Oxygen inhibition of photosynthesis

The effect of leaf temperature, O₂ and calculated O₂/CO₂ solubility ratio in the leaf on the quantum yield of photosynthesis was studied for the C₄ species, Zea mays L., and the C₃ species, Triticum aestivum L. Over a range of leaf temperatures of 16 to 35° C, the quantum yield of Z. mays was relatively constant and was similar under 1.5 and 21% O₂, being ca. 0.059 mol CO₂ mol^-1 quanta absorbed. Under 1.5% O₂ and atmospheric levels of CO₂, the quantum yield of T. aestivum was relatively constant (0.083 mol CO₂ mol^-1 quanta absorbed) at leaf temperatures from 15 to 35° C. Atmospheric levels of O₂ (21%) reduced the quantum yield of photosynthesis in T. aestivum and as leaf temperature increased, the quantum yield decreased from 0.062 at 15°C to 0.046 mol CO₂ mol^-1 quanta absorbed at 35°C. Increasing temperature decreases the solubility of CO₂ relatively more than the solubility of O₂, resulting in an increased solubility ratio of O₂/CO₂. Experimentally manipulating the atmospheric levels of O₂ or CO₂ to maintain a near-constant solubility ratio of O₂/CO₂ at varying leaf temperatures largely prevented the temperature-dependent decrease in quantum yield in t. aestivum. Thus, the decrease in quantum yield with increasing leaf temperature in C₃ species may be largely caused by a temperaturedependent change in the solubility ratio of O₂/CO₂.J and II=Ku and Edwards, 1977a, b     

Dependence of the Post-Illumination Burst of CO2 on Temperature, Light, CO2, and O2 Concentration in Wheat (Triticum aestivum)

The effect of environmental factors on the post-illumination burst of CO₂ (PIB) and O₂ inhibition of apparent photosynthesis (APS) in wheat (Triticum aestivum L.) was studied in an open gas exchange system utilizing the mathematics of non-steady-state systems. Two components of inhibition by O₂ are suggested: one is caused by photorespiration as measured from the maximum rate of the PIB, and the second is direct inhibition as taken as APS_2%O2— (APS_x%O2+ PIB_x%O2) where X is the oxygen concentration. A primary PIB which occurred from 16–28 s after the darkening of the foliage was attributed to photorespiration. No primary PIB was observed at 2% O₂. At a CO₂ concentration of 100 μ/1 in the atmosphere (about 2.5 μM based on leaf intercellular concentration) and at 30°C and 145 nE/cm² nE/cm²·s, APS decreased curve-linearly with increasing O₂ and reached an O₂ compensation point of 560 μM (48% by volume), above which there was a net loss of CO₂ in the light. The PIB increased with increasing O₂ and became saturated at about 500 μM O₂ but decreased above 900 μM O₂. Direct inhibition of photosynthesis by O₂ increased with increasing O₂ concentration. Decreasing CO₂ concentration had an effect on the magnitude of the PIB similar to that of increasing O₂. At 30°C and 21% O₂, the PIB increased with decreasing CO₂ down to the CO₂ compensation point (I) of 1.4 μM (47 μM/l). Below Γ, both PIB and CO₂ evolution into the air in the light (at 21% O₂) increased and then decreased at CO₂ below 0.8 μM. The ratio of the PIB to APS_{2% o} O₂ increased linearly with increasing O₂/CO₂ ratio where O₂ was held constant at 21% and CO₂ was varied from 1.4 to 8.5 μM, while direct inhibition of photosynthesis expressed as a proportion of APS_2%O2 remained constant over this range. At low CO₂ concentration photorespiration as estimated by the PIB is the major part of O₂ photosynthesis, while at atmospheric CO₂ levels, direct inhibition is the major component. The PIB and APS at 2% and 21% O₂ increased hyperbolically with increasing irradiance and all became light-saturated at about 65 nE/cm₂ s. The percentage total O₂ inhibition of photosynthesis remained constant with increasing irradiance as did the relative contribution of direct O₂ inhibition or photorespiration (PIB) to total O₂ inhibition. The PIB and APS at 21% O₂ had similar temperature optima of 30°C when experimental conditions were adjusted to provide a constant internal O₂/CO₂ solubility ratio at varying temperatures. However, with a constant external CO₂ concentration, the temperature optimum for the PIB shifted upward to 35°C while that for APS at 21% O₂ remained at 30°C, which may be due to an increased O₂/CO₂ concentration in the leaf with increasing temperature.     

