PHOTOSYNTHESIS AND PHOTOSYNTHESIS-COUPLED RESPIRATION IN NATURAL BIOFILMS QUANTIFIED WITH OXYGEN MICROSENSORS1

Photosynthesis and respiration were analyzed in natural biofilms by use of O₂ microsensors. Depth profiles of gross photosynthesis were obtained from the rate of decrease in O₂ concentration during the first few seconds following extinction of light, and net photosynthesis of the photic zone was calculated from O₂ concentration gradients measured at steady state. Respiration within the photic zone was calculated as the difference between gross and net photosynthesis. Two types of biofilms were investigated: one dominated by diatoms, and one dominated by cyanobacteria. High O₂/CO₂ ratios caused increased respiration especially within the diatom biofilm, which could indicate that photorespiration was a dominant O₂-consuming process. The rate of respiration was constant within both biofilms during the first 4.6 s following extinction of light, even when respiration was stimulated by high O₂/CO₂ ratio. The assumption of a constant rate of respiration during the dark period is an essential one for the determination of gross photosynthetic activity by use of O₂ microsensors. We here present the first evidence to substantiate this assumption. The results strongly suggest that gross photosynthesis as measured by use of O₂ microsensors may include carbon equivalents that are subsequently lost through photorespiration. Computer modeling of photosynthesis profiles measured after 1.1, 1.6, and 2.6 s of dark incubation illustrated how the actual photosynthesis profile could have appeared if it had been possible to do the determination at time 0. Diffusion of O₂ during the up to 4.6-s long dark incubations did not affect gross photosynthetic rate when integrated over all depths, but the apparent vertical distribution of the photosynthetic activity was strongly affected.     

Computational analysis of the oscillatory dynamics in the processes of CO₂ assimilation and photorespiration

The computational analysis of the model system consisting of the processes of CO₂ assimilation and photorespiration shows the appearance of sustained oscillations in the system which might reflect their presence in photosynthesizing cells. Concentrations of CO₂ and O₂ oscillate in opposite phases causing Rubisco switching continuously between the carboxylase (CO₂ assimilation) and the oxygenase (photorespiration) reactions. The results of modeling are consistent with carbon isotopic and other observed data. They show that the oscillation period varies from about 1 s to 3 s depending on the values of parameters taken. Too high concentrations of O₂ suppress the oscillations.     

Simultaneous measurement of oscillations in oxygen evolution and chlorophyll a fluorescence in leaf pieces

In spinach (Spinacia oleracea) and barley (Hordeum vulgare) leaves, chlorophyll a fluorescence and O₂ evolution have been measured simultaneously following re-illumination after a dark interval or when steady state photosynthesis has been perturbed by changes in the gas phase. In high CO₂ concentrations, both O₂ and fluorescence can display marked dampening oscillations that are antiparallel but slightly out of phase (a rise or fall in fluorescence anticipating a corresponding fall or rise in O₂ by about 10 to 15 seconds). Infrared gas analysis measurements showed that CO₂ uptake behaved like O₂ evolution both in the period of oscillation (about 1 minute) and in its relation to fluorescence. In the steady state, oscillations were initiated by increases in CO₂ or by increases or decreases in O₂. Oscillations in O₂ or CO₂ did not occur without associated oscillations in fluorescence and the latter were a sensitive indicator of the former. The relationship between such oscillations in photosynthetic carbon assimilation and chlorophyl a fluorescence is discussed in the context of the effect of ATP or NADPH consumption on known quenching mechanisms.     

CO(2) and O(2) Exchanges in the CAM Plant Ananas comosus (L.) Merr: Determination of Total and Malate-Decorboxylation-Dependent CO(2)-Assimilation Rates; Study of Light O(2)-Uptake

