Short-Term and Long-Term Responses of Crassulacean Acid Metabolism Plants to Elevated CO(2)

For the leaf succulent Agave deserti and the stem succulent Ferocactus acanthodes, increasing the ambient CO₂ level from 350 microliters per liter to 650 microliters per liter immediately increased daytime net CO₂ uptake about 30% while leaving nighttime net CO₂ uptake of these Crassulacean acid metabolism (CAM) plants approximately unchanged. A similar enhancement of about 30% was found in dry weight gain over 1 year when the plants were grown at 650 microliters CO₂ per liter compared with 350 microliters per liter. Based on these results plus those at 500 microliters per liter, net CO₂ uptake over 24-hour periods and dry weight productivity of these two CAM succulents is predicted to increase an average of about 1% for each 10 microliters per liter rise in ambient CO₂ level up to 650 microliters per liter.     

Day-Night Variations in Malate Concentration, Osmotic Pressure, and Hydrostatic Pressure in Cereus validus

Malate concentration and stem osmotic pressure concomitantly increase during nighttime CO₂ fixation and then decrease during the daytime in the obligate Crassulacean acid metabolism (CAM) plant, Cereus validus (Cactaceae). Changes in malate osmotic pressure calculated using the Van't Hoff relation match the changes in stem osmotic pressure, indicating that changes in malate level affected the water relations of the succulent stems. In contrast to stem osmotic pressure, stem water potential showed little day-night changes, suggesting that changes in cellular hydrostatic pressure occurred. This was corroborated by direct measurements of hydrostatic pressure using the Jülich pressure probe where a small oil-filled micropipette is inserted directly into chlorenchyma cells, which indicated a 4-fold increase in hydrostatic pressure from dusk to dawn. A transient increase of hydrostatic pressure at the beginning of the dark period was correlated with a short period of stomatal closing between afternoon and nighttime CO₂ fixation, suggesting that the rather complex hydrostatic pressure patterns could be explained by an interplay between the effects of transpiration and malate levels. A second CAM plant, Agave deserti, showed similar day-night changes in hydrostatic pressure in its succulent leaves. It is concluded that, in addition to the inverted stomatal rhythm, the oscillations of malate markedly affect osmotic pressures and hence water relations of CAM plants.     

Thermal Energy Exchange Model and Water Loss of a Barrel Cactus, Ferocactus acanthodes

The influences of various diurnal stomatal opening patterns, spines, and ribs on the stem surface temperature and water economy of a CAM succulent, the barrel cactus Ferocactus acanthodes, were examined using an energy budget model. To incorporate energy exchanges by shortwave and longwave irradiation, latent heat, conduction, and convection as well as the heat storage in the massive stem, the plant was subdivided into over 100 internal and external regions in the model. This enabled the average surface temperature to be predicted within 1 C of the measured temperature for both winter and summer days.     

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     

Diurnal patterns of carbon dioxide,water vapour,and energy fluxes in pineapple [<Emphasis Type="Italic">Ananas comosus</Emphasis> (L.) Merr. cv. Red Spanish] field using eddy covariance

