Estimation of Photorespiration Based on the Initial Rate of Postillumination CO(2) Release: I. A Nonsteady State Model for Measurement of CO(2) Exchange Transients

Although open systems have been used for the study of transients in leaf CO₂ exchange such as the postillumination burst, these systems frequently do not permit reliable estimates of transient rates due to their nonsteady state nature. A nonsteady state mathematical approach is described which predicts changes in CO₂ concentration in the leaf chamber and infrared gas analyzer measuring cell as a function of leaf CO₂ exchange rate in Nicotiana tabacum vars John Williams Broadleaf and Havana Seed. With the aid of a computer, a numerical formula simulates the mixing and dilution which occurs as CO₂ passes through the finite volume of the measuring cell of the analyzer. The method is presented with special relevance to photorespiration as manifested by the postillumination burst of CO₂. The latter is suggested to decline with the first order kinetics following darkening of a C₃ leaf. This approach provides a basis for reliable estimation of the initial and, hence, maximal rate of CO₂ evolution during the postillumination burst under a variety of environmental conditions.     

Effect of CO(2), O(2), and Light on Photosynthesis and Photorespiration in Wheat

Unidirectional O₂ fluxes were measured with ¹⁸O₂ in a whole plant of wheat cultivated in a controlled environment. At 2 or 21% O₂, O₂ uptake was maximum at 60 microliters per liter CO₂. At lower CO₂ concentrations, it was strongly inhibited, as was photosynthetic O₂ evolution. At 2% O₂, there remained a substantial O₂ uptake, even at high CO₂ level; the O₂ evolution was inhibited at CO₂ concentrations under 330 microliters per liter. The O₂ uptake increased linearly with light intensity, starting from the level of dark respiration. No saturation was observed at high light intensities. No significant change in the gas-exchange patterns occurred during a long period of the plant life. An adaptation to low light intensities was observed after 3 hours illumination. These results are interpreted in relation to the functioning of the photosynthetic apparatus and point to a regulation by the electron acceptors and a specific action of CO₂. The behavior of the O₂ uptake and the study of the CO₂ compensation point seem to indicate the persistence of mitochondrial respiration during photosynthesis.     

Quantitation of the O(2)-Dependent, CO(2)-Reversible Component of the Postillumination CO(2) Exchange Transient in Tobacco and Maize Leaves

The postillumination transient of CO₂ exchange and its relation to photorespiration has been examined in leaf discs from tobacco (Nicotiana tabacum) and maize (Zea mays). Studies of the transients observed by infrared gas analysis at 1, 21, and 43% O₂ in an open system were extended using the nonsteady state model described previously (Peterson and Ferrandino 1984 Plant Physiol 76: 976-978). Cumulative CO₂ exchange equivalents (i.e. nanomoles CO₂) versus time were derived from the analyzer responses of individual transients. In tobacco (C₃), subtraction of the time course of cumulative CO₂ exchange under photorespiratory conditions (21 or 43% O₂) from that obtained under nonphotorespiratory conditions (1% O₂) revealed the presence of an O₂-dependent and CO₂-reversible component within the first 60 seconds following darkening. This component was absent in maize (C₄) and at low external O₂:CO₂ ratios (i.e. <100) in tobacco. The size of the component in tobacco increased with net photosynthesis as irradiance was increased and was positively associated with inhibition of net photosynthesis by O₂. This relatively simple and rapid method of analysis of the transient is introduced to eliminate some uncertainties associated with estimation of photorespiration based on the maximal rate of postillumination CO₂ evolution. This method also provides a useful and complementary tool for detecting variation in photorespiration.     

Effects of O(2) Tension and Temperature during Light Uptake of CO(2) on the Dark Release of CO(2) by Excised C(3) and C(4) Leaves

A study was conducted on a C₄ (Panicum maximum) and a C₃ (Panicum bisulcatum) species to determine the nature of the dark release of ¹⁴CO₂ with respect to its responses to changes in temperature and O₂ tension during light CO₂ uptake of ¹⁴CO₂.     