Oxygen inhibition of photosynthesis and stimulation of photorespiration in soybean leaf cells

The occurrence of photorespiration in soybean (Glycine max [L.] Merr.) leaf cells was demonstrated by the presence of an O₂-dependent CO₂ compensation concentration, a nonlinear time course for photosynthetic ¹⁴CO₂ uptake at low CO₂ and high O₂ concentrations, and an O₂ stimulation of glycine and serine synthesis which was reversed by high CO₂ concentration. The compensation concentration was a linear function of O₂ concentration and increased as temperature increased. At atmospheric CO₂ concentration, 21% O₂ inhibited photosynthesis at 25 C by 27%. Oxygen inhibition of photosynthesis was competitive with respect to CO₂ and increased with increasing temperature. The Km (CO₂) of photosynthesis was also temperature-dependent, increasing from 12 μm CO₂ at 15 C to 38 μm at 35 C. In contrast, the Ki (O₂) was similar at all temperatures. Oxygen inhibition of photosynthesis was independent of irradiance except at 10 mm bicarbonate and 100% O₂, where inhibition decreased with increasing irradiance up to the point of light saturation of photosynthesis. Concomitant with increasing O₂ inhibition of photosynthesis was an increased incorporation of carbon into glycine and serine, intermediates of the photorespiratory pathway, and a decreased incorporation into starch. The effects of CO₂ and O₂ concentration and temperature on soybean cell photosynthesis and photorespiration provide further evidence that these processes are regulated by the kinetic properties of ribulose-1,5-diphosphate carboxylase with respect to CO₂ and O₂.     

Regulation of Soybean Net Photosynthetic CO(2) Fixation by the Interaction of CO(2), O(2), and Ribulose 1,5-Diphosphate Carboxylase

Kinetic properties of soybean net photosynthetic CO₂ fixation and of the carboxylase and oxygenase activities of purified soybean (Glycine max [L.] Merr.) ribulose 1, 5-diphosphate carboxylase (EC 4.1.1.39) were examined as functions of temperature, CO₂ concentration, and O₂ concentration. With leaves, O₂ inhibition of net photosynthetic CO₂ fixation increased when the ambient leaf temperature was increased. The increased inhibition of CO₂ fixation at higher temperatures was caused by a reduced affinity of the leaf for CO₂ and an increased affinity of the leaf for O₂. With purified ribulose 1,5-diphosphate carboxylase, O₂ inhibition of CO₂ incorporation and the ratio of oxygenase activity to carboxylase activity increased with increased temperature. The increased O₂ sensitivity of the enzyme at higher temperature was caused by a reduced affinity of the enzyme for CO₂ and a slightly increased affinity of the enzyme for O₂. The similarity of the effect of temperature on the affinity of intact leaves and of ribulose 1,5-diphosphate carboxylase for CO₂ and O₂ provides further evidence that the carboxylase regulates the O₂ response of photosynthetic CO₂ fixation in soybean leaves. Based on results reported here and in the literature, a scheme outlining the stoichiometry between CO₂ and O₂ fixation in vivo is proposed.     

O(2)-insensitive photosynthesis in c(3) plants : its occurrence and a possible explanation

Leaves of C₃ plants which exhibit a normal O₂ inhibition of CO₂ fixation at less than saturating light intensity were found to exhibit O₂-insensitive photosynthesis at high light. This behavior was observed in Phaseolus vulgaris L., Xanthium strumarium L., and Scrophularia desertorum (Shaw.) Munz. O₂-insensitive photosynthesis has been reported in nine other C₃ species and usually occurred when the intercellular CO₂ pressure was about double the normal pressure. A lack of O₂ inhibition of photosynthesis was always accompanied by a failure of increased CO₂ pressure to stimulate photosynthesis to the expected degree. O₂-insensitive photosynthesis also occurred after plants had been water stressed. Under such conditions, however, photosynthesis became O₂ and CO₂ insensitive at physiological CO₂ pressures. Postillumination CO₂ exchange kinetics showed that O₂ and CO₂ insensitivity was not the result of elimination of photorespiration.

