Effect of abscisic Acid on the gain of the feedback loop involving carbon dioxide and stomata

Gains of the feedback loops involving intercellular CO₂ concentration on one hand, and CO₂ assimilation and stomata on the other (= assimilation loop with gain [G_A] and conductance loop with gain [G_g]) were determined in detached leaves of Amaranthus powelli S. Wats., Avena sativa L., Gossypium hirsutum L., Xanthium strumarium L., and Zea mays in the absence and presence of 10⁻⁵ m (±) abscisic acid (ABA) in the transpiration stream. Determinations were made for an ambient CO₂ concentration of 300 microliters per liter. In the absence of ABA, stomata were insensitive to CO₂ (G_g between 0.00 and −0.02) in A. sativa, G. hirsutum, and X. strumarium, sensitive in A powelli (G_g = −0.46), and very sensitive in Z. mays (G_g = −3.6). Addition of ABA increased the absolute values of the gain of the conductance loop in A. powelli (G_g = −2.0), G. hirsutum (G_g = −0.31), and X. strumarium (G_g = −1.14). Stomata closed completely in A. sativa. In Z. mays, G_g decreased after application of ABA to a value of −0.86, but stomatal sensitivity to CO₂ increased for intercellular CO₂ concentrations < 100 microliters per liter. The gain of the assimilation loop increased after application of ABA in all cases, from values between 0.0 (A. powelli) and −0.21 (Z. mays) in the absence of ABA to values between −0.19 (A. powelli) and −0.43 (Z. mays) in the presence of ABA. In none of the species examined did ABA affect the photosynthetic capacity of the leaves.     

Internal CO(2) Measured Directly in Leaves : Abscisic Acid and Low Leaf Water Potential Cause Opposing Effects

Observations of nonuniform photosynthesis across leaves cast doubt on internal CO₂ partial pressures (p_i) calculated on the assumption of uniformity and can lead to incorrect conclusions about the stomatal control of photosynthesis. The problem can be avoided by measuring p_i directly because the assumptions of uniformity are not necessary. We therefore developed a method that allowed p_i to be measured continuously in situ for days at a time under growth conditions and used it to investigate intact leaves of sunflower (Helianthus annuus L.), soybean (Glycine max L. Merr.), and bush bean (Phaseolus vulgaris L.) subjected to high or low leaf water potentials (ψ_w) or high concentrations of abscisic acid (ABA). The leaves maintained a relatively constant differential (Δp) between ambient CO₂ and measured p_i throughout the light period when water was supplied. When water was withheld, ψ_w decreased and the stomata began to close, but measured p_i increased until the leaf reached a ψ_w of −1.76 (bush bean), −2.12 (sunflower) or −3.10 (soybean) megapascals, at which point Δp = 0. The increasing p_i indicated that stomata did not inhibit CO₂ uptake and a Δp of zero indicated that CO₂ uptake became zero despite the high availability of CO₂ inside the leaf. In contrast, when sunflower leaves at high ψ_w were treated with ABA, p_i did not increase and instead decreased rapidly and steadily for up to 8 hours even as ψ_w increased, as expected if ABA treatment primarily affected stomatal conductance. The accumulating CO₂ at low ψ_w and contrasting response to ABA indicates that photosynthetic biochemistry limited photosynthesis at low ψ_w but not at high ABA.     

ABA and BAP improve the accumulation of carbohydrates and alter carbon allocation in potato plants at elevated CO2

