Effects of long-term CO2 enrichment and nutrient availability in Norway spruce. I. Phenology and morphology of branches

 Branches of 30-year-old Norway spruce [Picea abies (L.) Karst.] trees were enclosed in ventilated, transparent plastic bags and flushed with air containing ambient (A≈370 μmol CO₂ mol^–1) or ambient plus 340 μmol CO₂ mol^–1 (EL). Light-saturated photosynthesis was on average 56% higher in EL compared to A. Branch phenology and morphology were strongly related to nitrogen concentration (mg g^–1 dry mass) in the foliage and to elevated temperatures in the bags, but no direct effect of EL was found. In 1995, budbreak occurred on average 4 days earlier in the bags compared to the control branches, which was partly explained by the temperature elevation in the bags. No nutrient or EL effect on budbreak was found. Increases in temperature and nitrogen supply increased shoot growth: together they explained 76% of the variation in the extension rate, 63% of the variation in extension duration and 65% of the variation in final length of leading shoots. Shoot morphology was altered both by increased nitrogen availability and by the enclosure induced environmental changes inside the bags, leading to reduced mutual shading between needles. Specific needle area (SNA) was lower in EL, but this was related to lower nitrogen concentrations. Total dry mass of the branches was unaffected by EL. It is concluded that treating individual branches of Norway spruce with elevated CO₂ does not increase branch growth. The nutrient status of the branch and climate determine its growth, i.e. its sink strength for carbon. Increased export of carbohydrates to the rest of the tree is probable in EL treated branches. Received: 20 July 1998 / Accepted 8 October 1998     

Ontogenetic patterns of leaf CO2 exchange, morphology and chemistry in Betula pendula trees

In order to explore ontogenetic variation in leaf-level physiological traits of Betula pendula trees, we measured changes in mass- (A _mass) and area-based (A _area) net photosynthesis under light-saturated conditions, mass- (RS_mass) and area-based (RS_area) leaf respiration, relative growth rate, leaf mass per area (LMA), total nonstructural carbohydrates (TNC), and macro- and micronutrient concentrations. Expanding leaves maintained high rates of A _area, but due to high growth respiration rates, net CO₂ fixation occurred only at irradiances >200 μmol photons m^–2 s^–1. We found that full structural leaf development is not a necessary prerequisite for maintaining positive CO₂ balance in young birch leaves. Maximum rates of A _area were realized in late June and early July, whereas the highest values of A _mass occurred in May and steadily declined thereafter. The maintenance respiration rate averaged ≈8 nmol CO₂ g^–1 s^–1, whereas growth respiration varied between 0 and 65 nmol CO₂ g^–1 s^–1. After reaching its lowest point in mid-June, leaf respiration increased gradually until the end of the growing season. Mass and area-based dark respiration were significantly positively correlated with LMA at stages of leaf maturity, and senescence. Concentrations of P and K decreased during leaf development and stabilized or increased during maturity, and concentrations of immobile elements such as Ca, Mn and B increased throughout the growing season. Identification of interrelations between leaf development, CO₂ exchange, TNC and leaf nutrients allowed us to define factors related to ontogenetic variation in leaf-level physiological traits and can be helpful in establishing periods appropriate for sampling birch leaves for diagnostic purposes such as assessment of plant and site productivity or effects of biotic or abiotic factors. Received: 29 December 1998 / Accepted: 26 July 1999     

Ultrahigh-cell-density culture of a marine green alga Chlorococcum littorale in a flat-plate photobioreactor

To test the feasibility of CO₂ remediation by microalgal photosynthesis, a modified type of flat-plate photobioreactor [Hu et al. (1996) Biotechnol Bioeng 51:51–60] has been designed for cultivation of a high-CO₂-tolerant unicellular green alga Chlorococcum littorale. The modified reactor has a narrow light path in which intensive turbulent flow is provided by streaming compressed air through perforated tubing into the culture suspension. The length of the reactor light path was optimized for the productivity of biomass. The interrelationship between cell density and productivity, as affected by incident light intensity, was quantitatively assessed. Cellular ultrastructural and biochemical changes in response to ultrahigh cell density were investigated. The potential of biomass production under extremely high CO₂ concentrations was also evaluated. By growing C. littorale cells in this reactor, a CO₂ fixation rate of 16.7 g CO₂ l⁻¹ 24 h⁻¹ (or 200.4 g CO₂ m⁻² 24 h⁻¹) could readily be sustained at a light intensity of 2000 μmol m⁻² s⁻¹ at 25 °C, and an ultrahigh cell density of well over 80 g l⁻¹ could be maintained by daily replacing the culture medium. Received: 20 October 1997 / Received revision: 19 December 1997 / Accepted: 24 January 1998     

