Moisture content and CO2 exchange of lichens

Summary Net photosynthesis (10 klx light intensity, 150 E m^-2 s^-1 PAR) and dark respiration of the lichen Ramalina maciformis at different temperatures are measured in relation to thallus water content. Both first increase with increasing hydration. Dark respiration then remains constant with increased water content until thallus saturation. In contrast, a further increase in water content leads to a depression of net photosynthesis, as shown in previous studies, after a maximum of CO₂ uptake has been attained. However, the extent of this depression depends strongly on temperature. In saturated thalli (160% water content in relation to lichen dry weight) the depression amounts to about 15% and 63% of the maximum unsaturated rate at 5°C and 25°C thallus temperature, respectively. The moisture compensation-point of net photosynthesis is also decisively determined by temperature (for 0°C at 20% water content; for 25°C at 15%), and the water content that allows maximum rates of CO₂ uptake (for 0°C at 80%; for 25°C at less than 40% water content). An electrical analogue of CO₂ exchange in a lichen thallus is presented, and it is suggested that the experimental results may be interpreted in terms of temperature-dependent CO₂ diffusion resistances in imbibed lichen thalli.     

Light, temperature and nutrients as factors in photosynthetic adjustment to an elevated concentration of carbon dioxide

The short-term stimulation of the net rate of carbon dioxide exchange of leaves by elevated concentrations of CO₂ usually observed in C₃ plants sometimes does not persist. Experiments were conducted to test whether the patterns of response to the environment during growth were consistent with the hypotheses that photosynthetic adjustment to elevated CO₂ concentration is due to (1) feedback inhibition or (2) nutrient stress. Soybean [Glycine max (L.) Merr. cv. Williams] and sugar beet (Best vulgaris L. cv. Mono Hye-4) were grown from seed at 350 and 700 μl^? CO₂, at 20 and 25°C, at a photon flux density of 0.5 and 1.0 mmol m^?2 S^?1 and with three nutrient regimes until the third trifoliolate leaf of soybean or the sixth leaf of sugar beet had finished expanding. Net rates of CO₂ exchange of the most recently expanded leaves were then measured at both 350 and 700 μl 1^?1 CO₂. Plants grown at the elevated CO₂ concentration had net rates of leaf CO₂ exchange which were reduced by 33% in sugar beet and 23% in soybean when measured at 350 μl 1^?1 CO₂ and when averaged over all treatments. Negative photosynthetic adjustment to elevated CO₂ concentration was not greater at 20 than at 25°C, was not greater at a photon flux density of 1.0 than at 0.5 mmol m^?2 S^?1 and was not greater with limiting nutrients. Furthermore, in soybean, negative photosynthetic adjustment could be induced by a single night at elevated CO₂ concentration, with net rates of CO₂ exchange the next day equal to those of leaves of plants grown from seed at the elevated concentration of CO₂. These patterns do not support either the feedback-inhibition or the nutrient-stress hypothesis of photosynthetic adjustment to elevated concentrations of CO₂.     

Evaluation of the light/dark C assay of photorespiration: tobacco leaf disk studies with glycidate and glyoxylate

Preincubation of illuminated tobacco (Nicotiana tabacum L.) leaf disks in glycidate (2,3-epoxypropionate) or glyoxylate inhibited photorespiration by about 40% as determined by the ratio of ¹⁴CO₂ evolved into CO₂-free air in light and in darkness. However, under identical preincubation conditions used for the light/dark ¹⁴C assays, the compounds failed to reduce photorespiration or stimulate net photosynthesis in tobacco leaf disks based on other CO₂ exchange parameters, including the CO₂ compensation concentration in 21% O₂, the inhibitory effect of 21% O₂ on net photosynthesis in 360 microliters per liter of CO₂ and the rate of net photosynthetic ¹⁴CO₂ uptake in air.

The effects of both glycidate and glyoxylate on the ¹⁴C assay are inconsistent with other measures of photorespiratory CO₂ exchange in tobacco leaf disks, and thus these data question the validity of the light to dark ratio of ¹⁴CO₂ efflux as an assay for relative rates of photorespiration (Zelitch 1968, Plant Physiol 43: 1829-1837). The results of this study specifically indicate that neither glycidate nor glyoxylate reduces photorespiration or stimulates net photosynthesis by tobacco leaf disks under physiological conditions of pO₂ and pCO₂, contrary to previous reports.