Elevated CO₂ interactions with other factors affects the plant performance. Regarding the differences between cultivars in response to CO₂ concentrations, identifying the cultivars that better respond to such conditions would maximize their potential benefits. Increasing the ability of plants to benefit more from elevated CO₂ levels alleviates the adverse effects of photoassimilate accumulation on photosynthesis and increases the productivity of plants. Despite its agronomic importance, there is no information about the interactive effects of elevated CO₂ concentration and plant growth regulators (PGRs) on potato (Solanum tuberosum L.) plants. Hence, the physiological response and source-sink relationship of potato plants (cvs. Agria and Fontane) to combined application of CO₂ levels (400 vs. 800 µmol mol⁻¹) and plant growth regulators (PGR) [6-benzylaminopurine (BAP) + Abscisic acid (ABA)] were evaluated under a controlled environment. The results revealed a variation between the potato cultivars in response to a combination of PGRs and CO₂ levels. Cultivars were different in leaf chlorophyll content; Agria had higher chlorophyll a, b, and total chlorophyll content by 23, 43, and 23%, respectively, compared with Fontane. The net photosynthetic rate was doubled at the elevated compared with the ambient CO₂. In Agria, the ratio of leaf intercellular to ambient air CO₂ concentrations [C_i:C_a] was declined in elevated-CO₂-grown plants, which indicated the stomata would become more conservative at higher CO₂ levels. On the other hand, the increased C_i:C_a in Fontane showed a stomatal acclimation to higher CO₂ concentration. The higher leaf dark respiration of the elevated CO₂-grown and BAP + ABA-treated plants was associated with a higher leaf soluble carbohydrates and starch content. Elevated CO₂ and BAP + ABA shifted the dry matter partitioning to the belowground more than the above-media organs. The lower leaf soluble carbohydrate content and greater tuber yield in Fontane might indicate a more efficient photoassimilate translocation than Agria. The results highlighted positive synergic effects of the combined BAP + ABA and elevated CO₂ on tuber yield and productivity of the potato plants.     

Limitations of Photosynthesis in Pinus taeda L. (Loblolly Pine) at Low Soil Temperatures

The relative importance of stomatal and nonstomatal limitations to net photosynthesis (A) and possible signals responsible for stomatal limitations were investigated in unhardened Pinus taeda seedlings at low soil temperatures. After 2 days at soil temperatures between 13 and 7°C, A was reduced by 20 to 50%, respectively. The reduction in A at these moderate root-chilling conditions appeared to be the result of stomatal limitations, based on the decrease in intercellular CO₂ concentrations (c_i). This conclusion was supported by A versus c_i analysis and measurements of O₂ evolution at saturating CO₂, which suggested increases in stomatal but not biochemical limitations at these soil temperatures. Nonuniform stomatal apertures, which were demonstrated with abscisic acid, were not apparent 2 days after root chilling, and results of our A versus c_i analysis appear valid. Bulk shoot water potential (ψ) declined as soil temperature dropped below 16°C. When half the root system of seedlings was chilled, shoot ψ and gas-exchange rates did not decline. Thus, nonhydraulic root-shoot signals were not implicated in stomatal limitations. The initial decrease in leaf conductance to water vapor after root chilling appeared to precede any detectable decrease in bulk fascicle ψ, but may be in response to a decrease in turgor of epidermal cells. These reductions in leaf conductance to water vapor, which occurred within 30 minutes of root chilling, could be delayed and temporarily reversed by reducing the leaf-to-air vapor-pressure deficit, suggesting that hydraulic signals may be involved in initiating stomatal closure. By independently manipulating the leaf-to-air vapor-pressure deficit of individual fascicles, we could induce uptake of water vapor through stomata, suggesting that nonsaturated conditions occur in the intercellular airspaces. There was an anomaly in our results on seedlings maintained for 2 days at soil temperatures below 7°C. Lower A appeared primarily the result of nonstomatal limitations, based on large increases in calculated c_i and A versus c_i analysis. In contrast, measurements of O₂ evolution at saturating CO₂ concentrations implied nonstomatal limitations per se did not increase at these temperatures. One explanation for this paradox is that calculations of c_i are unreliable at very low gas-exchange rates because of inadequate measurement resolution, and limitations of A are predominantly stomatal. An alternative interpretation is that increases in c_i are real and the results from O₂-evolution measurements are in error. The high CO₂ concentration used in O₂-evolution measurements (15%) may have overcome nonstomatal limitations by enzymes that were down-regulated by a feedback mechanism. In this scenario, carbohydrate feedback limitations may be responsible for nonstomatal reductions in A after 2 days at soil temperatures below 7°C.     

