Effects of SO(2) and O(3) on Allocation of C-Labeled Photosynthate in Phaseolus vulgaris

A series of laboratory exposures of two varieties of bush bean (Phaseolus vulgaris L., var 274 and var 290) was conducted to determine the sensitivity of [¹⁴C]photosynthate allocation patterns to alteration by SO₂ and O₃. Experiments with the pollution-resistant 274 variety demonstrated short-term changes in both ¹⁴C and biomass allocation to roots of ¹⁴CO₂-labeled plants but no significant effect on yield by up to 40 hours of exposure to SO₂ at 0.50 microliters per liter or 4 hours of O₃ at 0.40 microliters per liter. Subsequent experiments with the more sensitive 290 variety demonstrated significant alteration of photosynthesis, translocation, and partitioning of photosynthate between plant parts including developing pods. Significant increases in foliar retention of photosynthate (+40%) occurred after 8 hours of exposure to SO₂ at 0.75 microliters per liter (6.0 microliters per liter-hour) and 11 hours of exposure to O₃ at 0.30 microliters per liter-hour (3.3 microliters-hours). Time series sampling of labeled tissues after ¹⁴CO₂ uptake showed that the disruption of translocation patterns was persistent for at least 1 week after exposures ceased. Subsequent longer-term exposures at lower concentrations of both O₃ (0.0, 0.10, 0.15, and 0.20 microliters per liter) and SO₂ (0.0, 0.20, and 0.40 microliters per liter) demonstrated that O₃ more effectively altered allocation than SO₂, that primary leaves were generally more sensitive than trifoliates, and that responses of trifoliate leaves varied with plant growth stage. Altered rates of allocation of photosynthate by leaves were generally associated with alterations of similar magnitude and opposite direction in developing pods. Collectively, these experiments suggest that allocation patterns can provide sensitive indices of incipient growth responses of pollution-stressed vegetation.     

Photosynthetic responses of leaves to water stress, expressed by photoacoustics and related methods : I. Probing the photoacoustic method as an indicator for water stress in vivo

The effect of leaf desiccation on the photosynthetic activities in vivo was probed by the photoacoustic method. The aim of this research was: (a) To study the photoacoustic signal per se in varied conditions in order to develop this tool as a probe for stress conditions in vivo. (b) To obtain results pertaining to electron transport activities in vivo, and confirm conclusions based on work with isolated chloroplasts, which could otherwise be the result of nonspecific damage occurring during their isolation. Leaf discs from tobacco (Nicotiana tabacum L.) were routinely used, with other species tested also for comparison. Rapid leaf desiccation caused changes in the low frequency photoacoustic signal, attributed both to the mechanism of signal transduction, influenced by changes in the structural parameters of the leaf, and to the direct (nonstomatal) inhibition of gross photosynthesis. The dependence of the photothermal part of the signal on the frequency indicated the presence of two photothermal components, one of which persisted only at low modulation frequencies (below about 100 Hz) and which largely increased with the desiccation treatment. This component was ascribed to a thermal wave which reaches the leaf surface. The other nonvariable photothermal component was ascribed to a thermal wave propagating from the chloroplasts to the surface of the mesophyll cell. Only this component is considered in the ratio of the O₂ signal to the photothermal signal, which is used to estimate the quantum yield of photosynthesis. The specific dependence of the latter ratio on the frequency yielded a comparative quantum yield parameter from its extrapolation to zero frequency, and also indicated stress induced changes in the diffusion of O₂ through the mesophyll cell, reflected by changes in its characteristic slope. The (zero frequency extrapolated) quantum yield was markedly reduced with the progression of the water stress, indicating the inhibition of (gross) phototosynthetic electron transport in vivo. This result was expressed even more emphatically by the stronger inhibition of the photochemical energy storage, obtained by photoacoustic measurements at a high modulation frequency.     

