Leaf cavity CO2 concentrations and CO2 exchange in onion,Allium cepa L.

Onion (Allium cepa L.) plants were examined to determine the photosynthetic role of CO₂ that accumulates within their leaf cavities. Leaf cavity CO₂ concentrations ranged from 2250 L L^–1 near the leaf base to below atmospheric (<350 L L^–1) near the leaf tip at midday. There was a daily fluctuation in the leaf cavity CO₂ concentrations with minimum values near midday and maximum values at night. Conductance to CO₂ from the leaf cavity ranged from 24 to 202 mol m^–2 s^–1 and was even lower for membranes of bulb scales. The capacity for onion leaves to recycle leaf cavity CO₂ was poor, only 0.2 to 2.2% of leaf photosynthesis based either on measured CO₂ concentrations and conductance values or as measured directly by ¹⁴CO₂ labeling experiments. The photosynthetic responses to CO₂ and O₂ were measured to determine whether onion leaves exhibited a typical C₃-type response. A linear increase in CO₂ uptake was observed in intact leaves up to 315 L L^–1 of external CO₂ and, at this external CO₂ concentration, uptake was inhibited 35.4±0.9% by 210 mL L^–1 O₂ compared to 20 mL L^–1 O₂. Scanning electron micrographs of the leaf cavity wall revealed degenerated tissue covered by a membrane. Onion leaf cavity membranes apparently are highly impermeable to CO₂ and greatly restrict the refixation of leaf cavity CO₂ by photosynthetic tissue.Abbreviations C_a external CO₂ concentration - C_i intercellular CO₂ concentration - CO₂ compensation concentration - PPFR photosynthetic photon fluence rate     

Relationship Between Photosystem 2 Electron Transport and Photosynthetic CO2 Assimilation Responses to Irradiance in Young Apple Tree Leaves

The responses to irradiance of photosynthetic CO₂ assimilation and photosystem 2 (PS2) electron transport were simultaneously studied by gas exchange and chlorophyll (Chl) fluorescence measurement in two-year-old apple tree leaves (Malus pumila Mill. cv. Tengmu No.1/Malus hupehensis Rehd). Net photosynthetic rate (P _N) was saturated at photosynthetic photon flux density (PPFD) 600-1 100 (mol m^-2 s^-1, while the PS2 non-cyclic electron transport (P-rate) showed a maximum at PPFD 800 mol m^-2 s^-1. With PPFD increasing, either leaf potential photosynthetic CO₂ assimilation activity (F_d/F_s) and PS2 maximal photochemical activity (F_v/F_m) decreased or the ratio of the inactive PS2 reaction centres (RC) [(F_i – F_o)/(F_m – F_o)] and the slow relaxing non-photochemical Chl fluorescence quenching (q_s) increased from PPFD 1 200 mol m^-2 s^-1, but cyclic electron transport around photosystem 1 (RFp), irradiance induced PS2 RC closure [(F_s – F_o)/F_m – F_o)], and the fast and medium relaxing non-photochemical Chl fluorescence quenching (q_f and q_m) increased remarkably from PPFD 900 (mol m^-2 s^-1. Hence leaf photosynthesis of young apple leaves saturated at PPFD 800 mol m^-2 s^-1 and photoinhibition occurred above PPFD 900 mol m^-2 s^-1. During the photoinhibition at different irradiances, young apple tree leaves could dissipate excess photons mainly by energy quenching and state transition mechanisms at PPFD 900-1 100 mol m^-2 s^-1, but photosynthetic apparatus damage was unavoidable from PPFD 1 200 mol m^-2 s^-1. We propose that Chl fluorescence parameter P-rate is superior to the gas exchange parameter P _N and the Chl fluorescence parameter F_v/F_m as a definition of saturation irradiance and photoinhibition of plant leaves.     