Environmental Effects on Photorespiration of C(3)-C(4) Species : I. Influence of CO(2) and O(2) during Growth on Photorespiratory Characteristics and Leaf Anatomy

The possibility of altering CO₂ exchange of C₃-C₄ species by growing them under various CO₂ and O₂ concentrations was examined. Growth under CO₂ concentrations of 100, 350, and 750 micromoles per mole had no significant effect on CO₂ exchange characteristics or leaf anatomy of Flaveria pringlei (C₃), Flaveria floridana (C₃-C₄), or Flaveria trinervia (C₄). Carboxylation efficiency and CO₂ compensation concentrations in leaves of F. floridana developed under the different CO₂ concentrations were intermediate to F. pringlei and F. trinervia. When grown for 12 days at an O₂ concentration of 20 millimoles per mole, apparent photosynthesis was strongly inhibited in Panicum milioides (C₃-C₄) and to a lesser degree in Panicum laxum (C₃). In P. milioides, acute starch buildup was observed microscopically in both mesophyll and bundle sheath cells. Even after only 4 days exposure to 20 millimoles per mole O₂, the presence of starch was more pronounced in leaf cross-sections of P. milioides compared to those at 100 and 210 millimoles per mole. Even though this observation suggests that P. milioides has a different response to low O₂ with respect to translocation of photosynthate or sink activity than C₃ species, the concentration of total available carbohydrate increased in shoots of all species by 33% or more when grown at low O₂. This accumulation occurred even though relative growth rates of Festuca arundinacea (C₃) and P. milioides grown for 4 days at 210 millimoles per mole O₂, were inhibited 83 and 37%, respectively, when compared to plants grown at 20 millimoles per mole O₂.     

Quantum Yields for CO(2) Uptake in C(3) and C(4) Plants: Dependence on Temperature, CO(2), and O(2) Concentration

The quantum yields of C₃ and C₄ plants from a number of genera and families as well as from ecologically diverse habitats were measured in normal air of 21% O₂ and in 2% O₂. At 30 C, the quantum yields of C₃ plants averaged 0.0524 ± 0.0014 mol CO₂/absorbed einstein and 0.0733 ± 0.0008 mol CO₂/absorbed einstein under 21 and 2% O₂. At 30 C, the quantum yields of C₄ plants averaged 0.0534 ± 0.0009 mol CO₂/absorbed einstein and 0.0538 ± 0.0011 mol CO₂/absorbed einstein under 21 and 2% O₂. At 21% O₂, the quantum yield of a C₃ plant is shown to be strongly dependent on both the intercellular CO₂ concentration and leaf temperature. The quantum yield of a C₄ plant, which is independent of the intercellular CO₂ concentration, is shown to be independent of leaf temperature over the ranges measured. The changes in the quantum yields of C₃ plants are due to changes in the O₂ inhibition. The evolutionary significance of the CO₂ dependence of the quantum yield in C₃ plants and the ecological significance of the temperature effects on the quantum yields of C₃ and C₄ plants are discussed.     

Photorespiration and Internal CO(2) Accumulation in Chara corallina as Inferred from the Influence of DIC and O(2) on Photosynthesis