Photosynthesis and light O₂-uptake of the aerial portion of the CAM plant Ananas comosus (L.) merr. were studied by CO₂ and O₂ gas exchange measurements. The amount of CO₂ which was fixed during a complete day-night cycle was equal to the amount of total net O₂ evolved. This finding justifies the assumption that in each time interval of the light period, the difference between the rates of net O₂-evolution and of net light atmospheric CO₂-uptake give the rates of malate-decarboxylation-dependent CO₂ assimilation. Based upon this hypothesis, the following photosynthetic characteristics were observed: (a) From the onset of the light to midphase IV of CAM, the photosynthetic quotient (net O₂ evolved/net CO₂ fixed) was higher than 1. This indicates that malate-decarboxylation supplied CO₂ for the photosynthetic carbon reduction cycle during this period. (b) In phase III and early phase IV, the rate of CO₂ assimilation deduced from net O₂-evolution was 3 times higher than the maximum rate of atmospheric CO₂-fixation during phase IV. A conceivable explanation for this stimulation of photosynthesis is that the intracellular CO₂-concentration was high because of malate decarboxylation. (c) During the final hours of the light period, the photosynthetic quotient decreased below 1. This may be the result of CO₂-fixation by phosphoenolpyruvate-carboxylase activity and malate accumulation. Based upon this hypothesis, the gas exchange data indicates that at least 50% of the CO₂ fixed during the last hour of the light period was stored as malate. Light O₂-uptake determined with ¹⁸O₂ showed two remarkable characteristics: from the onset of the light until midphase IV the rate of O₂-uptake increased progressively; during the following part of the light period, the rate of O₂-uptake was 3.5 times higher than the maximum rate of CO₂-uptake. When malate decarboxylation was reduced or suppressed after a night in a CO₂-free atmosphere or in continuous illumination, the rate of O₂-uptake was higher than in the control. This supports the hypothesis that the low rate of O₂-uptake in the first part of the light period is due to the inhibition of photorespiration by increased intracellular CO₂ concentration because of malate decarboxylation. In view of the law of gas diffusion and the kinetic properties of the ribulose-1,5-bisphosphate carboxylase/oxygenase, O₂ and CO₂ gas exchange suggest that at the end of the light period the intracellular CO₂ concentration was very low. We propose that the high ratio of O₂-uptake/CO₂-fixation is principally caused by the stimulation of photorespiration during this period.     

Photosynthetic characteristics of coccoid marine cyanobacteria

Two strains of marine Synechococcus possessed a much greater potential for photorespiration than other marine algae we have studied. This conclusion was based on the following physiological and biochemical characteristics: a) a light-dependent O₂ inhibition of photosynthetic CO₂ assimilation at atmospheric O₂ concentrations. The degree of inhibition was dependent on the relative concentrations of dissolved O₂ and CO₂, being greatest at 100% O₂ with no extra bicarbonate added to the medium; b) actively photosynthesizing cells had high levels of ribulose-1,5-bisphosphate carboxylase compared with phosphoenolpyruvate carboxylase; ribulose-1,5-bisphosphate oxygenase activities were three times greater than ribulose-1,5-bisphosphate carboxylase activities; c) cells photosynthesizing in 21% O₂, showed significant ¹⁴C-labelling of phosphoglycolate and glycolate and the percentage of total carbon-14 incorporated into these two compounds increased when the O₂ concentration was 100%; d) at 100% O₂, there was a post-illumination enhanced rate of O₂ consumption, which was three times greater than dark respiration, and the rate declined with increasing bicarbonate concentrations. The inhibitory effect of O₂ on photosynthesis did not appear to be solely due to photorespiration, since O₂ inhibition of photosynthetic O₂ evolution was much greater than that of photosynthetic CO₂ assimilation. Also, O₂ inhibition of photosynthetic O₂ evolution declined only slightly with decreasing light intensities, while the inhibition of CO₂ assimilation declined rapidly with decreasing light intensity.     

Oscillatory character of carbon metabolism in photosynthesis: Arguments and facts

Plentiful experimental data on the primary photosynthetic assimilates, concerning mainly the carbon isotopic composition, were summarized. These data are considered based on the photosynthetic oscillatory model, according to which photosynthesis is an oscillatory process consisting of the CO₂ assimilation phase and the photorespiration phase. All the data considered and the results of numerical modeling are in agreement, thus confirming the validity of the model. They allow us to affirm the existence of sustained photosynthetic oscillations.     