Modelling continuous culture of Rhodospirillum rubrum in photobioreactor under light limited conditions

Rhodospirillum rubrum was grown continuously and photoheterotrophically under light limitation using a cylindrical photobioreactor in which the steady state biomass concentration was varied between 0.4 to 4 kg m^–3 at a constant radiant incident flux of 100 W m^–2. Kinetic and stoichiometric models for the growth are proposed. The biomass productivities, acetate consumption rate and the CO₂ production rate can be quantitatively predicted to a high level of accuracy by the proposed model calculations. Nomenclature: C _X, biomass concentration (kg m^–3) D, dilution rate (h^–1) Ea, mean mass absorption coefficient (m² kg^–1) I , total available radiant light energy (W m^–2) K, half saturation constant for light (W m^–2) R _W, boundary radius defining the working illuminated volume (m) r _X, local biomass volumetric rate (kg m^–3 h^–1) <r _X>, mean volumetric growth rate (kg m^–3 h^–1) V _W, illuminated working volume in the PBR (m^–3). Greek letters: , working illuminated fraction (–) _M, maximum quantum yield (–) bar, mean energetic yield (kg J^–1).     

Influence of CO2, light and watering on growth of Nostoc flagelliforme mats

The terrestrial blue-green alga (cyanobacterium), Nostoc flagelliforme, was cultured in air at variouslevels of CO₂, light and watering to see theireffects on its growth. The alga showed the highestrelative growth rate at the conditions of highCO₂ (1500 ppm), high light regime (219–414mol m^-2s^-1) and twice daily watering,but the lowest rate at the conditions of low light(58–114 mol m^-2s^-1) and daily twicewatering. Increased watering had little effect ongrowth rate at 350 ppm CO₂, but increased byabout 70% at 1500ppm CO₂ under high lightconditions. It was concluded that enriched CO₂could enhance the growth of N. flagelliformewhen sufficient light and water was supplied.     

Effect of changes in water content on photosynthesis,transpiration and discrimination against 13CO2 and C18O16O in Pleurozium and Sphagnum

Photosynthetic gas exchange characteristics of two common boreal forest mosses, Sphagnum (section acutifolia) and Pleurozium schreberi, were measured continuously during the time required for the moss to dry out from full hydration. Similar patterns of change in CO₂ assimilation with variation in water content occurred for both species. The maximum rates of CO₂ assimilation for Sphagnum (approx. 7 mol m^–2 s^–1) occurred at a water content of approximately 7 (fresh weight/dry weight) while for Pleurozium the maximum rate (approx. 2 mol m^–2 s^–1) occurred at a water content of approximately 6 (fresh weight/dry weight). Above and below these water contents CO₂ assimilation declined. In both species total conductance to water vapour (expressed as a percentage of the maximum rates) remained nearly constant at a water content above 9 (fresh weight/dry weight), but below this level declined in a strong linear manner. Short-term, on-line ¹³CO₂ and C¹⁸O¹⁶O discrimination varied substantially with changes in moss water content and associated changes in the ratio of chloroplast CO₂ to ambient CO₂ partial pressure. At full hydration (maximum water content) both Sphagnum and Pleurozium had similar values of ¹³CO₂ discrimination (approx. 15). Discrimination against ¹³CO₂ increased continuously with reductions in water content to a maximum of 27 in Sphagnum and 22 in Pleurozium. In a similar manner C¹⁸C¹⁶O discrimination increased from approximately 30 at full hydration in both species to a maximum of 150 in Sphagnum and 90 in Pleurozium, at low water content. The observed changes in C¹⁸O¹⁶O were strongly correlated to predictions of a mechanistic model of discrimination processes. Field measurements of moss water content suggested that photosynthetic gas exchange by moss in the understory of a black spruce forest was regularly limited by low water content.     

Effects of CO2 elevation on canopy development in the stands of two co-occurring annuals