Ethylene, Ethane, Acetaldehyde, and Ethanol Production By Plants under Stress

Red pine (Pinus resinosa Ait.) and paper birch (Betula papyrifera Marsh.) seedlings exposed to sulfur dioxide produced acetaldehyde and ethanol, and exhibited increased production of ethylene and ethane. Gas chromatographic measurement of head space gas from incubation tubes containing leaves or seedlings was a simple method of simultaneously measuring all four compounds. Increased ethylene production had two phases, a moderate increase from the beginning of the stress period and a large increase just prior to appearance of leaf lesions. Ethane production in SO₂-stressed plants did not increase until lesions appeared. Acetaldehyde and ethanol production began within 6 hours at 0.3 microliter per liter SO₂ and 24 hours at 0.1 microliter per liter SO₂ and continued throughout a 6-day fumigation. Production of acetaldehyde and ethanol continued when plants were removed to clean air for up to 2 days. A higher concentration of SO₂ (0.5 microliter per liter) induced acetaldehyde and ethanol production within 2 hours of the start of fumigation of birch and pine seedlings. A number of other stresses, including water deficit, freezing, and ozone exposure induced production of acetaldehyde and ethanol. Production of these compounds was not due to hypoxia, as the O₂ partial pressure in the incubation vessels did not decline. Increasing the O₂ partial pressure to 300 millimeters Hg did not affect production of these compounds. Production of ethylene, acetaldehyde, and ethanol declined when more than 80% of the leaf area became necrotic, while ethane production was linearly related to the percentage of necrosis. A number of woody and herbaceous plant species produced acetaldehyde and ethanol in response to freezing stress, while others did not. Measurement of these four compounds simultaneously in the gas phase may be a valuable method for monitoring plant stress, particularly air pollution stress.     

14C transfer between the spring ephemeral Erythronium americanum and sugar maple saplings via arbuscular mycorrhizal fungi in natural stands

We investigated in the field the carbon (C) transfer between sugar maple (Acer saccharum) saplings and the spring ephemeral Erythronium americanum via the mycelium of arbuscular mycorrhizal (AM) fungi. Sugar maple saplings and E. americanum plants were planted together in pots placed in the ground of a maple forest in 1999. Ectomycorrhizal yellow birches (Betula alleghaniensis) were added as control plants. In spring 2000, during leaf expansion of sugar maple saplings, the leaves of E. americanum were labelled with ¹⁴CO₂. Seven days after labelling, radioactivity was detected in leaves, stem and roots of sugar maples. Specific radioactivity in sugar maples was 13-fold higher than in yellow birches revealing the occurrence of a direct transfer of ¹⁴C between the AM plants. The quantity of ¹⁴C transferred to sugar maple saplings was negatively correlated with the percentage of ¹⁴C allocated to the storage organ of E. americanum. A second labelling was performed in autumn 2000 on sugar maple leaves during annual growth of E. americanum roots. Radioactivity was detected in 7 of 22 E. americanum root systems and absent in yellow birches. These results suggest that AM fungi connecting different understorey species can act as reciprocal C transfer bridges between plant species in relation with the phenology of the plants involved.     

Fusicoccin and Air Pollutant Injury to Plants : Evidence for Enhancement of SO(2) but Not O(3) Injury

Garden peas (Pisum sativum L. cv Alsweet) and a tomato mutant (Lycopersicon esculentum Mill. var flacca) were sprayed with fusicoccin, a fungal toxin affecting membrane transport properties, before exposure to SO₂ or O₃. Tomatoes treated with 10 micromolar fusicoccin and exposed to SO₂ (0.6 microliter per liter for 2 hours) exhibited twice as much foliar necrosis as untreated plants exposed to SO₂. Peas treated with fusicoccin and exposed to SO₂ (0.7 to 1.0 microliter per liter for 2 hours) exhibited 2 to 6 times more injury than untreated plants exposed to SO₂. Peas treated with fusicoccin and exposed to O₃ had less injury than untreated plants exposed to O₃ (0.1 to 0.3 microliter per liter for 2 hours). Several lines of evidence suggested that the fusicoccin enhancement of SO₂ injury is not the result of increased gas exchange, i.e. the tomato mutant has permanently open stomata under all conditions, and in peas fusicoccin had no effect on SO₂ or H₂O flux in plants exposed to 0.12 microliter per liter SO₂. However, a 21% greater leaf conductance in fusicoccin treated versus untreated plants indicated the possibility of some differences in gas exchange for peas exposed to 1.0 microliter per liter SO₂.     