     

Gas exchange and water‐use efficiency in plant canopies

In this review, I first address the basics of gas exchange, water‐use efficiency and carbon isotope discrimination in C₃ plant canopies. I then present a case study of water‐use efficiency in northern Australian tree species. In general, C₃ plants face a trade‐off whereby increasing stomatal conductance for a given set of conditions will result in a higher CO₂ assimilation rate, but a lower photosynthetic water‐use efficiency. A common garden experiment suggested that tree species which are able to establish and grow in drier parts of northern Australia have a capacity to use water rapidly when it is available through high stomatal conductance, but that they do so at the expense of low water‐use efficiency. This may explain why community‐level carbon isotope discrimination does not decrease as steeply with decreasing rainfall on the North Australian Tropical Transect as has been observed on some other precipitation gradients. Next, I discuss changes in water‐use efficiency that take place during leaf expansion in C₃ plant leaves. Leaf phenology has recently been recognised as a significant driver of canopy gas exchange in evergreen forest canopies, and leaf expansion involves changes in both photosynthetic capacity and water‐use efficiency. Following this, I discuss the role of woody tissue respiration in canopy gas exchange and how photosynthetic refixation of respired CO₂ can increase whole‐plant water‐use efficiency. Finally, I discuss the role of water‐use efficiency in driving terrestrial plant responses to global change, especially the rising concentration of atmospheric CO₂. In coming decades, increases in plant water‐use efficiency caused by rising CO₂ are likely to partially mitigate impacts on plants of drought stress caused by global warming.     

The Effects of N Nutrition on the Water Relations and Gas Exchange Characteristics of Wheat (Triticum aestivum L.)

The purpose of this study was to characterize leaf photosynthetic and stomatal responses of wheat (Triticum aestivum L.) plants grown under two N-nutritional regimes. High- and low-N regimes were imposed on growth-chamber-grown plants by fertilizing with nutrient solutions containing 12 or 1 millimolar nitrogen, respectively. Gas-exchange measurements indicated not only greater photosynthetic capacity of high-N plants under well-watered conditions, but also a greater sensitivity of CO₂ exchange rate and leaf conductance to CO₂ and leaf water potential compared to low-N plants. Increased sensitivity of high-N plants was associated with greater tissue elasticity, lower values of leaf osmotic pressure and greater aboveground biomass. These N-nutritional-related changes resulted in greater desiccation (lowered relative water content) of high-N plants as leaf water potential fell, and were implicated as being important in causing greater sensitivity of high-N leaf gas exchange to reductions in water potential. Water use efficiency of leaves, calculated as CO₂ exchange rate/transpiration, increased from 9.1 to 13 millimoles per mole and 7.9 to 9.1 millimoles per mole for high- and low-N plants as water became limiting. Stomatal oscillations were commonly observed in the low-N treatment at low leaf water potentials and ambient CO₂ concentrations, but disappeared as CO₂ was lowered and stomata opened.     

Long-term carbon exchange in a sparse, seasonally dry tussock grassland

Rainfall and its seasonal distribution can alter carbon dioxide (CO₂) exchange and the sustainability of grassland ecosystems. Using eddy covariance, CO₂ exchange between the atmosphere and a sparse grassland was measured for 2 years at Twizel, New Zealand. The years had contrasting distributions of rain and falls (446 mm followed by 933 mm; long‐term mean=646 mm). The vegetation was sparse with total above‐ground biomass of only 1410 g m^?2. During the dry year, leaf area index peaked in spring (November) at 0.7, but it was <0.2 by early summer. The maximum daily net CO₂ uptake rate was only 1.5 g C m^?2 day^?1, and it occurred before mid‐summer in both years. On an annual basis, for the dry year, 9 g C m^?2 was lost to the atmosphere. During the wet year, 41 g C m^?2 was sequestered from the atmosphere. The net exchange rates were determined mostly by the timing and intensity of spring rainfall. The components of ecosystem respiration were measured using chambers. Combining scaled‐up measurements with the eddy CO₂ effluxes, it was estimated that 85% of ecosystem respiration emanated from the soil surface. Under well‐watered conditions, 26% of the soil surface CO₂ efflux came from soil microbial activity. Rates of soil microbial CO₂ production and net mineral‐N production were low and indicative of substrate limitation. Soil respiration declined by a factor of four as the soil water content declined from field capacity (0.21 m³ m^?3) to the driest value obtained (0.04 m³ m^?3). Rainfall after periods of drought resulted in large, but short‐lived, respiration pulses that were curvilinearly related to the increase in root‐zone water content. Coupled with the low leaf area and high root : shoot ratio, this sparse grassland had a limited capacity to sequester and store carbon. Assuming a proportionality between carbon gain and rainfall during the summer, rainfall distribution statistics suggest that the ecosystem is sustainable in the long term.     