Leaf photosynthesis and conductance of selected triticum species at different water potentials

Leaf gas exchange characteristics of a desert annual (Triticum kotschyi [Boiss.] Bowden) and the wheat cultivar TAM W-101 (Triticum aestivum L. em Thell) were compared over a range of leaf water potentials from −0.50 to −2.9 megapascals. At an ambient [CO₂] of 330 microliters per liter, T. kotschyi had higher conductance and CO₂ assimilation (A) at a given water potential than T. aestivum. Under well watered conditions, A versus internal CO₂ concentration (C_i) response curves for both species were similar in shape and magnitude, and the higher A of T. kotschyi at an ambient [CO₂] of 330 microliters per liter was mostly related to the higher stomatal conductance of T. kotschyi. The higher conductance of T. kotschyi than T. aestivum under well watered conditions was associated with higher C_i and lower water use efficiency. Under water deficits, however, C_i at 330 microliters per liter ambient [CO₂] did not differ significantly between species. T. kotschyi had higher A under water deficits than T. aestivum primarily because its A versus C_i response curves had higher A at C_i values above about 150 microliters per liter. The results show that conductance played an important role in the high A of T. kotschyi under well watered conditions, but under water deficits the high A of T. kotschyi was related more to the maintenance of a higher capacity for mesophyll photosynthesis.     

Photosynthesis and Carbon Partitioning in Transgenic Tobacco Plants Deficient in Leaf Cytosolic Pyruvate Kinase

Whole-plant diurnal C exchange analysis provided a noninvasive estimation of daily net C gain in transgenic tobacco (Nicotiana tabacum L.) plants deficient in leaf cytosolic pyruvate kinase (PK_c−). PK_c− plants cultivated under a low light intensity (100 μmol m⁻² s⁻¹) were previously shown to exhibit markedly reduced root growth, as well as delayed shoot and flower development when compared with plants having wild-type levels of PK_c (PK_c+). PK_c− and PK_c+ source leaves showed a similar net C gain, photosynthesis over a range of light intensities, and a capacity to export newly fixed ¹⁴CO₂ during photosynthesis. However, during growth under low light the nighttime, export of previously fixed ¹⁴CO₂ by fully expanded PK_c− leaves was 40% lower, whereas concurrent respiratory ¹⁴CO₂ evolution was 40% higher than that of PK_c+ leaves. This provides a rationale for the reduced root growth of the PK_c− plants grown at low irradiance. Leaf photosynthetic and export characteristics in PK_c− and PK_c+ plants raised in a greenhouse during winter months resembled those of plants grown in chambers at low irradiance. The data suggest that PK_c in source leaves has a critical role in regulating nighttime respiration particularly when the available pool of photoassimilates for export and leaf respiratory processes are low.     

Controls of Evapotranspiration and CO2 Fluxes from Scots Pine by Surface Conductance and Abiotic Factors

Evapotranspiration (E) and CO₂ flux (F_c) in the growing season of an unusual dry year were measured continuously over a Scots pine forest in eastern Finland, by eddy covariance techniques. The aims were to gain an understanding of their biological and environmental control processes. As a result, there were obvious diurnal and seasonal changes in E, F_c, surface conductance (g_c), and decoupling coefficient (Ω), showing similar trends to those in radiation (PAR) and vapour pressure deficit (δ). The maximum mean daily values (24-h average) for E, F_c, g_c, and Ω were 1.78 mmol m⁻² s⁻¹, −11.18 µmol m⁻² s⁻¹, 6.27 mm s⁻¹, and 0.31, respectively, with seasonal averages of 0.71 mmol m⁻² s⁻¹, −4.61 µmol m⁻² s⁻¹, 3.3 mm s⁻¹, and 0.16. E and F_c were controlled by combined biological and environmental variables. There was curvilinear dependence of E on g_c and F_c on g_c. Among the environmental variables, PAR was the most important factor having a positive linear relationship to E and curvilinear relationship to F_c, while vapour pressure deficit was the most important environmental factor affecting g_c. Water use efficiency was slightly higher in the dry season, with mean monthly values ranging from 6.67 to 7.48 μmol CO₂ (mmol H₂O)⁻¹ and a seasonal average of 7.06 μmol CO₂ (μmol H₂O)⁻¹. Low Ω and its close positive relationship with g_c indicate that evapotranspiration was sensitive to surface conductance. Mid summer drought reduced surface conductance and decoupling coefficient, suggesting a more biotic control of evapotranspiration and a physiological acclimation to dry air. Surface conductance remained low and constant under dry condition, supporting that a constant value of surface constant can be used for modelling transpiration under drought condition.     