Production of Dissolved Organic Carbon in Canadian Forest Soils

To identify the controls on dissolved organic carbon (DOC) production, we incubated soils from 18 sites, a mixture of 52 forest floor and peats and 41 upper mineral soil samples, at three temperatures (3, 10, and 22°C) for over a year and measured DOC concentration in the leachate and carbon dioxide (CO₂) production from the samples. Concentrations of DOC in the leachate were in the range encountered in field soils (<2 to >50 mg l⁻¹). There was a decline in DOC production during the incubation, with initial rates averaging 0.03–0.06 mg DOC g⁻¹ soil C day⁻¹, falling to averages of 0.01 mg g⁻¹ soil C day⁻¹; the rate of decline was not strongly related to temperature. Cumulative DOC production rates over the 395 days ranged from less than 0.01 to 0.12 mg g⁻¹ soil C day⁻¹ (0.5–47.6 mg g⁻¹ soil C), with an average of 0.021 mg g⁻¹ soil C day⁻¹ (8.2 mg g⁻¹ soil C). DOC production rate was weakly related to temperature, equivalent to Q₁₀ values of 0.9 to 1.2 for mineral samples and 1.2 to 1.9 for organic samples. Rates of DOC production in the organic samples were correlated with cellulose (positively) and lignin (negatively) proportion in the organic matter, whereas in the mineral samples C and nitrogen (N) provided positive correlations. The partitioning of C released into CO₂–C and DOC showed a quotient (CO₂–C:DOC) that varied widely among the samples, from 1 to 146. The regression coefficient of CO₂–C:DOC production (log₁₀ transformed) ranged from 0.3 to 0.7, all significantly less than 1. At high rates of DOC production, a smaller proportion of CO₂ is produced. The CO₂–C:DOC quotient was dependent on incubation temperature: in the organic soil samples, the CO₂–C:DOC quotient rose from an average of 6 at 3 to 16 at 22°C and in the mineral samples the rise was from 7 to 27. The CO₂–C:DOC quotient was related to soil pH in the organic samples and C and N forms in the mineral samples.     

Atmospheric CO2 concentration, N availability, and water status affect patterns of ergastic substance deposition in longleaf pine (Pinus palustris Mill.) foliage

 Leaf chemistry alterations due to increasing atmospheric CO₂ will reflect plant physiological changes and impact ecosystem function. Longleaf pine was grown for 20 months at two levels of atmospheric CO₂ (720 and 365 μmol mol^–1), two levels of soil N (4 g m^–2 year^–1 and 40 g m^–2 year^–1), and two soil moisture levels (– 0.5 and – 1.5 MPa) in open top chambers. After 20 months of exposure, needles were collected and ergastic substances including starch grains and polyphenols were assessed using light microscopy, and calcium oxalate crystals were assessed using light microscopy, scanning electron microscopy, and transmission electron microscopy. Polyphenol content was also determined using the Folin-Denis assay and condensed tannins were estimated by precipitation with protein. Evaluation of phenolic content histochemically was compared to results obtained using the Folin-Denis assay. Total leaf polyphenol and condensed tannin content were increased by main effects of elevated CO₂, low soil N and well-watered conditions. Elevated CO₂ and low soil N decreased crystal deposition within needle phloem. Elevated CO₂ had no effect on the percentage of cells within the mesophyll, endodermis, or transfusion tissue which contained visible starch inclusions. With respect to starch accumulation in response to N stress, mesophyll > endodermis > transfusion tissue. The opposite was true in the case of starch accumulation in response to main effects of water stress: mesophyll < endodermis < transfusion tissue. These results indicate that N and water conditions significantly affect deposition of leaf ergastic substances in longleaf pine, and that normal variability in leaf tissue quality resulting from gradients in soil resources will be magnified under conditions of elevated CO₂. Received: 5 November 1996 / Accepted: 7 March 1997     

Chlorophyll fluorescence imaging of photosynthetic activity in sun and shade leaves of trees

