytic orchid, Schomburgkia humboldtiana formed smaller populations, and had large succulent leaves with uniform chlorenchyma and no distinct water‐storage parenchyma, unlike T. flexuosa. Both epiphytes were myrmecophilous. Leaf succulence (kg m−2) declined by ∼ 10% in the dry season for both plants. Both epiphytes showed reduced CO2 uptake during Phase I (dark period) and (dawn‐dusk) titratable acidity (ΔH+) in the dry season. Water‐use efficiency (WUE) was higher for S. humboldtiana (16.0 × 10−3 mol CO2 per mol H2O compared with 5.0 × 10 −3 for T. flexuosa) although WUE remained constant during rainy and dry season for each species. Sixty to seventy per cent of the dawn dusk titratable acidity was derived internally from respiratory CO2 (recycling) for both species, and in absolute terms, recycling decreased in the dry season, in contrast to the expected progression under drought stress. Recycling is an important facet of carbon balance for both species in both rainy and dry seasons. Leaf Na+ concentration was higher than values quoted for terrestrial salt stressed CAM plants. Fructose and glucose declined in leaf bases of T. flexuosa during the dark period, but not in the more distal regions of the leaf. S. humboldtiana showed a decrease in sucrose at night, and mannitol was also an important constituent of the leaves. Xylem sap tension maxima increased from 0.38 ± 0.09 MPa (rainy season) to only 0.55 ± 0.06 MPa (dry season) for T. flexuosa, these values being much lower than those found for terrestrial shrubs and herbs at the same site, the two epiphytes use CAM in conjunction with differing morphological adaptations to maintain growth throughout the year at the Ciénega el Ostional, but it would seem that T. flexuosa has better physiological characteristics for maintenance of carbon acquisition during the dry season. [ABSTRACT FROM AUTHOR]

Pitcairnia integrifolia is endemic to northern Trinidad and the Paria peninsula of Venezuela and is the only member of the bromeliad-subfamily Pitcairnioideae in Trinidad. It is terrestial with roots fully functional in water and solute uptake, grows on exposed steep rocky cliffs and can occur just above the spill zone of sea waves under continuous sea spray. Thus, it can be subject both to water stress, particularly during the dry season, and to salt stress. Gas exchange of P. integrifolia leaves was measured on a clear day in Trinidad. Uptake of CO2 and leaf conductance to diffusion of water vapour had two peaks during the light period, a larger one in the early morning and a smaller one in the late afternoon, which were separated by an extended midday depression of gas exchange. CO2 partial pressure in the leaf air-spaces increased during the midday depression. Leaf temperatures reached a maximum of 51.6°C and leaf-air water-vapour- concentration differences were also very high during the midday depression, when quantum fluxes were up to 2 mmol m-2 s-1 and higher. The midday depression is considered as a functional adaptation to temporary water stress. Although P. integrifolia is subject to sea-spray, an internal osmotic pressure of only 0.91 MPa indicated that NaCl is not accumulated. Leaf epidermis cells are thick-walled and have a prominent cuticle. The abaxial leaf surface with the trichomes is not wettable, and the trichomes apparently do not function in water and solute uptake. They cover the stomata densely and may create a favourable microenvironment around them. They also have a high reflectance. This does not prevent overheating of the leaves, but does reduce the photosynthetically active radiation penetrating to the mesophyll. The trichomes may thus contribute to the prevention of photoinhibition at high incident quantum flux. [ABSTRACT FROM AUTHOR]