The development of invertase activity in slices of the root of Beta vulgaris L. washed under aseptic conditions

1. When disks of root tissue from sugar or red beet (Beta vulgaris L.) are washed in running aerated tap water the sucrose contained in them disappears and glucose and fructose are formed. 2. Invertase activity in the disks has been measured by a polarimetric method. Freshly cut tissue has a very low activity, but a considerable increase occurs during the first 3–4 days of washing, the final activity being sufficient to hydrolyse the sucrose contained in the disk within a few hours. 3. Disks of red beet have been cut and shaken in water under aseptic conditions. Sucrose breakdown and invertase development still took place. Microbial contamination is therefore not responsible. 4. Trisaccharides that appear in sugar-beet disks during the washing process have been isolated and identified; their formation also suggests that a higher-plant invertase is acting. 5. The significance of these results is discussed in relation to protein synthesis in washed storage-tissue slices, and the occurrence of high invertase activity in growing plant cells.     

Low growth temperature effects a differential inhibition of photosynthesis in spring and winter wheat

In vivo room temperature chlorophyll a fluorescence coupled with CO₂ and O₂ exchange was measured to determine photosynthetic limitation(s) for spring and winter wheat (Triticum aestivum L.) grown at cold-hardening temperatures (5°C/5°C, day/night). Plants of comparable physiological stage, but grown at nonhardening temperatures (20°C/16°C, day/night) were used in comparison. Winter wheat cultivars grown at 5°C had light-saturated rates of CO₂ exchange and apparent photon yields for CO₂ exchange and O₂ evolution that were equal to or greater than those of winter cultivars grown at 20°C. In contrast, spring wheat cultivars grown at 5°C showed 35% lower apparent photon yields for CO₂ exchange and 25% lower light-saturated rates of CO₂ exchange compared to 20°C grown controls. The lower CO₂ exchange capacity is not associated with a lower efficiency of photosystem II activity measured as either the apparent photon yield for O₂ evolution, the ratio of variable to maximal fluorescence, or the level of reduced primary quinone electron acceptor maintained at steady-state photosynthesis, and is most likely associated with carbon metabolism. The lower CO₂ exchange capacity of the spring cultivars developed following long-term exposure to low temperature and did not occur following over-night exposure of nonhardened plants to 5°C.     

Moisture content and CO2 exchange of lichens. II. Depression of net photosynthesis in Ramalina maciformis at high water content is caused by increased thallus carbon dioxide diffusion resistance

Summary Thalli of Ramalina maciformis were moistened to their maximal water holding capacity, thus, simulating actual conditions following a heavy rainfall. Time courses of net photosynthesis at 17° C and 750 E m^-2 s^-1 light intensity (PAR) were obtained during drying of the thalli. At ambient CO₂ concentrations from 200 to 1,000 ppm, CO₂ uptake of the moist lichens was depressed at high water content. After a certain water loss, net photosynthesis increased to a maximal value and decreased again with further drying of the thalli. The degree of initial depression of photosynthesis decreased with increasing ambient CO₂ concentration, and it was fully absent at 1,600 ppm ambient CO₂. Under these conditions of CO₂ saturation, net photosynthesis remained constant at maximum for many hours and decreased only when substantial amounts of water had been lost. We conclude that the carboxylation capacity of the lichen is not affected by high contents of liquid water. Therefore, the depression of CO₂ uptake of the water saturated lichen at lower (e.g. natural) ambient CO₂ must be due exclusively to increased resistance to CO₂ diffusion from the external air to the sites of carboxylation.     