The Sequence of Change within the Photosynthetic Apparatus of Wheat following Short-Term Exposure to Ozone

The basis of inhibition of photosynthesis by single acute O₃ exposures was investigated in vivo using analyses based on leaf gas exchange measurements. The fully expanded second leaves of wheat plants (Triticum aestivum L. cv Avalon) were fumigated with either 200 or 400 nanomoles per mole O₃ for between 4 and 16 hours. This reduced significantly the light-saturated rate of CO₂ uptake and was accompanied by a parallel decrease in stomatal conductance. However, the stomatal limitation, estimated from the relationship between CO₂ uptake and the internal CO₂ concentration, only increased significantly during the first 8 hours of exposure to 400 nanomoles per mole O₃; no significant increase occurred for any of the other treatments. Analysis of the response of CO₂ uptake to the internal CO₂ concentration implied that the predominant factor responsible for the reduction in light-saturated CO₂ uptake was a decrease in the efficiency of carboxylation. This was 58 and 21% of the control value after 16 hours at 200 and 400 nanomoles per mole O₃, respectively. At saturating concentrations of CO₂, photosynthesis was inhibited by no more than 22% after 16 hours, indicating that the capacity for regeneration of ribulose bisphosphate was less susceptible to O₃. Ozone fumigations also had a less pronounced effect on light-limited photosynthesis. The maximum quantum yield of CO₂ uptake and the quantum yield of oxygen evolution showed no significant decline after 16 hours with 200 nanomoles per mole O₃, requiring 8 hours at 400 nanomoles per mole O₃ before a significant reduction occurred. The photochemical efficiency of photosystem II estimated from the ratio of variable to maximum chlorophyll fluorescence and the atrazine-binding capacity of isolated thylakoids demonstrated that photochemical reactions were not responsible for the initial inhibition of CO₂ uptake. The results suggest that the apparent carboxylation efficiency appears to be the initial cause of decline in photosynthesis in vivo following acute O₃ fumigation.     

Joint Action of O(3) and SO(2) in Modifying Plant Gas Exchange

The joint action of O₃ and SO₂ stress on plants was investigated by determining the quantitative relationship between air pollutant fluxes and effects on stomatal conductance. Gas exchange measurements of O₃, SO₂, and H₂O vapor were made for Pisum sativum L. (garden pea). Plants were grown under controlled environments, and O₃, SO₂, and H₂O vapor fluxes were evaluated with a whole-plant gas exchange chamber using the mass-balance approach. Maximum O₃ and SO₂ fluxes per unit area (2 sided) into leaves averaged 8 nanomoles per square meter per second with exposure to either O₃ or SO₂ at 0.1 microliters per liter. Internal fluxes of either O₃ or SO₂ were reduced by up to 50% during exposure to combined versus individual pollutants; the greatest reduction occurred with simultaneous versus sequential combinations of the pollutants. Stomatal conductance to H₂O was substantially altered by the pollutant exposures, with O₃ molecules twice as effective as SO₂ molecules in inducing stomatal closure. Stomatal conductance was related to the integrated dose of pollutants. The regression equations relating integrated dose to stomatal conductance were similar with O₃ alone, O₃ plus added SO₂, and O₃ plus SO₂ simultaneously; i.e. a dose of 100 micromoles per square meter produced a 39 to 45% reduction in conductance over nonexposed plants. With SO₂ alone, or SO₂ plus added O₃, a dose of 100 micromoles per square meter produced a 20 to 25% reduction in conductance. When O₃ was present at the start of the exposure, then stomatal response resembled that for O₃ more than the response for SO₂. This study indicated that stomatal responses with combinations of O₃ and SO₂ are not dependent solely on the integrated dose of pollutants, but suggests that a metabolic synergistic effect exists.     