It is proposed that O₂ and CO₂ insensitivity occurs when the concentration of phosphate in the chloroplast stroma cannot be both high enough to allow photophosphorylation and low enough to allow starch and sucrose synthesis at the rates required by the rest of the photosynthetic component processes. Under these conditions, the energy diverted to photorespiration does not adversely affect the potential for CO₂ assimilation.

     

Effect of carbon dioxide and temperature on photosynthetic CO2 uptake and photorespiratory CO2 evolution in sunflower leaves

Using an open gas-exchange system, apparent photosynthesis, true photosynthesis (TPS), photorespiration (PR) and dark respiration of sunflower (Helianthus annuus L.) leaves were determined at three temperatures and between 50 and 400 l/l external CO₂. The ratio of PR/TPS and the solubility ratio of O₂/CO₂ in the intercellular spaces both decreased with increasing CO₂. The rate of PR was not affected by the CO₂ concentration in the leaves and was independent of the solubility ratio of oxygen and CO₂ in the leaf cell. At photosynthesis-limiting concentrations of CO₂, the ratio of PR/TPS significantly increased from 18 to 30°C and the rate of PR increased from 4.3 mg CO₂ dm^-2 h^-1 at 18°C to 8.6 mg CO₂ dm^-2 h^-1 at 30°C. The specific activity of photorespired CO₂ was CO₂-dependent but temperature-independent, and the carbon traversing the glycolate pathway appeared to be derived both from recently fixed assimilate and from older reserve materials. It is concluded that PR as a percentage of TPS is affected by the concentrations of O₂ and CO₂ around the photosynthesizing cells, but the rate of PR may also be controlled by other factors.Abbreviations APS apparent photosynthesis (net CO₂ uptake) - PR photorespiration (CO₂ evolution in light) - RuBP ribulose-1,5-bisphosphate - TPS true photosynthesis (true CO₂ uptake)     

Control of Photosynthesis and Stomatal Conductance in Ricinus communis L. (Castor Bean) by Leaf to Air Vapor Pressure Deficit

Castor bean (Ricinus communis L.) has a high photosynthetic capacity under high humidity and a pronounced sensitivity of photosynthesis to high water vapor pressure deficit (VPD). The sensitivity of photosynthesis to varying VPD was analyzed by measuring CO₂ assimilation, stomatal conductance (g_s), quantum yield of photosystem II (_II), and nonphotochemical quenching of chlorophyll fluorescence (q_N) under different VPD. Under both medium (1000) and high (1800 micromoles quanta per square meter per second) light intensities, CO₂ assimilation decreased as the VPD between the leaf and the air around the leaf increased. The g_s initially dropped rapidly with increasing VPD and then showed a slower decrease above a VPD of 10 to 20 millibars. Over a temperature range from 20 to 40°C, CO₂ assimilation and g_s were inhibited by high VPD (20 millibars). However, the rate of transpiration increased with increasing temperature at either low or high VPD due to an increase in g_s. The relative inhibition of photosynthesis under photorespiring (atmospheric levels of CO₂ and O₂) versus nonphotorespiring (700 microbars CO₂ and 2% O₂) conditions was greater under high VPD (30 millibars) than under low VPD (3 millibars). Also, with increasing light intensity the relative inhibition of photosynthesis by O₂ increased under high VPD, but decreased under low VPD. The effect of high VPD on photosynthesis under various conditions could not be totally accounted for by the decrease in the intercellular CO₂ in the leaf (C_i) where C_i was estimated from gas exchange measurements. However, estimates of C_i from measurements of _II and q_N suggest that the decrease in photosynthesis and increase in photorespiration under high VPD can be totally accounted for by stomatal closure and a decrease in C_i. The results also suggest that nonuniform closure of stomata may occur in well-watered plants under high VPD, causing overestimates in the calculation of C_i from gas exchange measurements. Under low VPD, 30°C, high light, and saturating CO₂, castor bean (C₃ tropical shrub) has a rate of photosynthesis (61 micromoles CO₂ per square meter per second) that is about 50% higher than that of tobacco (C₃) or maize (C₄) under the same conditions. The chlorophyll content, total soluble protein, and ribulose-1,5-bisphosphate carboxylase/oxygenase level on a leaf area basis were much higher in castor bean than in maize or tobacco, which accounts for its high rates of photosynthesis under low VPD.     

