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.     

Water Relations of Growing Maize Coleoptiles : Comparison between Mannitol and Polyethylene Glycol 6000 as External Osmotica for Adjusting Turgor Pressure

Water relations of growing segments of maize (Zea mays L.) coleoptiles were investigated with osmotic methods using either mannitol (MAN) or polyethylene glycol 6000 (PEG) as external osmotica. Segments were incubated in MAN or PEG solutions at 0 to - 15 bar water potential (Ψ_o) and the effects were compared on elongation growth, osmotic shrinkage, cell sap osmolality (OC), and osmotic pressure (π_i). The nonpenetrating osmoticum PEG affects π_i in agreement with Boyle-Mariotte's law, i.e. the segments behave in principle as ideal osmometers. There is no osmotic adjustment in the Ψ_o range permitting growth (0 to −5 bar) nor in the Ψ_o range inducing osmotic shrinkage (−5 to −10 bar). Promoting growth by auxin (IAA) has no effect on the osmotic behavior of the tissue toward PEG. In contrast to PEG, MAN produces an apparent increase in π_i accompanied by anomalous effects on segment elongation and shrinkage leading to a lower value for Ψ_o which establishes a growth rate of zero and to an apparent recovery from osmotic shrinkage after 2 hours of incubation. These effects can be quantitatively attributed to uptake of MAN into the tissue. MAN is taken up into the apoplastic space and the symplast as revealed by a large temperature-dependent component of MAN uptake. It is concluded that MAN, in contrast to PEG, is unsuitable as an extemal osmoticum for the quantitative determination of water relations of growing maize coleoptiles.     

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.     

Relationships between Carbon Dioxide, Malate, and Nitrate Accumulation and Reduction in Corn (Zea mays L.) Seedlings

The observation that exposure of the leaf canopy to increasing concentrations of CO₂ (100-400 μl/l) decreases the influx of nitrate to the leaf blades, but not to the roots or stalks (largely leaf sheaths), was reconfirmed using ¹⁵NO₃⁻. Decreases in leaf nitrate supply were associated with decreases in induction of nitrate reductase, thus supporting the view that the influx of nitrate to a tissue is a major factor in regulation of the level of nitrate reductase. The whole plant ¹⁵N distribution data show that the CO₂ effects were due to decreased influx of nitrate into the leaf blade rather than CO₂-enhanced nitrate reduction. The decreases in nitrate accumulation by the leaf blade with increases in CO₂ concentration were only partially accounted for by differences in transpiration. Because the initial malate concentration of root tissue (detopped plants) had no subsequent effect on nitrate uptake, it seems unlikely that high levels of malate induced by CO₂ were responsible for the exclusion of nitrate from the leaf blades.     

Nature of the Inorganic Carbon Species Actively Taken Up by the Cyanobacterium Anabaena variabilis

The nature of the inorganic carbon (C_i) species actively taken up by cyanobacteria CO₂ or HCO₃⁻ has been investigated. The kinetics of CO₂ uptake, as well as that of HCO₃⁻ uptake, indicated the involvement of a saturable process. The apparent affinity of the uptake mechanism for CO₂ was higher than that for HCO₃⁻. Though the calculated V_max was the same in both cases, the maximum rate of uptake actually observed was higher when HCO₃⁻ was supplied. C_i uptake was far more sensitive to the carbonic anhydrase inhibitor ethoxyzolamide when CO₂ was the species supplied. Observations of photosynthetic rate as a function of intracellular C_i level (following supply of CO₂ or HCO₃⁻ for 5 seconds) led to the inference that HCO₃⁻ is the species which arrives at the inner membrane surface, regardless of the species supplied. When the two species were supplied simultaneously, mutual inhibition of uptake was observed.

On the basis of these and other results, a model is proposed postulating that a carboic anhydrase-like subunit of the C_i transport apparatus binds CO₂ and releases HCO₃⁻ at or near a membrane porter. The latter transports HCO₃⁻ ions to the cell interior.

     

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.     