Photorespiration-induced reduction of ribulose bisphosphate carboxylase activation level

Leaf photosynthesis and ribulose bisphosphate carboxylase activation level were inhibited in several mutants of the C₃ crucifer Arabidopsis thaliana which possess lesions in the photorespiratory pathway. This inhibition occurred when leaves were illuminated under a photorespiratory atmosphere (50% O₂, 350 microliters per liter CO₂, balance N₂), but not in nonphotorespiratory conditions (2% O₂, 350 microliters per liter CO₂, balance N₂). Inhibition of carboxylase activation level was observed in strains with deficient glycine decarboxylase, serine transhydroxymethylase, serine-glyoxylate aminotransferase, glutamate synthase, and chloroplast dicarboxylate transport activities, but inhibition did not occur in a glycolate-P phosphatase-deficient strain. Also, the photorespiration inhibitor aminoacetonitrile produced a decline in leaf and protoplast ribulose bisphosphate carboxylase activation level, but was without effect on intact chloroplasts. Fructose bisphosphatase, a light-activated enzyme which is strongly dependent on stromal pH and Mg²⁺ for regulation, was unaffected by conditions which caused inhibition of ribulose bisphosphate carboxylase. Thus, the mechanism of inhibition does not appear to involve changes in stromal Mg²⁺ and pH but rather is associated with metabolite flux through the photorespiratory pathway.     

Phytotoxicity of Air Pollutants: Evidence for the Photodetoxification of SO(2) but Not O(3)

Pisum sativum L. cv Alsweet (garden pea) and Lycopersicon esculentum flacca Mill. (tomato) were used to evaluate the phytotoxicity of SO₂ and O₃ in the light and dark. Plants were grown in controlled environment chambers and exposed to SO₂ or O₃ in the light or dark at the same environmental conditions at which they were grown. The pea plants were treated with fusicoccin to ensure open stomata in the dark; the stomata of the tomato mutant remained open in the dark. Both species exhibited 64% to 80% less foliar necrosis following exposure to SO₂ (0.5 to 1.0 microliter per liter for 2 hours) in the light than in the dark. The decrease in SO₂ injury for light versus dark exposed plants was greater in fully expanded than expanding leaves. Both species exhibited 30% greater foliar necrosis following exposure to O₃ (0.2 microliter per liter for 2 hours) in the light than dark. The increase in O₃ injury in the light versus dark was similar for leaves at all stages of expansion. Leaf conductance to water vapor was 7% to 11% and 23% higher in the light than dark for fusicoccin-treated peas and tomato plants, respectively, indicating greater foliar uptake of both pollutants in the light than dark. Thus, the decreased SO₂ toxicity in the light was not associated with pollutant uptake, but rather the metabolism of SO₂. In contrast, the increased toxicity of O₃ in the light was at least in part associated with increased uptake or could not be separated from it.     

A photoacoustic study of water infiltrated leaves

Photoacoustic measurements of photosynthetic energy storage were conducted on water infiltrated pea and sugar maple leaves. The samples were vacuum infiltrated with pure water or with a suitable buffer. The use of such methodology permitted an accurate determination of the energy storage parameter at low modulation frequencies, where in non-infiltrated leaves oxygen evolution dominates the photoacoustic signal and does not allow energy storage measurements. Differences between infiltration media were not essential, however the use of pure water as infiltration medium sometimes caused instability of the measured energy storage, particularly at longer experimental time. Values of energy storage in individual samples ranged mostly between 0.2 to 0.35. Measured as a function of the modulation frequency, energy storage was found to be constant from about 10 to 200 Hz for pea leaves. In sugar maple leaves, the energy storage slightly increased between 100 and 500 Hz. Obtaining an accurate value for energy storage also allowed an accurate estimation of the O₂ evolution contribution to the photoacoustic signal of an unfiltrated leaf. In a maple leaf its frequency dependence showed only the effect of diffusion in the entire frequency range (10–500 Hz). Energy storage transients were observed after long periods (ca. 1/4-2 hrs) of dark adaptation upon the transition to light. In this case the initial energy storage was roughly about 1/2 that of the steady state value indicating strong PS I activity, while PS II was transiently incompetent. Energy-storage increased during illumination in a way to correspond to photosynthetic induction events as previously measured by fluorescence and O₂ evolution. Transients in energy storage were also found following high light to low light transitions (i.e., switch off of the saturating background light), that paralleled similar transients in oxygen evolution, showing initial transient inactivation followed by progressive reactivation of PS II.Abbreviations ES energy storage - PA photoacoustic(s) - PTR photothermal radiometry     