We analyzed the eddy covariance measurements of momentum, mass, and energy taken daily throughout five consecutive seasonal courses (i.e. 840 d after planting) of a pineapple [Ananas comosus (L.) Merr. cv. Red Spanish] field growing in the Orinoco lowlands. This field provides an opportunity for micrometeorological studies because of the flat and windy site; the seasonal weather including ENSO effects and the Crassulacean Acid Metabolism (CAM) physiology of the crop were additional attributes. Soil CO₂ flux was quantified and added to the net ecosystem exchange in order to obtain the canopy flux (F_C). The canopy CO₂ flux partially followed the four phases of CAM sensu Osmond (1978). The daily pattern of gaseous exchange in pineapple showed a continuum spectrum in which a major proportion of CO₂ uptake occurring during the daytime was common and in which the CAM expression was related to day and nocturnal CO₂ uptake. However, the benefits of CO₂ uptake at low water cost were constrained by the limited nocturnal CO₂ uptake. Seasonal and ontogenetic changes affected the energy exchange as well as the partitioning of available energy into sensible (Q_H) and latent (Q_LE) heat. When the hourly net radiation (Q_Rn) reached its maximum value, latent heat flux (Q_LE) to available energy throughout the vegetative and reproductive stages was 0.65, 0.05, 0.30, 0.11, and 0.33 for the 1997 wet season, 1997/98 dry season, 1998 wet season, 1998/99 dry season, and 1999 wet season, respectively. Throughout the growth period, we found the pivotal role of surface conductance (g _S) in both Q_LE and F_C. Furthermore, the canopy responded to environmental changes. During the wet seasons the g _S was strongly influenced by humidity mole fraction deficit and was usually lower than aerodynamic conductance, whereas during the dry seasons, soil water deficit limited evapotranspiration and production rates. For the fully canopy cover, the hourly trend of marginal water cost of pineapple carbon gain in the dry seasons indicated that g _S became sufficiently efficient to reduce the amount of water transported per unit of carbon gain. In the wet season, the coupling of CO₂ uptake and stomatal conductance was more effective in maintaining a higher proportionality between Q_LE and g _S.     

Seasonal photosynthetic gas exchange and water-use efficiency in a constitutive CAM plant, the giant saguaro cactus (Carnegiea gigantea)

Crassulacean acid metabolism (CAM) and the capacity to store large quantities of water are thought to confer high water use efficiency (WUE) and survival of succulent plants in warm desert environments. Yet the highly variable precipitation, temperature and humidity conditions in these environments likely have unique impacts on underlying processes regulating photosynthetic gas exchange and WUE, limiting our ability to predict growth and survival responses of desert CAM plants to climate change. We monitored net CO₂ assimilation (A _net), stomatal conductance (g _s), and transpiration (E) rates periodically over 2 years in a natural population of the giant columnar cactus Carnegiea gigantea (saguaro) near Tucson, Arizona USA to investigate environmental and physiological controls over carbon gain and water loss in this ecologically important plant. We hypothesized that seasonal changes in daily integrated water use efficiency (WUE_day) in this constitutive CAM species would be driven largely by stomatal regulation of nighttime transpiration and CO₂ uptake responding to shifts in nighttime air temperature and humidity. The lowest WUE_day occurred during time periods with extreme high and low air vapor pressure deficit (D _a). The diurnal with the highest D _a had low WUE_day due to minimal net carbon gain across the 24 h period. Low WUE_day was also observed under conditions of low D _a; however, it was due to significant transpiration losses. Gas exchange measurements on potted saguaro plants exposed to experimental changes in D _a confirmed the relationship between D _a and g _s. Our results suggest that climatic changes involving shifts in air temperature and humidity will have large impacts on the water and carbon economy of the giant saguaro and potentially other succulent CAM plants of warm desert environments.     

Soil O2 and CO2 effects on root respiration of cacti

Through use of a recently developed technique that can measure CO₂ exchange by individual attached roots, the influences of soil O₂ and CO₂ concentrations on root respiration were determined for two species of shallow-rooted cacti that typically occur in porous, well-drained soils. Although soil O₂ concentrations in the rooting zone in the field were indistinguishable from that in the ambient air (21% by volume), the CO₂ concentrations 10 cm below the soil surface averaged 540 μLL⁻¹ for the barrel cactusFerocactus acanthodes under dry conditions and 2400 μLL⁻¹ under wet conditions in a loamy sand. For the widely cultivated platyopuntiaOpuntia ficus-indica in a sandy clay loam, the CO₂ concentration at 10 cm averaged 1080 μLL⁻¹ under dry conditions and 4170 μLL⁻¹ under wet conditions. For both species, the respiration rate in the laboratory was zero at 0% O₂ and increased to its maximum value at 5% O₂ for rain roots (roots induced by watering) and 16% O₂ for established roots. Established roots ofO. ficus-indica were slightly more tolerant of elevated CO₂ than were those ofF. acanthodes, 5000 μLL⁻¹ inhibiting respiration by 35% and 46%, respectively. For both species, root respiration was reduced to zero at 20,000 μLL⁻¹ (2%) CO₂. In contrast to the reversible effects of 0% O₂, inhibition by 2% CO₂ was irreversible and led to the death of cortical cells in established roots in 6 h. Although the restriction of various cacti and other CAM plants to porous soils has generally been attributed to their requirement for high O₂ concentrations, the present results indicate that susceptibility of root respiration to elevated soil CO₂ concentrations may be more important.     

