Photosynthetic acclimation to long-term exposure to elevated CO2 concentration in Pinus radiata D. Don. is related to age of needles

The effects of CO₂ enrichment on photosynthesis and ribulose-1,5-bisphosphate carboxylase/ oxygenase (Rubisco) in current year and 1-year-old needles on the same branch were studied on Pinus radiata D. Don. trees growing for 4 years in large, open-top chambers at ambient (36 Pa) and elevated (65 Pa) CO₂ partial pressures. At this age trees were 3·5–4 m tall. Measurements made late in the growing cycle (March) showed that photosynthetic rates at the growth CO₂ concentration [(pCO₂)_a] were lower in 1-year-old needles of trees grown at elevated CO₂ concentrations than in those of trees grown at ambient (pCO₂)_a. At elevated CO₂ concentrations V_cmax (maximum carboxylation rate) was reduced by 13% and J_max (RuBP regeneration capacity mediated by maximum electron transport rate) by 17%. This corresponded with photosynthetic rates at the growth (pCO₂)_a of 4·68 ± 0·41 μmol m^–2 s^–1 and 6·15 ± 0·46 μmol m^–2 s^–1 at 36 and 65 Pa, respectively (an enhancement of 31%). In current year needles photosynthetic rates at the growth (pCO₂)_a were 6·2 ± 0·72 μmol m^–2 s^–1 at 36 Pa and 10·15 ± 0·64 μmol m^–2 s^–1 at 65 Pa (an enhancement of 63%). The smaller enhancement of photosynthesis in 1-year-old needles at 65 Pa was accompanied by a reduction in Rubisco activity (39%) and content (40%) compared with that at 36 Pa. Starch and sugar concentrations in 1-year-old needles were not significantly different in the CO₂ treatments. There was no evidence in biochemical parameters for down-regulation at elevated (pCO₂)_a in fully fexpanded needles of the current year cohort. These data show that enhancement of photosynthesis continues to occur in needles after 4 years’ exposure to elevated CO₂ concentrations. Photosynthetic acclimation reduces the degree of this enhancement, but only in needles after 1 year of growth. Thus, responses to elevated CO₂ concentration change during the lifetime of needles, and acclimation may not be apparent in current year needles. This transitory effect is most probably attributable to the effects of developmental stage and proximity to actively growing shoots on sink strength for carbohydrates. The implications of such age-dependent responses are that older trees, in which the contribution of older needles to the photosynthetic biomass is greater than in younger trees, may become progressively more acclimated to elevated CO₂ concentration.     

Net ecosystem CO2 exchange in Mojave Desert shrublands during the eighth year of exposure to elevated CO2

Arid ecosystems, which occupy about 35% of the Earth's terrestrial surface area, are believed to be among the most responsive to elevated [CO₂]. Net ecosystem CO₂ exchange (NEE) was measured in the eighth year of CO₂ enrichment at the Nevada Desert Free‐Air CO₂ Enrichment (FACE) Facility between the months of December 2003–December 2004. On most dates mean daily NEE (24 h) (μmol CO₂ m^?2 s^?1) of ecosystems exposed to elevated atmospheric CO₂ were similar to those maintained at current ambient CO₂ levels. However, on sampling dates following rains, mean daily NEEs of ecosystems exposed to elevated [CO₂] averaged 23 to 56% lower than mean daily NEEs of ecosystems maintained at ambient [CO₂]. Mean daily NEE varied seasonally across both CO₂ treatments, increasing from about 0.1 μmol CO₂ m^?2 s^?1 in December to a maximum of 0.5–0.6 μmol CO₂ m^?2 s^?1 in early spring. Maximum NEE in ecosystems exposed to elevated CO₂ occurred 1 month earlier than it did in ecosystems exposed to ambient CO₂, with declines in both treatments to lowest seasonal levels by early October (0.09±0.03 μmol CO₂ m^?2 s^?1), but then increasing to near peak levels in late October (0.36±0.08 μmol CO₂ m^?2 s^?1), November (0.28±0.03 μmol CO₂ m^?2 s^?1), and December (0.54±0.06 μmol CO₂ m^?2 s^?1). Seasonal patterns of mean daily NEE primarily resulted from larger seasonal fluctuations in rates of daytime net ecosystem CO₂ uptake which were closely tied to plant community phenology and precipitation. Photosynthesis in the autotrophic crust community (lichens, mosses, and free‐living cyanobacteria) following rains were probably responsible for the high NEEs observed in January, February, and late October 2004 when vascular plant photosynthesis was low. Both CO₂ treatments were net CO₂ sinks in 2004, but exposure to elevated CO₂ reduced CO₂ sink strength by 30% (positive net ecosystem productivity=127±17 g C m^?2 yr^?1 ambient CO₂ and 90±11 g C m^?2 yr^?1 elevated CO₂, P=0.011). This level of net C uptake rivals or exceeds levels observed in some forested and grassland ecosystems. Thus, the decrease in C sequestration seen in our study under elevated CO₂– along with the extensive coverage of arid and semi‐arid ecosystems globally – points to a significant drop in global C sequestration potential in the next several decades because of responses of heretofore overlooked dryland ecosystems.     