The differences in pigment levels, photosynthetic activity and the chlorophyll fluorescence decrease ratio R _Fd (as indicator of photosynthetic rates) of green sun and shade leaves of three broadleaf trees (Platanus acerifolia Willd., Populus alba L., Tilia cordata Mill.) were compared. Sun leaves were characterized by higher levels of total chlorophylls a + b and total carotenoids x + c as well as higher values for the weight ratio chlorophyll (Chl) a/b (sun leaves 3.23–3.45; shade leaves: 2.74–2.81), and lower values for the ratio chlorophylls to carotenoids (a + b)/(x + c) (with 4.44–4.70 in sun leaves and 5.04–5.72 in shade leaves). Sun leaves exhibited higher photosynthetic rates P _N on a leaf area basis (mean of 9.1–10.1 μmol CO₂ m⁻² s⁻¹) and Chl basis, which correlated well with the higher values of stomatal conductance G _s (range 105–180 mmol m⁻² s⁻¹), as compared to shade leaves (G _s range 25–77 mmol m⁻² s⁻¹; P _N: 3.2–3.7 μmol CO₂ m⁻² s⁻¹). The higher photosynthetic rates could also be detected via imaging the Chl fluorescence decrease ratio R _Fd, which possessed higher values in sun leaves (2.8–3.0) as compared to shade leaves (1.4–1.8). In addition, via R _Fd images it was shown that the photosynthetic activity of the leaves of all trees exhibits a large heterogeneity across the leaf area, and in general to a higher extent in sun leaves than in shade leaves.     

Influence of net ecosystem metabolism in transferring riverine organic carbon to atmospheric CO<Subscript>2</Subscript> in a tropical coastal lagoon (Chilka Lake,India)

Studies on biogeochemical cycling of carbon in the Chilka Lake, Asia’s largest brackish lagoon on the east coast of India, revealed, for the first time, strong seasonal and spatial variability associated with salinity distribution. The lake was studied twice during May 2005 (premonsoon) and August 2005 (monsoon). It exchanges waters with the sea (Bay of Bengal) and several rivers open into the lake. The lake showed contrasting levels of dissolved inorganic carbon (DIC) and organic carbon (DOC) in different seasons; DIC was higher by ∼22% and DOC was lower by ∼36% in premonsoon than in monsoon due to seasonal variations in their supply from rivers and in situ production/mineralisation. The DIC/DOC ratios in the lake during monsoon were influenced by physical mixing of end member water masses and by intense respiration of organic carbon. A strong relationship between excess DIC and apparent oxygen utilisation showed significant control of biological processes over CO₂ production in the lake. Surface partial pressure of CO₂ (pCO₂), calculated using pH–DIC couple according to Cai and Wang (Limnol and Oceanogr 43:657–668, 1998), exhibited discernable gradients during monsoon through northern (1,033–6,522 μatm), central (391–2,573 μatm) and southern (102–718 μatm) lake. The distribution pattern of pCO₂ in the lake seems to be governed by pCO₂ levels in rivers and their discharge rates, which were several folds higher during monsoon than premonsoon. The net CO₂ efflux, based on gas transfer velocity parameterisation of Borges et al. (Limnol and Oceanogr 49(5):1630–1641, 2004), from entire lake during monsoon (141 mmolC m⁻² d⁻¹ equivalent to 2.64 GgC d⁻¹ at basin scale) was higher by 44 times than during premonsoon (9.8 mmolC m⁻² d⁻¹ ≈ 0.06 GgC d⁻¹). 15% of CO₂ efflux from lake in monsoon was contributed by its supply from rivers and the rest was contributed by in situ heterotrophic activity. Based on oxygen and total carbon mass balance, net ecosystem production (NEP) of lake (−308 mmolC m⁻² d⁻¹ ≈ −3.77 GgC d⁻¹) was found to be almost in consistent with the total riverine organic carbon trapped in the lake (229 mmolC m⁻² d⁻¹ ≈ 2.80 GgC d⁻¹) suggesting that the strong heterotrophy in the lake is mainly responsible for elevated fluxes of CO₂ during monsoon. Further, the pelagic net community production represented 92% of NEP and benthic compartment plays only a minor role. This suggests that Chilka lake is an important region in biological transformation of organic carbon to inorganic carbon and its export to the atmosphere.     

Heat and mass exchange processes between the surface of the human body and ambient air at various altitudes