RELATION BETWEEN MONTHLY GROWTH OF FEROCACTUS ACANTHODES AND AN ENVIRONMENTAL PRODUCTIVITY INDEX

Most productivity studies use destructive harvesting methods, prohibiting continuous plant monitoring under various environmental conditions. Here a nondestructive procedure involving the displacement of areoles from the apex was utilized to study the growth of Ferocactus acanthodes, a common barrel cactus of the Sonoran Desert. Net CO₂ uptake measured under controlled conditions of temperature, water, and photosynthetically active radiation in the laboratory was used to indicate how F. acanthodes would respond to field values of these parameters. For example, net CO₂ uptake over 24 hr was maximal at day/night air temperatures of 23 C/14 C, the mean annual values in the field, and was approximately halved at 11 C/5 C and 32 C/23 C, the monthly extreme values in the field. An environmental productivity index (EPI), constructed as the product of indices for the three environmental variables, indicated the fraction of maximal CO₂ uptake expected. The monthly production of areoles on 33 plants was highly correlated with EPI (r² = 0.81). Areole production for individual plants, however, tended to be in pulses represented by Fibonacci numbers. EPI predicted an annual stem growth of 8% compared with 9 ± 3% measured previously in the field. Thus, morphological and physiological studies can be usefully combined and applied to indicate field productivity of F. acanthodes and, by extension, of other plants.     

Spine influences on PAR interception, stem temperature, and nocturnal acid accumulation by cacti

Abstract. The influence of spines on nocturnal acid accumulation was examined for two Crassulacean acid metabolism plants, the barrel cactus Ferocactus acanthodes (Lemaire) Britton & Rose var. lecontei (Engelm.) Lindsay and the cylindropuntia Opuntia bigelovii Engelm. var. bigelovii. The ambient photo-synthetically active radiation (PAR) required for 90% saturation of nocturnal acid accumulation by O. bigelovii was 23 mol m^?2 d^?1 for stems that were 32% shaded by spines and 16 mol m^?2 d^?1 when the spines were removed. For F. acanthodes, 90% saturation occurred for an ambient PAR of 45 mol m^?2 d^?1 where the stem was 78% shaded by spines and 21 mol m^?2 d^?1 when the spines were removed. For comparison, the ambient PAR averaged 21 mol m^?2 d^?1 at various times of the year and stem positions for O. bigelovii fully exposed to solar irradiation. Although spines can reflect some PAR toward the stem surface, their presence decreases the PAR incident on the stem surface, thereby decreasing the nocturnal acid accumulation. For O. bigelovii, periodically removing the spines led to a 60% greater increase in stem volume over 2^1/2 years. The effect of variation in spine-shading on stem surface temperature was determined for F. acanthodes using computer simulations and an energy budget model. A substantial reduction in daytime stem temperatures and a smaller increase in night-time temperatures as the spine-shading increased had very little influence on nocturnal acid accumulation for both winter and summer days. Thus, the main effect of spines on the metabolism of cacti is apparently to reduce the PAR incident on the stem surface and thereby to reduce productivity.     

