Crassulacean acid metabolism in the epiphytic ferns Drymoglossum piloselloides and Pyrrosia longifolia: studies on responses to environmental signals

Abstract The paper reports the results of the comprehensive study of crassulacean acid metabolism in two epiphytic tropical ferns, Drymoglossum piloselloides and Pyrrosia longifolia. The plants were investigated under different light, temperature and water status. It was found that both species are obligate CAM plants. The diurnal acidity rhythm is due to the fluctuation in malic acid concentration, which accounts for the change in titratable acidity. Besides malic acid, shikimate and oxalate are found to be present, but not contributing to the CAM acid rhythm. The diurnal rhythm of malic acid content results in a corresponding rhythm in leaf water relations. Both Φ_Φ and Φ_total, were lowest at the end of the night, i.e. when the level of malic acid was highest. The effects of temperature on CO₂ exchange were inverse to those observed in other CAM plants. In both ferns studied, dark CO₂ fixation increased when the night temperature was increased. Increase in day temperature reduced CO₂ uptake during phase IV and during the following night. The observed responses of the ferns to temperature changes suggest that the in situ environmental conditions are optimal for their CAM performance. In weak light, the plants showed net CO₂ output during the midday deacidification period. Increases in light intensity reduced such CO₂ output. Under drought conditions, the CO₂ exchange in the ferns was reduced to zero within 5–6 d, indicating that the ferns studied are more susceptible to water deficiency than other CAM plants. This could be due to a higher cuticular conductance for water. The results are discussed, in particular, in relation to CAM performance of epiphytes growing in the wet tropics.     

Oxygen Budget of a River Rich in Submerged Macrophytes (River Zschopau in the South of the GDR)

The oxygen budget of the river Zschopau, a flowing water typical of hilly areas in the south of the GDR, was investigated in regard of seasonal variations in the activity of submerged macrophytes. Oxygen concentrations were continuously measured in the river by means of electrodes. Seasonal activity patterns of Ranunculus penicillatus, the dominant macrophyte in the river stretch investigated, were found by laboratory experiments. The oxygen budget was obtained by measuring CO₂ concentrations by infrared gas analysis (IRGA) and CO₂ and O₂ concentriations by means of electrodes.     

Effects of phytoplankton on the distribution of submerged macrophytes in a small canal

Submerged macrophytes have been disappearing from the Kanto Plain, Japan since the 1960s. This disappearance is usually attributable to the interaction between macrophytes and phytoplankton. Phytoplankton contributes to shading of the available light and changes the availability of inorganic carbon from free CO₂ to HCO ₃ ^? for use in photosynthesis. However, limited information is available about the interaction between carbon fraction and submerged macrophytes through phytoplankton abundance. In this short note, we observe the distribution of submerged macrophytes and phytoplankton in a small canal. We found that, despite high photosynthetically active radiation (PAR) in the downstream region, low free CO₂ concentration through phytoplankton abundance can deplete free CO₂ for submerged macrophytes. In contrast, the upstream region exhibited macrophytes in an environment with high free CO₂ concentration. The stable carbon isotope ratio of submerged macrophytes follows this pattern, with more positive values occurring in the downstream region and more negative values in the upstream region. It has been reported that phytoplankton limits light availability for submerged macrophytes, but carbon availability could also be a factor in the distribution of submerged macrophytes. Because the source of water for submerged macrophytes is groundwater, its preservation possibly plays a key role for the restoration of submerged macrophytes.     

Heavy metals contamination and accumulation in submerged macrophytes in an urban river in China

Deteriorating urban water quality has attracted considerable attention in China. We investigated the contamination levels and distribution of heavy metals (As, Cd, Cu, Ni, Pb, and Zn) in Yuxi River water and sediments, and assessed the heavy metal accumulation capability of five species of submerged macrophytes: Vallisneria natans (Lour.) Hara, Potamogeton pectinatus L., Hydrilla verticillata (L. f.) Royle, Myriophyllum spicatum L., and Potamogeton crispus L. Samples were collected from upstream and downstream locations in different season. The results showed that the levels of heavy metals in the downstream areas were higher than in the upstream areas. Heavy metal concentrations in the river water during the dry seasons were higher than those during the rainy seasons, and the opposite results appeared in sediments and submerged macrophytes. In general, the river was slightly contaminated by heavy metals, and the concentrations of Pb and Ni in this river should serve as a warning, while Cd and Zn pollution in the sediments desperately needs to be removed. Furthermore, Potamogeton pectinatus L. showed a higher accumulation capacity for these metals among the five native submerged macrophytes and could be defined as a hyperaccumulator for Cd. Therefore, the potential use of native aquatic plants in contaminated rivers is worth further exploration.     