 The rates of convection and evaporation at the interface between the human body and the surrounding air are expressed by the parameters convective heat transfer coefficient h _c, in W m^–2°C^–1 and evaporative heat transfer coefficient h _e, W m^–2 hPa^–1. These parameters are determined by heat transfer equations, which also depend on the velocity of the airstream around the body, that is still air (free convection) and moving air (forced convection). The altitude dependence of the parameters is represented as an exponential function of the atmospheric pressure p: h _c∼p ⁿ and h _e∼p ^1–n, where n is the exponent in the heat transfer equation. The numerical values of n are related to airspeed: n=0.5 for free convection, n=0.618 when airspeed is below 2.0 ms^–1 and n=0.805 when airspeed is above 2.0 ms^–1. This study considers the coefficients h _c and h _e with respect to the similarity of the two processes, convection and evaporation. A framework to explain the basis of established relationships is proposed. It is shown that the thickness of the boundary layer over the body surface increases with altitude. As a medium of the transfer processes, the boundary layer is assumed to be a layer of still air with fixed insulation which causes a reduction in the intensity of heat and mass flux propagating from the human body surface to its surroundings. The degree of reduction is more significant at a higher altitude because of the greater thickness of the boundary layer there. The rate of convective and evaporative heat losses from the human body surface at various altitudes in otherwise identical conditions depends on the following factors: (1) during convection – the thickness of the boundary layer, plus the decrease in air density, (2) during evaporation (mass transfer) – the thickness of the boundary layer, plus the increase with altitude in the diffusion coefficient of water vapour in the air. The warming rate of the air volume due to convection and evaporation is also considered. Expressions for the calculation of altitude dependences h _c (p) and h _e (p) are suggested. Received: 23 June 1998 / Accepted: 10 February 1999     

A free-air system for long-term stable carbon isotope labeling of adult forest trees

Stable carbon (C) isotopes, in particular employed in labeling experiments, are an ideal tool to broaden our understanding of C dynamics in trees and forest ecosystems. Here, we present a free-air exposure system, named isoFACE, designed for long-term stable C isotope labeling in the canopy of 25 m tall forest trees. Labeling of canopy air was achieved by continuous release of CO₂ with a δ¹³C of −46.9‰. To this end, micro-porous tubes were suspended at c. 1 m distance vertically through the canopy, minimizing CO₂ gradients from the exterior to the interior and allowing for C labeling exposure during periods of low wind speed. Target for CO₂ concentration ([CO₂]) increase was ambient +100 μmol mol⁻¹. Canopy [CO₂] stayed within 10% of the target during more than 57% of the time and resulted in a drop of δ¹³C in canopy air by 7.8‰. After 19 labeling days about 50% of C in phloem sugars and stem CO₂ efflux were turned over and 20–30% in coarse root CO₂ efflux and soil CO₂. The isoFACE system successfully altered δ¹³C of canopy air for studying turn-over of C pools in forest trees and soils, highlighting their slow turn-over rates.     

Interseasonal comparison of CO2 concentrations, isotopic composition, and carbon dynamics in an Amazonian rainforest (French Guiana)

Canopy CO₂ concentrations in a tropical rainforest in French Guiana were measured continuously for 5 days during the 1994 dry season and the 1995 wet season. Carbon dioxide concentrations ([CO₂]) throughout the canopy (0.02–38 m) showed a distinct daily pattern, were well-stratified and decreased with increasing height into the canopy. During both seasons, daytime [CO₂] in the upper and middle canopy decreased on average 7–10 μmol mol⁻¹ below tropospheric baseline values measured at Barbados. Within the main part of the canopy (≥ 0.7 m), [CO₂] did not differ between the wet and dry seasons. In contrast, [CO₂] below 0.7 m were generally higher during the dry season, resulting in larger [CO₂] gradients. Supporting this observation, soil CO₂ efflux was on average higher during the dry season than during the wet season, either due to diffusive limitations and/or to oxygen deficiency of root and microbial respiration. Soil respiration rates decreased by 40% after strong rain events, resulting in a rapid decrease in canopy [CO₂] immediately above the forest floor of about 50␣μmol mol⁻¹. Temporal and spatial variations in [CO₂]_canopy were reflected in changes of δ¹³C_canopy and δ¹⁸O_canopy values. Tight relationships were observed between δ¹³C and δ¹⁸O of canopy CO₂ during both seasons (r ² > 0.86). The most depleted δ¹³C_canopy and δ¹⁸O_canopy values were measured immediately above the forest floor (δ¹³C = −16.4‰; δ¹⁸O = 39.1‰ SMOW). Gradients in the isotope ratios of CO₂ between the top of the canopy and the forest floor ranged between 2.0‰ and 6.3‰ for δ¹³C, and between 1.0‰ and 3.5‰ for δ¹⁸O. The δ¹³C_leaf and calculated c _i/c _a of foliage at three different positions were similar for the dry and wet seasons indicating that the canopy maintained a constant ratio of photosynthesis to stomatal conductance. About 20% of the differences in δ¹³C_leaf within the canopy was accounted for by source air effects, the remaining 80% must be due to changes in c _i/c _a. Plotting 1/[CO₂] vs. the corresponding δ¹³C ratios resulted in very tight, linear relationships (r ² = 0.99), with no significant differences between the two seasons, suggesting negligible seasonal variability in turbulent mixing relative to ecosystem gas exchange. The intercepts of these relationships that should be indicative of the δ¹³C of respired sources were close to the measured δ¹³C of soil respired CO₂ and to the δ¹³C of litter and soil organic matter. Estimates of carbon isotope discrimination of the entire ecosystem, Δ_e, were calculated as 20.3‰ during the dry season and as 20.5‰ during the wet season. Received: 3 March 1996 / Accepted: 19 October 1996     

pH and buffer capacities of apoplastic and cytoplasmic cell compartments in leaves