A chloroplast pump model for the CO2 concentrating mechanism in the diatom Phaeodactylum tricornutum

Prior analysis of inorganic carbon (C_i) fluxes in the diatom Phaeodactylum tricornutum has indicated that transport of C_i into the chloroplast from the cytoplasm is the major C_i flux in the cell and the primary driving force for the CO₂ concentrating mechanism (CCM). This flux drives the accumulation of C_i in the chloroplast stroma and generates a CO₂ deficit in the cytoplasm, inducing CO₂ influx into the cell. Here, the “chloroplast pump” model of the CCM in P. tricornutum is formalized and its consistency with data on CO₂ and HCO₃ ^? uptake rates, carbonic anhydrase (CA) activity, intracellular C_i concentration, intracellular pH, and RubisCO characteristics is assessed. The chloroplast pump model can account for the major features of the data. Analysis of photosynthetic and C_i uptake rates as a function of external C_i concentration shows that the model has the most difficulty obtaining sufficiently low cytoplasmic CO₂ concentrations to support observed CO₂ uptake rates at low external C_i concentrations and achieving high rates of photosynthesis. There are multiple ways in which model parameters can be varied, within a plausible range, to match measured rates of photosynthesis and CO₂ uptake. To increase CO₂ uptake rates, CA activity can be increased, kinetic characteristics of the putative chloroplast pump can be enhanced to increase HCO₃ ^? export, or the cytoplasmic pH can be raised. To increase the photosynthetic rate, the permeability of the pyrenoid to CO₂ can be reduced or RubisCO content can be increased.     

Carbonyl sulfide: an inhibitor of inorganic carbon transport in cyanobacteria

Cells of a high CO₂-requiring mutant (E₁) and wild type of Synechococcus PCC7942 were incubated with COS in the light, then suspended in COS-free medium and their CO₂ exchange was measured using an open gas-analysis system under the conditions where photosynthetic CO₂ fixation is inhibited. When the suspension of cells untreated with COS was illuminated, the rate of CO₂ uptake was high and addition of carbonic anhydrase during illumination released a large amount of CO₂ from the medium into the gas phase. The COS treatment in the light markedly reduced the rate of CO₂ uptake by the cells and the amount of CO₂ released by carbonic anhydrase. Incubation of cells with COS in the dark had no effect on the CO₂-exchange profile. The COS concentration required for 50% inhibition of CO₂ uptake was about 25 micromolar when the concentration of inorganic carbon (C_i) in the medium was 60 micromolar; higher C_i concentrations reduced the inhibitory effect of COS. Measurement of C_i uptake in E₁ cells by a silicone oil centrifugation method also indicated marked reduction of the activities of ¹⁴CO₂ and H¹⁴CO₃⁻ uptake in the cells treated with COS in the light. The results demonstrated that COS is a potent inhibitor of C_i transport.     

Potassium Transport in Corn Roots : III. Perturbation by Exogenous NADH and Ferricyanide

It has recently been reported that plasmalemma electron transport may be involved in the generation of H⁺ gradients and the uptake of ions into root tissue. We report here on the influence of extracellular NADH and ferricyanide on K⁺ (⁸⁶Rb⁺) influx, K⁺ (⁸⁶Rb⁺) efflux, net apparent H⁺ efflux, and O₂ consumption in 2-centimeter corn (Zea mays [A632 × Oh43]) root segments and intact corn roots. In freshly excised root segments, NADH had no effect on O₂ consumption and K⁺ uptake. However, after the root segments were given a 4-hour wash in aerated salt solution, NADH elicited a moderate stimulation in O₂ consumption but caused a dramatic inhibition of K⁺ influx. Moreover, net apparent H⁺ efflux was significantly inhibited following NADH exposure in 4-hour washed root segments.

Exogenous ferricyanide inhibited K⁺ influx in a similar fashion to that caused by NADH, but caused a moderate stimulation of net H⁺ efflux. Additionally, both reagents substantially altered K⁺ efflux at both the plasmalemma and tonoplast.

These complex results do not lend themselves to straightforward interpretation and are in contradiction with previously published results. They suggest that the interaction between cell surface redox reactions and membrane transport are more complex than previously considered. Indeed, more than one electron transport system may operate in the plasmalemma to influence, or regulate, a number of transport functions and other cellular processes. The results presented here suggest that plasmalemma redox reactions may be involved in the regulation of ion uptake and the `wound response' exhibited by corn roots.

     

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.     

