Asymmetric responses of adaxial and abaxial stomata to elevated CO2: impacts on the control of gas exchange by leaves

The response of adaxial and abaxial stomatal conductance in Rumex obtusifolius to growth at elevated atmospheric concentrations of CO₂ (250 μmol mol^?1 above ambient) was investigated over two growing seasons. The conductance of both the adaxial and abaxial leaf surfaces was found to be reduced by elevated concentrations of CO₂. Elevated CO₂ caused a much greater reduction in conductance for the adaxial surface than for the abaxial surface. The absence of effects upon stomatal density indicated that the reductions were probably the result of changes in stomatal aperture. Partitioning of gas exchange between the leaf surfaces revealed that increased concentrations of CO₂ caused increased rates of photosynthesis only via the abaxial surface. Additionally, leaf thickness was found to increase during growth at elevated concentrations of CO₂. The tendency for these amphistomatous leaves to develop a distribution of conductance approaching that of hypostomatous leaves clearly reduced their maximum photosynthetic potential. This conclusion was supported by measurements of stomatal limitation, which showed greater values for the adaxial surfaces, and greater values at elevated CO₂. This reduction in photosynthesis may in part be caused by higher diffusive limitations imposed because of increased leaf thickness. In an uncoupled canopy, asymmetrical stomatal responses of the kind identified here may appreciably reduce transpiration. Species which show symmetrical responses are less likely to show reduced transpirational rates, and a redistribution of water loss between species may occur. The implications of asymmetrical stomatal responses for photosynthesis and canopy transpiration are discussed.     

Photosynthetic CO2 uptake byZea mays leaves as influenced by unilateral irradiation of adaxial and abaxial leaf surfaces

A study was made on the effect of increasing photon fluence rate (I) at a unilateral irradiation of adaxial (normal leaf position) and abaxial (inverse leaf position) blade surface of maize leaves of various insertion levels on net photosynthetic CO₂ uptake (P _n ) by the leaves, as well as the contribution of individual surfaces toP _n of the leaves, and the significance of, or relationship between the stomatal (g _s ) and intracellular (gm) conductances at the CO₂ transport.P _n of leaves of various age according to their insertion level was unaffected by the direction of incident irradiation. Upon irradiation of the leaves in normal and inverse position the contribution of the adaxial and abaxial surfaces toP _n ,g _s and g_m was different. On irradiating the leaves in normal position, the contribution of the irradiated adaxial surface to the characteristics mentioned made on the average 55% of total values, the contribution of the abaxial surface irradiated in inverse position made on the average 70% inP _n andg _m , and 80% ing _s . At lowerI’s g _m was higher thang _s both in irradiated and non-irradiated surfaces. The ratio ofg _s to g_m gradually got square with increasingI. In the irradiated adaxial surface the equilibrium (g _s /g _m = 1.0) took place at the highestI’s, in the irradiated abaxial surface between 500 to 1000 μmol m⁻² s⁻¹. The significance of the ratiog _m in the CO₂ transport through the individual surfaces is discussed.     

Elevated CO<Subscript>2</Subscript> and temperature differentially affect photosynthesis and resource allocation in flag and penultimate leaves of wheat

Differences in acclimation to elevated growth CO₂ (700 μmol mol⁻¹, EC) and elevated temperature (ambient +4 °C, ET) in successive leaves of wheat were investigated in field chambers. At a common measurement CO₂, EC increased photosynthesis and the quantum yield of electron transport (Φ) early on in the growth of penultimate leaves, and later decreased them. In contrast, EC did not change photosynthesis, and increased Φ at later growth stages in the flag leaf. Contents of chlorophyll (Chl), ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO), and total soluble protein were initially higher and subsequently lower in penultimate than flag leaves. EC decreased RuBPCO protein content relative to soluble protein and Chl contents throughout the development of penultimate leaves. On the other hand, EC initially increased the RuBPCO:Chl and Chl a/b ratios, but later decreased them in flag leaves. In the flag leaves but not in the penultimate leaves, ET initially decreased initial and specific RuBPCO activities at ambient CO₂ (AC) and increased them at EC. Late in leaf growth, ET decreased Chl contents under AC in both kinds of leaves, and had no effect or a positive one under EC. Thus the differences between the two kinds of leaves were due to resource availability, and to EC-increased allocation of resources to photon harvesting in the penultimate leaves, but to increased allocation to carboxylation early on in growth, and to light harvesting subsequently, in the flag leaves.     