After opening the stomata in CO₂-free air, darkened leaves of several plant species were titrated with CO₂ at concentrations between 1 and 16%, in air in order to reversibly decrease cellular pH values and to calculate buffer capacities from pH changes and bicarbonate accumulation using both gas-exchange and fluorescence methods for analysis. After equilibration with CO₂ for times ranging between 4.4 and 300 s, fast CO₂ release from bicarbonate indicated catalysis by highly active carbonic anhydrase. Its time constant was below 2.5 s. Additional CO₂ was released with time constants of about 5, 15 and approximately 300 s. With CO₂ as the acidifying agent, calculated buffer capacities depend on assumptions regarding initial pH in the absence of an acid load. At an initial stroma pH of 7.7, the stromal buffer capacity was about 20 mM pH-unit⁻¹ in darkened spinach leaves. At an initial pH of 7.5 it would be only 12 mM pH-unit⁻¹, i.e. not higher than expected solely on the basis of known stromal concentrations of phosphate and phosphate esters, disregarding the contribution of other solutes. At a concentration of 16%, CO₂ reduced the stromal pH by about 1 pH unit. Buffering of the cytosol was measured by the CO₂-dependent quenching of the fluorescence of pyranine which was fed to spinach leaves via the petiole. Brief exposures to high CO₂ minimized interference by effective cytosolic pH regulation. Cytosolic buffering appeared to be similar to or only somewhat higher than chloroplast buffering if the initial cytosolic pH was assumed to be 7.25, which is in accord with published cytosolic pH values. The difference from chloroplast pH values indicates the existence of a pH gradient across the chloroplast envelope even in darkened leaves. Apoplastic buffering was weak as measured by the CO₂-dependent quenching of dextran-conjugated fluorescein isothiocyanate which was infiltrated together with sodium vanadate into potato leaves. In the absence of vanadate, the kinetics of apoplastic fluorescence quenching were more complex than in its presence, indicating fast apoplastic pH regulation which strongly interfered with the determination of apoplastic buffering capacities. At an apoplastic pH of 6.1 in potato leaves, apoplastic buffering as determined by CO₂ titration with and without added buffer was somewhat below 4 mM pH-unit⁻¹. Thus the apoplastic and cytosolic pH responses to additions of CO₂ indicated that the observed cytoplasmic pH regulation under acid stress involves pumping of protons from the cytosol into the vacuole of leaf cells, but not into the apoplast. Received: 27 November 1998 / Accepted: 22 March 1999     

Interactions of culture vessels, media volume, culture density, and carbon dioxide levels on lettuce and spearmint shoot growth in vitro

 The influence of culture chamber capacity, medium volume and culture density on the growth yields of lettuce (Lactuca sativa L.) and spearmint (Mentha spicata L.) shoots were determined in an environment containing either 350 or 10,000 μmol mol^–1 CO₂ after 8 weeks of incubation. High positive correlations occurred between the culture vessel capacity and spearmint fresh weight, leaf number, root number, and shoot number. Similarly, high positive correlations occurred between culture vessel capacity and lettuce fresh weight, leaf number, and root number. Higher fresh weights, leaf numbers, and root numbers were obtained from lettuce and spearmint shoots when cultured in 1-quart Mason jars containing 100- or 150-ml aliquots of medium compared to jars containing 25- or 50-ml aliquots of medium within an environment containing either 350 or 10,000 μmol mol^–1 CO₂. High culture density decreased growth yields, and this phenomenon could only be slightly off-set by the employment of an elevated CO₂ environment or larger culture vessels. Received: 22 December 1998 / Revision received: 2 July 1999 / Accepted: 12 July 1999     

Gas-exchange analysis of chloroplastic fructose-1,6-bisphosphatase antisense potatoes at different air humidities and at elevated CO2