An investigation was carried out into the water relations of CAM and C3 bromeliads in their natural habitat during the dry season in Trinidad. Measurements were made of xylem tension with the pressure chamber and of cell-sap osmotic pressure and titratable acidity on crushed leaf samples. A steady-state CO2 and H2O-vapour porometer was also used so that changes in leaf water relations during individual day-night cycles could be directly related to gas-exchange patterns in situ. Xylem tension changed in parallel with transpiration rate and in general reached its maximum value in CAM bromeliads at night and in C2 bromeliads during the day. In addition, large nocturnal increases in cell-sap osmotic pressure and titratable acidity (ΔH+) typically occurred in the CAM bromeliads. The C3-CAM intermediate Guzmania monostachia showed slight nocturnal acidification, but had higher values of xylem tension during the day. Very high values of ΔH+ were observed in the CAM species when the tanks of the epiphytic bromeliads contained water: Aechmea nudicaulis showed a mean maximum ΔH+ of 474 mol m[SUP-3], the highest value so far observed for CAM plants. On some nights dew formed on the leaf surfaces of the epiphytes, partially curtailing gas exchange and leading to a marked decrease in xylem tension in both C3 and CAM species. Between-site comparisons were also made for a wide range of habitats from arid coastal scrub to montane rain forest. Compared with values characteristic of other life-forms, xylem tension and cell-sap osmotic pressure were low for all bromeliads, and did not differ significantly in co-occurring CAM and C3 bromeliads. Mean maximum xylem tension (10 species in total) ranged from 0.29 Mpa at the montane sites to 0.67 Mpa at the most arid site, and mean minimum osmotic pressure (17 species) from 0.51 to 0.97 Mpa. At the arid sites the bromeliads were exclusively CAM species, two of which (Aechmea aquilega and Bromelia plumieri) grew terrestrially in the undergrowth of the coastal scrub. Xylem tension in these species was low enough to indicate that they must be functionally independent of the substratum during the dry season. In the wetter part of Trinidad, no between-site differences in leaf water relations were found along an altitudinal gradient in the Northern Mountain Range; seasonal differences in this area were also small. Overall, leaf water strongly affected both by short-term changes in water availability and by longer-term climatic differences in the various regions of the island. [ABSTRACT FROM AUTHOR]

Field measurements of the gas exchange of epiphytic bromeliads were made during the dry season in Trinidad in order to compare carbon assimilation with water use in CAM and C3 photosynthesis. The expression of CAM was found to be directly influenced by habitat and microclimate. The timing of nocturnal CO2 uptake was restricted to the end of the dark period in the plants found at drier habitats, and stomatal conductance in the two CAM species was found to respond directly to humidity or temperature. Total night-time CO2 uptake, when compared with malic-acid formation (measured as the dawn-dusk difference in acidity, ΔH+), could only account for 10-40% of the total ΔH+ accumulated. The remaining malic acid must have been derived from the refixation of respired CO2 (recycling). Within the genus Aechmea (12 samples from four species), recycling was significantly correlated with night temperature at the six sample sites. Recycling was lowest in A. fendleri (54% of ΔH+ derived from respired CO2), a CAM bromeliad with little water-storage parenchyma that is restricted to wetter, cooler regions of Trinidad. Gas-exchange rated of C3 bromeliads were found to be similar to those of the CAM bromeliads, with CO2 uptake from 1 to 3 μmol m-2 s-1 and stomatal conductances generally up to 100 mmol m-2 s-1. The midday depression of photosynthesis occurred in exposed habitats, although photosynthetically active radiation (PAR) limited photosynthetically in shaded habitats. CO2 uptake of the C3 bromeliad Guzmania lingulata was saturated at around 500 μmol m-2 s-1 PAR, suggesting that epiphytic plants found in the shaded forest understorey are shade-tolerant rather than shade-demanding. Transpiration ratios (TR) during CO2 fixation in CAM (Phase I and IV) and C3 bromeliads were compared at different sites in order to assess the efficiency of water utilization. For the epiphytes displaying marked uptake of CO2, TR were found to be lower than many previously published values. In addition, the average TR values were very similar for dark CO2 uptake in CAM (42 ± 41, n = 12), Phase IV of CAM (69 ± 36, n = 3) and for C3 photosynthesis (99 ± 73, n = 4) in theses plants. It appears that recycling of respired CO2 by CAM bromeliads and efficient use of water in all phases of CO2 uptake are physiological adaptations of bromeliads to arid microclimates in the humid tropics. [ABSTRACT FROM AUTHOR]