Photosynthetic acclimation to CO2 enrichment related to ribulose-1,5-bisphosphate carboxylation limitation in wheat

Net photosynthetic rate (P _N) measured at the same CO₂ concentration, the maximum in vivo carboxylation rate, and contents of ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (RuBPCO) and RuBPCO activase were significantly decreased, but the maximum in vivo electron transport rate and RuBP content had no significant change in CO₂-enriched [EC, about 200 μmol mol⁻¹ above the ambient CO₂ concentration (AC)] wheat leaves compared with those in AC grown wheat leaves. Hence photosynthetic acclimation in wheat leaves to EC is largely due to RuBP carboxylation limitation.     

Compensatory effects of elevated CO<Subscript>2</Subscript> concentration on the inhibitory effects of high temperature and irradiance on photosynthetic gas exchange in carrots

We determined the interactive effects of irradiance, elevated CO₂ concentration (EC), and temperature in carrot (Daucus carota var. sativus). Plants of the cv. Red Core Chantenay (RCC) were grown in a controlled environmental plant growth room and exposed to 3 levels of photosynthetically active radiation (PAR) (400, 800, 1 200 μmol m⁻² s⁻¹), 3 leaf chamber temperatures (15, 20, 30 °C), and 2 external CO₂ concentrations (C _a), AC and EC (350 and 750 μmol mol⁻¹, respectively). Rates of net photosynthesis (P _N) and transpiration (E) and stomatal conductance (g _s ) were measured, along with water use efficiency (WUE) and ratio of internal and external CO₂ concentrations (C _i/C _a). P _N revealed an interactive effect between PAR and C _a. As PAR increased so did P _N under both C _a regimes. The g _s showed no interactive effects between the three parameters but had singular effects of temperature and PAR. E was strongly influenced by the combination of PAR and temperature. WUE was interactively affected by all three parameters. Maximum WUE occurred at 15 °C and 1 200 μmol m⁻² s^{− 1} PAR under EC. The C _i /C _a was influenced independently by temperature and C _a. Hence photosynthetic responses are interactively affected by changes in irradiance, external CO₂ concentration, and temperature. EC significantly compensates the inhibitory effects of high temperature and irradiance on P _N and WUE.     

Ontogenetic changes in stomatal size and conductance of sunflowers

The ontogenetic changes in stomatal size, frequency and conductance (g_s) on abaxial and adaxial leaf surfaces of sunflower plants (Helianthus annuus L. Russian Mammoth) were examined under controlled environmental conditions. The stomatal frequency on the adaxial and abaxial leaf surfaces decreased with leaf ontogeny and insertion level. The ratio of adaxial to abaxial stomatal frequency did not change with leaf ontogeny and insertion level, and 42–44% of total stomata was apportioned to the adaxial surface. Ontogenetic changes in stomatal pore length were detected and increased with ontogenesis. The stomatal length of both leaf surfaces had linear relationships with leaf area. Ontogenetic changes in g_s were similar between the two surfaces. However the adaxial g_s was lower than abaxial g_s in leaves of higher insertion levels. Conductance had a linear relationship with width x frequency but not with pore area.     