Estimation of Photorespiration Based on the Initial Rate of Postillumination CO(2) Release: II. Effects of O(2), CO(2), and Temperature

An open system associated with an infrared gas analyzer was employed to study transients in CO₂ exchange generated upon darkening preilluminated leaf discs of tobacco (Nicotiana tabacum vars John Williams Broadleaf and Havana Seed). An empirical formula presented previously enabled prediction of the analyzer response under nonsteady state conditions as a function of time and of the leaf CO₂ exchange rate. A computer was used to evaluate parameters of the leaf CO₂ release rate to provide an estimate of the initial rate of postillumination CO₂ evolution and to produce maximal agreement between predicted and observed analyzer responses. In 21% O₂, the decline in rate of CO₂ evolution upon darkening followed first order kinetics. Initial rates of CO₂ evolution following darkening were relatively independent of the prior ambient CO₂ concentrations. However, rates of photorespiration expressed as a fraction of net photosynthesis declined rapidly with increasing external CO₂ concentration at 21% O₂. Under normal atmospheric conditions, photorespiration was 45 to 50% of the net CO₂ fixation rate at 32°C and high irradiance. The rapid initial CO₂ evolution observed upon darkening at 21% O₂ was absent in 3% O₂. Rates of photorespiration under normal atmospheric concentrations of CO₂ and O₂ as measured by the postillumination burst were highly dependent upon temperature (observed activation energy = 30.1 kilocalories per mole). The results are discussed with respect to previously published estimates of photorespiration in C₃ leaf tissue.     

Photorespiration in Air and High CO(2)-Grown Chlorella pyrenoidosa

Oxygen inhibition of photosynthesis and CO₂ evolution during photorespiration were compared in high CO₂-grown and air-grown Chlorella pyrenoidosa, using the artificial leaf technique at pH 5.0. High CO₂ cells, in contrast to air-grown cells, exhibited a marked inhibition of photosynthesis by O₂, which appeared to be competitive and similar in magnitude to that in higher C₃ plants. With increasing time after transfer to air, the photosynthetic rate in high CO₂ cells increased while the O₂ effect declined. Photorespiration, measured as the difference between ¹⁴CO₂ and ¹²CO₂ uptake, was much greater and sensitive to O₂ in high CO₂ cells. Some CO₂ evolution was also present in air-grown algae; however, it did not appear to be sensitive to O₂. True photosynthesis was not affected by O₂ in either case. The data indicate that the difference between high CO₂ and air-grown algae could be attributed to the magnitude of CO₂ evolution. This conclusion is discussed with reference to the oxygenase reaction and the control of photorespiration in algae.     

Selection and characterization of tobacco plants with novel o(2)-resistant photosynthesis

Plants were obtained with novel O₂-resistant photosynthetic characteristics. At low CO₂ (250-350 μL CO₂ L⁻¹) and 30°C when O₂ was increased from 1% to 21% to 42%, the ratio of net CO₂ uptake in O₂-resistant whole plants or leaf discs compared to wild type increased progressively, and this was not related to stomatal opening. Dihaploid plantlets regenerated from anther culture were initially screened and selected for O₂-resistant growth in 42% O₂/160 μL CO₂ L⁻¹ and 0.18% of the plantlets showed O₂-resistant photosynthesis. About 30% of the progeny (6 of 19 plants) of the first selfing of a fertile plant derived from a resistant dihaploid plant had O₂-resistant photosynthesis, and after a second selfing this increased to 50% (6 of 12 plants). In 21% O₂ and low CO₂, net photosynthesis of the resistant plants was about 15% greater on a leaf area basis than wild type. Net photosynthesis was compared in leaf discs at 30 and 38°C in 21% O₂, and at the higher temperature O₂-resistant plants showed still greater photosynthesis than wild type. The results suggest that the O₂-resistant photosynthesis described here is associated with a decreased stoichiometry of CO₂ release under conditions of rapid photorespiration. This view was supported by the finding that leaves of O₂-resistant plants averaged 40% greater catalase activity than wild type.     