Oxygen Stimulation of Apparent Photosynthesis in Flaveria linearis

A plant was found in the C₃-C₄ intermediate species, Flaveria linearis, in which apparent photosynthesis is stimulated by atmospheric O₂ concentrations. A survey of 44 selfed progeny of the plant showed that the O₂ stimulation of apparent photosynthesis was passed on to the progeny. When leaves equilibrated at 210 milliliters per liter O₂ were transferred to 20 milliliters per liter O₂ apparent photosynthesis was initially stimulated, but gradually declined so that at 30 to 40 minutes the rate was only about 80 to 85% of that at 210 milliliters per liter O₂. Switching from 20 to 210 milliliters per liter caused the opposite transition in apparent photosynthesis. All other plants of F. linearis reached steady rates within 5 minutes after switching O₂ that were 20 to 24% lower in 210 than in 20 milliliters per liter O₂. At low intercellular CO₂ concentrations and low irradiances, O₂ inhibition of apparent photosynthesis of the aberrant plant was similar to that in normal plants, but at an irradiance of 2 millimoles quanta per square meter per second and near 300 microliters per liter CO₂ apparent photosynthesis was consistently higher at 210 than at 20 milliliters per liter O₂. In morphology and leaf anatomy, the aberrant plant is like the normal plants in F. linearis. The stimulation of apparent photosynthesis at air levels of O₂ in the aberrant plant is similar to other literature reports on observations with C₃ plants at high CO₂ concentrations, high irradiance and/or low temperatures, and may be related to limitation of photosynthesis by triose phosphate utilization.     

Interaction between light and chilling temperature on the inhibition of photosynthesis in chilling-sensitive plants*

Abstract. Fully expanded leaves of 25°C grown Phaseolus vulgaris and six other species were exposed for 3 h to chilling temperatures at photon flux densities equivalent to full sunlight. In four of the species this treatment resulted in substantial inhibition of the subsequent quantum yield of CO₂ uptake, indicating reduction of the photochemical efficiency of photosynthesis. The extent of inhibition was dependent on the photon flux density during chilling and no inhibition occurred when chilling occurred at a low photon flux density. No inhibition occurred at temperatures above 11.5°C, even in the presence of the equivalent of full sunlight. This interaction between chilling and light to cause inhibition of photosynthesis was promoted by the presence of oxygen at normal air partial pressures and was unaffected by the CO₂ partial pressure present when chilling occurred in air. When chilling occurred at low O₂ partial pressures, CO₂ was effective in reducing the degree of inhibition. Apparently, when leaves of chilling-sensitive plants are exposed to chilling temperatures in air of normal composition then light is instrumental in inducing rapid damage to the photochemical efficiency of photosynthesis.     