Effects of Various Levels of CO(2) on the Induction of Crassulacean Acid Metabolism in Portulacaria afra (L.) Jacq

In response to water stress, Portulacaria afra (L.) Jacq. (Portulacaceae) shifts its photosynthetic carbon metabolism from the Calvin-Benson cycle for CO₂ fixation (C₃) photosynthesis or Crassulacean acid metabolism (CAM)-cycling, during which organic acids fluctuate with a C₃-type of gas exchange, to CAM. During the CAM induction, various attributes of CAM appear, such as stomatal closure during the day, increase in diurnal fluctuation of organic acids, and an increase in phosphoenolpyruvate carboxylase activity. It was hypothesized that stomatal closure due to water stress may induce changes in internal CO₂ concentration and that these changes in CO₂ could be a factor in CAM induction. Experiments were conducted to test this hypothesis. Well-watered plants and plants from which water was withheld starting at the beginning of the experiment were subjected to low (40 ppm), normal (ca. 330 ppm), and high (950 ppm) CO₂ during the day with normal concentrations of CO₂ during the night for 16 days. In water-stressed and in well-watered plants, CAM induction as ascertained by fluctuation of total titratable acidity, fluctuation of malic acid, stomatal conductance, CO₂ uptake, and phosphoenolpyruvate carboxylase activity, remained unaffected by low, normal, or high CO₂ treatments. In well-watered plants, however, both low and high ambient concentrations of CO₂ tended to reduce organic acid concentrations, low concentrations of CO₂ reducing the organic acids more than high CO₂. It was concluded that exposing the plants to the CO₂ concentrations mentioned had no effect on inducing or reducing the induction of CAM and that the effect of water stress on CAM induction is probably mediated by its effects on biochemical components of leaf metabolism.     

Environmental Influences on Open Stomates of a Crassulacean Acid Metabolism Plant, Agave deserti

The major short term stomatal response of Agave deserti was to temperature; increases in leaf temperature led to decreases in water vapor conductance for stomatal opening during the daytime (C₃ mode) as well as at night (Crassulacean acid metabolism or CAM mode). Hourly changes in the water vapor concentration drop from leaf to air had no significant stomatal effect in either mode. Stomatal responses to external CO₂ levels up to 800 microliters per liter were not significant after 15 minutes and only moderate after a few hours, suggesting that CO₂ effects on open stomates of this succulent were indirect in both CAM and C₃ modes.     

The role of CAM in high rainfall cloud forests: an in situ comparison of photosynthetic pathways in Bromeliaceae

Crassulacean acid metabolism (CAM), an advanced photosynthetic pathway conferring water conservation to plants in arid habitats, has enigmatically been reported in some species restricted to extremely wet tropical forests. Of these, epiphytic Bromeliaceae may possess absorbent foliar trichomes that hinder gas‐exchange when wetted, imposing further limitations on carbon dioxide (CO₂) uptake. The hypothesis that the metabolic plasticity inherent to CAM confers an ecological advantage over conventional C₃ plants, when constant rainfall and mist might inhibit gas‐exchange was investigated. Gas‐exchange, fluorometry and organic acid and mineral nutrient contents were compared for the bromeliads Aechmea dactylina (CAM) and Werauhia capitata (C₃) in situ at the Cerro Jefe cloud forest, Panama (annual rainfall > 4 m). Daily carbon gain and photosynthetic nutrient use efficiencies were consistently higher for A. dactylina, due to a greater CO₂ uptake period, recycling of CO₂ from respiration and a dynamic response of CO₂ uptake to wetting of leaf surfaces. During the dry season CAM also had water conserving and photoprotective roles. A paucity of CAM species at Cerro Jefe suggests a recent radiation of this photosynthetic pathway into the wet cloud forest, with CAM extending diversity in form and function for epiphytes.     

Crassulacean acid metabolism,CO2-recycling,and tissue desiccation in the Mexican epiphyte Tillandsia schiedeana Steud (Bromeliaceae)

After 23 days without water in a greenhouse, rates of nocturnal CO₂ uptake in Tillandsia schiedeana decreased substantially and maximum rates occurred later in the dark period eventually coinciding with the onset of illumination. Nocturnal CO₂ uptake accounted for less than half the total nighttime increase in acidity measured in well-watered plants. With increased tissue desiccation, only 11–12% of measured acid accumulation was attributable to atmospheric CO₂ uptake. Plants desiccated for 30 days regained initial levels of nocturnal acid accumulation and CO₂ uptake after rehydration for 10h. These results stress the importance of CO₂ recycling via CAM in this epiphytic bromeliad, especially during droughts.Partially supported by Biomedical Sciences Support Grant RR07037.     