Gas-exchange measurements were performed to analyze the leaf conductances and assimilation rates of potato (Solanum tuberosum L. cv. Desireé) plants expressing an antisense construct against chloroplastic fructose-1,6-bisphosphatase (FBPase, EC 3.1.3.11) in response to increasing photon flux densities, different relative air humidities and elevated CO₂ concentrations. Assimilation rates (A) and transpiration rates (E) were observed during a stepwise increase of photon flux density. These experiments were carried out under atmospheric conditions and in air containing 500 μmol mol⁻¹ CO₂. In both gas atmospheres, two levels of relative air humidity (60–70% and 70–80%) were applied in different sets of measurements. Intercellular CO₂ concentration, leaf conductance, air-to-leaf vapour pressure deficit, and instantaneous water-use efficiency (A/E) were determined. As expected, assimilation rates of the FBPase antisense plants were significantly reduced as compared to the wild type. Saturation of assimilation rates in transgenic plants occurred at a photon flux density of 200 μmol m⁻² s⁻¹, whereas saturation in wild type plants was observed at 600 μmol m⁻² s⁻¹. Elevated ambient CO₂ levels did not effect assimilation rates of transgenic plants. At 70–80% relative humidity and atmospheric CO₂ concentration the FBPase antisense plants had significantly higher leaf conductances than wild-type plants while no difference emerged at 60–70%. These differences in leaf conductance vanished at elevated levels of ambient CO₂. Stomatal response to different relative air humidities was not affected by mesophyll photosynthetic activity. It is suggested that the regulation of stomatal opening upon changes in photon flux density is merely mediated by a signal transmitted from mesophyll cells, whereas the intercellular CO₂ concentration plays a minor role in this kind of stomatal response. The results are discussed with respect to stomatal control by environmental parameters and mesophyll photosynthesis. Received: 24 September 1998 / Accepted: 9 February 1999     

Effect of carbon dioxide on cell growth and saponin production in suspension cultures of Panax ginseng

The effects of carbon dioxide supply within the range of 1–5 % (along with purified air), on cell culture of Panax ginseng were investigated in a balloon type bubble bioreactor containing 4 dm³ of Murashige and Skoog (MS) medium supplemented with 7.0 mg dm⁻³ indolebutyric acid, 0.5 mg dm⁻³ kinetin and 30 g dm⁻³ sucrose. A 1 % CO₂ supply was found beneficial for the production of cell mass; however, increasing CO₂ concentration to 2.5 and 5 % decreased the biomass accumulation. CO₂ enrichment was not beneficial for saponin production and 1, 2.5, and 5 % CO₂ supply resulted in decrease in saponin accumulation up to 11.6, 19.5, and 50.6 %, respectively.     

Atmospheric conditions required for the growth ofGaleola septentrionalis seedlings

The seedling of an achlorophyllous orchid,Galeola septentrionalis, requires for its early growth anomalous atmospheric conditions appropriate to each process of development. The most favorable atmosphere for the enlargement of protocorm consists of 10% O₂, 6% CO₂ and 84% N₂ at a pressure of 1.4 kg/cm². Tolerable ranges of atmospheric pressure, concentrations of O₂ and CO₂ were 1.1–2.0 kg/cm², 5–12% and 2–10%, respectively. Such a range for culture temperatures was 20–26 C. Subsequent development of the seedlings was little influenced by atmospheric pressure (1.0–1.8 kg/cm²) and concentration of CO₂ (0–8%), but influenced by O₂ concentration. Optimum and tolerable concentrations of O₂ were 10–15% and 5–19.7%, respectively. These atmospheres are discussed hypothetically as the conditions required to pass an inevitable process specific to the epigenetic ontogeny of scobiform (sawdusty) seeds.     

Increases in Desert Shrub Productivity under Elevated Carbon Dioxide Vary with Water Availability