An O₂ electrode system with a specially designed chamber for `whorl' cell complexes of Chara corallina was used to study the combined effects of inorganic carbon and O₂ concentrations on photosynthetic O₂ evolution. At pH = 5.5 and 20% O₂, cells grown in HCO₃⁻ medium (low CO₂, pH ≥ 9.0) exhibited a higher affinity for external CO₂ (K_½(CO₂) = 40 ± 6 micromolar) than the cells grown for at least 24 hours in high-CO₂ medium (pH = 6.5), (K_½(CO₂) = 94 ± 16 micromolar). With O₂ ≤ 2% in contrast, both types of cells showed a high apparent affinity (K_½(CO₂) = 50 − 52 micromolar). A Warburg effect was detectable only in the low affinity cells previously cultivated in high-CO₂ medium (pH = 6.5). The high-pH, HCO₃⁻-grown cells, when exposed to low pH (5.5) conditions, exhibited a response indicating an ability to fix CO₂ which exceeded the CO₂ externally supplied, and the reverse situation has been observed in high-CO₂-grown cells. At pH 8.2, the apparent photosynthetic affinity for external HCO₃⁻ (K_½[HCO₃⁻]) was 0.6 ± 0.2 millimolar, at 20% O₂. But under low O₂ concentrations (≤2%), surprisingly, an inhibition of net O₂ evolution was elicited, which was maximal at low HCO₃⁻ concentrations. These results indicate that: (a) photorespiration occurs in this alga and can be revealed by cultivation in high-CO₂ medium, (b) Chara cells are able to accumulate CO₂ internally by means of a process apparently independent of the plasmalemma HCO₃⁻ transport system, (c) molecular oxygen appears to be required for photosynthetic utilization of exogenous HCO₃⁻: pseudocyclic electron flow, sustained by O₂ photoreduction, may produce the additional ATP needed for the HCO₃⁻ transport.     

Effects of O(2) and CO(2) Concentration on the Steady-State Fluorescence Yield of Single Guard Cell Pairs in Intact Leaf Discs of Tradescantia albiflora: Evidence for Rubisco-Mediated CO(2) Fixation and Photorespiration in Guard Cells

A procedure for following changes in the steady-state yield of chlorophyll a fluorescence (F_s) from single guard cell pairs in variegated leaves of Tradescantia albiflora is described. As an indicator of photosynthetic electron transport, F_s is a very sensitive indirect measure of the balance of adenosine 5′-triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH), producing reactions with the sink reactions that utilize those light-generated products. We found that F_s under constant light is sensitive to manipulation of ambient CO₂ concentrations, as would be expected if either phosphoenolpyruvate carboxylase or ribulose-1, 5 bisphosphate carboxylase/oxygenase (Rubisco)-dependent CO₂ fixation is the sink for photosynthetic ATP and NADPH in guard cells. However, we also found that changing O₂ concentration had a strong effect on fluorescence yield, and that O₂ sensitivity was only evident when the concentration of CO₂ was low. This finding provides evidence that both O₂ and CO₂ can serve as sinks for ATP and NADPH produced by photosynthetic electron transport in guard cell chloroplasts. Identical responses were observed with mesophyll cell chloroplasts in intact leaves. This finding is difficult to reconcile with the view that guard cell chloroplasts have fundamentally different pathways of photosynthetic metabolism from other chloroplasts in C₃ plants. Indeed, Rubisco has been detected at low levels in guard cell chloroplasts, and our studies indicate that it is active in the pathways for photosynthetic carbon reduction and photorespiration in guard cells.     

Effects of CO(2) and O(2) on the Photosynthetic O(2) Evolution of Spirodela polyrrhiza Turions

Net photosynthetic rates of Spirodela polyrrhiza turions, at low O₂ levels, were 6.2 and 38.8 micromoles O₂ per gram fresh weight per hour at 1 millimolar HCO₃⁻ and CO₂ saturation, respectively, and much lower in a regular low-pH growth solution. Air equilibration O₂ concentrations decreased rates considerably, except at CO₂ saturation. The surfacing rate of turions in various inorganic carbon surroundings correlated positively with their photosynthetic rates, but were the same at high and low O₂ levels. The relevance of these findings in relation to environmental conditions conductive to germination of autotrophically growing turions is discussed.     