Patterns of rhizosphere carbon flux in sugar maple (Acer saccharum) and yellow birch (Betula allegheniensis) saplings

Despite its importance in the terrestrial C cycle rhizosphere carbon flux (RCF) has rarely been measured for intact root–soil systems. We measured RCF for 8‐year‐old saplings of sugar maple (Acer saccharum) and yellow birch (Betula allegheniensis) collected from the Hubbard Brook Experimental Forest (HBEF), NH and transplanted into pots with native soil horizons intact. Five saplings of each species were pulse labeled with ¹³CO₂ at ambient CO₂ concentrations for 4–6 h, and the ¹³C label was chased through rhizosphere and bulk soil pools in organic and mineral horizons for 7 days. We hypothesized yellow birch roots would supply more labile C to the rhizosphere than sugar maple roots based on the presumed greater C requirements of ectomycorrhizal roots. We observed appearance of the label in rhizosphere soil of both species within the first 24 h, and a striking difference between species in the timing of ¹³C release to soil. In sugar maple, peak concentration of the label appeared 1 day after labeling and declined over time whereas in birch the label increased in concentration over the 7‐day chase period. The sum of root and rhizomicrobial respiration in the pots was 19% and 26% of total soil respiration in sugar maple and yellow birch, respectively. Our estimate of the total amount of RCF released by roots was 6.9–7.1% of assimilated C in sugar maple and 11.2–13.0% of assimilated C in yellow birch. These fluxes extrapolate to 55–57 and 90–104 g C m^?2 yr^?1 from sugar maple and yellow birch roots, respectively. These results suggest RCF from both arbuscular mycorrhizal and ectomycorrhizal roots represents a substantial flux of C to soil in northern hardwood forests with important implications for soil microbial activity, nutrient availability and C storage.     

Effects of Exogenous γ-Aminobutyric Acid (GABA) on Photosynthesis and Antioxidant System in Pepper (Capsicum annuum L.) Seedlings Under Low Light Stress

The effects of γ-aminobutyric acid (GABA) treatment on parameters of photosynthesis and antioxidant defense system were measured in pepper (Capsicum annuum L.) leaves under low-light (LL) stress. Seedlings exposed to LL stress showed increased chlorophyll content as well as decreased net photosynthetic rate (P _n), stomatal conductance (g _s), maximum quantum yield of PSII (F _v/F _m), actual PSII photochemical efficiency (Φ_PSII), electron transport rates and photochemical quenching coefficient (q _p). However, almost all the photosynthetic parameters above were enhanced markedly in seedlings treated with GABA under LL stress. Moreover, LL stress increased malondialdehyde (MDA) content, superoxide anion radical (O₂ ^·?) and hydrogen peroxide (H₂O₂) production. GABA-treated, LL-stressed seedlings exhibited lower MDA, O₂ ^·? and H₂O₂ production, and showed an activated antioxidant defense system, including increased activities of superoxide dismutase, catalase, ascorbate peroxidase, glutathione peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, ascorbate and glutathione. Moreover, seedlings subjected to LL stress showed increased endogenous GABA levels, and the level was further improved by application of exogenous GABA. These results suggest that GABA mitigates the LL-induced stress via regulating the antioxidant defense system and maintaining a high level of photochemical efficiency in pepper seedlings.

     

The effect of acid rain on ultrastructure and functional parameters of photosynthetic apparatus in pea leaves

The ultrastructure and functional parameters of the photosynthetic apparatus in leaves of 14-day-old pea seedlings were studied in conditions of laboratory simulated acid rain (SAR). Pea seedlings were sprayed with an aqueous solution containing NaNO₃ (0.2 mM) and Na₂SO₄ (0.2 mM) (pH 5.6, a control variant), or with the same solution, which was acidified to pH 2.5 (acid variant). Functional characteristics were determined by chlorophyll fluorescence analysis. There was reduction in the efficiency of the photosynthetic electron transport by 25% accompanied by an increase in the quantum yield of thermal dissipation of excess light quanta by 85% without significant change in maximum quantum yield of PSII photochemistry (F_v/F_m). Ultrastructural changes in chloroplasts were revealed by transmission electron microscopy (TEM) 2 days after the SAR treatment of pea leaves. In this case, changes in the structure of the grana and heterogeneity of the thylakoids packing in the granum, namely, an increase in thylakoid intraspace widths and thickness of granal thylakoids compared to the control, were found. It was shown also that carbonic anhydrase activity was significantly inhibited in chloroplast preparations isolated from SAR-treated pea leaves. We hypothesize possible involvement of chloroplast carbonic anhydrase in thylakoid granal structure maintenance. The structural disturbances and the inhibition of photochemical activity of chloroplasts are possible consequences of the carbonic anhydrase inactivation by SAR treatment leading to violation of HCO₃ ^?–CO₂ equilibrium. The data obtained suggest that acid rains negatively affect the photosynthetic apparatus by disrupting the membrane system of the chloroplast.     