Fluxes of carbon, water and energy over Brazilian cerrado: an analysis using eddy covariance and stable isotopes

We present the energy and mass balance of cerrado sensu stricto (a Brazilian form of savanna), in which a mixture of shrubs, trees and grasses forms a vegetation with a leaf area index of 1·0 in the wet season and 0·4 in the dry season. In the wet season the available energy was equally dissipated between sensible heat and evaporation, but in the dry season at high irradiance the sensible heat greatly exceeded evaporation. Ecosystem surface conductance g_s in the wet season rose abruptly to 0·3 mol m^?2 s^?1 and fell gradually as the day progressed. Much of the total variation in g_s was associated with variation in the leaf-to-air vapour pressure deficit of water and the solar irradiance. In the dry season the maximal g_s values were only 0·1 mol m^?2 s^?1. Maximal net ecosystem fluxes of CO₂ in the wet and dry season were –10 and –15 μmol CO₂ m^?2 s^?1, respectively (sign convention: negative denotes fluxes from atmosphere to vegetation). The canopy was well coupled to the atmosphere, and there was rarely a significant build-up of respiratory CO₂ during the night. For observations in the wet season, the vegetation was a carbon dioxide sink, of maximal strength 0·15 mol m^?2 d^?1. However, it was a source of carbon dioxide for a brief period at the height of the dry season. Leaf carbon isotopic composition showed all the grasses except for one species to be C₄, and all the palms and woody plants to be C₃. The CO₂ coming from the soil had an isotopic composition that suggested 40% of it was of C₄ origin.     

The influence of elevated CO2 and temperature on biomass production of continuously defoliated white clover

Clonal plants of white clover (Trifolium repens L.), grown singly in pots of Perlite and solely dependent for nitrogen on root nodule N₂ fixation, were maintained in controlled environments which provided four environments: 18/13 °C day/night temperature at 340 and 680 μmol mol^?1 CO₂ and 20·5/15·5°C day/night temperature at 340 and 680 μmol mol^?1 CO₂. The daylength was 12 h and the photon flux density 500±25 μmol m^?2 s^?1 (PFD). All plants were defoliated for about 80d, nominally every alternate day, to leave the youngest expanded leaf intact on 50% of stolons, plus expanding leaves (simulated grazing). Elevated CO₂ increased the yield of biomass removed at defoliation by a constant 45% during the second 40d of the experiment and by a varying amount in the first half of the experiment. Elevated temperature had little effect on biomass yield. Nitrogen, as a proportion of the harvested biomass, was only fractionally affected by elevated CO₂ or temperature. In contrast, N₂ fixation increased in concert with the promoting effect of elevated CO₂ on biomass production. The increased yield of biomass harvested in 680 μmol mol^?1 CO₂ was primarily due to the early development and continued maintenance of more stolons. However, the stolons of plants grown in elevated CO₂ also developed leaves which were heavier and slightly larger in area than their counterparts in ambient CO₂. The conclusion is that, when white clover plants are maintained at constant mass by simulated grazing, they continue to respond to elevated CO₂ in terms of a sustained increase in biomass production.     

The cyanobacterium Synechococcus R-2 (Anacystis nidulans, S. leopoliensis) PCC 7942 has a sodium-dependent chloride transporter