An improved method for the simultaneous determination of photosynthetic O2 evolution and CO2 consumption in Rhizophora mucronata leaves

The photosynthetic gas-exchange has been assessed traditionally either as O₂ evolution or CO₂ consumption. In this study, we used a liquid-phase O₂ electrode combined with CO₂ optodes to examine simultaneously photosynthesis in intact leaves of mangrove Rhizophora mucronata. We verified suitable conditions for leaf photosynthetic rates by assessing pH levels and NaHCO₃ concentrations and compared these to the gas-exchange method at various PAR levels. The photosynthetic rate in response to pH exhibited a similar pattern both for O₂ evolution and CO₂ consumption, and higher rates were associated with intermediate pH compared with low and high pH values. The net photosynthetic quotient (PQ) of R. mucronata leaves ranged from 1.04–1.28. The PQ values, which were never lesser than 1, suggested that photorespiration did not occur in R. mucronata leaves under aqueous conditions. The similar maximum photosynthetic rates suggested that all measurements had a high capacity to adjust the photosynthetic apparatus under a light saturation condition. The simultaneous measurements of O₂ evolution and CO₂ consumption using the Clark oxygen electrode polarographic sensor with the CO₂ optode sensor provided a simple, stable, and precise measurement of PQ under aqueous and saturated light conditions.     

Photorespiratory Properties of Mesophyll Protoplasts of Nicotiana plumbaginifolia

The photorespiratory activity of mesophyll protoplasts of Nicotiana plumbaginifolia has been clearly demonstrated by the presence of a Warburg-effect, the occurrence of an important CO₂-sensitive O₂ uptake and the effect of some photorespiratory inhibitors on photosynthetic activity. At a nonsaturating dissolved inorganic carbon (DIC) concentration (0.1 millimolar), we observed that the rate of CO₂ fixation was 60% lower at 50% O₂ compared to that measured at 2% O₂. Using ¹⁸O₂ and mass spectrometry, we measured O₂ exchange as a function of light intensity and of DIC concentration. Oxygen uptake measured at the CO₂ compensation point (47.4 micromoles O₂ per hour per milligram chlorophyll) was three-fold higher than that measured at a saturating CO₂ concentration. Cyanide or iodoacetamide, inhibitors of the Calvin cycle, were found to reduce the O₂ uptake to the same extent as CO₂ saturation. We conclude from these results that the major part of the CO₂-sensitive O₂ uptake is due to photorespiration. Further, we investigated the effect on net photosynthesis of some inhibitors of the glycolate pathway. At CO₂ saturation (10 millimolar DIC), 5 millimolar aminoacetonitrile (AAN), and 1 millimolar aminooxyacetate (AOA) did not cause any significant decrease in net photosynthesis. However, when these two inhibitors were added under a period of active photorespiration (10 minutes at the CO₂ compensation point at 20% O₂), we observed a decrease in the rate of net photosynthesis at 10 millimolar DIC measured afterward (respectively, 18 and 29%). This inhibition did not appear at 2% O₂, but was stronger at 50% O₂ (40% for AAN and 47% for AOA). With 0.05 millimolar butyl 2-hydroxy-3-butynoate (BHB) or 0.5 millimolar l-methionine-dl-sulfoximine (l-MSO), rates of net photosynthesis at 10 millimolar DIC were decreased by 10 to 15%. Additional decreases were observed after a period at the CO₂ compensation point at 20% O₂ (30% for BHB and 20% for l-MSO). From the sites of action of the four inhibitors tested, we suggest the inhibition of photosynthesis occurring after a period of active photorespiration to be due to the toxic accumulation of nonmetabolized phosphoglycolate.     

Effect of oxygen on photosynthesis, photorespiration and respiration in detached leaves. I. Soybean

The effect of O₂ on the CO₂ exchange of detached soybean leaves was measured with a Clark oxygen electrode and infrared carbon dioxide analysers in both open and closed systems.

The rate of apparent photosynthesis was inhibited by O₂ while the steady rate of respiration after a few minutes in the dark was not affected. Part of the inhibition of apparent photosynthesis was shown to be a result of increased photorespiration. This stimulation of photorespiration by O₂ was manifested by an increase in the CO₂ compensation point.