Effect of glycidate on glycolate formation and photosynthesis in isolated spinach chloroplasts

Glycidate (2,3-epoxypropionate) increased CO₂ photoassimilation in intact spinach (Spinacia oleracea L.) chloroplasts in the presence of various inhibitors of photosynthesis, including O₂, arsenite, azide, iodo-acetamide, and carbonylcyanide 3-chlorophenylhydrazone. Although the mechanism by which glycidate enhances photosynthesis is obscure, the stimulatory effect cannot be ascribed to either an inhibition of glycolate formation, a specific interaction with the O₂ inhibition of photosynthesis, or a direct effect on the ribulose 1,5-bisphosphate carboxylase (EC 4.1.1.39) reaction. The lack of a differential effect of glycidate on photosynthesis and glycolate formation in the isolated chloroplast was confirmed in whole leaf studies by the CO₂ compensation concentration assay. These results are at variance with the report that glycidate stimulates net photosynthesis in tobacco leaf disks by irreversibly inhibiting glycolate formation and thus photorespiration (Zelitch, I., 1974, Arch. Biochem. Biophys. 163: 367-377).     

Photosynthetic Characteristics of C(3)-C(4) Intermediate Flaveria Species : I. Leaf Anatomy, Photosynthetic Responses to O(2) and CO(2), and Activities of Key Enzymes in the C(3) and C(4) Pathways

Four species of the genus Flaveria, namely F. anomala, F. linearis, F. pubescens, and F. ramosissima, were identified as intermediate C₃-C₄ plants based on leaf anatomy, photosynthetic CO₂ compensation point, O₂ inhibition of photosynthesis, and activities of C₄ enzymes. F. anomala and F. ramosissima exhibit a distinct Kranz-like leaf anatomy, similar to that of the C₄ species F. trinervia, while the other C₃-C₄ intermediate Flaveria species possess a less differentiated Kranz-like leaf anatomy. Photosynthetic CO₂ compensation points of these intermediates at 30°C were very low relative to those of C₃ plants, ranging from 7 to 14 microliters per liter. In contrast to C₃ plants, net photosynthesis by the intermediates was not sensitive to O₂ concentrations below 5% and decreased relatively slowly with increasing O₂ concentration. Under similar conditions, the percentage inhibition of photosynthesis by 21% O₂ varied from 20% to 25% in the intermediates compared with 28% in Lycopersicon esculentum, a typical C₃ species. The inhibition of carboxylation efficiency by 21% O₂ varied from 17% for F. ramosissima to 46% for F. anomala and were intermediate between the C₄ (2% for F. trinervia) and C₃ (53% for L. esculentum) values. The intermediate Flaveria species, especially F. ramosissima, have substantial activities of the C₄ enzymes, phosphoenolpyruvate carboxylase, pyruvate, orthophosphate dikinase, NADP-malic enzyme, and NADP-malate dehydrogenase, indicating potential for C₄ photosynthesis. It appears that these Flaveria species may be true biochemical C₃-C₄ intermediates.     

Rapid isolation of mesophyll cells from leaves of soybean for photosynthetic studies

Mesophyll cells were rapidly isolated from soybean (Glycine max [L.]) leaves using a combined Macerase enzyme-stirring technique. About 50% to 70% of the leaf cells on a chlorophyll basis from 3 grams of leaves could be isolated in 15 minutes. The cells obtained by this method were capable of high rates of photosynthesis even after storage in the dark for periods of up to 9 hours. The CO₂-saturated rate of photosynthesis increased from 5 μm CO₂/mg Chl·hour at 5 C to 170 μm CO₂/mg Chl·hour at 40 C. At atmospheric CO₂ concentration, the rate varied from 5 to 55 μm CO₂/mg Chl·hour over this temperature range. The reduced temperature response of photosynthesis at low CO₂ concentration was due to an increased Km(CO₂) of the cells with increasing temperature. The products of photosynthesis in the isolated cells were similar to the products of leaf photosynthesis.     

Effects of Irradiance on the in Vivo CO(2):O(2) Specificity Factor in Tobacco Using Simultaneous Gas Exchange and Fluorescence Techniques

The effects of gas phase O₂ concentration (1%, 20.5%, and 42.0%, v/v) on the quantum yield of net CO₂ fixation and fluorescence yield of chlorophyll a are examined in leaf tissue from Nicotiana tabacum at normal levels of CO₂ and 25 to 30°C. Detectable decreases in nonphotochemical quenching of absorbed excitation occurred at the higher O₂ levels relative to 1% O₂ when irradiance was nearly or fully saturating for photosynthesis. Photochemical quenching was increased by high O₂ levels only at saturating irradiance. Simultaneous measurements of CO₂ and H₂O exchange and fluorescence yield permit estimation of partitioning of linear photosynthetic electron transport between net CO₂ fixation and O₂-dependent, dissipative processes such as photorespiration as a function of leaf internal CO₂ concentration. Changes in the in vivo CO₂:O₂ `specificity factor' (K_sp) with increasing irradiance are examined. The magnitude K_sp was found to decline from a value of 85 at moderate irradiance to 68 at very low light, and to 72 at saturating photon flux rates. The results are discussed in terms of the applicability of the ribulose bisphosphate carboxylase/oxygenase enzyme model to photosynthesis in vivo.     