Synechococcus R-2 (PCC 7942) actively accumulated Cl^? in the light and dark, under control conditions (BG-11 media: pH_o, 7·5; [Na⁺]_o, 18 mol m^?3; [Cl^?]_o, 0·508 molm^?3). In BG-11 medium [Cl^?], was 17·2±0·848 mol m^?3 (light), electrochemical potential of Cl^? (ΔμCl^?_i,o) =+211±2mV; [Cl^?]_i= 1·24±0·11 mol m^?3(dark), ΔμCl^?_i,o=+133±4mV. Cl^? fluxes, but not permeabilities, were much higher in the light: ?Cl^?_i,o= 4·01±5·4 nmol m^?2 s^?1, ^PCl^?_i,o= 47±5pm s^?1 (light); ?Cl^?_i,o= 0·395±0·071 nmol m^?2 s^?1, _PCl^?_i,o= 69±14 pm s^?1 (dark). Chloride fluxes are inhibited by acid pH_o (pH_o 5; ?Cl^?_i,o= 0·14±0·04 nmol m^?2 s^?1); optimal at pH_o 7·5 and not strongly inhibited by alkaline pH_o (pH_o 10; ?Cl^?1_i,o= 1·7±0·14 nmol m^?2 s^?1). A Cl^?_in/2H⁺_in coporter could not account for the accumulation of Cl^? alkaline pH_o. Permeability of Cl^? is very low, below 100pm s^?1 under all conditions used, and appears to be maximal at pH_o 7·5 (50–70 pm s^?1) and minimal in acid pH_o (20pm s^?1). DCCD (dicyclohexyl-carbodiimide) inhibited ?Cl^?_i,o in the light about 75% and [Cl^?]_i fell to 2·2±0·26 (4) mol m^?3. Valinomycin had no effect but monensin severely inhibited Cl^? uptake ([Cl^?]_i= 1·02±0·32 mol m^?3; ?Cl^?_i,o= 0·20±0·1 nmol m^?2 s^?1). Vanadate (200 mmol m^?3) accelerated the Cl^? flux (?Cl^?_i,o= 5·28±0·64 nmol m^?2 s^?1) but slightly decreased accumulation of Cl^? ([Cl^?], = 13·9±1·3 mol m^?3) in BG-11 medium but had no significant effect in Na⁺-free media. DCMU (dichlorophenyldimethylurea) did not reduce [Cl^?], or ?Cl^?_i,o to that found in the dark ([Cl^?]_i= 8·41±0·76 mol m^?3; ?Cl^?_i,o= 2·06±0·36 nmol m^?2 s^?1). Synechococcus also actively accumulated Cl^? in Na⁺-free media, [Cl^?]_i was lower but ΔΨ_i,o hyperpolarized in Na⁺-free media and so the ΔμCl^?_i,o was little changed ([Cl^?]_i= 7·98±0·698 mol m^?3; ΔμCl^?_i,o=+203±3 mV). Net Cl^? uptake was stimulated by Na⁺; Li⁺ acted as a partial analogue for Na⁺. Synechococcus has a Na⁺ activated Cl^? transporter which is probably a primary 2Cl^?/ATP pump. The Cl^? pump is voltage sensitive. ΔμCl^?_i,o is directly proportional to ΔΨ_i,o(P»0·01%): ΔμCl^?_i,o= -1·487 (±0·102) ×ΔΨ_i,o, r= -0·983, n= 31. The ΔμCl^?_i,o increased (more positive) as the Δμ_i,o became more negative. The ΔμCl^?_i,o has no known function, but might provide a driving force for the uptake of micronutrients.     

Stomatal acclimation to increased CO2 concentration in a Florida scrub oak species Quercus myrtifolia Willd

Native scrub‐oak communities in Florida were exposed for three seasons in open top chambers to present atmospheric [CO₂] (approx. 350 μmol mol^?1) and to high [CO₂] (increased by 350 μmol mol^?1). Stomatal and photosynthetic acclimation to high [CO₂] of the dominant species Quercus myrtifolia was examined by leaf gas exchange of excised shoots. Stomatal conductance (g_s) was approximately 40% lower in the high‐ compared to low‐[CO₂]‐grown plants when measured at their respective growth concentrations. Reciprocal measurements of g_s in both high‐ and low‐[CO₂]‐grown plants showed that there was negative acclimation in the high‐[CO₂]‐grown plants (9–16% reduction in g_s when measured at 700 μmol mol^?1), but these were small compared to those for net CO₂ assimilation rate (A, 21–36%). Stomatal acclimation was more clearly evident in the curve of stomatal response to intercellular [CO₂] (c_i) which showed a reduction in stomatal sensitivity at low c_i in the high‐[CO₂]‐grown plants. Stomatal density showed no change in response to growth in high growth [CO₂]. Long‐term stomatal and photosynthetic acclimation to growth in high [CO₂] did not markedly change the 2·5‐ to 3‐fold increase in gas‐exchange‐derived water use efficiency caused by high [CO₂].     