Whole Plant CO(2) Exchange Measurements for Nondestructive Estimation of Growth

A computer controlled semiclosed net CO₂ exchange measurement system, employing an infrared gas analyzer and mass flow controllers to inject pure CO₂ at preset rates, has been developed for measuring whole plant net CO₂ exchange and net C gain in a controlled environment (i.e. CO₂, light, and temperature). Data for tomato (Lycoperscicon esculentum cv Campbell 19 VF) and rose (Rosa hybrida cv Samantha) plants grown for 4 and 17 day periods, respectively, clearly show that net C gain measured and computed using nondestructive CO₂ analysis equaled the increase in C content determined by chemical analysis following destruction of the test plants. The analysis of C gain based on CO₂ exchange allows estimation of biomass production and growth of a single population of plants under varying light and CO₂ conditions without physically handling the test plants.     

Limiting Factors in Photosynthesis: IV. Iron Stress-Mediated Changes in Light-Harvesting and Electron Transport Capacity and its Effects on Photosynthesis in Vivo

Using iron stress to reduce the total amount of light-harvesting and electron transport components per unit leaf area, the influence of light-harvesting and electron transport capacity on photosynthesis in sugar beet (Beta vulgaris L. cv F58-554H1) leaves was explored by monitoring net CO₂ exchange rate (P) in relation to changes in the content of Chl.

In most light/CO₂ environments, and especially those with high light (≥1000 microeinsteins photosynthetically active radiation per square meter per second) and high CO₂ (≥300 microliters CO₂ per liter air), P per area was positively correlated with changes in Chl (a + b) content (used here as an index of the total amount of light-harvesting and electron transport components). This positive correlation of P per area with Chl per area was obtained not only with Fe-deficient plants, but also over the normal range of variation in Chl contents found in healthy, Fe-sufficient plants. For example, light-saturated P per area at an ambient CO₂ concentration close to normal atmospheric levels (300 microliters CO₂ per liter air) increased by 36% with increase in Chl over the normal range, i.e. from 40 to 65 micrograms Chl per square centimeter. Iron deficiency-mediated changes in Chl content did not affect dark respiration rate or the CO₂ compensation point. The results suggest that P per area of sugar beet may be colimited by light-harvesting and electron transport capacity (per leaf area) even when CO₂ is limiting photosynthesis as occurs under field conditions.

     

The subdominant status of Echinocereus viridiflorus and Mammillaria vivipara in the shortgrass prairie: The role of temperature and water effects on gas exchange

Summary The subdominant CAM species, Echinocereus viridiflorus and Mammillaria vivipara, collected from the shortgrass prairie in northeastern Colorado were pretreated and analyzed for gas exchange under cool temperatures (20/15°C) and warm temperatures (35/15°C). Well watered plants of both species under a 35/15°C thermoperiod fixed atmospheric CO₂ during the night and early moring. Echinocereus viridiflorus grown and analyzed at 20/15°C fixed CO₂ during the night, early morning and late afternoon but total carbon gain over a 24 h period is less than when grown and analyzed under the 35/15°C thermoperiod. Mammillaria vivipara grown and analyzed at 20/15°C assimilates CO₂ at low rates during all parts of a 24 h period with the greatest CO₂ fixation rates occuring from midday to late afternoon. The total carbon gain under the 20/15°C thermoperiod is less than that for this species under the 35/15°C thermoperiod. Decreasing the night temperature of plants grown under the warm conditions to 10°C or 5°C results in a depression of the night CO₂ fixation in both species. E. viridiflorus from the cool growth conditions showed an enhancement of the CO₂ uptake during the night, early morning and late afternoon when subjected to the cooler night temperatures (10°C and 5°C). The CO₂ uptake of M. vivipara grown at 20/15°C shows an enhancement during the night and early morning while the CO₂ fixation during midday and late afternoon is slightly depressed under cool night temperatures (10° and 5°C). Under the 35/15°C thermoperiod both species exhibit depressed rates of CO₂ fixation during the night and early morning when water stressed. Plants of both species grown under the 20/15°C thermoperiod exhibit no net CO₂ fixation following five weeks of water deprivation. Upon rewatering, E. viridiflorus begins to recover its capacity for CO₂ fixation within 24 h under both the warm and cool temperature regimes. However, M. vivipara did not show recovery within 48 h following rewatering under the warm or cool temperature regime. Contrasting the patterns of gas exchange of the subdominant species, E. viridiflorus and M. vivipara, with a dominant CAM species of the shortgrass prairie, Opuntia polyacantha reveals significant differences that may well dictate the role of these species in this ecosystem. E. viridiflorus and M. vivipara have a lower capacity of carbon gain and recovery from water stress than O. polyacantha mainly due to their lack of late afternoon CO₂ uptake. This study suggests that carbon gain plays an important role in limiting E. viridiflorus and M. vivipara in the shortgrass prairie ecosystem.     