Elevated CO₂ may increase dry mass production of canopies directly through increasing net assimilation rate of leaves and also indirectly through increasing leaf area index (LAI). We studied the effects of CO₂ elevation on canopy productivity and development in monospecific and mixed (1:1) stands of two co-occurring C₃ annual species, Abutilon theophrasti, and Ambrosia artemisiifolia. The stands were established in the glasshouse with two CO₂ levels (360 and 700 l/l) under natural light conditions. The planting density was 100 per m² and LAI increased up to 2.6 in 53 days of growth. Root competition was excluded by growing each plant in an individual pot. However, interference was apparent in the amount of photons absorbed by the plants and in photon absorption per unit leaf area. Greater photon absorption by Abutilon in the mixed stand was due to different canopy structures: Abutilon distributed leaves in the upper layers in the canopy while Ambrosia distributed leaves more to the lower layers. CO₂ elevation did not affect the relative performance and light interception of the two species in mixed stands. Total aboveground dry mass was significantly increased with CO₂ elevation, while no significant effects on leaf area development were observed. CO₂ elevation increased dry mass production by 30–50%, which was mediated by 35–38% increase in the net assimilation rate (NAR) and 37–60% increase in the nitrogen use efficiency (NUE, net assimilation rate per unit leaf nitrogen). Since there was a strong overall correlation between LAI and aboveground nitrogen and no significant difference was found in the regression of LAI against aboveground nitrogen between the two CO₂ levels, we hypothesized that leaf area development was controlled by the amount of nitrogen taken up from the soil. This hypothesis suggests that the increased LAI with CO₂ elevation observed by several authors might be due to increased uptake of nitrogen with increased root growth.     

Leaf and canopy responses to elevated CO2 in a pine forest under free-air CO2 enrichment

Physiological responses to elevated CO₂ at the leaf and canopy-level were studied in an intact pine (Pinus taeda) forest ecosystem exposed to elevated CO₂ using a free-air CO₂ enrichment (FACE) technique. Normalized canopy water-use of trees exposed to elevated CO₂ over an 8-day exposure period was similar to that of trees exposed to current ambient CO₂ under sunny conditions. During a portion of the exposure period when sky conditions were cloudy, CO₂-exposed trees showed minor (7%) but significant reductions in relative sap flux density compared to trees under ambient CO₂ conditions. Short-term (minutes) direct stomatal responses to elevated CO₂ were also relatively weak (5% reduction in stomatal aperture in response to high CO₂ concentrations). We observed no evidence of adjustment in stomatal conductance in foliage grown under elevated CO₂ for nearly 80 days compared to foliage grown under current ambient CO₂, so intrinsic leaf water-use efficiency at elevated CO₂ was enhanced primarily by direct responses of photosynthesis to CO₂. We did not detect statistical differences in parameters from photosynthetic responses to intercellular CO₂ (A _net-C _i curves) for Pinus taeda foliage grown under elevated CO₂ (550 mol mol^–1) for 50–80 days compared to those for foliage grown under current ambient CO₂ from similar-sized reference trees nearby. In both cases, leaf net photosynthetic rate at 550 mol mol^–1 CO₂ was enhanced by approximately 65% compared to the rate at ambient CO₂ (350 mol mol^–1). A similar level of enhancement under elevated CO₂ was observed for daily photosynthesis under field conditions on a sunny day. While enhancement of photosynthesis by elevated CO₂ during the study period appears to be primarily attributable to direct photosynthetic responses to CO₂ in the pine forest, longer-term CO₂ responses and feedbacks remain to be evaluated.     

Long-term steady-state labelling of wheat plants by use of natural 13CO2/12CO2 mixtures in an open,rapidly turned-over system

A photosynthate labelling method is presented which takes advantage of the natural difference in carbon-isotope composition () which exists between atmospheric CO₂ (-8) and commercially available compressed CO₂. Carbon dioxide with -4.0 and –27.9%., respectively, has been used for labelling. A plant growth cabinet served as the labelling compartment. CO₂-free air was continuously injected at a rate of up to 54m³·h^–1. Dilution of cabinet CO₂ by CO₂-free air was counterbalanced by addition of CO₂ with known constant . Since the labelling-cabinet atmosphere was continuously exchanged at a high rate, photosynthetic carbon-isotope discrimination was fully expressed. In order to study the distribution of carbon acquired by the plant during a defined growth period, the of CO₂ was modified by replacing, for example, atmospheric CO₂ by CO₂ with –27.9%. and the weight and 5 of plant carbon pools was monitored over time. In such an experiment the change of CO₂ was followed by a rapid change of the of sucrose in mature flag-leaf blades of wheat (Triticum aestivum L.). The 5 of sucrose stabilized near –51%., indicating complete exchange by current photosynthate. In contrast 83% of the total carbon in mature flag-leaf blades was not exchanged after 14 d continuous labelling. Differential labelling of pre- and post-anthesis photosynthate indicated that 13% of grain carbon originated from pre-anthesis photosynthesis. Carbon-isotope discrimination and its consideration in experimentation and labelling data evaluation are discussed in detail. Since the air supplied to the labelling cabinet is dry and free of CO₂, carbon-isotope discrimination and carbon turnover and partitioning can be studied over a wide range of CO₂ concentrations (0–2600 cm³ · m^–3) and vapor-pressure deficits.Abbreviation and Symbol PPFD photosynthetic photon flux density - carbon-isotope composition Dr. G. Schleser (Forschungszentrum Jülich, FRG) and Professor S. Hoernes (Mineralogisch-Petrologisches Institut, Universität Bonn) for valuable help and advice during the initial stages of the project and Professor W. Kühbauch (Institut für Pflanzenbau, Universität Bonn) for continuing support. Technical assistance of Ute Labusch, Petra Biermann, Ludwig Schmitz and Thomas Gebbing is gratefully acknowleged.