The Effect of pH, O(2), and Temperature on the CO(2) Compensation Point of Isolated Asparagus Mesophyll Cells

The effect of pH, O₂ concentration, and temperature on the CO₂ compensation point (Г[CO₂]) of isolated Asparagus sprengeri Regel mesophyll cells has been determined in a closed, aqueous environment by a sensitive gas-chromatographic technique. Measured values range between 10 and 100 microliters per liter CO₂ depending upon experimental conditions. The Г(CO₂) increases with increasing temperature. The rate of increase is dependent upon the O₂ concentration and is more rapid at high (250-300 micromolar), than at low (30-60 micromolar), O₂ concentrations. The differential effect of temperature on Г(CO₂) is more pronounced at pH 6.2 than at pH 8.0, but this pH-dependence is not attributable to a direct, differential effect of pH on the relative rates of photosynthesis and photorespiration, as the O₂-sensitive component of Г(CO₂) remains constant over this range. The Г(CO₂) of Asparagus cells at 25°C decreases by 50 microliters per liter when the pH is raised from 6.2 to 8.0, regardless of the prevailing O₂ concentration. It is suggested that the pH-dependence of Г(CO₂) is related to the ability of the cell to take up CO₂ from the aqueous environment. The correlation between high HCO₃⁻ concentrations and low Г(CO₂) at alkaline pH indicates that extracellular HCO₃⁻ facilitates the uptake of CO₂, possibly by increasing the flux of inorganic carbon from the bulk medium to the cell surface. The strong O₂− and temperature-dependence of Г(CO₂) indicates that isolated Asparagus mesophyll cells lack an efficient means for concentrating intracellular CO₂ to a level sufficient to reduce or suppress photorespiration.     

C4 Photosynthesis (The CO2-Concentrating Mechanism and Photorespiration)

Despite previous reports of no apparent photorespiration in C4 plants based on measurements of gas exchange under 2 versus 21% O2 at varying [CO2], photosynthesis in maize (Zea mays) shows a dual response to varying [O2]. The maximum rate of photosynthesis in maize is dependent on O2 (approximately 10%). This O2 dependence is not related to stomatal conductance, because measurements were made at constant intercellular CO2 concentration (Ci); it may be linked to respiration or pseudocyclic electron flow. At a given Ci, increasing [O2] above 10% inhibits both the rate of photosynthesis, measured under high light, and the maximum quantum yield, measured under limiting light ([phi]CO2). The dual effect of O2 is masked if measurements are made under only 2 versus 21% O2. The inhibition of both photosynthesis and [phi]CO2 by O2 (measured above 10% O2) with decreasing Ci increases in a very similar manner, characteristically of O2 inhibition due to photorespiration. There is a sharp increase in O2 inhibition when the Ci decreases below 50 [mu]bar of CO2. Also, increasing temperature, which favors photorespiration, causes a decrease in [phi]CO2 under limiting CO2 and 40% O2. By comparing the degree of inhibition of photosynthesis in maize with that in the C3 species wheat (Triticum aestivum) at varying Ci, the effectiveness of C4 photosynthesis in concentrating CO2 in the leaf was evaluated. Under high light, 30[deg]C, and atmospheric levels of CO2 (340 [mu]bar), where there is little inhibition of photosynthesis in maize by O2, the estimated level of CO2 around ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the bundle sheath compartment was 900 [mu]bar, which is about 3 times higher than the value around Rubisco in mesophyll cells of wheat. A high [CO2] is maintained in the bundle sheath compartment in maize until Ci decreases below approximately 100 [mu]bar. The results from these gas exchange measurements indicate that photorespiration occurs in maize but that the rate is low unless the intercellular [CO2] is severely limited by stress.     