The differential effects of O₂ on dark respiration (no effect) and photorespiration (stimulation) indicated that these were 2 different processes.

Moreover the extrapolation of the CO₂ compensation point to zero at zero O₂ indicated that dark respiration was suppressed in the light at least at zero O₂ concentration.

     

Nitrogen assimilation and transpiration: key processes conditioning responsiveness of wheat to elevated [CO2] and temperature

Although climate scenarios have predicted an increase in [CO₂] and temperature conditions, to date few experiments have focused on the interaction of [CO₂] and temperature effects in wheat development. Recent evidence suggests that photosynthetic acclimation is linked to the photorespiration and N assimilation inhibition of plants exposed to elevated CO₂. The main goal of this study was to analyze the effect of interacting [CO₂] and temperature on leaf photorespiration, C/N metabolism and N transport in wheat plants exposed to elevated [CO₂] and temperature conditions. For this purpose, wheat plants were exposed to elevated [CO₂] (400 vs 700 µmol mol^?1) and temperature (ambient vs ambient + 4°C) in CO₂ gradient greenhouses during the entire life cycle. Although at the agronomic level, elevated temperature had no effect on plant biomass, physiological analyses revealed that combined elevated [CO₂] and temperature negatively affected photosynthetic performance. The limited energy levels resulting from the reduced respiratory and photorespiration rates of such plants were apparently inadequate to sustain nitrate reductase activity. Inhibited N assimilation was associated with a strong reduction in amino acid content, conditioned leaf soluble protein content and constrained leaf N status. Therefore, the plant response to elevated [CO₂] and elevated temperature resulted in photosynthetic acclimation. The reduction in transpiration rates induced limitations in nutrient transport in leaves of plants exposed to elevated [CO₂] and temperature, led to mineral depletion and therefore contributed to the inhibition of photosynthetic activity.     

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.     

In situ estimation of net CO2 assimilation, photosynthetic electron flow and photorespiration in Turkey oak (Q. cerris L.) leaves: diurnal cycles under different levels of water supply

Diurnal time courses of net CO₂ assimilation rates, stomatal conductance and light-driven electron fluxes were measured in situ on attached leaves of 30-year-old Turkey oak trees (Quercus cerris L.) under natural summer conditions in central Italy. Combined measurements of gas exchange and chlorophyll a fluorescence under low O₂ concentrations allowed the demonstration of a linear relationship between the photochemical efficiency of PSII (fluorescence measurements) and the apparent quantum yield of gross photosynthesis (gas exchange). This relationship was used under normal O₂ to compute total light-driven electron fluxes, and to partition them into fractions used for RuBP carboxylation or RuBP oxygenation. This procedure also yielded an indirect estimate of the rate of photorespiration in vivo. The time courses of light-driven electron flow, net CO₂ assimilation and photorespiration paralleled that of photosynthetic photon flux density, with important afternoon deviations as soon as a severe drought stress occurred, whereas photochemical efficiency and maximal fluorescence underwent large but reversible diurnal decreases. The latter observation indicated the occurrence of a large non-photochemical energy dissipation at PSII. We estimated that less than 60% of the total photosynthetic electron flow was used for carbon assimilation at midday, while about 40% was devoted to photorespiration. The rate of carbon loss by photorespiration (R₁) reached mean levels of 56% of net assimilation rates. The potential application of this technique to analysis of the relative contributions of thermal de-excitation at PSII and photorespiratory carbon recycling in the protection of photosynthesis against stress effects is discussed.     

Conversion of photosynthetic products in the light in CO2-free O2 and N2 in leaves of Zea mays L. and Phaseolus vulgaris L.