---

     

Growth,photosynthesis and photorespiration of Lemna gibba: response to variations in CO2 and O2 concentrations and photon flux density

Dry weight and Relative Growth Rate of Lemna gibba were significantly increased by CO₂ enrichment up to 6000 l CO₂ l^–1. This high CO₂ optimum for growth is probably due to the presence of nonfunctional stomata. The response to high CO₂ was less or absent following four days growth in 2% O₂. The Leaf Area Ratio decreased in response to CO₂ enrichment as a result of an increase in dry weight per frond. Photosynthetic rate was increased by CO₂ enrichment up to 1500 l CO₂ l^–1 during measurement, showing only small increases with further CO₂ enrichment up to 5000 l CO₂ l^–1 at a photon flux density of 210 mol m^–2 s^–1 and small decreases at 2000 mol m^–1 s^–1. The actual rate of photosynthesis of those plants cultivated at high CO₂ levels, however, was less than the air grown plants. The response of photosynthesis to O₂ indicated that the enhancement of growth and photosynthesis by CO₂ enrichment was a result of decreased photorespiration. Plants cultivated in low O₂ produced abnormal morphological features and after a short time showed a reduction in growth.     

H2 oxidation,O2 uptake and CO2 fixation in hydrogen treated soils

In many legume nodules, the H₂ produced as a byproduct of N₂ fixation diffuses out of the nodule and is consumed by the soil. To study the fate of this H₂ in soil, a H₂ treatment system was developed that provided a 300 cm³ sample of a soil:silica sand (2:1) mixture with a H₂ exposure rate (147 nmol H₂ cm^–3hr^–1) similar to that calculated exist in soils located within 1–4 cm of nodules (30–254 nmol H₂ cm^–3hr^–1). After 3 weeks of H₂ pretreatment, the treated soils had a K_m and V_max for H₂ uptake (1028 ppm and 836 nmol cm^–3 hr^–1, respectively) much greater than that of control, air-treated soil (40.2 ppm and 4.35 nmol cm^–3 hr^–1, respectively). In the H₂ treated soils, O₂, CO₂ and H₂ exchange rates were measured simultaneously in the presence of various pH₂. With increasing pH₂, a 5-fold increase was observed in O₂ uptake, and CO₂ evolution declined such that net CO₂ fixation was observed in treatments of 680 ppm H₂ or more. At the H₂ exposure rate used to pretreat the soil, 60% of the electrons from H₂ were passed to O₂, and 40% were used to support CO₂ fixation. The effect of H₂ on the energy and C metabolism of soil may account for the well-known effect of legumes in promoting soil C deposition.     

The quantum yield for CO2 uptake in C3 and C4 grasses

The quantum yield for CO₂ uptake was measured in C₃ and C₄ monocot species from several different grassland habitats. When the quantum yield was measured in the presence of 21% O₂ and 340 cm³ m^-3 CO₂, values were very similar in C₃ monocots, C₃ dicots, and C₄ monocots (0.045–0.056 mole CO₂ · mole^-1 quanta absorbed). In the presence of 2% O₂ and 800 cm³ m^-3 CO₂, enhancements of the quantum yield values occurred for the C₃ plants (both monocots and dicots), but not for C₄ monocots. A dependence of the quantum yield on leaf temperature was observed in the C₃ grass, Agropyron smithii, but not in the C₄ grass, Bouteloua gracilis, in 21% O₂ and 340 cm³ m^-3 CO₂. At leaf temperatures between 22–25°C the quantum yield values were approximately equal in the two species.     