Photosynthetic Responses to Dynamic Light Environments by Hawaiian Trees : Time Course of CO(2) Uptake and Carbon Gain during Sunflecks

Gas exchange responses to rapid changes in light were studied in a C₃ tree, Claoxylon sandwicense Muell-Arg and a C₄ tree, Euphorbia forbesii Sherff that are native to the understory of a mesic Hawaiian forest. When light was increased to 500 micromoles per meter per second following a 2 hour preexposure at 22 micromoles per meter per second, net CO₂ uptake rates and stomatal conductance gradually increased for over 1 hour in C. sandwicense but reached maximum values within 30 minutes in E. forbesii. Calculation of the intercellular CO₂ pressures indicated that the primary limitation to CO₂ uptake during this induction was nonstomatal in both species. The photosynthetic response to simulated sunflecks (lightflecks) was strongly dependent on the induction state of the leaf. Total CO₂ uptake during a lightfleck was greater and the response was faster after exposure of the leaf to high light than when the leaf had been exposed only to low light for the previous 2 hours. During a series of lightflecks, induction resulted in increased CO₂ uptake in successive lightflecks. Significant postillumination CO₂ fixation was evident and contributed substantially to the total carbon gain, especially for lightflecks of 5 to 20 seconds' duration.     

Energy Sources for HCO3? and CO2 Transport in Air-Grown Cells of Synechococcus UTEX 625

Light-dependent inorganic C (C_i) transport and accumulation in air-grown cells of Synechococcus UTEX 625 were examined with a mass spectrometer in the presence of inhibitors or artificial electron acceptors of photosynthesis in an attempt to drive CO₂ or HCO₃⁻ uptake separately by the cyclic or linear electron transport chains. In the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, the cells were able to accumulate an intracellular C_i pool of 20 mm, even though CO₂ fixation was completely inhibited, indicating that cyclic electron flow was involved in the C_i-concentrating mechanism. When 200 μm N,N-dimethyl-p-nitrosoaniline was used to drain electrons from ferredoxin, a similar C_i accumulation was observed, suggesting that linear electron flow could support the transport of C_i. When carbonic anhydrase was not present, initial CO₂ uptake was greatly reduced and the extracellular [CO₂] eventually increased to a level higher than equilibrium, strongly suggesting that CO₂ transport was inhibited and that C_i accumulation was the result of active HCO₃⁻ transport. With 3-(3,4-dichlorophenyl)-1,1-dimethylurea-treated cells, C_i transport and accumulation were inhibited by inhibitors of CO₂ transport, such as COS and Na₂S, whereas Li⁺, an HCO₃⁻-transport inhibitor, had little effect. In the presence of N,N-dimethyl-p-nitrosoaniline, C_i transport and accumulation were not inhibited by COS and Na₂S but were inhibited by Li⁺. These results suggest that CO₂ transport is supported by cyclic electron transport and that HCO₃⁻ transport is supported by linear electron transport.     

Investigation on photorespiration with a sensitive C-assay

A leaf disk assay for photorespiration has been developed based on the rate of release of recently fixed ¹⁴CO₂ in light in a rapid stream of CO₂-free air at 30° to 35°. In tobacco leaves (Havana Seed) photorespiration with this assay is 3 to 5 times greater than the ¹⁴CO₂ output in the dark. In maize, photorespiration is only 2% of that in tobacco.

The importance of open leaf stomata, rapid flow rates of CO₂-free air, elevated temperatures, and oxygen in the atmosphere in order to obtain release into the air of a larger portion of the ¹⁴CO₂ evolved within the tissue in the light was established in tobacco. Photorespiration, but not dark respiration, was inhibited by α-hydroxy-2-pyridinemethanesulfonic acid, an inhibitor of glycolate oxidase, and by 3-(4-chlorophenyl)-1,1-dimethylurea (CMU), an inhibitor of photosynthetic electron transport, under conditions which did not affect the stomata. These experiments show that the substrates of photorespiration and dark respiration differ and also provide additional support for the role of glycolate as a major substrate of photorespiration. It was also shown that at 35° the quantity of ¹⁴CO₂ released in the assay may represent only 33% of the gross ¹⁴CO₂ evolved in the light, the remainder being recycled within the tissue.

It was concluded that maize does not evolve appreciable quantities of CO₂ in the light and that this largely accounts for the greater efficiency of net photosynthesis exhibited by maize. Hence low rates of photorespiration may be expected to be correlated with a high rate of CO₂ uptake at the normal concentrations of CO₂ found in air and at higher light intensities.