CO2 is the main inorganic C species entering photosynthetically active leaf protoplasts of the freshwater macrophyte Ranunculus penicillatus ssp. pseudofluitans

Submerged aquatic macrophytes growing in water where free CO₂ is unavailable (above pH 8·2) must use mechanisms to supply external dissolved inorganic carbon in a form available to chloroplasts (CO₂). Active transport of HCO₃^– across the plasmalemma has not been proven to be widespread in aquatic macrophytes and catalytic conversion of HCO₃^– to CO₂ is the usual supply mechanism in submerged macrophytes. The interaction of leaf form and function in this respect was investigated in the linear, submerged leaves of Ranunculus penicillatus (Dumort.) Bab ssp. pseudofluitans (Syme) S.Webster. Viable protoplasts were isolated using a mixture of cell wall degrading enzymes optimized for this species. Protoplast viabilities greater than 80% after 5 h of isolation were achieved. Photosynthetic rates of isolated protoplasts were comparable with that of intact plant tissue. Results of carbon isotopic disequilibrium experiments showed that CO₂ was the preferred species of dissolved inorganic carbon for photosynthesis by protoplasts and that HCO₃^– which predominates in the plant’s natural environment mainly contributes by supplying CO₂ outside the cells.     

Recycling of CO2 via Crassulacean acid metabolism in the rock outcrop succulent Sedum pulchellum Michx. (Crassulaceae)

Under well-watered conditions in the laboratory, Sedum pulchellum assimilated CO₂ only during the day, yet exhibited small nocturnal increases in tissue acid content followed by deacidification in the light (CAM-cycling). When drought-stressed, little CO₂ was fixed in the day and none at night, yet even greater acid fluctuations were observed (CAM-idling). Calculations indicate that water savings associated with CAM-cycling when water is available are small. Water saving is more likely to be significant during CAM-idling when water supply is limited and stomata are closed day and night. Thus, in this species, CAM-idling may be of greater benefit to the plant, relative to CAM-cycling, in surviving habitats prone to frequent drought stress.Abbreviations A CO₂ exchange rate - CAM Crassulacean acid metabolism - c_i shoot internal CO₂ concentration - g_c shoot conductance to CO₂ - PPFD photosynthetic photon flux density - WUE water-use efficiency Supported by National Science Foundation Grant No. DMB 8506093.     

Regulation of CAM and Respiratory Recycling by Water Supply in Higher Poikilohydric Plants — Haberlea rhodopensis Friv. and Ramonda serbica Panč, at Transition from Biosis to Anabiosis and Vice Versa

In this paper the expression of C₃ and CAM in the resurrection plants Haberlea rhodopensis Friv. and Ramonda serbica Pan?, during the transition from biosis to anabiosis and Wee versa is reported for the first time. The transition from predominantly C₃ metabolism to net dark fixation of CO₂ occurred in leaves of R.serbica during desiccation. Desiccated plants of H. rhodopensis react by reducing light assimilation of CO₂. When watering was resumed night time fixation of CO₂ by R. serbica was observed within 24 hours. The recovery of CO₂ fixation by H. rhodopensis was not seen until the 8 th day. Desiccated and rehydrated plants of H. rhodopensis recapture a higher proportion of respiratory CO₂ than well-watered plants. Since both species have little capacity for water conservation in their tissues, the early onset of high recycling of CO₂ following drought could be an important mechanism for potentially saving water.     

Major changes in CO<Subscript>2</Subscript> efflux when shallow lakes shift from a turbid to a clear water state

Lakes can be sources or sinks of carbon, depending on local conditions. Recent studies have shown that the CO₂ efflux increases when lakes recover from eutrophication, mainly as a result of a reduction in phytoplankton biomass, leading to less uptake of CO₂ by producers. We hypothesised that lake restoration by removal of coarse fish (biomanipulation) or invasion of mussels would have a similar effect. We studied 14–22 year time series of five temperate Danish lakes and found profound effects on the calculated CO₂ efflux of major shifts in ecosystem structure. In two lakes, where limited colonisation of submerged macrophytes occurred after biomanipulation or invasion of zebra mussels (Dreissena polymorpha), the efflux increased significantly with decreasing phytoplankton chlorophyll a. In three lakes with major interannual variation in macrophyte abundance, the efflux declined with increasing macrophyte abundance in two of the lakes, while no relation to macrophytes or chlorophyll a was found in the third lake, likely due to high groundwater input to this lake. We conclude that clearing water through invasive mussels or lake restoration by biomanipulation may increase the CO₂ efflux from lakes. However, if submerged macrophytes establish and form dense beds, the CO₂ efflux may decline again.     