Effects of Ozone and Sulfur Dioxide on Phyllosphere Fungi from Three Tree Species

Short-term effects of ozone (O₃) on phyllosphere fungi were studied by examining fungal populations from leaves of giant sequoia (Sequoiadendron giganteum (Lindl.) Buchholz) and California black oak (Quercus kelloggii Newb.). Chronic effects of both O₃ and sulfur dioxide (SO₂) were studied by isolating fungi from leaves of mature Valencia orange (Citrus sinensis L.) trees. In this chronic-exposure experiment, mature orange trees were fumigated in open-top chambers at the University of California, Riverside, for 4 years with filtered air, ambient air plus filtered air (1:1), ambient air, or filtered air plus SO₂ at 9.3 parts per hundred million. Populations of Alternaria alternata (Fr.) Keissler and Cladosporium cladosporioides (Fres.) de Vries, two of the four most common fungi isolated from orange leaves, were significantly reduced by chronic exposure to ambient air. In the short-term experiments, seedlings of giant sequoia or California black oak were fumigated in open-top chambers in Sequoia National Park for 9 to 11 weeks with filtered air, ambient air, or ambient air plus O₃. These short-term fumigations did not significantly affect the numbers of phyllosphere fungi. Exposure of Valencia orange trees to SO₂ at 9.3 parts per hundred million for 4 years reduced the number of phyllosphere fungi isolated by 75% compared with the number from the filtered-air treatment and reduced the Simpson diversity index value from 3.3 to 2.5. A significant chamber effect was evident since leaves of giant sequoia and California black oak located outside of chambers had more phyllosphere fungi than did seedlings within chambers. Results suggest that chronic exposure to ambient ozone or SO₂ in polluted areas can affect phyllosphere fungal communities, while short-term exposures may not significantly disturb phyllosphere fungi.     

Growth and Mitochondrial Respiration of Mungbeans (Phaseolus aureus Roxb.) Germinated at Low Pressure

Mungbean (Phaseolus aureus Roxb.) seedlings were grown hypobarically to assess the effects of low pressure (21-24 kilopascals) on growth and mitochondrial respiration. Control seedlings grown at ambient pressure (101 kilopascals) were provided amounts of O₂ equivalent to those provided experimental seedlings at reduced pressure to factor out responses to O₂ concentration and to total pressure. Respiration was assayed using washed mitochondria, and was found to respond only to O₂ concentration. Regardless of total pressure, seedlings grown at 2 millimoles O₂ per liter had higher state 3 respiration rates and decreased percentages of alternative respiration compared to ambient (8.4 millimoles O₂ per liter) controls. In contrast, seedling growth responded to total pressure but not to O₂ concentration. Seedlings were significantly larger when grown under low pressure. While low O₂ (2 millimoles O₂ per liter) diminished growth at ambient pressure, growth at low pressure in the same oxygen concentration was enhanced. Respiratory development and growth of mungbean seedlings under low pressure is unimpaired whether oxygen or air is used as the chamber gas, and further, low pressure can improve growth under conditions of poor aeration.     