Seasonal response to drought and rewatering in Portulacaria afra (L.) Jacq.

Summary Gas exchange characteristics of droughted and rewatered Portulacaria afra were studied during the seasonal shift from CAM to C₃ photosynthesis. ¹⁴CO₂ uptake, stomatal conductance, and total titratable acidity were determined for both irrigated and 2, 4, and 7.5 month waterstressed plants from summer 1984 to summer 1985. Irrigated P. afra plants were utilizing the CAM pathway throughout the summer and shifted to C₃ during the winter and spring. Beginning in September, P. afra plants shifted from CAM to CAM-idling after 2 months of water-stress. When water-stress was initiated later in the fall, exogenous CO₂ uptake was still measurable after 4 months of drought. After 7.5 months of stress, exogenous CO₂ uptake was absent. The shift from CAM to CAM-idling or C₃ in the fall and winter was related to when water stress was initiated and not to the duration of the stress. Gas exchange resumed within 24 h of rewatering regardless of the duration of the drought. In the winter and spring, rewatering resulted in a full resumption of daytime CO₂ uptake. Whereas during the summer, rewatering quickly resulted in early morning CO₂ uptake, but nocturnal CO₂ uptake through the CAM pathway was observed after 7 days. Gas exchange measurements, rewatering characteristics, and transpirational water loss support the hypothesis that the C₃ pathway was favored during the winter and spring. The CAM pathway was functional during the summer when potential for water loss was greater. Our investigations indicate that P. afra has a flexible photosynthetic system that can withstand long-term drought and has a rapid response to rewatering.     

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.     

Patterns of gas exchange and organic acid oscillations in tropical trees of the genus Clusia

Summary Gas exchange patterns and nocturnal acid accumulation were examined in four species of Clusia under simulated field conditions in the laboratory. Clusia alata and C. major had midday stomatal closure, substantial net CO₂ exchange ( ) during the night, and the highest water use efficiency (WUE). C. venosa showed a pattern similar to a C₃ plant, with nighttime stomatal closure, while C. minor maintained positive continuously throughout a 24-h period. However, large changes in titratable acidity, which closely matched changes in citrate and malate levels, indicated that Crassulacean acid metabolism (CAM) is active in all four species. C. venosa showed dawn-dusk oscillations in titratable acidity that were higher than the values reported for other C₃-CAM intermediates, while the nighttime acid accumulation of 998 mol m^–3 observed in C. major is unsurpassed by any other CAM plant. Moreover, the dawn-dusk changes in citrate levels of over 65 mol m^–3 in C. alata and C. minor, and over 120 mol m^–3 in C. major, are 3–6 times higher than values reported for other CAM plants. Although these oscillations in citrate levels were quite large, and the nighttime dark respiration rates were high, the O₂ budget analysis suggestes that only part of the reducing power generated by the synthesis of citric acid enters the respiratory chain. Dawn-dusk changes in malate levels were just over 50 mol m^–3 for C. venosa but over 300 mol m^–3 for C. major. Between 28% (C. major) and 89% (C. venosa) of the malate accumulated during the night was derived from recycled respiratory CO₂. These daily changes in malate and citrate levels also contributed significantly to changes in leaf sap osmolality. This variability in CO₂ uptake patterns, the recycling of nighttime respiratory CO₂, and the high WUE may have contributed to the successful invasion of Clusia into a wide range of habitats in the tropics.     