Productivity of aridland plants is predicted to increase substantially with rising atmospheric carbon dioxide (CO₂) concentrations due to enhancement in plant water-use efficiency (WUE). However, to date, there are few detailed analyses of how intact desert vegetation responds to elevated CO₂. From 1998 to 2001, we examined aboveground production, photosynthesis, and water relations within three species exposed to ambient (around 38 Pa) or elevated (55 Pa) CO₂ concentrations at the Nevada Desert Free-Air CO₂ Enrichment (FACE) Facility in southern Nevada, USA. The functional types sampled—evergreen (Larrea tridentata), drought-deciduous (Ambrosia dumosa), and winter-deciduous shrubs (Krameria erecta)—represent potentially different responses to elevated CO₂ in this ecosystem. We found elevated CO₂ significantly increased aboveground production in all three species during an anomalously wet year (1998), with relative production ratios (elevated:ambient CO₂) ranging from 1.59 (Krameria) to 2.31 (Larrea). In three below-average rainfall years (1999–2001), growth was much reduced in all species, with only Ambrosia in 2001 having significantly higher production under elevated CO₂. Integrated photosynthesis (mol CO₂ m⁻² y⁻¹) in the three species was 1.26–2.03-fold higher under elevated CO₂ in the wet year (1998) and 1.32–1.43-fold higher after the third year of reduced rainfall (2001). Instantaneous WUE was also higher in shrubs grown under elevated CO₂. The timing of peak canopy development did not change under elevated CO₂; for example, there was no observed extension of leaf longevity into the dry season in the deciduous species. Similarly, seasonal patterns in CO₂ assimilation did not change, except for Larrea. Therefore, phenological and physiological patterns that characterize Mojave Desert perennials—early-season lags in canopy development behind peak photosynthetic capacity, coupled with reductions in late-season photosynthetic capacity prior to reductions in leaf area—were not significantly affected by elevated CO₂. Together, these findings suggest that elevated CO₂ can enhance the productivity of Mojave Desert shrubs, but this effect is most pronounced during years with abundant rainfall when soil resources are most available.     

Effects of atmospheric CO2 enrichment on δ 13C, δ 15N values and turnover times of soil organic matter pools isolated by thermal techniques

CO₂ applied for Free-Air CO₂ Enrichment (FACE) experiments is strongly depleted in ¹³C and thus provides an opportunity to study C turnover in soil organic matter (SOM) based on its δ ¹³C value. Simultaneous use of ¹⁵N labeled fertilizers allows N turnover to be studied. Various SOM fractionation approaches (fractionation by density, particle size, chemical extractability etc.) have been applied to estimate C and N turnover rates in SOM pools. The thermal stability of SOM coupled with C and N isotopic analyses has never been studied in experiments with FACE. We tested the hypothesis that the mean residence time (MRT) of SOM pools is inversely proportional to its thermal stability. Soil samples from FACE plots under ambient (380 ppm) and elevated CO₂ (540 ppm; for 3 years) treatments were analyzed by thermogravimetry coupled with differential scanning calorimetry (TG-DSC). Based on differential weight losses (TG) and energy release or consumption (DSC), five SOM pools were distinguished. Soil samples were heated up to the respective temperature and the remaining soil was analyzed for δ ¹³C and δ ¹⁵N by IRMS. Energy consumption and mass losses in the temperature range 20–200°C were mainly connected with water volatilization. The maximum weight losses occurred from 200–310°C. This pool contained the largest amount of carbon: 61% of the total soil organic carbon in soil under ambient treatment and 63% in soil under elevated CO₂, respectively. δ ¹³C values of SOM pools under elevated CO₂ treatment showed an increase from −34.3‰ of the pool decomposed between 20–200°C to −18.1‰ above 480°C. The incorporation of new C and N into SOM pools was not inversely proportional to its thermal stability. SOM pools that decomposed between 20–200 and 200–310°C contained 2 and 3% of the new C, with a MRT of 149 and 92 years, respectively. The pool decomposed between 310–400°C contained the largest proportion of new C (22%), with a MRT of 12 years. The amount of fertilizer-derived N after 2 years of application in ambient and elevated CO₂ treatments was not significantly different in SOM pools decomposed up to 480°C having MRT of about 60 years. In contrast, the pool decomposed above 480°C contained only 0.5% of new N, with a MRT of more than 400 years in soils under both treatments. Thus, the separation of SOM based on its thermal stability was not sufficient to reveal pools with contrasting turnover rates of C and N. Responsible Editor: Bernard Nicolardot.     

Comparison of leaf water use efficiency of oak and sycamore in the canopy over two growing seasons

The seasonal trends in water use efficiency of sun and shade leaves of mature oak (Quercus robur) and sycamore (Acer pseudoplatanus) trees were assessed in the upper canopy of an English woodland. Intrinsic water use efficiency (net CO₂ assimilation rate/leaf conductance, A/g) was measured by gas exchange and inferred from C isotope discrimination (δ¹³C) methods. Shade leaves had consistently lower δ¹³C than sun leaves (by 1–2‰), the difference being larger in sycamore. Buds had distinct sun and shade isotopic signatures before bud break and received an influx of ¹³C-rich C before becoming net autotrophs. After leaf full expansion, δ¹³C declined by 1–2‰ gradually through the season, emphasising the importance of imported carbon in the interpretation of leaf δ¹³C values in perennial species. There was no significant difference between the two species in the value of intrinsic water use efficiency for either sun or shade leaves. For sun leaves, season-long A/g calculated from δ¹³C (72–78 μmol CO₂ [mol H₂O]⁻¹) was 10–16% higher than that obtained from gas exchange and in situ estimates of leaf boundary layer conductance. For shade leaves, the gas exchange–derived values were low, only 10–18% of the δ¹³C-derived values. This is ascribed to difficulties in obtaining a comprehensive sample of gas exchange measurements in the rapidly changing light environment.     