CO2 environment,microclimate and photosynthetic characteristics of the moss Hylocomium splendens in a subarctic habitat

Summary In order to document the natural CO₂ environment of the moss Hylocomium splendens, and ascertain whether or not the moss was adapted to this, and its interactions with other microenvironmental factors, two studies were carried out. Firstly, the seasonal variations of CO₂ concentration, photosynthetically active radiation (PAR), tissue water content and temperature were measured in the natural microenvironment of H. splendens in a subarctic forest during the summer period (July–September). Secondly, the photosynthetic responses of the species to controlled CO₂ concentrations, PAR, temperature, and hydration were measured in the laboratory. CO₂ concentrations around the upper parts of the plant, when PAR was above the compensation point (30 mol m^–2 s^–1), were mostly between 400 and 450 ppm. They occasionally increased up to 1143 ppm for short periods. PAR flux densities below saturating light levels for photosynthesis (100 mol m^–2 s^–1), occurred during 65% (July), 76% (August) and 96% (September) of the hours of the summer period. The temperature optimum of photosynthesis was 20° C: this temperature coincided with PAR above the compensation point during 5%, 6% and 0% of the time in July, August and September, respectively. Optimal hydration of tissues was infrequent. Hence PAR, temperature and water limit CO₂ uptake for most of the growing season. Our data suggest that the higher than normal ambient CO₂ concentration in the immediate environment of the plant counteracts some of the limitations in PAR supply that it experiences in its habitat. This species already experiences concentrations of atmospheric CO₂ predicted to occur over the next 50 years.     

Net CO2 assimilation of taro and cocoyam as affected by shading and leaf age

Taro and cocoyam were grown outdoors in either full sun or under 40% shade. Leaves were tagged as they emerged and the effect of leaf age on net CO₂ assimilation rate (A) was determined. The effects of shading on A, transpiration (E), stomatal conductance for CO₂ (g_c) and H₂O (g_s), and water use efficiency (WUE) were also determined for leaves of a single age for each species. The effect of leaf age on A was similar for both species. Net CO₂ assimilation rates increased as leaf age increased up to 28 days with the exception of a sharp decline in A for 21 day-old leaves which corresponded to unusually low temperatures during the period of leaf expansion. A generally decreased as leaves aged beyond 28 days. Cocoyam had higher A rates than taro. Leaves of shade-grown plants had higher rates of A and E for both species at photosynthetic photon flux densities (PPFD) up to 1600 mol s^–1 m^–2. Shade-grown leaves of cocoyam had greater leaf dry weights per area (LW/A) and a trend toward higher g_c and g_s than sun-grown leaves. Shade leaves of taro had greater g_c and g₃ rates than sun-grown leaves. The data suggest that taro and cocoyam are highly adapted to moderate shade conditions.     

Effects of CO<Subscript>2</Subscript> concentration on acclimatization and physiological responses of two cultivars of carob tree

This study reports survival and physiological responses of micropropagated Ceratonia siliqua L. cvs. Galhosa and Mulata plants during ex vitro acclimatization under ambient (AC; 330 mol mol^–1) or elevated (EC; 810 mol mol^–1) CO₂ concentration and a photosynthetic photon flux density of 125 mol m^–2 s^–1. CO₂ enrichment during acclimatization did not improve survival rate that was around 80 % for both treatments. Eight weeks after ex vitro transplantation, photosynthetic capacity and apparent quantum yield in acclimatized leaves were higher in comparison with those in in vitro-grown leaves, without any significant difference between CO₂ treatments. Chlorophyll content increased after acclimatization. However, EC led to a decrease in the total amount of chlorophyll in new leaves of both cultivars, compared to those grown at AC. Soluble sugars and starch contents were not markedly affected by growth EC, although starch had significantly increased after transfer to ex vitro conditions. EC induced an increase in the stem elongation and in the effective life of leaves, and a decrease in the number of new leaves.     