Irrigation magnifies CAM-photosynthesis in Opuntia basilaris (Cactaceae)

Summary Measurements of acid metabolism and gas exchange were carried out four times during a year to assess the relative importance of temperature and the accompanying seasonal change to the carbon metabolism of Opuntia basilaris Engelm. & Bigel. plants growing in situ under irrigated and natural (control) conditions. Our experiments showed that this cactus did not change its pattern of carbon assimilation when continuously irrigated under field conditions. Non-irrigated cacti had maximum acid accumulation after periods of precipitation. Maximum acid accumulation in irrigated cacti occurred when there was a large difference between day/night temperatures (i.e., 16°C), and when nighttime temperatures were moderate (14C). Irrigated cacti had greater duration of stomatal opening and lower resistance to ¹⁴CO₂ uptake. When temperatures were low, daytime stomatal resistance to ¹⁴CO₂ uptake decreased (to 20–40 s cm^-1), but never to the level of the nocturnal resistances (5–10 s cm^-1). During periods of drought, nonirrigated cycti changed to a pattern in which organic acids fluctuated. Irrigated cacti continued to have ¹⁴CO₂ uptake when nighttime temperatures were as high as 33°C. ¹³C/¹²C isotope composition ratios, determined after two years of irrigation, were near -12 in irrigated and non-irrigated plants. Therefore, under conditions of continual irrigation, seasonal and temperature changes affected the degree of dark CO₂ fixation and acid metabolism, but these cacti did not change from CAM to CO₂ fixation in the light.Abbreviations C₃ reductive pentose phosphate cycle - C₄ dicarboxylic acid cacle - CAE carbon assimilation efficiency - CAM Crassulacean acid metabolism - THO tritiated water - T/P transpiration ratio - vpd vapor pressure deficit - water potential This study was supported in part by National Science Foundation grant OIP 74-15673. Under the auspice of this grant a cooperative research project was carried out between the Australian National University, Canberra, and the Philip L. Boyd Deep Canyon Desert Research Center, University of California, Riverside. The studies involved a comparison of the photosynthetic pathways employed by succulents during growth in their native environment (the Sonoran Desert of southeastern California) and in a favorable introduced environment (Queensland and New South Wales, Australia). Studies carried out in Australia are under the direction of Dr. C.B. Osmond     

Comparative ecophysiology of CAM and C3 bromeliads. III. Environmental influences on CO2 assimilation and transpiration

Abstract 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 plants found at drier habitats, and stomatal conductance in 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 rates 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 photosynthesis 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 these 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.     

Influences of minimum stem temperatures on ranges of cacti in southwestern United States and central Chile

Summary A computer model predicted the minimum stem surface temperature, which generally occurred at the apex, for various species of cacti under a particular set of environmental conditions. Based on stem mass, spines, and apical pubescence for the four Ferocactus species found in the southwestern United States, F. acanthodes had the highest minimum apical temperatures and hence was predicted to range the furthest north, F. wizlizenii next, then F. covillei, and finally F. viridescens, in agreement with field observations. Direct measurement of apical temperatures at night showed that F. viridescens was about 2°C colder than a side-by-side F. acanthodes, in agreement with the model. The simulated apical temperature of Trichocereus chilensis increased about 0.3°C for each 50 cm increase in height up to 2 m; observations at a high elevation site in central Chile showed that the freezing damage progressively halved over this sequence of height intervals. The upper elevational limit of Eriosyce ceratistes and T. chilensis at different latitudes from 29°S to 35°S indicated that the populations were responsive to changes of only 0.1°C. Such temperature sensitivities underscore the importance of morphological differences in establishing the low temperature limits on the ranges of cacti in particular and plants in general.     

Stomata of the CAM plant Tillandsia recurvata respond directly to humidity

Summary Under controlled conditions, CO₂ exchange of Tillandsia recurvata showed all characteristics of CAM. During the phase of nocturnal CO₂ fixation stomata of the plant responded sensitively to changes in ambient air humidity. Dry air resulted in an increase, moist air in a decrease of diffusion resistance. The evaporative demand of the air affected the level of stomatal resistance during the entire night period. Due to stomatal closure, the total nocturnal water loss of T. recurvata was less at low than at high humidity. It is concluded that stomata respond directly to humidity and not via bulk tissue water conditions of the leaves. Such control of transpiration may optimize water use efficiency for this almost rootless, extreme epiphyte.