Effect of Temperature, CO(2) Concentration, and Light Intensity on Oxygen Inhibition of Photosynthesis in Wheat Leaves

The effect of 21% O₂ and 3% O₂ on the CO₂ exchange of detached wheat leaves was measured in a closed system with an infrared carbon dioxide analyzer. Temperature was varied between 2° and 43°, CO₂ concentration between 0.000% and 0.050% and light intensity between 40 ft-c and 1000 ft-c. In most conditions, the apparent rate of photosynthesis was inhibited in 21% O₂ compared to 3% O₂. The degree of inhibition increased with increasing temperature and decreasing CO₂ concentration. Light intensity did not alter the effect of O₂ except at light intensities or CO₂ concentrations near the compensation point. At high CO₂ concentrations and low temperature, O₂ inhibition of apparent photosynthesis was absent. At 3% O₂, wheat resembled tropical grasses in possessing a high rate of photosynthesis, a temperature optimum for photosynthesis above 30°, and a CO₂ compensation point of less than 0.0005% CO₂. The effect of O₂ on apparent photosynthesis could be ascribed to a combination of stimulation of CO₂ production during photosynthesis, and inhibition of photosynthesis itself.     

Estimation of Mesophyll Conductance to CO(2) Flux by Three Different Methods

The resistance to diffusion of CO₂ from the intercellular airspaces within the leaf through the mesophyll to the sites of carboxylation during photosynthesis was measured using three different techniques. The three techniques include a method based on discrimination against the heavy stable isotope of carbon, ¹³C, and two modeling methods. The methods rely upon different assumptions, but the estimates of mesophyll conductance were similar with all three methods. The mesophyll conductance of leaves from a number of species was about 1.4 times the stomatal conductance for CO₂ diffusion determined in unstressed plants at high light. The relatively low CO₂ partial pressure inside chloroplasts of plants with a low mesophyll conductance did not lead to enhanced O₂ sensitivity of photosynthesis because the low conductance caused a significant drop in the chloroplast CO₂ partial pressure upon switching to low O₂. We found no correlation between mesophyll conductance and the ratio of internal leaf area to leaf surface area and only a weak correlation between mesophyll conductance and the proportion of leaf volume occupied by air. Mesophyll conductance was independent of CO₂ and O₂ partial pressure during the measurement, indicating that a true physical parameter, independent of biochemical effects, was being measured. No evidence for CO₂-accumulating mechanisms was found. Some plants, notably Citrus aurantium and Simmondsia chinensis, had very low conductances that limit the rate of photosynthesis these plants can attain at atmospheric CO₂ level.     

CO2 photoassimilation and chlorophyll fluorescence in two clover species showing different response to O3

The current study evaluated photosynthetic processes in two clover species based on their performance under high O₃ conditions (150 nl l^–1 for 3 h). These species are Trifolium repens L. and Trifolium pratense L., which are well known for their different sensitivity to ozone. Ozone affected the two clover species very differently. In T. pratense, ozone induced visible symptoms of damage even though CO₂ photoassimilation, stomatal conductance and the electron transport rate were not affected. A decrease in the optimal quantum yield was observed in T. pratense immediately after the end of the period of O₃ stress but it reverted to a value similar to the control 24 h after removing the stress, indicating that non-irreversible photoinhibition had occurred. Data obtained for symptomatic T. pratense indicate that still-green living tissue of the leaf is able to carry out CO₂ assimilation; the only parameter found to be affected by O₃ was the efficiency of excitation capture. In T. repens, acute O₃ fumigation induced inhibition of photosynthetic activity, enhanced stomatal closure and increased the reduction state of the PSII primary acceptor. A possible explanation for the inhibition of photosynthesis could reside in inhibition of the Calvin cycle over-reducing PSII, thus increasing (1 – q_P) and increasing non-photochemical quenching.     