Ulva rigida (Ulvales,Chlorophyta) tank culture as biofilters for dissolved inorganic nitrogen from fishpond effluents

Ulva rigida was cultivated in 7501 tanks at different densities with direct and continuous inflow (at 2, 4, 8 and 12 volumes d^–1) of the effluents from a commercial marine fishpond (40 metric tonnes, Tm, of Sparus aurata, water exchange rate of 16 m³ Tm^–1) in order to assess the maximum and optimum dissolved inorganic nitrogen (DIN) uptake rate and the annual stability of the Ulva tank biofiltering system. Maximum yields (40 g DW m^–2 d^–1) were obtained at a density of 2.5 g FW 1^–1 and at a DIN inflow rate of 1.7 g DIN m^–2 d^–1. Maximum DIN uptake rates were obtained during summer (2.2 g DIN M^–2 d^–1), and minimum in winter (1.1 g DIN m^–2 d^–1) with a yearly average DIN uptake rate of 1.77 g DIN m^–2 d^–1 At yearly average DIN removal efficiency (2.0 g DIN m^–2 d^–1, if winter period is excluded), 153 m² of Ulva tank surface would be needed to recover 100% of the DIN produced by 1 Tm of fish.Abbreviations DIN= dissolved inorganic nitrogen (NH _inf4 ^sup+ + NO _inf3 ^sup– + NO _inf2 ^sup– ); - FW= fresh weight; - DW= dry weight; - PFD= photon flux density; - V= DIN uptake rate     

Analysing the responses of photosynthetic CO2 assimilation to long-term elevation of atmospheric CO2 concentration

Understanding how photosynthetic capacity acclimatises when plants are grown in an atmosphere of rising CO₂ concentrations will be vital to the development of mechanistic models of the response of plant productivity to global environmental change. A limitation to the study of acclimatisation is the small amount of material that may be destructively harvested from long-term studies of the effects of elevation of CO₂ concentration. Technological developments in the measurement of gas exchange, fluorescence and absorption spectroscopy, coupled with theoretical developments in the interpretation of measured values now allow detailed analyses of limitations to photosynthesisin vivo. The use of leaf chambers with Ulbricht integrating spheres allows separation of change in the maximum efficiency of energy transduction in the assimilation of CO₂ from changes in tissue absorptance. Analysis of the response of CO₂ assimilation to intercellular CO₂ concentration allows quantitative determination of the limitation imposed by stomata, carboxylation efficiency, and the rate of regeneration of ribulose 1:5 bisphosphate. Chlorophyll fluorescence provides a rapid method for detecting photoinhibition in heterogeneously illuminated leaves within canopies in the field. Modulated fluorescence and absorption spectroscopy allow parallel measurements of the efficiency of light utilisation in electron transport through photosystems I and IIin situ.Abbreviations A net rate of CO₂ uptke per unit leaf area (µmol m^–2 s^–1) - A_sat light-saturated A - A₈₂₀ change in absorptance of PSI on removal of illumination (OD) - c CO₂ concentration in air (µmol mol^–1) - c_a c in the bulk air; c_i, c in the intercellular spaces - ce carboxylation efficiency (mol m^–2 s^–1) - E transpiration per unit leaf area (mol m^–2 s^–1) - F fluorescence emission of PSII (relative units) - F_m maximal level of F - F_o minimal level of F upon illumination when PSII is maximally oxidised - F_s the steady-state F following the m peak - F_v the difference between F_m and F_o - F'_m maximal F' generated after the m peak by addition of a saturating light pulse - F'_o the minimal level of F' after the m peak determined by re-oxidising PSII by far-red light - g₁ leaf conductance to CO₂ diffusion in the gas phase (mol m^–2 s^–1) - g'₁ leaf conductance to water vapour diffusion in the gas phase (mol m^–2 s^–1) - k_c and k_o the Michaelis constants for CO₂ and O₂, respectively, (µmol mol^–1); - J_max the maximum rate of regeneration of rubP (µmol m^–2 s^–1) - l stomatal limitation to CO₂ uptake (dimensionless, 0–1) - LCP light compensation point of photosynthesis (µmol m^–2 s^–1) - o_i the intercellular O₂ concentration (mmol mol^–1) - Pi cytosol inorganic phosphate concentration - PSI photosystem I - PSII photosystem II - Q photon flux (µmol m^–2 s^–1) - Q_abs Q absorbed by the leaf - rubisCO ribulose 1:5 bisphosphate carboxylase/oxygenase; rubP, ribulose 1:5 bisphosphate; s, projected surface area of a leaf (m²) - V_c,max is the maximum rate of carboxylation (µmol m^–2 s^–1) - W_c the rubisCO limited rate of carboxylation (µmol m^–2 s¹) - W_j the electron transport limited rate of regeneration of rubP (µmol m^–2 s^–1) - W_p the inorganic phosphate limited rate of regeneration of rubP (µmol m^–2 s^–1) - absorptance of light (dimensionless, 0–1) - _a of standard black absorber ₁, of leaf - _s of integrating sphere walls - , CO₂ compensation point of photosynthesis (µmol mol^–1) - the specificity factor for rubisCO carboxylation (dimensionless) - , convexity of the response of A to Q (dimensionless 0–1) - the quantum yield of photosynthesis on an absorbed light basis (A/Q_abs; dimensionless) - the quantum yield of photosynthesis on an incident light basis (A/Q; dimensionless) - _app the maximum - _m the maximum - _m,app the photochemical efficiency of PSII (dimensionless, 0–1) - _PSII,m the maximum      