Interactive effect of CO2 enrichment and temperature on the photosynthesis of field-grown hinoki cypress (Chamaecyparis obtusa) branches

Two branches of a field-grown Chamaecy-paris obtusa tree were enclosed in chambers of an open gas exchange system for continuous CO₂ exchange measurements. One branch was subjected to ambient air (CO₂, 370 μmol mol^–1) and the other was subjected to CO₂-enriched air (800 μmol mol^–1). The CO₂ exchange rate of the branches, air temperature and photosynthetic photon flux density were recorded every 4 min by a computer during the two experimental periods of July 1994 to June 1995 (experiment 1) and April 1996 to August 1997 (experiment 2). The response of CO₂ gas exchange rate to light changed with the seasonal temperature. The highest saturated rate of net photosynthesis on a leaf area basis was observed in May and October in both CO₂ treatments when the mean daytime temperature was about 18–19°C. This temperature was almost equal to the yearly mean daytime temperature. Above and below this temperature, the saturated net photosynthesis rate decreased. The net photosynthesis rate was usually higher in the elevated CO₂ treatment. The ratio of monthly net photosynthesis rate in elevated CO₂ to that in ambient CO₂ was linearly related to the monthly mean daytime temperature. This ratio increased by 3.3% for each 1°C increase in the monthly mean daytime temperature; the highest ratio of 1.8 occurred in August. When the ratio was 1.0, the temperature was about 5–6°C, which was close to the mean daytime temperature of the coldest month. Elevated CO₂ increased per unit area net photosynthesis by 38.5% and 43.7% in experiments 1 and 2, respectively. Received: 29 March 1999 / Accepted: 22 October 1999     

Leaf δ13C in Pinus resinosa trees and understory plants: variation associated with light and CO2 gradients

 Our objective was to evaluate the relative importance of gradients in light intensity and the isotopic composition of atmospheric CO₂ for variation in leaf carbon isotope ratios within a Pinus resinosa forest. In addition, we measured photosynthetic gas exchange and leaf carbon isotope ratios on four understory species (Dryopteris carthusiana, Epipactus helleborine, Hieracium floribundum, Rhamnus frangula), in order to estimate the consequence of the variation in the understory light microclimate for carbon gain in these plants. During midday, CO₂ concentration was relatively constant at vertical positions ranging from 15 m to 3 m above ground. Only at positions below 3 m was CO₂ concentration significantly elevated above that measured at 15 m. Based on the strong linear relationship between changes in CO₂ concentration and δ¹³C values for air samples collected during a diurnal cycle, we calculated the expected vertical profile for the carbon isotope ratio of atmospheric CO₂ within the forest. These calculations indicated that leaves at 3 m height and above were exposed to CO₂ of approximately the same isotopic composition during daylight periods. There was no significant difference between the daily mean δ¹³C values at 15 m (–7.77‰) and 3 m (–7.89‰), but atmospheric CO₂ was significantly depleted in ¹³C closer to the ground surface, with daily average δ¹³C values of –8.85‰ at 5 cm above ground. The light intensity gradient in the forest was substantial, with average photosynthetically active radiation (PAR) on the forest floor approximately 6% of that received at the top of the canopy. In contrast, there were only minor changes in air temperature, and so it is likely that the leaf-air vapour pressure difference was relatively constant from the top of the canopy to the forest floor. For red pine and elm tree samples, there was a significant correlation between leaf δ¹³C value and the height at which the leaf sample was collected. Leaf tissue sampled near the forest floor, on average, had lower δ¹³C values than samples collected near the top of the canopy. We suggest that the average light intensity gradient through the canopy was the major factor influencing vertical changes in tree leaf δ¹³C values. In addition, there was a wide range of variation (greater than 4‰) among the four understory plant species for average leaf δ¹³C values. Measurements of leaf gas exchange, under natural light conditions and with supplemental light, were used to estimate the influence of the light microclimate on the observed variation in leaf carbon isotope ratios in the understory plants. Our data suggest that one species, Epipactus helleborine, gained a substantial fraction of carbon during sunflecks. Received: 21 March 1996 / Accepted: 13 August 1996