RESPONSE OF TWO OLD FIELD PERENNIALS TO INTERACTIONS OF CO2 ENRICHMENT AND DROUGHT STRESS

Aster pilosus Willd. (aster, C₃) and Andropogon virginicus L. (broomsedge, C₄) were grown in growth chambers at 26/20 C day/night temperatures with a PPFD of 1,000 μmol s^–1 m^–2. Water was withheld for a 2-wk drought period under three CO₂ concentrations (approximately 380, 500, and 650 μl 1^–1). There were significant effects of CO₂ enrichment on aster so that drought stress did not occur in plants grown with CO₂ enrichment. Non-watered plants with CO₂ enrichment had greater leaf water potentials, greater photosynthetic rates, and greater total dry wt than non-watered plants grown at 380 μl 1^–1 CO₂. The response of broomsedge to drought was the same in all CO₂ treatments and there was no significant interaction of CO₂ enrichment and drought. The decreased severity of drought stress and the increased growth of aster with CO₂ enrichment may increase its competitive ability during droughts, allowing it to persist for longer periods during succession in abandoned fields.     

Physiological responses of Opuntia ficus-indica to growth temperature

The influences of various day/night air temperatures on net CO₂ uptake and nocturnal acid accumulation were determined for Opuntia ficus-indica, complementing previous studies on the water relations and responses to photosynthetically active radiation (PAR) for this widely cultivated cactus. As for other Crassulacean acid metabolism (CAM) plants, net nocturnal CO₂ uptake had a relatively low optimal temperature, ranging from 11°C for plants grown at day/night air temperatures of 10°C/0°C to 23°C at 45°C/35°C. Stomatal opening, which occurred essentially only at night and was measured by changes in water vapor conductance, progressively decreased as the measurement temperature was raised. The CO₂ residual conductance, which describes chlorenchyma properties, had a temperature optimum a few degrees higher than the optimum for net CO₂ uptake at all growth temperatures. Nocturnal CO₂ uptake and acid accumulation summed over the whole night were maximal for growth temperatures near 25°C/15°C, CO₂ uptake decreasing more rapidly than acid accumulation as the growth temperature was raised. At day/night air temperatures that led to substantial nocturnal acid accumulation (25°C/15°C.). 90% saturation of acid accumulation required a higher total daily PAR than at non-optimal growth temperatures (10°C/0°C and 35°C/25°C). Also, the optimal temperature of net CO₂ uptake shifted downward when the plants were under drought conditions at all three growth temperatures tested, possibly reflecting an increased fractional importance of respiration at the higher temperatures during drought. Thus, water status, ambient PAR, and growth temperatures must all be considered when predicting the temperature response of gas exchange for O. ficus-indica and presumably for other CAM plants.     

The Effect of Atmospheric Carbon Dioxide Elevation on Plant Growth in Freshwater Ecosystems

We developed a dynamic model to investigate the effect of atmospheric carbon dioxide (CO₂) increase on plant growth in freshwater ecosystems. Steady-state simulations were performed to analyze the response of phytoplankton and submerged macrophytes to atmospheric CO₂ elevation from 350 to 700 ppm. We studied various conditions that may affect this response, such as alkalinity, the air–water exchange rate of CO₂, the community respiration rate, and the phosphorus (P) supply rate. The increase in atmospheric CO₂ could affect submerged plant growth only under relatively eutrophic conditions and at a low community respiration rate. Alkalinity had little effect on the response of the different species. When the air–water exchange was low, the proportional effect of the CO₂ increase on plant growth was higher. Under eutrophic conditions, algae and macrophytes using CO₂ and HCO₃^– may double their growth rate due to atmospheric CO₂ elevation, while the growth of macrophytes restricted to CO₂ assimilation may be threefold. The differences in response of the species under various conditions indicate that the elevation of atmospheric CO₂ may induce drastic changes in the productivity and species dominance in freshwater systems.     