Correlation of Stomatal Conductance with Photosynthetic Capacity of Cotton Only in a CO2-Enriched Atmosphere: Mediation by Abscisic Acid?

Some evidence indicates that photosynthetic rate (A) and stomatal conductance (g) of leaves are correlated across diverse environments. The correlation between A and g has led to the postulation of a “messenger” from the mesophyll that directs stomatal behavior. Because A is a function of intercellular CO₂ concentration (c_i), which is in turn a function of g, such a correlation may be partially mediated by c_i if g is to some degree an independent variable. Among individual sunlit leaves in a cotton (Gossypium hirsutum L.) canopy in the field, A was significantly correlated with g (r² = 0.41, n = 63). The relative photosynthetic capacity of each leaf was calculated as a measure of mesophyll properties independent of c_i. This approach revealed that, in the absence of c_i effects, mesophyll photosynthetic capacity was unrelated to g (r² = 0.06). When plants were grown in an atmosphere enriched to about 650 microliters per liter of CO₂, however, photosynthetic capacity remained strongly correlated with g even though the procedure discounted any effect of variable c_i. This “residual” correlation implies the existence of a messenger in CO₂-enriched plants. Enriched CO₂ also greatly increased stomatal response to abscisic acid (ABA) injected into intact leaves. The data provide no evidence for a messenger to coordinate g with A at ambient levels of CO₂. In a CO₂-enriched atmosphere, though, ABA may function as such a messenger because the sensitivity of the system to ABA is enhanced.     

Separation and measurement of direct and indirect effects of light on stomata

Conductance for water vapor, assimilation of CO₂, and intercellular CO₂ concentration of leaves of five species were determined at various irradiances and ambient CO₂ concentrations. Conductance and assimilation were then plotted as functions of irradiance and intercellular CO₂ concentration. The slopes of these curves allowed us to estimate infinitesimal changes in conductance (and assimilation) that occurred when irradiance changed and intercellular CO₂ concentration was constant, and when CO₂ concentration changed and irradiance was constant. On leaves of Xanthium strumarium L., Gossypium hirsutum L., Phaseolus vulgaris L., and Perilla frutescens (L.), Britt., the stomatal response to light was determined to be mainly a direct response to light and to a small extent only a response to changes in intercellular CO₂ concentration. This was also true for stomata of Zea mays L., except at irradiances < 150 watts per square meter, when stomata responded primarily to the depletion of the intercellular spaces of CO₂ which in turn was caused by changes in the assimilation of CO₂.     

The boundary layer and the apparent responses of stomatal conductance to wind speed and to the mole fractions of CO2 and water vapour in the air

The experiments and simulations reported in this paper show that, for stomata sensitive to both CO₂ and water vapour concentrations, responses of stomatal conductance (g^w_s) to boundary layer thickness have two components, one resulting from changes in intercellular CO₂ concentration (χ^c_i) and another from changes in leaf surface water vapour saturation deficit (D^w_s). The experiments and simulations also show that the boundary layer conductance (g^w_b) can significantly alter the apparent response of g^w_s to ambient air CO₂ mole fraction (χ^c_a) and water vapour mole fraction (χ^w_a). Because of the feedback loop involved the responses of g^w_s for χ^c_a and χ^w_a each include responses to both χ^c_i and D^w_s. The boundary layer alters the state of the variables sensed by the guard cells—i.e. χ^c_i and D^w_s—and so it is a source of feedback. Thus, when scaling up from responses of stomata to the response of g^w_s for a whole leaf, the effect of the boundary layer must be considered. The results indicate that, for given responses of g^w_s to χ^c_i and D^w_s, the apparent responses of g^w_s to D^w_a and χ^c_a depend on the size of the leaf and wind speed, showing that this effect of the boundary layer should be considered when comparing data measured under different conditions, or with different methods.     