Photosynthetic Characteristics of Sun and Shade Leaves in the Canopy of Arbutus unedo L. Trees Exposed to In Situ Long-Term Elevated CO2

Photosynthetic parameters were studied in Arbutus unedo L. trees growing at either ambient (AC) or elevated EC (mean 465 μmol mol^-1) CO₂ concentration near a natural CO₂ vent in Orciatico, Italy Diurnal courses of net photosynthetic rate (P _N), ratio of variable to maximum chlorophyll fluorescence (F_v/F_m), and quantum yield of electron transport through photosystem 2 (Φ₂) were measured on sun and shade leaves. The contents of N, C, Ca, K, P, and chlorophyll (Chl) and specific leaf area (SLA) in these leaf categories were also determined. A morning peak and midday depression of P _N were found for both AC and EC sun leaves. Long-term EC caused little or no down-acclimation of P _N in sum leaves. The estimate of total daily CO₂ uptake was lower in AC leaves than in EC leaves. In shade leaves, it reached up to 70 % of the value of sun leaves. The F_v/F_m ratio showed decreasing trend in the morning, reached a minimum at midday (90 % of dawn value), and then increased in the afternoon. The EC had no effect on F_v/F_m either in sun or shade leaves. Plants grown near the CO₂ spring had lower Chl content, higher SLA, and higher Ca and K contents than plants grown under AC. This revised version was published online in June 2006 with corrections to the Cover Date.     

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.     

Elevated CO2 concentration enhances the role of the ear to the flag leaf in determining grain yield of wheat

Net photosynthetic rate (P _N) of ear and flag leaf during grain filling stage and grain yield of plants with non-darkened or darkened flag leaf or darkened ear were examined in two different CO₂ concentrations: ambient (AC) and AC+200 μmol mol⁻¹ (EC). Ear showed much higher enhancement (56 %) of P _N than flag leaf (23 %) under EC. Moreover, CO₂ enrichment shortened the photosynthetic duration of flag leaf relative to ear. In this way the ratio of ear to flag leaf contribution to grain yield increased from 1.18 (AC) to 1.39 (EC).     

Photosynthetic response of wheat and sunflower cultivars to long-term exposure of elevated carbon dioxide concentration

Wheat (Triticum aestivum L. cv. HD 2329 and DL 1266-5) and sunflower (Helianthus annuus L. cv. MSFH 17 and MRSF 1754) plants were grown in field under atmospheric (360±10 cm³ m⁻³, AC) and elevated (650±50 cm³ m⁻³, EC) CO₂ concentrations in open top chambers for entire period of growth and development till maturity. Net photosynthetic rate (P _N) of wheat cvs. when compared at the same internal CO₂ concentration (C _i), by generating P _N/C _i curves, showed lower P _N in EC plants than in AC ones. EC-grown wheat cultivars also showed a lesser response to irradiance than AC plants. In sunflower cultivars, P _N/C _i curves and irradiance response curves were not significantly different in AC and EC plants. CO₂ and irradiance responses of photosynthesis, therefore, further revealed a down-regulation of _{P N} in wheat but not so in sunflower under long-term CO₂ enrichment. Wheat cvs. accumulated in leaves mostly sugars, whereas sunflower accumulated mainly starch. This further strengthened the view that accumulation of excess assimilates in the leaves under EC as starch is not inhibitory to _{P N}.     

Intrinsic changes in photosynthetic parameters of carrot leaves under increasing CO<Subscript>2</Subscript> concentrations and soil moisture regimes

A controlled growth chamber experiment was conducted to investigate the short-term water use and photosynthetic responses of 30-d-old carrot seedlings to the combined effects of CO₂ concentration (50–1 050 μmol mol⁻¹) and moisture deficits (−5, −30, −55, and −70 kPa). The photosynthetic response data was fitted to a non-rectangular hyperbola model. The estimated parameters were compared for effects of moisture deficit and elevated CO₂ concentration (EC). The carboxylation efficiency (α) increased in response to mild moisture stress (−30 kPa) under EC when compared to the unstressed control. However, moderate (−55 kPa) and extreme (−70 kPa) moisture deficits reduced α under EC. Maximum net photosynthetic rate (P _Nmax) did not differ between mild water deficit and unstressed controls under EC. Moderate and extreme moisture deficits reduced P _Nmax by nearly 85 % compared to controls. Dark respiration rate (R _D) showed no consistent response to moisture deficit. The CO₂ compensation concentration (Γ) was 324 μmol mol⁻¹ for −75 kPa and ranged 63–93 μmol mol⁻¹ for other moisture regimes. Interaction between moisture deficit and EC was noticed for P _N, ratio of intercellular and ambient CO₂ concentration (C _i/C _a), stomatal conductance (g _s ), and transpiration rate (E). P _N was maximum and C _i/C _a was minimum at −30 kPa moisture deficit and at C _a of 350 μmol mol⁻¹. The g _s and E showed an inverse relationship at all moisture deficit regimes and EC. Water use efficiency (WUE) increased with moisture deficit up to −55 kPa and declined thereafter. EC showed a positive influence towards sustaining P _N and increasing WUE only under mild moisture stress, and no beneficial effects of EC were noticed at moderate or extreme moisture deficits.     