     

Characterisation of carbon dioxide and bicarbonate transport during steady-state photosynthesis in the marine cyanobacterium Synechococcus strain PCC7002

Net O₂ evolution, gross CO₂ uptake and net HCO _inf3 ^su– uptake during steady-state photosynthesis were investigated by a recently developed mass-spectrometric technique for disequilibrium flux analysis with cells of the marine cyanobacterium Synechococcus PCC7002 grown at different CO₂ concentrations. Regardless of the CO₂ concentration during growth, all cells had the capacity to transport both CO₂ and HCO _inf3 ^su– ; however, the activity of HCO _inf3 ^su– transport was more than twofold higher than CO₂ transport even in cyanobacteria grown at high concentration of inorganic carbon (C_i = CO₂ + HCO _inf3 ^su– ). In low-C_i cells, the affinities of CO₂ and HCO _inf3 ^su– transport for their substrates were about 5 (CO₂ uptake) and 10 (HCO _inf3 ^su– uptake) times higher than in high-C_i cells, while air-grown cells formed an intermediate state. For the same cells, the intracellular accumulated C_i pool reached 18, 32 and 55 mM in high-C_i, air-grown and low-C_i cells, respectively, when measured at 1 mM external C_i. Photosynthetic O₂ evolution, maximal CO₂ and HCO _inf3 ^su– transport activities, and consequently their relative contribution to photosynthesis, were largely unaffected by the CO₂ provided during growth. When the cells were adapted to freshwater medium, results similar to those for artificial seawater were obtained for all CO₂ concentrations. Transport studies with high-C_i cells revealed that CO₂ and HCO _inf3 ^su– uptake were equally inhibited when CO₂ fixation was reduced by the addition of glycolaldehyde. In contrast, in low-C_i cells steady-state CO₂ transport was preferably reduced by the same inhibitor. The inhibitor of carbonic anhydrase ethoxyzolamide inhibited both CO₂ and HCO _inf3 ^su– uptake as well as O₂ evolution in both cell types. In high-C_i cells, the degree of inhibition was similar for HCO _inf3 ^su– transport and O₂ evolution with 50% inhibition occurring at around 1 mM ethoxyzolamide. However, the uptake of CO₂ was much more sensitive to the inhibitor than HCO _inf3 ^su– transport, with an apparent I₅₀ value of around 250 M ethoxyzolamide for CO₂ uptake. The implications of our results are discussed with respect to C_i utilisation in the marine Synechococcus strain.Abbreviations Chl chlorophyll - C_i inorganic carbon (CO₂ + HCO _inf3 ^su– ) - CA carbonic anhydrase - CCM CO₂-concentrating mechanism - EZA ethoxyzolamide - GA glycolaldehyde - K_1/2 concentration required for half-maximal response - Rubisco ribulose-1,5,-bisphosphate carboxylase-oxygenase D.S. is a recipient of a research fellowship from the Deutsche Forschungsgemeinschaft (D.F.G.). In addition, we are grateful to Donald A. Bryant, Department of Molecular and Cell Biology and Center of Biomolecular Structure Function, Pennsylvania State University, USA, for sending us the wild-type strain of Synechococcus PCC7002.     

Metabolism of alpha-Ketoglutarate by Roots of Woody Plants

The uptake and metabolism of α-ketoglutarate-5-¹⁴C by peach, apple, and privet root tissues were studied over various time intervals. As much as 80% of the absorbed ¹⁴C appeared as ¹⁴CO₂ in 320 minutes in peach roots. Apple and privet roots were less effective in this conversion with the bulk of the ¹⁴C found in the organic acid fraction. This indicates differences in organic acid metabolism among species of woody plants.

The ¹⁴C accumulated in malate earlier and in larger quantities than in citrate. Both glutamate and aspartate were labeled in 10 minutes and glutamate was labeled as early as 3 minutes. The labeling pattern does not clearly distinguish between the synthesis of glutamate by glutamic dehydrogenase or by transamination with oxaloacetate.

The rapid metabolism of α-ketoglutarate to glutamate by the 3 species studied indicates the presence of enzyme systems important in amino acid synthesis in the roots of woody plants.

     

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.     