Carbon Dioxide and Light Responses of Photosynthesis in Cowpea and Pigeonpea during Water Deficit and Recovery

Greenhouse-grown pigeonpea (Cajanus cajan, [L.] Millsp.; cultivar UW-10) and cowpea (Vigna unguiculata, [L.] Walp.; cultivar California No. 5) were well-watered (control) or subjected to low water potential by withholding water to compare their modes of adaptation to water-limited conditions. Leaf CO₂ exchange rate (CER), leaf diffusive conductance to CO₂ (g_l), and CO₂ concentration in the leaf intercellular air space (C_i) were determined at various CO₂ concentrations and photon flux densities (PFD) of photosynthetically active radiation (400 to 700 nanometer). In cowpea, g_l declined to less than 15% of controls and total water potential (ψ_w) at midafternoon declined to −0.8 megapascal after 5 days of withholding water, whereas g_l in pigeonpea was about 40% of controls even though midafternoon ψ_w was −1.9 megapascal. After 8 days of withholding water, ψ_w at midafternoon declined to −0.9 and −2.4 megapascals in cowpea and pigeonpea, respectively. The solute component of water potential (ψ_s) decreased substantially less in cowpea than pigeonpea. Photosynthetic CER at saturation photon flux density (PFD) and ambient external CO₂ concentration (360 microliters per liter) on day 5 of withholding decreased by 83 and 55% in cowpea and pigeonpea, respectively. When measured at external, CO₂ concentration in bulk air of 360 microliters per liter, the CER of cowpea had fully recovered to control levels 3 days after rewatering; however, at 970 microliters per liter the PFD-saturated CERs of both species were substantially lower than in controls, indicating residual impairment. In stressed plants of both species the CER responses to C_i from 250 to 600 microliters per liter indicated that a substantial nonstomatal inhibition of CER had occurred. Although the sensitivity of g_l to water limitation in cowpea suggested a dehydration avoidance response, parallel measurements of CER at various C_i and PFD indicated that photosynthetic activity of cowpea mesophyll was substantially inhibited by the water-limited treatment.     

Ethoxyzolamide Inhibition of CO(2) Uptake in the Cyanobacterium Synechococcus PCC7942 without Apparent Inhibition of Internal Carbonic Anhydrase Activity

In high inorganic carbon grown (1% CO₂ [volume/volume]) cells of the cyanobacterium Synechococcus PCC7942, the carbonic anhydrase (CA) inhibitor, ethoxyzolamide (EZ), was found to inhibit the rate of CO₂ uptake and to reduce the final internal inorganic carbon (C_i) pool size reached. The relationship between CO₂ fixation rate and internal C_i concentration in high C_i grown cells was little affected by EZ. This suggests that in intact cells internal CA activity was unaffected by EZ. High C_i grown cells readily took up CO₂ but had little or no capacity for HCO₃⁻ uptake. These cells appear to possess a CO₂ utilizing C_i pump that has a CA-like function associated with the transport step such that HCO₃⁻ is the species delivered to the cell interior. This CA-like step may be the site of inhibition by EZ. Low C_i grown cells possess both CO₂ uptake and HCO₃⁻ uptake activities and EZ inhibited both activities to a similar degree, suggesting that a common step in CO₂ and HCO₃⁻ uptake (such as the C_i pump) may have been affected. The inhibitor had no apparent effect on internal CO₂/HCO₃⁻ equilibria (internal CA function) in low C_i grown cells.     