Relative enhancement of photosynthesis and growth at elevated CO2 is greater under sunflecks than uniform irradiance in a tropical rain forest tree seedling

The survivorship of dipterocarp seedlings in the deeply shaded understorey of South‐east Asian rain forests is limited by their ability to maintain a positive carbon balance. Photosynthesis during sunflecks is an important component of carbon gain. To investigate the effect of elevated CO₂ upon photosynthesis and growth under sunflecks, seedlings of Shorealeprosula were grown in controlled environment conditions at ambient or elevated CO₂. Equal total daily photon flux density (PFD) (～7·7 mol m^?2 d^?1) was supplied as either uniform irradiance (～170 µmol m^?2 s^?1) or shade/fleck sequences (～30 µmol m^?2 s^?1/～525 µmol m^?2 s^?1). Photosynthesis and growth were enhanced by elevated CO₂ treatments but lower under flecked irradiance treatments. Acclimation of photosynthetic capacity occurred in response to elevated CO₂ but not flecked irradiance. Importantly, the relative enhancement effects of elevated CO₂ were greater under sunflecks (growth 60%, carbon gain 89%) compared with uniform irradiance (growth 25%, carbon gain 59%). This was driven by two factors: (1) greater efficiency of dynamic photosynthesis (photosynthetic induction gain and loss, post‐irradiance gas exchange); and (2) photosynthetic enhancement being greatest at very low PFD. This allowed improved carbon gain during both clusters of lightflecks (73%) and intervening periods of deep shade (99%). The relatively greater enhancement of growth and photosynthesis at elevated CO₂ under sunflecks has important potential consequences for seedling regeneration processes and hence forest structure and composition.     

Elevated concentrations of CO2 may double methane emissions from mires

The potential impact of an increase in methane emissions from natural wetlands on climate change models could be very large. We report a profound increase in methane emissions from cores of mire peat and vegetation as a direct result of increasing the CO₂ concentration from 355 to 550 μol mol^?1 (a 60% increase). Increased CH₄ fluxes were observed throughout the four month period of study. Seasonal variation in CH₄ flux, consistent with that seen in the field, was observed under both ambient and elevated CO₂. Under ambient CO₂, methane fluxes rose from 0.02 μol m_-2 s^?1 in May to 0.11 μol m^?2 s^?3 in July before declining again in August. Under elevated CO₂ methane fluxes were at least 100% greater throughout the experiment, rising from 0.05 μol m_-2 s^?1 in May to a peak of 0.27 μol m^?2 s^?1 in July. The stimulation of CO₄ emissions was accompanied by a 100% increase in rates of photosynthesis from 4.6 (± 0.3) under ambient CO₂ to 9.3 (± 0.7) μol m^?2 s^?1. Root and shoot biomass were unaffected.     

Soil-surface carbon dioxide efflux and microbial biomass in relation to tree density 13 years after a stand replacing fire in a lodgepole pine ecosystem

The effects of fire on soil‐surface carbon dioxide (CO₂) efflux, F_S, and microbial biomass carbon, C_mic, were studied in a wildland setting by examining 13‐year‐old postfire stands of lodgepole pine differing in tree density (< 500 to > 500 000 trees ha^?1) in Yellowstone National Park (YNP). In addition, young stands were compared to mature lodgepole pine stands (～110‐year‐old) in order to estimate ecosystem recovery 13 years after a stand replacing fire. Growing season F_S increased with tree density in young stands (1.0 µmol CO₂ m^?2 s^?1 in low‐density stands, 1.8 µmol CO₂ m^?2 s^?1 in moderate‐density stands and 2.1 µmol CO₂ m^?2 s^?1 in high‐density stands) and with stand age (2.7 µmol CO₂ m^?2 s^?1 in mature stands). Microbial biomass carbon in young stands did not differ with tree density and ranged from 0.2 to 0.5 mg C g^?1 dry soil over the growing season; C_mic was significantly greater in mature stands (0.5–0.8 mg C g^?1 dry soil). Soil‐surface CO₂ efflux in young stands was correlated with biotic variables (above‐ground, below‐ground and microbial biomass), but not with abiotic variables (litter and mineral soil C and N content, bulk density and soil texture). Microbial biomass carbon was correlated with below‐ground plant biomass and not with soil carbon and nitrogen, indicating that plant activity controls not only root respiration, but C_mic pools and overall F_S rates as well. These findings support recent studies that have demonstrated the prevailing importance of plants in controlling rates of F_S and suggest that decomposition of older, recalcitrant soil C pools in this ecosystem is relatively unimportant 13 years after a stand replacing fire. Our results also indicate that realistic predictions and modeling of terrestrial C cycling must account for the variability in tree density and stand age that exists across the landscape as a result of natural disturbances.     