Effect of CO(2) Concentration on Glycine and Serine Formation during Photorespiration

Amount and products of photosynthesis during 10 minutes were measured at different ¹⁴CO₂ concentrations in air. With tobacco (Nicotiana tabacum L. cv. Maryland Mammoth) leaves the percentage of ¹⁴C in glycine plus serine was highest (42%) at 0.005% CO₂, and decreased with increasing CO₂ concentration to 7% of the total at 1% CO₂ in air. However, above 0.03% CO₂ the total amount of ¹⁴C incorporated into the glycine and serine pool was about constant. At 0.005% or 0.03% CO₂ the percentage and amount of ¹⁴C in sucrose was small but increased greatly at higher CO₂ levels as sucrose accumulated as an end product. Relatively similar data were obtained with sugar beet (Beta vulgaris L. cv. US H20) leaves. The results suggest that photorespiration at high CO₂ concentration is not inhibited but that CO₂ loss from it becomes less significant.     

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.     

C4 Photosynthesis (The Effects of Leaf Development on the CO2-Concentrating Mechanism and Photorespiration in Maize)

The effect of O2 on photosynthesis was determined in maize (Zea mays) leaves at different developmental stages. The optimum level of O2 for maximum photosynthetic rates was lower in young and senescing tissues (2-5 kPa) than in mature tissue (9 kPa). Inhibition of photosynthesis by suboptimal levels of O2 may be due to a requirement for functional mitochondria or to cyclic/pseudocyclic photophosphorylation in chloroplasts; inhibition by supraoptimal levels of O2 is considered to be due to photorespiration. Analysis of a range of developmental stages (along the leaf blade and at different leaf ages and positions) showed that the degree of inhibition of photosynthesis by supraoptimal levels of O2 increased rapidly once the ribulose-1,5-bisphosphate carboxylase/oxygenase and chlorophyll contents were below a critical level and was similar to that of C3 plants. Tissue having a high sensitivity of photosynthesis to O2 may be less effective in concentrating CO2 in the bundle sheath cells due either to limited function of the C4 cycle or to higher bundle sheath conductance to CO2. An analysis based on the kinetic properties of ribulose-1,5-bisphosphate carboxylase/oxygenase was used to predict the maximum CO2 level concentrated in bundle sheath cells at a given degree of inhibition of photosynthesis by supraoptimal levels of 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.     

Effects of Hydroxylamine on Photosystem II: II. Photoreversal of the NH(2)OH Destruction of O(2) Evolution

The inactivation of O₂-evolving centers by NH₂OH extraction was shown to be reversible. This reversal required light and manganese. This light-induced restoration of active O₂-evolving centers was analyzed using three green algae and the blue-green alga, Anacystis nidulans. The following results were obtained: [List: see text]     

Assimilatory Power (Postillumination CO(2) Uptake) in Leaves: Measurement, Environmental Dependencies, and Kinetic Properties

Assimilatory power was measured in ten C₃ species by means of a rapid-response gas exchange device as the total amount of CO₂ fixed in N₂-CO₂ atmosphere after switching the light off. Different steady-state levels of the assimilatory power were obtained by varying light intensity and O₂ and CO₂ concentrations during the preexposition periods in the leaf chamber.

Within the limits of the linear part of the CO₂ curve of photosynthesis in N₂, the assimilatory power is constant, being sufficient for the assimilation of about 20 nanomoles CO₂ per square centimeter leaf. The pool starts to decrease with the onset of the CO₂ saturation of photosynthesis. Increase in O₂ concentration from 0 to 100% at 350 microliters CO₂ per liter produces a considerable decrease in the assimilatory power.

The mesophyll conductance (M) was found to be proportional to the assimilatory power (A): M = mA. The most frequently occurring values of the proportionality constant (m) (called the specific efficiency of carboxylation) were concentrated between 0.03 and 0.04 centimeter per second per nanomole A per square centimeter but the measured extreme values were 0.01 and 0.06 centimeter per second per nanomole A per square centimeter. The specific rate of carboxylation (the rate per unit A) showed a hyperbolic dependence on CO₂ conentration with the most frequent values of K_m (CO₂) ranging from 25 to 35 micromolar in the liquid phase of mesophyll cells (extremes 23 and 100 micromolar).