The Stoichiometry between CO(2) and H Fluxes Involved in the Transport of Inorganic Carbon in Cyanobacteria

The pH of the medium during CO₂ uptake into the intracellular inorganic carbon (C_i) pool of a high CO₂-requiring mutant (E₁) and wild type of Anacystis nidulans R2 was measured. Experiments were performed under conditions where photosynthetic CO₂ fixation is inhibited. There was an acidification of the medium during CO₂ uptake in the light and an alkalization during CO₂ efflux after darkening. A one to one stoichiometry existed between the amounts of H⁺ appearing in the medium and CO₂ taken up into the intracellular C_i pool, regardless of the carbon species transported. The results indicate that (a) CO₂ is taken up simultaneously with an efflux of equimolar H⁺, probably produced as a result of CO₂ hydration during transport and (b) HCO₃⁻ produced by hydration of CO₂ in the medium was transported into the cells without accompanying net flux of H⁺ or OH⁻. The influx and efflux of C_i during C_i transport produced nonequilibrium between CO₂ and HCO₃⁻ in the medium, with the concentration of HCO₃⁻ being higher than that expected under equilibrium conditions. The nonequilibrium was present even under the conditions where the influx of C_i is compensated by its efflux. The direction of this nonequilibrium suggested that efflux of HCO₃⁻ occurs during uptake of C_i.     

Intracellular Carbonic Anhydrase Activity Sensitizes Cancer Cell pH Signaling to Dynamic Changes in CO2 Partial Pressure

Carbonic anhydrase (CA) enzymes catalyze the chemical equilibration among CO₂, HCO₃⁻ and H⁺. Intracellular CA (CA_i) isoforms are present in certain types of cancer, and growing evidence suggests that low levels correlate with disease severity. However, their physiological role remains unclear. Cancer cell CA_i activity, measured as cytoplasmic CO₂ hydration rate (k_f), ranged from high in colorectal HCT116 (∼2 s⁻¹), bladder RT112 and colorectal HT29, moderate in fibrosarcoma HT1080 to negligible (i.e. spontaneous k_f = 0.18 s⁻¹) in cervical HeLa and breast MDA-MB-468 cells. CA_i activity in cells correlated with CAII immunoreactivity and enzymatic activity in membrane-free lysates, suggesting that soluble CAII is an important intracellular isoform. CA_i catalysis was not obligatory for supporting acid extrusion by H⁺ efflux or HCO₃⁻ influx, nor for maintaining intracellular pH (pH_i) uniformity. However, in the absence of CA_i activity, acid loading from a highly alkaline pH_i was rate-limited by HCO₃⁻ supply from spontaneous CO₂ hydration. In solid tumors, time-dependence of blood flow can result in fluctuations of CO₂ partial pressure (pCO₂) that disturb cytoplasmic CO₂-HCO₃⁻-H⁺ equilibrium. In cancer cells with high CA_i activity, extracellular pCO₂ fluctuations evoked faster and larger pH_i oscillations. Functionally, these resulted in larger pH-dependent intracellular [Ca²⁺] oscillations and stronger inhibition of the mTORC1 pathway reported by S6 kinase phosphorylation. In contrast, the pH_i of cells with low CA_i activity was less responsive to pCO₂ fluctuations. Such low pass filtering would “buffer” cancer cell pH_i from non-steady-state extracellular pCO₂. Thus, CA_i activity determines the coupling between pCO₂ (a function of tumor perfusion) and pH_i (a potent modulator of cancer cell physiology).     

Resistance Analysis of Nocturnal Carbon Dioxide Uptake by a Crassulacean Acid Metabolism Succulent, Agave deserti

Nocturnal CO₂ uptake by a Crassulacean acid metabolism succulent, Agave deserti Engelm. (Agavaceae), was measured so that the resistance properties of the mesophyll chlorenchyma cells and their CO₂ concentrations could be determined. Two equivalents of acidity were produced at night per mole of CO₂ taken up. The nocturnal CO₂ uptake became light-saturated at 3.5 mEinsteins cm⁻² of photosynthetically active radiation (400-700 nm) incident during the preceding day; at least 46 Einsteins were required per mole of CO₂ fixed. Variations in the daytime leaf temperature between 20 and 37 C had little effect on nocturnal CO₂ uptake. After the first few hours in the dark, the leaf liquid phase CO₂ resistance (r^liq_CO2) and the CO₂ concentration in the chlorenchyma cells (cⁱ_CO2) both increased, the latter usually reaching the ambient external CO₂ level at the end of the dark period. Increasing the leaf surface temperature above 15 C at night markedly increased the stomatal resistance, r^liq_CO2, and cⁱ_CO2.