Foliar maintenance respiration of subalpine and boreal trees and shrubs in relation to nitrogen content

A nitrogen-based model of maintenance respiration (R_m) would link R_m with nitrogen-based photosynthesis models and enable simpler estimation of dark respiration flux from forest canopies. To test whether an N-based model of R_m would apply generally to foliage of boreal and subalpine woody plants, I measured R_m (CO₂ efflux at night from fully expanded foliage) for foliage of seven species of trees and shrubs in the northern boreal forest (near Thompson, Manitoba, Canada) and seven species in the subalpine montane forest (near Fraser, Colorado, USA). At 10°C, average R_m for boreal foliage ranged from 0.94 to 6.8μmol kg^?1 s^?1 (0.18–0.58 μmol m^?2 s^?1) and for subalpine foliage it ranged from 0.99 to 7.6 μmol kg^?1 s^?1 (0.28–0.64μmol m^?2 s^?1). CO₂ efflux at 10°C for the samples was only weakly correlated with sample weight (r = 0.11) and leaf area (r = 0.58). However, CO₂ efflux per unit foliage weight was highly correlated with foliage N concentration [r = 0.83, CO₂ flux at 10°C (mol kg^?1 s^?1) = 2.62 × foliage N (mol kg^?1)J, and slopes were statistically similar for the boreal and subalpine sites (P=0.28). CO₂ efflux per unit of foliar N was 1.8 times that reported for a variety of crop and wildland species growing in warmer climates.     

A SIMPLE METHOD FOR ISOLATING FILAMENTS AS “ALGAL SEED STOCK” FROM MONOSTROMA LATISSIMUM (CHLOROPHYTA) GERMLINGS,AND APPLICATION FOR MASS CULTIVATION1

Germlings were grown from Monostroma latissimum Wittr. reproductive cells on nylon ropes. Holdfast threads and some uniseriate filaments were observed to have penetrated the fibers of the dispersed ropes. The algal filaments were easily isolated and prepared for cultivation, in comparison to the methods of enzymatically isolated algal protoplasts. Under low light (60–100 μmol photons · m^?2 · s^?1), the algal filaments grew to form a filamentous mass. When cultivated under stronger light (300–600 μmol photons · m^?2 · s^?1), they grew to initially form tubular thalli and then, when cultivated under light intensities >700 μmol photons · m^?2 · s^?1, formed foliaceous thalli. Consequently, the filaments were homogenized into small sections and then sewed on the nylon rope for algal mass cultivation. Under high‐intensity natural light, they grew to form leafy thalli.     

COMPARISON OF DEVELOPMENT AND PHOTOSYNTHETIC GROWTH FOR FILAMENT CLUMPS AND REGENERATED MICROPLANTLET CULTURES OF AGARDHIELLA SUBULATA (RHODOPHYTA, GIGARTINALES)

Two axenic, in vitro liquid suspension cultures were established for Agardhiella subulata (C. Agardh) Kraft et Wynne, and their growth characteristics were compared. This study illustrated how reliable routes for the development of suspension cultures of macrophytic red algae of terete thallus morphology can be achieved for biotechnology applications. Undifferentiated filament clumps of 2–8 mm diameter were established by induction of callus-like tissue from thallus explants, and lightly branched microplantlets of 2–10 mm length were established by regeneration of filament clumps. The filament clumps were susceptible to regeneration. Adventitious shoot formation was reliably induced from 40% to 70% of the filament clumps by gentle mixing at 100 rev min^?1 on an orbital shaker. The specific growth rate of the microplantlets was higher than the filament clumps in nonagitated well plate culture (4%–6% per day for microplantlets vs. 2%–3% per day for filament clumps) at 24° C and 8–36 μmol photons·m^?2·s^?1 irradiance (10:14 h LD cycle) when grown on ASP12 artificial seawater medium at pH 8.6–8.9 with 20%–25% per day medium replacement. Oxygen evolution rate vs. irradiance measurements showed that relative to the filament clumps, microplantlets had a higher maximum specific oxygen evolution rate (P_o,max= 0.181 ± 0.035 vs. 0.130 ± 0.023 mmol O₂·g^?1 dry cell mass·h^?1), but comparable respiration rate (Q_o= 0.040 ± 0.013 vs. 0.033 ± 0.017 mmol O₂·g^?1 dry cell mass·h^?1), compensation point (I_c= 3.8 ± 2.4 vs. 5.7 ± 1.2 μmol photons·m^?2·s^?1), and light intensity at 63.2% of saturation (I_k= 17.5 ± 3.9 vs. 14.9 ± 2.6 μmol photons·m^?2·s^?1). The microplantlet culture was more suitable for suspension culture development than the filament clump culture because it was morphologically stable and exhibited higher growth rates.     