Floating Aquatic Macrophytes Can Substantially Offset Open Water CO<Subscript>2</Subscript> Emissions from Tropical Floodplain Lake Ecosystems

Tropical floodplain lake ecosystems are recognized as important sources of carbon (C) from the water to the atmosphere. They receive large amounts of organic matter and nutrients from the watershed, leading to intense net heterotrophy and carbon dioxide (CO₂) emission from open waters. However, the role of extensive stands of floating macrophytes colonizing floodplains areas is still neglected in assessments of net ecosystem exchange of CO₂ (NEE). We assessed rates of air-lake CO₂ flux using static chambers in both open waters and waters covered by the widespread floating aquatic macrophyte (water hyacinth; Eichornia sp.) in two tropical floodplain lakes in Pantanal, Brazil during different hydrological seasons. In both lakes, areas colonized by floating macrophytes were a net CO₂ sink during all seasons. In contrast, open waters emitted CO₂, with higher emissions during the rising and high water periods. Our results indicate that the lake NEE can be substantially overestimated (fivefold or more in the studied lakes) if the carbon fixation by macrophytes is not considered. The contribution of these plants can lead to neutral or negative NEE (that is, net uptake of CO₂) on a yearly basis. This highlights the importance of floating aquatic macrophytes for the C balance in shallow lakes and extensive floodplain areas.     

The Underwater Life of Secondarily Aquatic Plants: Some Problems and Solutions

The freshwater secondarily aquatic plants, most of which are higher plants, are those returned to the water environment after spending a period of time living on land. The readaptation to living underwater has made it necessary for these plants to put in place morphological and functional strategies to cope with some major problems due to features of the aquatic environment, but also deriving from the specialized organization of their “terrestrial” bodies. The poor O₂ availability underwater accounted for the evolution of wide aerenchyma tissues throughout the plant organs to improve the photosynthetic O₂ flux from the shoot to the roots buried in anoxic sediments and to the neighboring rhizosphere. This favors sediment oxygenation, sustains the aerobic metabolism of roots, and improves the availability and uptake of mineral nutrients, whose delivery to the entire plants, without a transpirational flux, is ensured by an acropetal mass transport depending on root pressure, guttation from hydathodes and channeling by apoplast closure around the vascular tissues. A great expansion of leaf surfaces and an enhanced surface:volume ratio of chloroplast-rich photosynthetic cells help to contact the water medium and to increase the cell/environment exchanges to gain inorganic carbon. Furthermore, different physiological mechanisms operate to cope with the scarce availability of CO₂ and the prevalence of HCO₃ ^? as inorganic carbon form in water. Some of them, like cell wall acidification through H⁺ extrusion by a light-dependent APTase or activation of an apoplastic carbonic anhydrase, operate outside the cells, leading to a conversion of HCO₃ ^? to CO₂, which then diffuses into the cells. Others, on the contrary, act inside the cells to load the active site of Rubisco with CO₂, thus favoring photosynthesis and lowering photorespiration. Aquatic macrophytes with isoetid life form, moreover, can obtain most ot the fixed CO₂ from sediments. In submerged species, in additin to the C₃ cycle, the C₄ and CAM-like photosynthetic metabolisms can also operate, and are modulated by the environmental inorganic carbon availability and the plant photosynthetic demand. Interestingly, in the aquatic plants the C₄ pathway, which can be concomitant with the C₃ one, does not depend on the Kranz anatomy of leaves, but relies on the intracellular compartmentation of carboxylative and decarboxylative enzymes. The CAM-like pathway, defined AAM, which also coexists with the C₃, allows the submerged plants to fix CO₂ in the dark, thus exploiting the higher CO₂ availability in the water medium during the night, and extending to 24?h the period of inorganic carbon assimilation. In almost all the aquatic macrophytes the AAM is only expressed in the submersion state, whereas it is quickly inactivated in emerging leaves in a cell by cell way.     

Temperature and water regulation of gas exchange of Opuntia polyacantha

Summary Opuntia polyacantha was collected from the shortgrass prairie in Colorado. Carbon dioxide and water vapor exchange was monitored in plants pretreated and analyzed under cool temperatures (20/15°C) and warm temperatures (35/15°C). Well watered plants under a 35/15 thermoperiod supported the fixation of atmospheric CO₂ during the night, early morning, and late afternoon. Plants under a 20/15 thermoperiod exhibited CO₂ uptake only during the afternoon. The fixation of CO₂ at night could be stimulated or induced by decreasing the night temperature. Plants from which water was withheld two or four weeks showed a decline in CO₂ fixation with the uptake at night exhibiting the greatest sensitivity. Under conditions of water stress the enhancement of CO₂ uptake at night by cool night temperatures was largely lost. Plants water stressed for 4 weeks recovered rapidly upon rewatering under warm or cool temperatures. Rates of CO₂ fixation were comparable to well watered plants within 24 h. The effects of temperature and water stress on gas exchange are compared to the in situ growth pattern of O. polyacantha and contrasted with the regulation of gas exchange observed in C₃ and C₄ grasses of the shortgrass prairie.This research was supported by funds from NSF Grants BMS 74-07894, GB-31862X, and GB-41233X     