Effects of extreme high temperature,drought and elevated CO2 on photosynthesis of the Mojave Desert evergreen shrub,Larrea tridentata

The interaction of extreme temperature events with future atmospheric CO₂ concentrations may have strong impacts on physiological performance of desert shrub seedlings, which during the critical establishment phase often endure temperature extremes in conjunction with pronounced drought. To evaluate the interaction of drought and CO₂ on photosynthesis during heat stress, one-year-old Larrea tridentata[DC] Cov. seedlings were exposed to nine days of heat with midday air temperature maxima reaching 53 °C under three atmospheric CO₂ concentrations (360, 550 and 700 mol mol^–1) and two water regimes (well-watered and droughted). Photosynthetic gas exchange, chlorophyll fluorescence and water potential responses were measured prior to, during and one week following the high temperature stress event. Heat stress markedly decreased net photosynthetic rate (A _net), stomatal conductance (g _s), and the photochemical efficiency of photosystem II (F _v/F _m) in all plants except for well-watered L. tridentata grown in 700 mol mol^–1 CO₂. A _net and g _s remained similar to pre-stress levels in these plants. In droughted L. tridentata, A _net was ca. 2× (in 550 mol mol^–1 CO₂) to 3× (in 700 mol mol^–1 CO₂) higher than in ambient-CO₂-grown plants, while g _s and F _v/F _m were similar and low in all CO₂ treatments. Following heat stress, g _s in all well-watered plants rose dramatically, exceeding pre-stress levels by up to 100%. In droughted plants, g _s and A _net rose only in plants grown at elevated CO₂ following release from heat. This recovery response was strongest at 700 mol mol^–1 CO₂, which returned to A _net and g _s values similar to pre-heat following several days of recovery. Extreme heat diminished the photosynthetic down-regulation response to growth at elevated CO₂ under well-watered conditions, similar to the action of drought. Ambient-CO₂-grown L. tridentata did not show significant recovery of photosynthetic capacity (A _\max and CE) after alleviation of temperature stress, especially when exposed to drought, while plants exposed to elevated CO₂ appeared to be unaffected. These findings suggest that elevated CO₂ could promote photosynthetic activity during critical periods of seedling establishment, and enhance the potential for L. tridentata to survive extreme high temperature events.     