Combined effects of elevated CO2 and air temperature on carbon assimilation of Pinus taeda trees

Branches of 22-year-old loblolly pine (Pinus taeda, L.) trees growing in a plantation were exposed to ambient CO₂, ambient + 165 μmol mol^?1 CO₂ or ambient + 330 μmol mol^?1 CO₂ concentrations in combination with ambient or ambient + 2°C air temperatures for 3 years. Field measurements in the third year indicated that net carbon assimilation was enhanced in the elevated CO₂ treatments in all seasons. On the basis of A/C_i, curves, there was no indication of photosynthetic down-regulation. Branch growth and leaf area also increased significantly in the elevated CO₂ treatments. The imposed 2°C increase in air temperature only had slight effects on net assimilation and growth. Compared with the ambient CO₂ treatment, rates of net assimilation were ～1·6 times greater in the ambient + 165 μmol mol^?1 CO₂ treatment and 2·2 times greater in the ambient + 330 μmol mol^?1 CO₂ treatment. These ratios did not change appreciably in measurements made in all four seasons even though mean ambient air temperatures during the measurement periods ranged from 12·6 to 28·2°C. This indicated that the effect of elevated CO₂ concentrations on net assimilation under field conditions was primarily additive. The results also indicated that the effect of elevated CO₂ (+ 165 or + 330 μmol mol^?1) was much greater than the effect of a 2°C increase in air temperature on net assimilation and growth in this species.     

Variations in leaf respiration across different seasons for Mediterranean evergreen species

Leaf respiration (R _L) of evergreen species co-occurring in the Mediterranean maquis developing along the Latium coast was analyzed. The results on the whole showed that the considered evergreen species had the same R _L trend during the year, with the lowest rates [0.83 ± 0.43 μmol(CO₂) m^?2 s^?1, mean value of the considered species] in winter, in response to low air temperatures. Higher R _L were reached in spring [2.44 ± 1.00 μmol(CO₂) m^?2 s^?1, mean value] during the favorable period, and in summer [3.17 ± 0.89 μmol(CO₂) m^?2 s^?1] during drought. The results of the regression analysis showed that 42% of R _L variations depended on mean air temperature and 13% on total monthly rainfall. Among the considered species, C. incanus, was characterized by the highest R _L in drought [4.93 ± 0.27 μmol(CO₂) m^?2 s^?1], low leaf water potential at predawn (Ψ_pd= ?1.08 ± 0.18 MPa) and midday (Ψ_md = ?2.75 ± 0.11 MPa) and low relative water content at predawn (RWC_pd = 80.5 ± 3.4%) and midday (RWC_md = 67.1 ± 4.6%). Compared to C. incanus, the sclerophyllous species (Q. ilex, P. latifolia, P. lentiscus, A. unedo) and the liana (S. aspera), had lower R _L [2.72 ± 0.66 μmol(CO₂) m^?2 s^?1, mean value of the considered species], higher RWC_pd (91.8 ± 1.8%), RWC_md (82.4 ± 3.2%), Ψ_pd (?0.65 ± 0.28 MPa) and Ψ_md (?2.85 ± 1.20 MPa) in drought. The narrow-leaved species (E. multiflora, R. officinalis, and E. arborea) were in the middle. The coefficients, proportional to the respiration increase for each 10°C rise (Q₁₀), ranging from 1.49 (E. arborea) to 1.98 (A. unedo) were indicative of the different sensitivities of the considered species to air temperature variation.     

ACCLIMATION OF SYNECHOCOCCUS STRAIN WH7803 TO AMBIENT CO2 CONCENTRATION AND TO ELEVATED LIGHT INTENSITY1

A CO₂ concentrating mechanism has been identified in the phycoerythrin-possessing Synechococcus sp. WH7803 and has been observed to be severely inhibited by short exposure to elevated light intensities. A light treatment of 300–2000 μmol quanta·m^?2·s^?1 resulted in a considerable decay in the variable fluorescence of PSII with time, suggesting decreased efficiency of energy transfer from the phycobilisomes, direct damage to the reaction center II, or both. Measurements of the activity of PSII and changes in fluorescence emission spectra during a light treatment of 1000 μmol quanta·m^?2·s^?1 indicated considerable reduction in the energy flow from the phycocyanin to the phycobilisome terminal acceptor and chlorophyll a. Consequently, whereas the maximal photosynthetic rate, at saturating light and Co₂ concentration, was hardly affected by a light treatment of 1000 μmol quanta·m^?2·s^?1 for 2 h, the light intensity required to reach that maximum increased with the duration of the light treatment.     