Effects of CO(2) Enrichment and Carbohydrate Content on the Dark Respiration of Soybeans

During the period of most active leaf expansion, the foliar dark respiration rate of soybeans (Glycine max cv Williams), grown for 2 weeks in 1000 microliters CO₂ per liter air, was 1.45 milligrams CO₂ evolved per hour leaf density thickness, and this was twice the rate displayed by leaves of control plants (350 microliters CO₂ per liter air). There was a higher foliar nonstructural carbohydrate level (e.g. sucrose and starch) in the CO₂ enriched compared with CO₂ normal plants. For example, leaves of enriched plants displayed levels of nonstructural carbohydrate equivalent to 174 milligrams glucose per gram dry weight compared to the 84 milligrams glucose per gram dry weight found in control plant leaves. As the leaves of CO₂ enriched plants approached full expansion, both the foliar respiration rate and carbohydrate content of the CO₂ enriched leaves decreased until they were equivalent with those same parameters in the leaves of control plants. A strong positive correlation between respiration rate and carbohydrate content was seen in high CO₂ adapted plants, but not in the control plants.

Mitochondria, isolated simultaneously from the leaves of CO₂ enriched and control plants, showed no difference in NADH or malate-glutamate dependent O₂ uptake, and there were no observed differences in the specific activities of NAD⁺ linked isocitrate dehydrogenase and cytochrome c oxidase. Since the mitochondrial O₂ uptake and total enzyme activities were not greater in young enriched leaves, the increase in leaf respiration rate was not caused by metabolic adaptations in the leaf mitochondria as a response to long term CO₂ enrichment. It was concluded, that the higher respiration rate in the enriched plant's foliage was attributable, in part, to a higher carbohydrate status.

     

Changes in Sugars, Enzymic Activities and Acid Phosphatase Isoenzyme Profiles of Bananas Ripened in Air or Stored in 2.5% O(2) with and without Ethylene

This study investigates the effect of 2.5% O₂, both alone and in combination with ethylene, on respiration, sugar accumulation and activities of pectin methylesterase and acid phosphatase during ripening of bananas (Musa paradisiaca sapientum). In addition, the changes in the phosphatase isoenzyme profiles are also analyzed. Low oxygen diminished respiration and slowed down the accumulation of sugars and development of the yellow color. Furthermore, low O₂ prevented the rise in acid phosphatase activities and this suppression was not reversed by the inclusion of 100 microliters per liter ethylene in 2.5% O₂ atmosphere. Gel electrophoresis of both the soluble and particulate cell-free fractions under nondenaturing conditions revealed the presence of 8 and 9 isoenzymes in the soluble and particulate fractions, respectively. Low O₂ suppressed the appearance of all isoenzymes, and the addition of 500 microliters per liter ethylene to the low oxygen atmosphere did not reverse this effect. Similarly, the decline in pectin methylesterase that was observed in air-ripened fruits was prevented by 2.5% O₂ alone and in combination with 500 microliters per liter ethylene.     

Effects of water vapor pressure deficit on photochemical and fluorescence yields in tobacco leaf tissue

The relationship between photochemical quantum yield (_s) and fluorescence yield have been investigated in leaf tissue from Nicotiana tabacum using CO₂ exchange and a modulated fluorescence measuring system. The quantum yield of CO₂ fixation at 1.6% (v/v) O₂ and limiting irradiance was reduced 20% by increasing the mean H₂O vapor pressure deficit (VPD) from 9.2 to 18.6 mbars. As [CO₂] and irradiance were varied, the intrinsic quantum yield of open photosystem II units (_s/q_Q where q_Q is the photochemical fluorescence quenching coefficient) declined linearly with the degree of nonphotochemical fluorescence quenching. The slope and y-intercept values for this function were significantly reduced when the mean VPD was 18.4 millibars relative to 8.9 millibars. Susceptibility of the leaf tissue to photoinhibition was unaffected by VPD. Elevated O₂ concentrations (20.5% v/v) reduced the intrinsic quantum yield of net CO₂ uptake due to the occurrence of O₂-reducing processes. However, the relative effect of high VPD compared to low VPD on intrinsic quantum yield was not dependent on the O₂ level. This suggests that the Mehler reaction does not mediate the response of quantum yield to elevated VPD. The results are discussed with regard to the possible role of transpiration stress in regulating dissipation of excitation by electron transport pathways other than noncyclic electron flow supporting reduction of CO₂ and/or O₂.