Rice grain yield and quality responses to free‐air CO2 enrichment combined with soil and water warming

Rising air temperatures are projected to reduce rice yield and quality, whereas increasing atmospheric CO₂ concentrations ([CO₂]) can increase grain yield. For irrigated rice, ponded water is an important temperature environment, but few open‐field evaluations are available on the combined effects of temperature and [CO₂], which limits our ability to predict future rice production. We conducted free‐air CO₂ enrichment and soil and water warming experiments, for three growing seasons to determine the yield and quality response to elevated [CO₂] (+200 μmol mol^?1, E‐[CO₂]) and soil and water temperatures (+2 °C, E‐T). E‐[CO₂] significantly increased biomass and grain yield by approximately 14% averaged over 3 years, mainly because of increased panicle and spikelet density. E‐T significantly increased biomass but had no significant effect on the grain yield. E‐T decreased days from transplanting to heading by approximately 1%, but days to the maximum tiller number (MTN) stage were reduced by approximately 8%, which limited the panicle density and therefore sink capacity. On the other hand, E‐[CO₂] increased days to the MTN stage by approximately 4%, leading to a greater number of tillers. Grain appearance quality was decreased by both treatments, but E‐[CO₂] showed a much larger effect than did E‐T. The significant decrease in undamaged grains (UDG) by E‐[CO₂] was mainly the result of an increased percentage of white‐base grains (WBSG), which were negatively correlated with grain protein content. A significant decrease in grain protein content by E‐[CO₂] accounted in part for the increased WBSG. The dependence of WBSG on grain protein content, however, was different among years; the slope and intercept of the relationship were positively correlated with a heat dose above 26 °C. Year‐to‐year variation in the response of grain appearance quality demonstrated that E‐[CO₂] and rising air temperatures synergistically reduce grain appearance quality of rice.     

Cellular Localization of CO(2) Fixation and Translocation of Metabolites

Leaves of maize (Zea mays L.) and sugar beet (Beta vulgaris L.) were enclosed in an illuminated chamber in air for 30 min after which time ¹⁴CO₂ was released into the chamber. Two min after the ¹⁴CO₂ was released, the leaves were removed from the chamber, and small sections were cut from them. The sections were put in small wire baskets and frozen in isopentane cooled by liquid nitrogen. Approximately 1.5 min elapsed from the removal of the leaf from the illuminated chamber until the tissue was frozen. The tissue was freeze-dried, embedded in paraffin and the cellular location of the isotopic activity was determined by radiography of leaf cross sections. Isotopic activity in maize leaves was localized in bundle sheath parenchyma. In contrast, the label in sugar beet leaves was generally distributed in the mesophyll cells. The bundle sheath cells in maize contain specialized chloroplasts which appear to have a unique capacity to incorporate CO₂. Translocation from leaves of maize was 3-fold as rapid as from sugar beet leaves in the same environment. Low light intensity did not alter the distribution pattern of fixed CO₂.     

Carbon fixation in eucalypts in the field

Summary The rate of CO₂ assimilation at light saturation and an intercellular CO₂ concentration of 350 l l^-1 (photosynthetic capacity), measured in leaves of Eucalyptus pauciflora, E. behriana, E. delegatensis and Acacia melanoxylon, declined over the course of cloudless days under naturally varying environmental conditions as well as under constant optimal conditions for high CO₂ uptake. Since the capacity did not recover during the light period, it was different from the midday depression of gas exchange. The change appeared to be caused neither by the diurnal variation of total leaf water potential, by photoinhibition of redox-reaction centres in photosystems nor by changes in the intrinsic properties of Ribulose-bisphosphate carboxylase-oxygenase. The decline was more pronounced in winter than in summer. It was related to the duration of illumination or the cumulative carbon gain. It was reversible in the following dark phase, and it did not occur on changeable days with short peaks of high light.Despite the decline in photosynthetic capacity, the initial slope of the CO₂ response of net photosynthesis, as obtained at low intercellular CO₂ concentrations, remained constant during the day, but declined at night when photosynthetic capacity recovered. In all cases stomatal conductance varied in parallel with photosynthetic capacity. The relevance of changes in photosynthetic capacity for the intercellular CO₂ concentration is discussed.Abbreviations and symbols A CO₂ assimilation - ABA abscisic acid - A_c350 photosynthetic capacity at c_i=350l l^-1 - c_i intercellular CO₂ concentration - g leaf conductance to water vapour - I photon flux density (irradiance) - P air pressure - P_i inorganic phosphate - R_d net CO₂ release at _* - Rubisco Ribulose-bisphosphate carboxylase-oxygenase - RuBP Ribulose-bisphosphate - T leaf temperature - w leaf-to-air water vapour concentration difference - A/c_i carboxylation efficiency at low c_i - _* light-independent CO₂ compensation point - total leaf water potential     