Structural and metabolic properties of green tissue cultures from a CAM plant, Kalanchoë blossfeldiana hybr. Montezuma

Abstract Crassulacean acid metabolism (CAM) was studied in mixotrophic callus tissue cultures of Kalanchoë blossfeldiana hybr. Montezuma and compared with plants propagated from the calli. The ultrastructural properties of the green callus cells are similar to mesophyll cells of CAM plants except that occasionally abnormal mitochondria were observed. There was permanent net CO₂ output by the calli in light and darkness, which was lower in darkness than in light. The calli exhibited a diurnal rhythm of malic acid, with accumulation during the night and depletion during the day. ¹⁴C previously incorporated by dark CO₂ fixation into malate was transferred upon subsequent illumination into end products of photosynthesis. All these data indicate that CAM operates in the calli tissue. The results revealed that the capacity for CAM is obviously lower in the calli compared with plantlets developing from the calli, or with ‘adult’ plants. The data suggest also that CAM in the calli was not limited by the activities of CAM enzymes.     

The Response of Leaf Water Potential and Crassulacean Acid Metabolism to Prolonged Drought in Sedum rubrotinctum

Plants of Sedum rubrotinctum R. T. Clausen were studied in a green-house over a 2-year period without watering. Only the apical leaves survived and were turgid at the end of the experiment. The midday leaf water potential of these apical leaves was −1.20 megapascals, while the leaf water potential of comparable leaves on well-watered control plants was −0.20 megapascals. The unwatered plants appear to have maintained turgor by means of an osmotic adjustment. After 2 years without water the plants no longer exhibited a nocturnal accumulation of titratable acidity. However, the daytime levels of titratable acidity of the unwatered plants were more than 2-fold greater than the levels in well-watered control plants. Well-watered plants of S. rubrotinctum exhibited seasonal shifts in biomass stble carbon isotope ratios, indicating a greater proportion of day versus night CO₂ uptake in the winter than in the summer. The imposition of water stress prevented the expression of this seasonal rhythm and restricted the plants to dark CO₂ uptake.     

A global perspective of ground level, ‘ambient’ carbon dioxide for assessing the response of plants to atmospheric CO2

For most studies involving the response of plants to future concentrations of atmospheric carbon dioxide (CO₂), a current concentration of 360–370 μatm is assumed, based on recent data obtained from the Mauna Loa observatory. In the present study, average seasonal diurnal values of ambient CO₂ obtained at ground level from three global locations (Australia, Japan and the USA) indicated that the average CO₂ (at canopy height) can vary from over 500 μatm at night to 350 μatm during the day with average 24‐h values ranging from 390 to 465 μatm. At all sites sampled, ambient CO₂ rose to a maximum value during the pre‐dawn period (03.00–06.00 hours); at sunrise, CO₂ remained elevated for several hours before declining to a steady‐state concentration between 350 and 400 μatm by mid‐morning (08.00–10.00 hours). Responses of plant growth to simulations of the observed variation of in situ CO₂ were compared to growth at a constant CO₂ concentration in controlled environment chambers. Three diurnal patterns were used (constant 370 μatm CO₂, constant 370 during the day (07.00–19.00 hours), high CO₂ (500 μatm) at night; or, high CO₂ (500 μatm) at night and during the early morning (07.00–09.00 hours) decreasing to 370 μatm by 10.00 hours). Three plant species ? soybean (Glycine max, L (Merr.), velvetleaf (Abutilon theophrasti L.) and tomato (Lycopersicon esculentum L.) ? were grown in each of these environments. For soybean, high night‐time CO₂ resulted in a significant increase in net assimilation rate (NAR), plant growth, leaf area and biomass relative to a constant ambient value of CO₂ by 29 days after sowing. Significant increases in NAR for all three species, and significant increases in leaf area, growth and total biomass for two of the three C3 species tested (velvetleaf and soybean) were also observed after 29 days post sowing for the high night/early morning diurnal pattern of CO₂. Data from these experiments suggest that the ambient CO₂ concentration experienced by some plants is higher than the Mauna Loa average, and that growth of some agricultural species at in situ CO₂ levels can differ significantly from the constant CO₂ value used as a control in many CO₂ experiments. This suggests that a reassessment of control conditions used to quantify the response of plants to future, elevated CO₂ may be required.     