In situ litter decomposition and litter quality in a Mojave Desert ecosystem: effects of elevated atmospheric CO2 and interannual climate variability

Rising atmospheric CO₂ has been predicted to reduce litter decomposition as a result of CO₂‐induced reductions in litter quality. However, available data have not supported this hypothesis in mesic ecosystems, and no data are available for desert or semi‐arid ecosystems, which account for more than 35% of the Earth's land area. The objective of our study was to explore controls on litter decomposition in the Mojave Desert using elevated CO₂ and interannual climate variability as driving environmental factors. In particular, we sought to evaluate the extent to which decomposition is modulated by litter chemistry (C:N) and litter species and tissue composition. Naturally senesced litter was collected from each of nine 25 m diameter experimental plots, with six plots exposed to ambient [CO₂] or 367 μL CO₂ L^?1 and three plots continuously fumigated with elevated [CO₂] (550 μL CO₂ L^?1) using FACE technology beginning in April 1997. All litter collected in 1998 (a wet, or El Niño year; 306 mm precipitation) was pooled as was litter collected in 1999 (a dry year; 94 mm). Samples were allowed to decompose for 4 and 12 months starting in May 2001 in mesh litterbags in the locations from which litter was collected. Decomposition of litter produced under elevated CO₂ and ambient CO₂ did not differ. Litter produced in the wetter year showed more rapid initial decomposition (over the first 4 months) than that produced in the drier year (27±2% yr^?1 or 7.8±0.7 g m^?2 yr^?1 for 1998 litter; 18±3% yr^?1 or 2.2±0.4 g m^?2 yr^?1 for 1999 litter). C:N ratios of litter produced under elevated CO₂ (wet year: 37±0.5; dry year: 42±2.5) were higher than those of litter produced under ambient CO₂ (wet year: 34±1.1; dry year: 35±1.4). Litter production in the wet year (amb. CO₂: 25.1±1.1 g m^?2 yr^?1; elev. CO₂: 35.0±1.1 g m^?2 yr^?1) was more than twice as high as that in the dry year (amb. CO₂: 11.6±1.7 g m^?2, elev. CO₂: 13.3±3.4 g m^?2), and contained a greater proportion of Lycium pallidum and a lower proportion of Larrea tridentata than litter produced in the dry year. Decomposition, viewed across all treatments, decreased with increasing C:N ratios, decreased with increasing proportions of Larrea tridentata and increased with increasing proportions of Lycium pallidum and Lycium andersonii. Because litter C:N did not vary by litter production year, and CO₂ did not alter decomposition or litter species/tissue composition, it is likely that the impact of year‐to‐year variation in precipitation on the proportion of key plant species in the litter may be the most important way in which litter decomposition will be modulated in the Mojave Desert under future rising atmospheric CO₂.     

Kinetics of chloride transport in the cyanobacterium Synechococcus R-2 (Anacystis nidulans, S. leopoliensis) PCC 7942

The kinetics of the light-driven Cl^? uptake pump of Synechococcus R-2 (PCC 7942) were investigated. The kinetics of Cl^? uptake were measured in BG-11 medium (pH_o, 7·5; [K⁺]_o, 0·35 mol m^?3; [Na⁺]_o, 18 mol m^?3; [Cl^?]_o, 0·508 mol m^?3) or modified media based on the above. Net³⁶Cl^? fluxes (?Cl^?_o,i) followed Michaelis-Menten kinetics and were stimulated by Na⁺ [18 mol m^?3 Na⁺ BG-11 ?Cl^?_max= 3·29±0·60 (49) nmol m^?2 s^?1 versus Na⁺-free BG-11 ?Cl^?_max= 1·02±0·13 (54) nmol m^?2 s^?1] but the Km was not significantly different in the presence or absence of Na⁺ at pH_o 10; the Km was lower, but not affected by the presence or absence of Na⁺ [Km = 22·3±3·54 (20) mmol m^?3]. Na⁺ is a non-competitive activator of net ?Cl^?_o,i. High [K⁺]_o (18 mol m^?3) did not stimulate net ?Cl^?_o,i or change the Km in Na⁺-free medium. High [K⁺]_o (18 mol m^?3) added to Na⁺ BG-11 medium decreased net ?Cl^?_o,i [18 mol m^?3K⁺ BG-11; ?Cl^?_max= 2·50±0·32 (20) nmol m^?2 s^?1 versus BG-11 medium; ?Cl^?_max= 3·35±0·56 (20) nmol m^?2 s^?1] but did not affect the Km 55·8±8·100 (40) mmol m^?3]. Na⁺-stimulation of net ?Cl^?_o,i followed Michaelis-Menten kinetics up to 2–5 mol m^?3 [Na⁺]_o but higher concentrations were inhibitory. The Km for Na⁺-stimulation of net ?Cl^?_o,i [K_1/2(Na⁺)] was different at 47 mmol m^?3 [Cl^?]_o (K_1/2[Na⁺] = 123±27 (37) mmol m^?3]. Li⁺ was only about one-third as effective as Na⁺ in stimulating Cl^? uptake but the activation constant was similar [K_1/2(Li⁺) = 88±46 (16) mmol m^?3]. Br^? was a competitive inhibitor of Cl^? uptake. The inhibition constant (Ki) was not significantly different in the presence and absence of Na⁺. The overall Ki was 297±23 (45) mmol m^?3. The discrimination ratio of Cl^? over Br^? (δCl^?/δBr^?) was 6·38±0·92 (df = 147). Synechococcus has a single Na⁺-stimulated Cl^? pump because the Km of the Cl^? transporter and its discrimination between Cl^? and Br^? are not significantly different in the presence and absence of Na⁺. The Cl^? pump is probably driven by ATP.     