It is concluded that the CO_2⁻ and light-saturated rate of photosynthesis is limited by the reactions of the formation of the assimilatory power and not by ribulose-1,5-bisphosphate carboxylase. O₂ is a competitive consumer of the assimilatory power, and the inhibitory effect of O₂ on photosynthesis is caused mainly by a decrease in the pool of the assimilatory power at high O₂ concentrations. In intact leaves, the kinetic properties of ribulose-1,5-bisphosphate carboxylase seem to be variable.

     

Effects of O(2) and CO(2) Concentrations on Quantum Yields of Photosystems I and II in Tobacco Leaf Tissue

The interactive effects of irradiance and O₂ and CO₂ levels on the quantum yields of photosystems I and II have been studied under steady-state conditions at 25°C in leaf tissue of tobacco (Nicotiana tabacum). Assessment of radiant energy utilization in photosystem II was based on changes in chlorophyll fluorescence yield excited by a weak measuring beam of modulated red light. Independent estimates of photosystem I quantum yield were based on the light-dark in vivo absorbance change at 830 nanometers, the absorption band of P700⁺. Normal (i.e. 20.5%, v/v) levels of O₂ generally enhanced photosystem II quantum yield relative to that measured under 1.6% O₂ as the irradiance approached saturation. Photorespiration is suspected to mediate such positive effects of O₂ through increases in the availability of CO₂ and recycling of orthophosphate. Conversely, at low intercellular CO₂ concentrations, 41.2% O₂ was associated with lower photosystem II quantum yield compared with that observed at 20.5% O₂. Inhibitory effects of 41.2% O₂ may occur in response to negative feedback on photosystem II arising from a build-up in the thylakoid proton gradient during electron transport to O₂. Covariation between quantum yields of photosystems I and II was not affected by concentrations of either O₂ or CO₂. The dependence of quantum yield of electron transport to CO₂ measured by gas exchange upon photosystem II quantum yield as determined by fluorescence was unaffected by CO₂ concentration.     

Photorespiration and carbon concentrating mechanisms: two adaptations to high O2, low CO2 conditions

This review presents an overview of the two ways that cyanobacteria, algae, and plants have adapted to high O₂ and low CO₂ concentrations in the environment. First, the process of photorespiration enables photosynthetic organisms to recycle phosphoglycolate formed by the oxygenase reaction catalyzed by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Second, there are a number of carbon concentrating mechanisms that increase the CO₂ concentration around Rubisco which increases the carboxylase reaction enhancing CO₂ fixation. This review also presents possibilities for the beneficial modification of these processes with the goal of improving future crop yields.     

Estimation of Photorespiration of Green Plants and of their Mesophyll Resistance to CO2 Uptake

An estimate of photorespiration is obtained from the relationshipbetween the net exchange of CO₂ of the leaf and the internalCO₂ concentration, i.e. within the mesophyll intercellular spaces.The latter is obtained by calculation, taking into account thecombined epidermal and boundary-layer resistances between thebulk atmosphere and the mesophyll intercellular spaces. Thelinear part of this relationship (at low CO₂ concentrations)is extrapolated to zero internal concentration, at which noneof the intercellular photorespired CO₂ is available for reassimilation.The calculated output of CO₂ under such conditions providesan estimate of photorespiration, but, by failing to take intoaccount intracellular reassimilation of photorespired CO₂ underestimatesactual photorespiration. As the slope of this linear relationshiprepresents the mesophyll (intracellular) resistance to CO₂ uptake,this procedure was used to recalculate published data on effectsof light intensity and of oxygen concentration on net photosynthesis.The analysis showed that increased oxygen concentration anddecreased light intensity reduced photosynthesis largely byincreasing mesophyll resistance to CO₂ uptake. It is suggestedthat the CO₂ compensation point () is a function of both photorespiration(L) and mesophyll resistance (r_m): = L. r_m.