Degree of C(4) Photosynthesis in C(4) and C(3)-C(4)Flaveria Species and Their Hybrids : II. Inhibition of Apparent Photosynthesis by a Phosphoenolpyruvate Carboxylase Inhibitor

Hybrids have been made between species of Flaveria exhibiting varying levels of C₄ photosynthesis. The degree of C₄ photosynthesis expressed in four interspecific hybrids (Flaveria trinervia [C₄] × F. linearis [C₃-C₄], F. brownii [C₄-like] × F. linearis, and two three-species hybrids from F. trinervia × [F. brownii × F. linearis]) was estimated by inhibiting phosphoenolpyruvate carboxylase in vivo with 3,3-dichloro-2-dihydroxyphosphinoylmethyl-2-propenoate (DCDP). The inhibitor was fed to detached leaves at a concentration of 4 mm, and apparent photosynthesis was measured at atmospheric levels of CO₂ and at 20 and 210 mL L⁻¹ of O₂. Photosynthesis at 210 mL L⁻¹ of O₂ was inhibited 32% by DCDP in F. linearis, by 60% in F. brownii, and by 87% in F. trinervia. Inhibition in the hybrids ranged from 38 to 52%. The inhibition of photosynthesis by 210 mL L⁻¹ of O₂ was increased when DCDP was used, except in the C₄ species, F. trinervia, in which photosynthesis was insensitive to O₂. Except for F. trinervia, control plants with less O₂ sensitivity (more C₄-like) exhibited a progressively greater change in O₂ inhibition of photosynthesis when treated with DCDP. This increased O₂ inhibition probably resulted from decreased CO₂ concentrations in bundle sheath cells due to inhibition of phosphoenolpyruvate carboxylase. The inhibition of photosynthesis by DCDP is concluded to underestimate the degree of C₄ photosynthesis in the interspecific hybrids because increased direct assimilation of atmospheric CO₂ by ribulose bisphosphate carboxylase may compensate for inhibition of phosphoenolpyruvate carboxylase.     

Inhibition of photosynthesis by ethylene-a stomatal effect

Ethylene at hormonally significant levels inhibited net photosynthesis of the cultivated peanut (Arachis hypogaea L.) as measured by gas analysis. Upon the removal of ethylene, the inhibition was naturally overcome at the concentration-exposure duration combinations tested. Increased length of exposure of 1 microliter of ethylene per liter of air up to 6 hours increased the degree of net photosynthesis inhibition (68% reduction after 6-hour exposure). Significantly greater inhibition of photosynthesis by ethylene was detected on peanut genotypes having higher photosynthetic efficiency. In contrast to peanut, hormonal concentrations of ethylene only moderately inhibited sweet potato, Jerusalem artichoke, and sunflower photosynthesis and was without effect on beans, peas, Irish potato, Mimosa pudica, and white clover. No inhibition could be found by ethylene on ribulose 1,5-biphosphate carboxylase activity in vitro. Photosynthesis was lowered at all CO₂ concentrations below ambient at an O₂ concentration of 1.5%, indicating that the action of ethylene was not affected by low O₂; concomitantly, an increase in the CO₂ compensation point occurred. Diffusion resistance measurements of leaf water vapor loss made on ethylene-treated peanut leaves showed a measurable decrease in leaf conductance which correlated with net photosynthesis decrease. Ethylene influenced the conductance of abaxial stomata more so than adaxial.     

Photorespiration and Oxygen Inhibition of Photosynthesis in Chlorella pyrenoidosa

The inhibition of photosynthesis by O₂ in air-grown Chlorella pyrenoidosa was investigated using three experimental techniques (artificial leaf, aqueous method, and O₂ electrode) to measure carbon assimilation. CO₂ response curves were determined under different O₂, pH, and temperature conditions. Regardless of the experimental technique and condition, O₂ inhibition was not evident until a concentration of 50% was reached; V_max values were reduced whereas K_m (CO₂) values were unaffected by the increasing O₂ concentration. The response of photosynthesis to O₂ was independent of CO₂ and HCO₃⁻ concentrations as well as temperature. Relative rates of photosynthesis showed a 4 to 5% stimulation in 2% O₂, a 12% inhibition in 50% O₂, and a 24% inhibition in 100% O₂. The inhibition by 50% O₂ was still reversible after 20 minutes exposure whereas 100% O₂ caused irreversible inhibition after only 4 minutes.