Limitations due to water stress on leaf net photosynthesis of Quercus coccifera in the Portuguese evergreen scrub

Summary Gas exchange characteristics in leaves of the sclerophyll shrub Quercus coccifera were studied in the natural habitat in Portugal during spring and during the summer dry period. Compared to other sclerophyll species growing at the same site, photosynthesis in leaves of Quercus coccifera was less affected by water stress. Moderate water stress after six weeks of drought led to large decreases in stomatal conductance but no change in mesophyll photosynthetic capacity as compared to late spring. Leaf internal CO₂ pressure remained near 220 bar during diurnal courses in the spring. On midsummer days, leaf internal CO₂ decreased from a late morning value of 200 bar to a late afternoon value of approximately 150 bar. In contrast to Quercus suber (Tenhunen et al. 1984), restriction of CO₂ supply due to stomatal closure reduced net CO₂ uptake at midday and in the afternoon during midsummer. A decrease in leaf carboxylation efficiency and an increase in CO₂ compensation point at midday also played an important role in determining the diurnal course of net photosynthesis. During the late stages of drought in September, severe water stress led to reduction in mesophyll photosynthetic capacity and further reduction in leaf conductance. The observed decrease in mesophyll photosynthetic capacity was correlated with decrease in the daily minimum leaf water potential to greater negative values than-30 bar. At this time, CO₂ saturated photosynthetic rates decreased as much as 50% over the course of a day when measured at constant saturating light, 32° C leaf temperature, and a water vapor mole fraction difference between leaf and air of 30 mbar bar^-1.     

Genetic manipulation of 6-phosphofructokinase in potato tubers

The aim of this work was to discover whether genetic manipulation of 6-phosphofructokinase [EC 2.7.1.11; PFK(ATP)] influenced the rate of respiration of tuber tissue of Solanum tuberosum L. Transgenic plants were produced that contained the coding sequence of the Escherichia coli pfkA gene linked to a patatin promoter. Expression of this chimaeric gene in tubers resulted in a 14to 21-fold increase in the maximum catalytic activity of PFK(ATP) without affecting the activities of the other glycolytic enzymes. Tubers, and aged disks of tuber tissue, from transformed plants showed no more than a 30% fall in the content of hexose 6-monophosphates; the other intermediates of glycolysis increased threeto eightfold. Fructose-2,6-bisphosphate was barely detectable in aged disks of transformed tubers. The relative rates of ¹⁴CO₂ production from [1-¹⁴C]-and [6-¹⁴C]-glucose supplied to disks of transformed and control tubers were similar. Oxygen uptake and CO₂ production by aged disks of transformed tubers did not differ significantly from those from control tubers. The same was true of CO₂ production, in air, and in nitrogen, for tuber tissue. It is concluded that PFK(ATP) does not dominate the control of respiration in potato tubers.Abbreviations Fru2,6bisP fructose-2,6-bisphosphate - FW freshweight - GUS -glucuronidase - PFK(ATP) 6-phosphofructokinase - PFK(PPi) pyrophosphate: fructose-6-phosphate 1-phosphotransferase     