Responses of Two Olive Tree (Olea Europaea L.) Cultivars to Elevated CO2 Concentration in the Field

Five-year-old plants of two olive cultivars (Frantoio and Moraiolo) grown in large pots were exposed for 7 to 8 months to ambient (AC) or elevated (EC) CO₂ concentration in a free-air CO₂ enrichment (FACE) facility. Exposure to EC enhanced net photosynthetic rate (P _N) and decreased stomatal conductance, leading to greater instantaneous transpiration efficiency. Stomata density also decreased under EC, while the ratio of intercellular (C _i) to atmospheric CO₂ concentration and chlorophyll content did not differ, except for the cv. Moraiolo after seven months of exposure to EC. Analysis of the relationship between photosynthesis and C _i indicated no significant change in carboxylation efficiency of ribulose-1,5-bisphosphate carboxylase/oxygenase after five months of exposure to EC. Based on estimates derived from the P _N-C _i relationship, there were no apparent treatment differences in daytime respiration, CO₂ compensation concentration, CO₂-saturated photosynthetic rate, or photosynthetic rate at the mean C _i, but there was a reduction in stomata limitation to P _N at EC. Thus 5-year-old olive trees did not exhibit down regulation of leaf-level photosynthesis in their response to EC, though some indication of adjustment was evident for the cv. Frantoio with respect to the cv. Moraiolo.     

Photosynthesis and fluorescence responses of C<Subscript>4</Subscript> plant <Emphasis Type="Italic">Andropogon gerardii</Emphasis> acclimated to temperature and carbon dioxide

Increase in both atmospheric CO₂ concentration [CO₂] and associated warming are likely to alter Earths’ carbon balance and photosynthetic carbon fixation of dominant plant species in a given biome. An experiment was conducted in sunlit, controlled environment chambers to determine effects of atmospheric [CO₂] and temperature on net photosynthetic rate (P _N) and fluorescence (F) in response to internal CO₂ concentration (C _i) and photosynthetically active radiation (PAR) of the C₄ species, big bluestem (Andropogon gerardii Vitman). Ten treatments were comprised of two [CO₂] of 360 (ambient, AC) and 720 (elevated, EC) μmol mol⁻¹ and five day/night temperature of 20/12, 25/17, 30/22, 35/27 and 40/32 °C. Treatments were imposed from 15 d after sowing (DAS) through 130 DAS. Both F-P _N/C _i and F-P _N/PAR response curves were measured on top most fully expanded leaves between 55 and 75 DAS. Plants grown in EC exhibited significantly higher CO₂-saturated net photosynthesis (P _sat), phosphoenolpyruvate carboxylase (PEPC) efficiency, and electron transport rate (ETR). At a given [CO₂], increase in temperature increased P _sat, PEPC efficiency, and ETR. Plants grown at EC did not differ for dark respiration rate (R _D), but had significantly higher maximum photosynthesis (P _max) than plants grown in AC. Increase in temperature increased Pmax, R _D, and ETR, irrespective of the [CO₂]. The ability of PEPC, ribulose-1,5-bisphosphate carboxylase/oxygenase, and photosystem components, derived from response curves to tolerate higher temperatures (>35 °C), particularly under EC, indicates the ability of C₄ species to sustain photosynthetic capacity in future climates.     