Summary After 10 min illumination of segments of bean (Phaseolus vulgaris L.) or maize (Zea mays L.) leaves in air with ¹⁴CO₂, the atmosphere was changed to CO₂-free O₂ or N₂ and conversion of photosynthetic products in the light was investigated. The experiments have shown that after the ¹⁴CO₂ assimilation period the bean leaves contain the pool of weakly fixed ¹⁴C (WF-¹⁴C) which is converted into stable products during the subsequent period of illumination in CO₂-free N₂. In O₂ atmosphere the WF-¹⁴C pool is initially the main source of CO₂ evolved. The marked decrease in radioactivity of sucrose and starch during illumination of bean leaves in O₂ atmosphere indicates that these compounds were also the source of CO₂ evolved in the light. The total amount of previously fixed ¹⁴C remained almost on the same level during illumination of maize leaves in N₂ as well as in O₂. However, oxygen changed the distribution of ¹⁴C in photosynthetic products, which is suggested to be the consequence of the photorespiration process in maize.Abbreviation WF-¹⁴C weakly fixed ¹⁴C     

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.     

Post‐illumination transient O2‐uptake is driven by photorespiration in tobacco leaves

This study aims to elucidate the molecular mechanism for the transient increase in the O₂‐uptake rate in tobacco (Nicotiana tabacum cv Xanthi) leaves after turning off actinic lights (ALs). The photosynthetic O₂ evolution rate reaches a maximum shortly after the onset of illumination with ALs and then decreases to zero in atmospheric CO₂/O₂ conditions. After turning off the ALs, tobacco leaves show a transient increase in the O₂‐uptake rate, the post‐illumination transient O₂‐uptake, and thereafter, the O₂‐uptake rate decreases to the level of the dark‐respiration rate. Photosynthetic linear electron flow, evaluated as the quantum yield of photosystem II [Y(II)], maintained a steady‐state value distinct from the photosynthetic O₂‐evolution rate. In high‐[CO₂] conditions, the photosynthetic O₂‐evolution rate and Y(II) showed a parallel behavior, and the post‐illumination transient O₂‐uptake was suppressed. On the other hand, in maize leaves (a C4 plant), even in atmospheric CO₂/O₂ conditions, Y(II) paralleled the photosynthetic O₂‐evolution rate and the post‐illumination transient O₂‐uptake was suppressed. Hypothesizing that the post‐illumination transient O₂‐uptake is driven by C3 plant photorespiration in tobacco leaves, we calculated both the ribulose 1,5‐bisphosphate carboxylase‐ and oxygenase‐rates (Vc and Vo) from photosynthetic O₂‐evolution and the post‐illumination transient O₂‐uptake rates. These values corresponded to those estimated from simultaneous chlorophyll fluorescence/O₂‐exchange analysis. Furthermore, the H⁺‐consumption rate for ATP synthesis in both photosynthesis and photorespiration, calculated from both Vc and Vo that were estimated from chlorophyll fluorescence/CO₂‐exchange analysis, showed a positive linear relationship with the dissipation rate of the electrochromic shift signal. Thus, these findings support our hypothesis.     

Compartmental modelling of photorespiration and carbon metabolism of water stressed leaves

Abstract Carbon fluxes in photosynthesis and photorespiration of water stressed leaves have been analysed in a steady state model based on the ribulose diphosphate carboxylase (RuDP carboxylase) and RuDP oxygenase enzyme activities and the CO₂ and O₂ concentrations in the leaf. Agreement between predicted and observed photorespiration (Lawlor & Fock, 1975) and C flux in the glycollate pathway is good over much of the range of water stress, but not at severe stress. An alternative source of respiratory CO₂ is suggested to explain the discrepancy. The model suggests that resistance to CO₂ fixation is mainly in the carboxylation reactions, not in CO₂ transport. Using the steady state model, the kinetics of ¹⁴C incorporation into photosynthetic and photorespiratory intermediates are simulated. The predicted rate of ¹⁴C incorporation is faster than observed and delay terms in the model are used to simulate the slow rates of mixing and metabolic reactions. Inactive pools of glycine and serine are suggested to explain the observed specific activities of glycine and serine. Three models of carbon flux between the glycollate pathway, the photosynthetic carbon reduction cycle and sucrose synthesis are considered. The most satisfactory simulation is for glycollate pathway carbon feeding into the PCR cycle pool of 3-phosphoglyceric acid which provides the carbon for sucrose synthesis. Simulation of the specific activity of CO₂ released in photorespiration suggests that a source of unlabelled carbon may contribute to photorespiration.     