The minimum r^liq_CO2 at night was about 1.6 seconds cm⁻¹. Based on the ratio of chlorenchyma surface area to total leaf surface area of 82, this r^liq_CO2 corresponded to a minimum cellular resistance of approximately 130 seconds cm⁻¹, comparable to values for mesophyll cells of C₃ plants. The contribution of the carboxylation reaction and/or other biochemical steps to r^liq_CO2 may increase appreciably as the nighttime temperature shifts a few degrees from the optimum or after a few hours in the dark, both of which caused large increases in r^liq_CO2. This necessitates a large internal leaf area for CO₂ diffusion into the chlorenchyma to support moderate nocturnal CO₂ uptake rates by these succulent leaves.

     

Do Stomata Respond to CO(2) Concentrations Other than Intercellular?

Most studies on stomatal responses to CO₂ assume that guard cells respond only to intercellular CO₂ concentration and are insensitive to the CO₂ concentrations in the pore and outside the leaf. If stomata are sensitive to the CO₂ concentration at the surface of the leaf or in the stomatal pore, the stomatal response to intercellular CO₂ concentration will be incorrect for a `normally' operating leaf (where ambient CO₂ concentration is a constant). In this study asymmetric CO₂ concentrations for the two surfaces of amphistomatous leaves were used to vary intercellular and leaf surface CO₂ concentrations independently in Xanthium strumarium L. and Helianthus annuus L. The response of stomata to intercellular CO₂ concentration when the concentration at the leaf surface was held constant was found to be the same as the response when the surface concentration was varied. In addition, stomata did not respond to changes in leaf surface CO₂ concentration when the intercellular concentration for that surface was held constant. It is concluded that stomata respond to intercellular CO₂ concentration and are insensitive to the CO₂ concentration at the surface of the leaf and in the stomatal pore.     

Mechanism of Photosynthetic Carbon Dioxide Uptake by the Red Macroalga, Chondrus crispus

The aim of this study was to determine how Chondrus crispus, a marine red macroalga, acquires the inorganic carbon (C_i) it utilizes for photosynthetic carbon fixation. Analyses of C_i uptake were done using silicone oil centrifugation (using multicellular fragments of thallus), infrared gas analysis, and gas chromatography. Inhibitors of carbonic anhydrase (CA), the band 3 anion exchange protein and Na⁺/K⁺ exchange were used in the study. It was found that: (a) C. crispus does not accumulate C_i internally above the concentration attainable by diffusion; (b) the initial C_i fixtion rate of C. crispus fragments saturates at approximately 3 to 4 millimolar C_i; (c) CA is involved in carbon uptake; its involvement is greatest at high HCO₃⁻ and low CO₂ concentration, suggesting its participation in the dehydration of HCO₃⁻ to CO₂; (d) C. crispus has an intermediate C_i compensation point; and (e) no evidence of any active or facilitated mechanism for the transport of HCO₃⁻ was detected. These data support the view that photosynthetic C_i uptake does not involve active transport. Rather, CO₂, derived from HCO₃⁻ catalyzed by external CA, passively diffuses across the plasma membrane of C. crispus. Intracellular CA also enhances the fixation of carbon in C. crispus.     

Relationship between stomatal conductance and light intensity in leaves of Zea mays L., derived from experiments using the mesophyll as shade

Attached leaves of Zea mays were illuminated with monochromatic light, with either the upper or the lower epidermis facing the light source. The mesophyll absorbed between 99.5 and 99.6% of the red or blue light used. An inversion of the light direction therefore caused a 200- to 250-fold change in the quantum flux into each epidermis. This variation in quantum flux did not affect stomatal conductance. Stomatal conductance was however correlated with intercellular CO₂ concentration, c_i, and the relationship between stomatal conductance and c_i appeared also to remain the same if changes in c_i were brought about by changes in atmospheric CO₂ concentration instead of light. A close inspection of the data showed that stomata of the upper (adaxial) epidermis exhibited a small increase in conductance (<0.1 cm s^-1) in response to blue light that was superimposed on the dominating response to c_i.     