Effect of Elevated CO2 on Photosynthesis and Chlorophyll Fluorescence of Rose Plants Grown at High Temperature and High Photosynthetic Photon Flux Density

Gas exchange and chlorophyll (Chl) fluorescence were measured on young mature leaves of rose plants (Rosa hybrida cvs. First Red and Twingo) grown in two near-to-tight greenhouses, one under control ambient CO₂ concentration, AC (355 µmol mol^–1) and one under CO₂ enrichment, EC (700 µmol mol^–1), during four flushes from late June to early November. Supply of water and mineral elements was non-limiting while temperature was allowed to rise freely during daytime. Leaf diffusive conductance was not significantly reduced at EC but net photosynthetic rate increased by more than 100 %. Although the concentration of total non-structural saccharides was substantially higher in the leaves from the greenhouse with EC, _PS2 (quantum efficiency of radiation use) around noon was not significantly reduced at EC indicating that there was no down-regulation of electron transport. Moreover, CO₂ enrichment did not cause any increase in the risk of photo-damage, as estimated by the 1 – q_P parameter. Non-photochemical quenching was even higher in the greenhouse with EC during the two summer flushes, when temperature and photosynthetic photon flux density (PPFD) were the highest. Hence rose photosynthesis benefits strongly from high concentrations of atmospheric CO₂ at both high and moderate temperatures and PPFD.     

Independent and contrasting effects of elevated CO2 and N-fertilization on root architecture in Pinus ponderosa

The effects of elevated CO₂ and N-fertilization on the architecture of Pinus ponderosa Dougl. ex P. Laws & C. Laws fine roots and their associated mycorrhizal symbionts were measured over a 4-year period using minirhizotron tubes. The study was conducted in open-top field-exposure chambers located near Placerville, Calif. A replicated (3 replicates), 3×3 factorial experimental design with three CO₂ concentrations [ambient air (354 mol mol^–1), 525 mol mol^–1, and 700 mol mol^–1] and three rates of N-fertilization (0, 100 and 200 kg ha^–1 year^–1) was used. Elevated CO₂ and N treatment had contrasting effects on the architecture of fine roots and their associated mycorrhizae. Elevated CO₂ increased both fine root extensity (degree of soil exploration) and intensity (extent that roots use explored areas) but had no effect on mycorrhizae. In contrast, N-fertilization had no effect on fine root extensity or intensity but increased mycorrhizal extensity and intensity. To better understand and model the responses of systems to increasing CO₂ concentrations and N deposition/fertilization it is necessary to consider these contrasting root architectural responses.     

Gas Exchange of Carrot Leaves in Response to Elevated CO2 Concentration

Short-term responses of four carrot (Daucus carota) cultivars: Cascade, Caro Choice (CC), Oranza, and Red Core Chantenay (RCC) to CO₂ concentrations (C _a) were studied in a controlled environment. Leaf net photosynthetic rate (P _N), intercellular CO₂ (C _i), stomatal conductance (g _s), and transpiration rate (E) were measured at C _a from 50 to 1 050 mol mol^–1. The cultivars responded similarly to C _a and did not differ in all the variables measured. The P _N increased with C _a until saturation at 650 mol mol^–1 (C _i= 350–400 mol mol^–1), thereafter P _N increased slightly. On average, increasing C _a from 350 to 650 and from 350 to 1 050 mol mol^–1 increased P _N by 43 and 52 %, respectively. The P _N vs. C _i curves were fitted to a non-rectangular hyperbola model. The cultivars did not differ in the parameters estimated from the model. Carboxylation efficiencies ranged from 68 to 91 mol m^–2 s^–1 and maximum P _N were 15.50, 13.52, 13.31, and 14.96 mol m^–2 s^–1 for Cascade, CC, Oranza, and RCC, respectively. Dark respiration rate varied from 2.80 mol m^–2 s^–1 for Oranza to 3.96 mol m^–2 s^–1 for Cascade and the CO₂ compensation concentration was between 42 and 46 mol mol^–1. The g _s and E increased to a peak at C _a= 350 mol mol^–1 and then decreased by 17 and 15 %, respectively when C _a was increased to 650 mol mol^–1. An increase from 350 to 1 050 mol mol^–1 reduced g _s and E by 53 and 47 %, respectively. Changes in g _s and P _N maintained the C _i:C _a ratio. The water use efficiency increased linearly with C _a due to increases in P _N in addition to the decline in E at high C _a. Hence CO₂ enrichment increases P _N and decreases g _s, and can improve carrot productivity and water conservation.