Two photosynthetic mechanisms mediating the low photorespiratory state in submersed aquatic angiosperms

The submersed angiosperms Myriophyllum spicatum L. and Hydrilla verticillata (L.f.) Royal exhibited different photosynthetic pulse-chase labeling patterns. In Hydrilla, over 50% of the ¹⁴C was initially in malate and aspartate, but the fate of the malate depended upon the photorespiratory state of the plant. In low photorespiration Hydrilla, malate label decreased rapidly during an unlabeled chase, whereas labeling of sucrose and starch increased. In contrast, for high photorespiration Hydrilla, malate labeling continued to increase during a 2-hour chase. Thus, malate formation occurs in both photorespiratory states, but reduced photorespiration results when this malate is utilized in the light. Unlike Hydrilla, in low photorespiration Myriophyllum, ¹⁴C incorporation was via the Calvin cycle, and less than 10% was in C₄ acids.

Ethoxyzolamide, a carbonic anhydrase inhibitor and a repressor of the low photorespiratory state, increased the label in glycolate, glycine, and serine of Myriophyllum. Isonicotinic acid hydrazide increased glycine labeling of low photorespiration Myriophyllum from 14 to 25%, and from 12 to 48% with high photorespiration plants. Similar trends were observed with Hydrilla. Increasing O₂ increased the per cent [¹⁴C]glycine and the O₂ inhibition of photosynthesis in Myriophyllum. In low photorespiration Myriophyllum, glycine labeling and O₂ inhibition of photosynthesis were independent of the CO₂ level, but in high photorespiration plants the O₂ inhibition was competitively decreased by CO₂. Thus, in low but not high photorespiration plants, glycine labeling and O₂ inhibition appeared to be uncoupled from the external [O₂]/[CO₂] ratio.

These data indicate that the low photorespiratory states of Hydrilla and Myriophyllum are mediated by different mechanisms, the former being C₄-like, while the latter resembles that of low CO₂-grown algae. Both may require carbonic anhydrase to enhance the use of inorganic carbon for reducing photorespiration.

     

Inhibition of photorespiration and increase of net photosynthesis in isolated maize bundle sheath cells treated with glutamate or aspartate

Net photosynthetic ¹⁴CO₂ fixation by isolated maize (Zea mays) bundle sheath strands was stimulated 20 to 35% by the inclusion of l-glutamate or l-aspartate in the reaction mixture. Maximal stimulation occurred at a 7.5 mm concentration of either amino acid. Since the photosynthetic rate and the glutamate-dependent stimulation in the rate were equally sensitive to a photosynthetic electron transport inhibitor, 3-(p-chlorophenyl)-1,1-dimethylurea, it was concluded that glutamate did not stimulate CO₂ fixation by supplying needed NADPH (NADH) through glutamate dehydrogenase. Treatment of the bundle sheath strands with glutamate inhibited glycolate synthesis by 59%. Photorespiration in this tissue, measured as the O₂ inhibition of CO₂ fixation (the Warburg effect), was inhibited by treatment with glutamate. The stimulation in net photosynthetic CO₂ fixation probably results from the decrease in photorespiratory CO₂ loss. This metabolic regulation of the rate of glycolate synthesis and photorespiration observed with isolated bundle sheath strands could account for the inability to detect rapid photorespiration in the mature intact maize leaf.     

A Transient Burst of CO(2) from Geranium Leaves during Illumination at Various Light Intensities as a Measure of Photorespiration

A transient CO₂ burst is exhibited by irradiated leaves of the C₃ plant geranium (Pelargonium X hortorum, Bailey) after the irradiance is quickly lowered. The light CO₂ burst appears to be related to photorespiration because of its irradiance dependency and its sensitivity to other environmental components such as CO₂ and O₂ concentration. The term post-lower-irradiance CO₂ burst or PLIB is used to describe the phenomenon. The PLIB appears to be a quantitative measurement of photorespiration with intact geranium leaves. The PLIB has been observed with intact leaves of other C₃ plants but not with C₄ leaves. Therefore, it is proposed that, after maximizing intact leaf photosynthetic rates and leaf chamber gas measuring conditions, photorespiration can be measured with intact C₃ leaves such as geranium as a transient post-lower-irradiance CO₂ burst.