Gain of the feedback loop involving carbon dioxide and stomata: theory and measurement

The physiological and physical components of the feedback loop involving intercellular CO₂ concentration (c_i) and stomata are identified. The loop gain (G) is a measure of the degree of homeostasis in a negative feedback loop [the expression 1/(1-G) represents the fraction to which feedback reduces a perturbance]. Estimates are given for the effects of G on responses of stomata and c_i to changes in ambient CO₂ concentration, light intensity, and perturbations in the water relations of a leaf. At normal ambient CO₂ concentration, the gain of the loop involving stomatal conductance and c_i was found to be −2.2 in field-grown Zea mays, −3.6 if plants of this species were grown in a growth chamber, and zero in well watered Xanthium strumarium in the vegetative state.     

Potassium Transport in Corn Roots : IV. Characterization of the Linear Component

A detailed examination was conducted on the linear, or first-order kinetic component for K⁺(⁸⁶Rb⁺) influx into root segments of both low- and high-salt grown corn seedlings (Zea mays [A632 × Oh 43]). In tissue from both low- and high-salt grown roots, replacement of Cl⁻ in the uptake solution by either SO₄²⁻, H₂PO₄⁻, or NO₃⁻ caused a significant (50-60%) and specific inhibition of the linear component of K⁺ influx. The anion transport inhibitor, 4,4′-diisothiocyano-2,2′-disulfonic acid, was found to abolish saturable Cl⁻ influx in corn roots while causing a significant (50-60%) and specific inhibition of the linear K⁺ uptake system; this inhibition was identical to that observed when Cl⁻ was replaced by other anions in the K⁺ uptake solution. Additionally, the quaternary ammonium cation, tetraethylammonium, which has been shown to block K⁺ channels in nerve axons, also caused a dramatic (70%) and specific inhibition of the linear component of K⁺ influx, but this was obtained only in high-salt roots. The reasons for this difference are discussed with respect to the differing abilities of low- and high-salt roots to absorb tetraethylammonium.

Our present results indicate that the linear component of K⁺ influx may occur by a passive process involving transmembrane K⁺ channels. Fluxes through these K⁺ channels may be partly coupled to a saturating Cl⁻ influx mechanism.

     

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.

     

Osmotic adjustment, symplast volume, and nonstomatally mediated water stress inhibition of photosynthesis in wheat

At low water potential (ψ_w), dehydration reduces the symplast volume of leaf tissue. The effect of this reduction on photosynthetic capacity was investigated. The influence of osmotic adjustment on this relationship was also examined. To examine these relationships, comparative studies were undertaken on two wheat cultivars, one that osmotically adjusts in response to water deficits (`Condor'), and one that lacks this capacity (`Capelle Desprez'). During a 9-day stress cycle, when water was withheld from plants grown in a growth chamber, the relative water content of leaves declined by 30% in both cultivars. Leaf osmotic potential (ψ_s) declined to a greater degree in Condor plants. Measuring ψ_s at full turgor indicated that osmotic adjustment occurred in stressed Condor, but not in Capelle plants. Two methods were used to examine the degree of symplast (i.e. protoplast) volume reduction in tissue rapidly equilibrated to increasingly low ψ_w. Both techniques gave similar results. With well-watered plants, symplast volume reduction from the maximum (found at high ψ_w for each cultivar) was the same for Condor and Capelle. After a stress cycle, volume was maintained to a greater degree at low ψ_w in Condor leaf tissue than in Capelle. Nonstomatally controlled photosynthesis was inhibited to the same degree at low ψ_w in leaf tissue prepared from well-watered Condor and Capelle plants. However, photosynthetic capacity was maintained to a greater degree at low ψ_w in tissue prepared from stressed Condor plants than in tissue from stressed Capelle plants. Net CO₂ uptake in attached leaves was monitored using an infrared gas analyzer. These studies indicated that in water stressed plants, photosynthesis was 106.5% higher in Condor than Capelle at ambient [CO₂] and 21.8% higher at elevated external [CO₂]. The results presented in this report were interpreted as consistent with the hypothesis that there is a causal association between protoplast (and presumably chloroplast) volume reduction at low ψ_w and low ψ_w inhibition of photosynthesis. Also, the data indicate that osmotic adjustment allows for maintenance of relatively greater volume at low ψ_w, thus reducing low ψ_w inhibition of chloroplast photosynthetic potential.