Persistent circadian rhythms in the phosphorylation state of phosphoenolpyruvate carboxylase from Bryophyllum fedtschenkoi leaves and in its sensitivity to inhibition by malate

Phosphoenolpyruvate carboxylase (EC 4.1.1.31; PEPCase) from Bryophyllum fedtschenkoi leaves has previously been shown to exist in two forms in vivo. During the night the enzyme is phosphorylated and relatively insensitive to feedback inhibition by malate whereas during the day the enzyme is dephosphorylated and more sensitive to inhibition by malate. These properties of PEPCase have now been investigated in leaves maintained under constant conditions of temperature and lighting. When leaves were maintained in continuous darkness and CO₂-free air at 15°C, PEPCase exhibited a persistent circadian rhythm of interconversion between the two forms. There was a good correlation between periods during which the leaves were fixing respiratory CO₂ and periods during which PEPCase was in the form normally observed at night. When leaves were maintained in continuous light and normal air at 15°C, starting at the end of a night or the end of a day, a circadian rhythm of net uptake of CO₂ was observed. Only when these constant conditions were applied at the end of a day was a circadian rhythm of interconversions between the two forms of PEPCase observed and the rhythms of enzyme interconversion and CO₂ uptake did not correlate in phase or period.Abbreviations CAM Crassulacean acid metabolism - FW fresh weight - PEPCase phosphoenolpyruvate carboxylase - RuBPCase ribulose-1,5-bisphosphate carboxylase To whom correspondence should be addressed.     

Increased night temperature reduces the stimulatory effect of elevated carbon dioxide concentration on methane emission from rice paddy soil

To determine how elevated night temperature interacts with carbon dioxide concentration ([CO₂]) to affect methane (CH₄) emission from rice paddy soil, we conducted a pot experiment using four controlled‐environment chambers and imposed a combination of two [CO₂] levels (ambient: 380 ppm; elevated: 680 ppm) and two night temperatures (22 and 32 °C). The day temperature was maintained at 32 °C. Rice (cv. IR72) plants were grown outside until the early‐reproductive growth stage and then transferred to the chambers. After onset of the treatment, day and night CH₄ fluxes were measured every week. The CH₄ fluxes changed significantly with the growth stage, with the largest fluxes occurring around the heading stage in all treatments. The total CH₄ emission during the treatment period was significantly increased by both elevated [CO₂] (P=0.03) and elevated night temperature (P<0.01). Elevated [CO₂] increased CH₄ emission by 3.5% and 32.2% under high and low night temperature conditions, respectively. Elevated [CO₂] increased the net dry weight of rice plants by 12.7% and 38.4% under high and low night temperature conditions, respectively. These results imply that increasing night temperature reduces the stimulatory effect of elevated [CO₂] on both CH₄ emission and rice growth. The CH₄ emission during the day was larger than at night even under the high‐night‐temperature treatment (i.e. a constant temperature all day). This difference became larger after the heading stage. We observed significant correlations between the night respiration and daily CH₄ flux (P<0.01). These results suggest that net plant photosynthesis contributes greatly to CH₄ emission and that increasing night temperature reduces the stimulatory effect of elevated [CO₂] on CH₄ emission from rice paddy soil.     

CO2 exchange of CAM exhibiting suceulents in the southern Namib desert in relation to microclimate and water stress

The responses of CO₂ exchange and overnight malate accumulation of leaf and stem succulent CAM-plants to water stress and the particular climatic conditiens of fog and föhn in the southern Namib desert have been investigated. In most of the investigated CAM plants a long term water stress gradually attenuated any uptake of external CO₂ and led to CO₂ release throughout day and night. No CAM-idling was observed. Rainfall or irrigation immediately restored daytime CO₂ uptake while the recovery of the noctural CO₂ uptake was delayed. Dawn peak of photosynthesis was only found in well watered plants but was markedly reduced by the short term water stress of a föhn-storm. Morning fog with its higher diffuse light intensity compared with clear days increased photosynthetic CO₂ uptake considerably. Even in well watered plants noctural CO₂ uptake and malate accumulation were strongly affected by föhn indicating that the water vapour pressure deficit during the night determines the degree of acidification.