Leaf Conductance in Relation to Rate of CO(2) Assimilation: I. Influence of Nitrogen Nutrition, Phosphorus Nutrition, Photon Flux Density, and Ambient Partial Pressure of CO(2) during Ontogeny

Plants of Zea mays were grown with different concentrations of nitrate (0.6, 4, 12, and 24 millimolar) and phosphate (0.04, 0.13, 0.53, and 1.33 millimolar) supplied to the roots, photon flux densities (0.12, 0.5, and 2 millimoles per square meter per second), and ambient partial pressures of CO₂ (305 and 610 microbars). Differences in mineral nutrition and irradiance led to a large variation in rate of CO₂ assimilation per unit leaf area (A, 11 to 58 micromoles per square meter per second) when measured under standard conditions. The variation was shown, with the plants that had received different amounts of nitrate, to be related to variations in the nitrogen and chlorophyll contents, and phosphoenolpyruvate and ribulose-1,5-bisphosphate carboxylase activities per unit leaf area. Irrespective of growth treatment, A and leaf conductance to CO₂ transfer (g), measured under standard conditions were in almost constant proportion, implying that intercellular partial pressure of CO₂ (p_i), was almost constant at 95 microbars. The same proportionality was maintained as A and g increased in an initially nitrogen-deficient plant that had been supplied with abundant nitrate. It was shown that p_i measured at a given ambient partial pressure was not affected by the ambient partial pressure at which the plants had been grown, although it was different when measured at different ambient partial pressures. This suggests that the close coupling between A and g in these experiments is not associated with sensitivity of stomata to change in p_i.

Similar, though less comprehensive, experiments were done with Gossypium hirsutum, and yielded similar conclusions, except that the proportionality between A and g at normal ambient partial pressure of CO₂ implied P_i ≈ 200 microbars.

     

Causes of decreased photosynthetic rate and metabolic capacity in water-deficient leaf cells: a critical evaluation of mechanisms and integration of processes

Background

Water deficit (WD) decreases photosynthetic rate (A) via decreased stomatal conductance to CO₂ (g_s) and photosynthetic metabolic potential (A_pot). The relative importance of g_s and A_pot, and how they are affected by WD, are reviewed with respect to light intensity and to experimental approaches.

Scope and Conclusions

With progressive WD, A decreases as g_s falls. Under low light during growth and WD, A is stimulated by elevated CO₂, showing that metabolism (A_pot) is not impaired, but at high light A is not stimulated, showing inhibition. At a given intercellular CO₂ concentration (C_i) A decreases, showing impaired metabolism (A_pot). The C_i and probably chloroplast CO₂ concentration (C_c), decreases and then increases, together with the equilibrium CO₂ concentration, with greater WD. Estimation of C_c and internal (mesophyll) conductance (g_i) is considered uncertain. Photosystem activity is unaffected until very severe WD, maintaining electron (e⁻) transport (ET) and reductant content. Low A, together with photorespiration (PR), which is maintained or decreased, provides a smaller sink for e⁻_, causing over-energization of energy transduction. Despite increased non-photochemical quenching (NPQ), excess energy and e⁻ result in generation of reactive oxygen species (ROS). Evidence is considered that ROS damages ATP synthase so that ATP content decreases progressively with WD. Decreased ATP limits RuBP production by the Calvin cycle and thus A_pot. Rubisco activity is unlikely to determine A_pot. Sucrose synthesis is limited by lack of substrate and impaired enzyme regulation. With WD, PR decreases relative to light respiration (R_L), and mitochondria consume reductant and synthesise ATP. With progressing WD at low A, R_L increases C_i and C_c. This review emphasises the effects of light intensity, considers techniques, and develops a qualitative model of photosynthetic metabolism under WD that explains many observations: testable hypotheses are suggested.Key words: Water stress, photosynthesis, photorespiration, stomata, ATP synthase, ATP, photoinhibition, electron transport, Rubisco, fluorescence, sucrose, mesophyll conductance