Relationships between the CO2 compensation concentration,the slope of CO2 curves of net photosynthetic rate and the energy of irradiance

Relationships between the CO₂ compensation concentration, the slope of CO₂ curves of P_n and the energy of irradiance were studied in adult leaves ofFagus silvatica L. seedlings. A gazometric method of quantitative analysis of air flow, in a closed system, was applied. P_n and the slope of CO₂ curve of P_n increased with increasing irradiance at constant temperature of the abaxial leaf surface (19 ± 0.5 °C), however, the CO₂ compensation concentration decreased. This physiological characteristic changed relatively slightly at irradiance close to saturation irradiance (235 Wm⁻²).     

The Effect of CO2 Enrichment on the Growth of Nodulated and Non-Nodulated Isogenic Types of Soybean Raised Under Two Nitrogen Concentrations

To find the effects of CO₂ enrichment on plant development and photosynthetic capacity of nodulated (line A62-1) and non-nodulated (line A62-2) isogenic lines of soybean (Glycine max Merr.), we examined the interactions among two CO₂ treatments (36±3 Pa = AC and 70±5 Pa = EC), and two nitrogen concentrations [0 g(N) m⁻²(land area) = 0N; 30 g(N) m⁻²(land area) = 30N]. Nodules were found in both CO₂ treatments in 0N of A62-1 where the number and dry mass of nodules increased from AC to EC. While the allocation of dry mass to root and shoot and the amount of N in each organ did not differ between the growth CO₂ concentrations, there was larger N allocation to roots in 0N than in 30N for A62-2. The CO₂-dependence of net photosynthetic rate (P _N) for A62-1 was unaffected by both CO₂ and N treatments. In contrast, the CO₂-dependence of P _N was lower in 0N than in 30N for A62-2, but it was independent of CO₂ treatment. P _N per unit N content was unaffected by CO₂ concentrations. The leaf area of both soybean lines grown in 30N increased in EC. But in 0N, only the nodulated A62-1 showed an increase in leaf area in EC. Nitrogen use efficiency of plants, NUE [(total dry mass of the plant)/(amount of N accumulated in the plant)] in 30N was unaffected by CO₂ treatments. In 0N, NUE in EC was lower than in AC in A62-1, and was higher than that at AC in A62-2. Hence, the larger amount and/or rate of N fixation with the increase of the sink-size of symbiotic microorganisms supplied adequate N to the plant under EC. In EC, N deficiency caused the down-regulation of the soybean plant. This revised version was published online in June 2006 with corrections to the Cover Date.     

Photosynthetic Response of Carrots to Varying Irradiances

Response to irradiance of leaf net photosynthetic rates (P _N) of four carrot cultivars: Cascade, Caro Choice (CC), Oranza, and Red Core Chantenay (RCC) were examined in a controlled environment. Gas exchange measurements were conducted at photosynthetic active radiation (PAR) from 100 to 1 000 μmol m⁻² s⁻¹ at 20 °C and 350 μmol (CO₂) mol⁻¹(air). The values of P _N were fitted to a rectangular hyperbolic nonlinear regression model. P _N for all cultivars increased similarly with increasing PAR but Cascade and Oranza generally had higher P _N than CC. None of the cultivars reached saturation at 1 000 μmol m⁻² s⁻¹. The predicted P _N at saturation (P _Nmax) for Cascade, CC, Oranza, and RCC were 19.78, 16.40, 19.79, and 18.11 μmol (CO₂) m⁻² s⁻¹, respectively. The compensation irradiance (I _c) occurred at 54 μmol m⁻² s⁻¹ for Cascade, 36 μmol m⁻² s⁻¹ for CC, 45 μmol m⁻² s⁻¹ for Oranza, and 25 μmol m⁻² s⁻¹ for RCC. The quantum yield among the cultivars ranged between 0.057–0.033 mol(CO₂) mol⁻¹(PAR) and did not differ. Dark respiration varied from 2.66 μmol m⁻² s⁻¹ for Cascade to 0.85 μmol m⁻² s⁻¹ for RCC. As P _N increased with PAR, intercellular CO₂ decreased in a non-linear manner. Increasing PAR increased stomatal conductance and transpiration rate to a peak between 600 and 800 μmol m⁻² s⁻¹ followed by a steep decline resulting in sharp increases in water use efficiency. This revised version was published online in August 2006 with corrections to the Cover Date.     