Carbon exchange of grazed pasture on a drained peat soil

Land‐use changes have contributed to increased atmospheric CO₂ concentrations. Conversion from natural peatlands to agricultural land has led to widespread subsidence of the peat surface caused by soil compaction and mineralization. To study the net ecosystem exchange of carbon (C) and the contribution of respiration to peat subsidence, eddy covariance measurements were made over pasture on a well‐developed, drained peat soil from 22 May 2002 to 21 May 2003. The depth to the water table fluctuated between 0.02 m in winter 2002 to 0.75 m during late summer and early autumn 2003. Peat soil moisture content varied between 0.6 and 0.7 m³ m^?3 until the water table dropped below 0.5 m, when moisture content reached 0.38 m³ m^?3. Neither depth to water table nor soil moisture was found to have an effect on the rate of night‐time respiration (ranging from 0.4–8.0 μmol CO₂ m^?2 s^?1 in winter and summer, respectively). Most of the variance in night‐time respiration was explained by changes in the 0.1 m soil temperature (r²=0.93). The highest values for daytime net ecosystem exchange were measured in September 2002, with a maximum of ?17.2 μmol CO₂ m^?2 s^?1. Grazing events and soil moisture deficiencies during a short period in summer reduced net CO₂ exchange. To establish an annual C balance for this ecosystem, non‐linear regression was used to model missing data. Annually integrated (CO₂) C exchange for this peat–pasture ecosystem was 45±500 kg C ha^?1 yr^?1. After including other C exchanges (methane emissions from cows and production of milk), the net annual C loss was 1061±500 kg C ha^?1 yr^?1.     

Carbon dioxide exchange in Cladina lichens from subarctic and temperate habitats

Summary The survival potential of lichens in a given habitat is determined by the response of CO₂ exchange to photosynthetically active radiation (PhAR), thallus temperature, and thallus relative water content (RWC). Therefore morphologically similar lichens from contrasting climatic environments 1) should differ in their CO₂ exchange responses, and 2) these differences should reflect adaptations to their climatic regimes. The CO₂ exchange responses of a subarctic (55°N, 67°W) Cladina stellaris (Opiz) Brodo population and a temperate (29°N, 82°W) Cladina evansii (Abb.) Hale and W. Culb, population were used to test these two related hypotheses.Infrared gas analysis with lichens collected in September–October 1975 established that the two populations differed in their responses to incident PhAR, thallus temperature, and thallus RWC. Net photosynthesis in C. stellaris had an optimum at a lower temperature and a greater relative photosynthetic capacity at low temperatures than did C. evansii. Cladina evansii maintained net photosynthesis above 35°C thallus temperature; C. stellaris did not. In both species the optimum temperature for net photosynthesis increased with increasing irradiance. The C. stellaris light saturation point was consistently lower than that of C. evansii. Both species had maximal rates of net photosynthesis at 70–80% relative water content. In C. evansii the CO₂ exchange rates, expressed as percentages of the maximum rate, declined more rapidly under suboptimal conditions. The absolute CO₂ exchange rates of C. evansii were greater than those of C. stellaris. At 20°C and 90–95% RWC, resaturation respiration occurred in both species and continued until 6–7 h after wetting.Contrasts in the temporal patterns of thallus condition at each collection site suggest that not all differences in the two response surfaces reflect climatic adaptation. The two populations appear well adapted to incident PhAR and thallus temperature regimes but the 70–80% RWC optimum for net photosynthesis common to both species is puzzling since their water regimes differ markedly. The overall adaptedness of the CO₂ exchange responses in the two species cannot be judged without a comprehensive quantitative analysis of carbon balance under differing climatic regimes.     

Effects of gibberellic acid and L-methionine on C1 metabolism in aerated carrot disk

Aeration of carrot storage tissue disks in water was accompanied by net folate synthesis and by changes in the specific activities of key folate-dependent enzymes. Disks aerated in 0.1 mM gibberellic acid (GA₃) for 48 hr contained higher concentrations of methyltetrahydrofolates but aeration in 5 mM L-methionine reduced net folate synthesis. Gibberellic acid also increased the specific activities of 5,10-methylenetetrahydrofolate reductase (E.C. 1.1.1.68), serine hydroxymethyltransferase (E.C. 2.1.2.1) and 5-methyltetrahydrofolate: homocysteine transmethylase. The levels of these enzymes in disks aerated in L-methionine (5 mM) were comparable or slightly higher than those of disks aerated in water. Activity of the reductase and 10-formyltetrahydrofolate synthetase (E.C. 6.3.4.3) was inhibited by L-methionine in vitro. Aeration increased ability to incorporate formate [¹⁴C] into serine, glycine and methionine. Disks aerated for 36 hr in 0.1 mM GA₃ incorporated greater amounts of ¹⁴C into free methionine but those aerated in L-methionine (5 mM) had less ability to metabolize formate and the specific radioactivities of free glycine, serine and methionine were low.