Effect of Light Intensity on Growth, CO2 Fixation, and Chlorophyll and Polar Lipid Production in Germinating Polytrichum commune Spores

The dry matter production in Polytrichum commune protonemata was increased when the light intensity was increased from 0 to 160 μE m^?2 s^?1, and at 160 μE m^?2 s^?1 production was about 200% of that found at 17 μE m^?2 s^?1. Production of chlorophyll (Chl) was increased by increasing light intensity from 0 to 17 μE m^?2 s^?1, but decreasing at light intensities above 17 μE m^?2 s^?1. At 160 μE m^?2 s^?1 the production of Chl was only about 50% of that at 17 μE m^?2 s^?1. The rate of CO₂ fixation was low (0.31 μg CO₂/mg Chi × h) at the light intensity of 17 μE m^?2 s^?1 as compared with that at 160 μE m^?2 s^?1 (0.83 μg CO₂/mg Chi × h). Production of mono- (MGDG) and diglycosyl diglycerides (DGDG) was closely associated with that of chlorophylls. At the higher light intensity (160 μE m^?2 s^?1) production of glycolipids was about 60% of that at 17 μE m^?2 s^?1. Production of more polar lipids was less affected by light intensity. Light intensity also affected the fatty acid pattern of the lipid fractions. The effect was most pronounced in the MGDG fraction, where the proportion of C 18: 3ω3 + C 16: 3ω3 was higher at the higher light intensity.     

EFFECTS OF HARVEST STAGE AND LIGHT ON THE BIOCHEMICAL COMPOSITION OF THE DIATOM THALASSIOSIRA PSEUDONANA1

The marine diatom Thalassiosira pseudonana (Hustedt, clone 3H) Hasle and Heimdal was cultured under three different light regimes: 100 μmol photon · m^?2· s^?1 on 12:12 h light : dark (L:D) cycles; 50 μmol photon · m^?2· s^?2 on 24:0 h L:D; and 100 μmol photon · m^?2· s^?1 on 24:0 h L:D. It was harvested during logarithmic and stationary phases for analysis of biochemical composition. Across the different light regimes, protein (as % of organic weight) was highest in cells during logarithmic phase, whereas carbohydrate and lipid were highest during stationary phase. Carbohydrate concentrations were most affected by the different light regimes; cells grown under 12:12 h L:D contained 37–44% of the carbohydrate of cells grown under 24:0 h L:D. Cells in logarithmic phase had high proportions of polar lipids (79 to 89% of total lipid) and low triacylglycerol (≤10% of total lipid). Cells in stationary phase contained less polar lipid (48 to 57% of total lipid) and more triacylglycerol (22 to 45% of total lipid). The fatty acid composition of logarithmic phase cells grown under 24:0 h L:D were similar, but the 100 μmol photon · m^?2· s^?1 (12:12 h L:D) cells at the same stage contained a higher proportion of polyunsaturated fatty acids (PUFAs) and a lower proportion of saturated and monounsaturated fatty acids due to different levels of 16:0, 16:1(n-7), 16:4(n-1), 18:4(n-3), and 20:5(n-3). With the onset of stationary phase, cells grown at 100 μmol photon · m^?2· s^?1 (both 12:12 and 24:0 h L:D) increased in proportions of saturated and monounsaturated fatty adds and decreased in PUFAs. Concentrations (% organic or dry weight) of 14:0, 16:0, 16:1(n-7), 20:5(n-3), and 22:6(n-3) increased in cells of all cultures during stationary phase. The amino acid compositions of cells were similar irrespective of harvest stage and light regime. For mariculture, the recommended light regime for culturing T. pseudonana will depend on the nutritional requirements of the animal to which the alga is fed. For rapidly growing bivalve mollusc larvae, stationary-phase cultures grown under a 24:0 h L:D regime may provide more energy by virtue of their higher percentage of carbohydrate and high proportions and concentrations of energy-rich saturated fatty acids.