Photosynthetic Acclimation to Elevated CO2 in Relation to Leaf Saccharide Constituents in Wheat and Sunflower

Wheat (T. durum cvs. HD 4502 and B 449, T. aestivum cvs. Kalyansona and Kundan) and sunflower (Helianthus annuus L. cv. Morden) were grown under atmospheric (360±10 cm³ m^–3, AC) and elevated CO₂ (650±50 cm³ m^–3, EC) concentration in open top chambers for entire period of growth and development till maturity. Leaf net photosynthetic rate (P _N) of EC-grown plants of wheat measured at EC was significantly decreased in comparison with AC-plants of wheat measured at EC. Sunflower, however, showed no significant depression in P _N in EC-plants. There was a decrease in ribulose-1,5-bisphosphate carboxylase (RuBPC) activity, its activation state and amount in EC-plants of wheat, whereas no significant decrease was observed in sunflower. The above different acclimation to EC in wheat and sunflower was related with saccharide constituents accumulated in the leaves. Under EC, sunflower accumulated in the leaves more starch, whereas wheat accumulated more sugars.     

Nitrogen and carbon partitioning in soybean under variable nitrogen supplies and acclimation to the prolonged action of elevated CO2

This study was conducted to determine reciprocal effects of low to high doses of nitrogenous fertilizer (N₃₀, N₄₀, N₅₀, N₆₀ and N₇₀ — 30, 40, 50, 60 and 70 kg ha⁻¹ respectively) and CO₂ enriched environment on C and N partitioning in soybean (Glycine max (L.) Merril cv JS-335). Plants were grown from seedling emergence to maturity inside open top chambers under ambient, AC (350±50 mol mol⁻¹) and elevated, EC (600±50 mol mol⁻¹) CO₂ and analyzed at seedling, vegetative, flowering, pod setting and maturity stages. Soybean responded to both CO₂ enrichment and N supply. Leaves, stem and root reserves at different growth stages were analyzed for total C and N contents. Consistent increase in the C contents of the leaf, stem and root was observed under EC than in AC. N contents in the different plant parts were found to be decreased under EC-grown plants specially at seedling and vegetative stage despite providing N doses to the soil. Significant increase observed for C to N dry mass ratio under EC in the root, stems and leaves at seedling and vegetative stage was decreased in the middle and later growth stages possibly due to combined impact of N doses to the soil and increased N₂ fixing activities due to EC conditions. Critical analysis of our findings reveals that the composition and partitioning of C and N of soybean under variable rates of N supply and CO₂ enrichment alter according to need under altered metabolic process. These changes eventually may lead to alteration in uptake of not only N but other essential nutrients also under changing atmosphere.     

Photosynthesis and nutrient composition of spinach and fenugreek grown under elevated carbon dioxide concentration

The effect of elevated carbon dioxide concentration on the changes in the biomass, photosynthesis and nutrient composition was investigated in two leafy vegetables. Spinach (Spinacia oleracea L.) and fenugreek (Trigonella foenum-graecum L.) plants were grown in open top chambers under either ambient (ACO₂, 350 ± 50 μmol mol⁻¹) or elevated (ECO₂, 600 ± 50 μmol mol⁻¹) CO₂ concentration and analyzed 40, 60 and 80 days after exposure. The plants grown in ECO₂ had higher net photosynthetic rate and lower stomatal conductance when compared with the plants grown in ACO₂. ECO₂ also changed the nutrient composition: a lower N, Mg and Fe contents and higher C and Ca contents were observed in the leaves of plants exposed to ECO₂ than in those grown at ACO₂.