Evidence for the occurrence of photorespiration in synurophyte algae

The fluxes of CO(2) and oxygen during photosynthesis by cell suspensions of Tessellaria volvocina and Mallomonas papillosa were monitored mass spectrometrically. There was no rapid uptake of CO(2,) only a slow drawdown to compensation concentrations of 26 μM for T. volvocina and 18 μM for M. papillosa, when O(2) evolution ceased, indicating a lack of active bicarbonate uptake by the cells. Darkening of the cells after a period of photosynthesis did not cause rapid release of CO(2), indicating the absence of an intracellular inorganic carbon pool. However, upon darkening a brief burst of CO(2) was observed similar to the post-illumination burst characteristic of C(3) higher plants. Treatment of the cells of both species with the membrane-permeable carbonic anhydrase inhibitor ethoxyzolamide had no adverse effect on photosynthetic rate, but stimulated the dark CO(2) burst indicating the dark oxidation of a compound formed in the light. In the absence of any active accumulation of inorganic carbon photosynthesis in these species should be inhibited by O(2). This was investigated in four synurophyte species T. volvocina, M. papillosa, Synura petersenii, and Synura uvella: photosynthetic O(2) evolution rates in all four algae, measured by O(2) electrode, were significantly higher (40-50%) in media at low O(2) (4%) than in air-equilibrated (21% O(2)) media, indicating an O(2) inhibition of photosynthesis (Warburg effect) and thus the occurrence of photorespiration in these species.     

Inorganic Carbon-Stimulated O2 Photoreduction Is Suppressed by NO2- Assimilation in Air-Grown Cells of Synechococcus UTEX 625

The effect of NO2- assimilation on O2 exchange and CO2 fixation of the cyanobacterium, Synechococcus UTEX 625, was studied mass spectrometrically. Upon addition of 1 mM inorganic carbon to the medium, inorganic carbon pools developed and accelerated O2 photoreduction 5-fold when CO2 fixation was inhibited. During steady-state photosynthesis at saturating light, O2 uptake represented 32% of O2 evolution and balanced that portion of O2 evolution that could not be accounted for by CO2 fixation. Under these conditions, NO2- assimilation reduced O2 uptake by 59% but had no influence on CO2 fixation. NO2- assimilation decreased both CO2 fixation and O2 photoreduction at low light and and increased net O2 evolution at all light intensities. The increase in net O2 evolution observed during simultaneous assimilation of carbon and nitrogen over carbon alone was due to a suppression of O2 photoreduction by NO2- assimilation. When CO2 fixation was precluded, NO2- assimilation inhibited O2 photoreduction and stimulated O2 evolution. When the electron supply was limiting (low light), competition among O2, CO2, and NO2- for electrons could be observed, but when the electron supply was not limiting (saturating light), O2 photoreduction and/or NO2- reduction caused electron transport that was additive to that for maximum CO2 fixation.     

A long-lasting photorespiration in CO2-free air, measured as the postirradiation CO2 burst, indicates mobilization of storage photosynthates

In CO2-free air, the CO2 postirradiation burst (PIB) in wheat leaves was measured with an IRGA in an open gas exchange system to ascertain its potential role in alleviating photoinhibition of photorespiratory carbon oxidation (PCO) under a CO2 deficiency. A pre-photosynthesized leaf having been transferred into CO2-free air exhibited a typical CO2 PIB following darkening which could last, with a rate substantially higher than that of dark respiration, over a long time period (at least more than 2 h) of continuously alternate irradiation (2 min)-dark (2 min)-light transitions. The rate and the time of PIB maintenance, although unaffected by the exogenous dark respiration inhibitor iodoacetic acid, were stimulated largely by increasing irradiance and O2 level, and suppressed by DCMU and N-ethyl-maleimide (NEM). They also showed a large photosynthates-loading dependence. In a darkened leaf, the irradiation-induced PIB in the CO2-free air was clearly revealed and it was characterized by an initial net uptake of respiratory CO2. The light-induced PIB was accelerated by increasing irradiance, and delayed by prolonging the period of darkening the leaves. Hence, the origin of carbon needed for a long-term CO2 evolution in the CO2-free air might not only be derived directly from the pool of intermediates in the Calvin cycle, but it might also arise indirectly from a remotely fixed reserve of photosynthates in the leaf via a PCO-mediated, yet to be further clarified, mobilization process. Such mobilization of photosynthates probably exerted an important role in coordination of photochemical reactions and carbon assimilation during photosynthesis in C3 plants under the photoinhibitory conditions.     

Effect of local irradiance on CO(2) transfer conductance of mesophyll in walnut

The acclimation responses of walnut leaf photosynthesis to the irradiance microclimate were investigated by characterizing the photosynthetic properties of the leaves sampled on young trees (Juglans nigraxregia) grown in simulated sun and shade environments, and within a mature walnut tree crown (Juglans regia) in the field. In the young trees, the CO(2) compensation point in the absence of mitochondrial respiration (Gamma*), which probes the CO(2) versus O(2) specificity of Rubisco, was not significantly different in sun and shade leaves. The maximal net assimilation rates and stomatal and mesophyll conductances to CO(2) transfer were markedly lower in shade than in sun leaves. Dark respiration rates were also lower in shade leaves. However, the percentage inhibition of respiration by light during photosynthesis was similar in both sun and shade leaves. The extent of the changes in photosynthetic capacity and mesophyll conductance between sun and shade leaves under simulated conditions was similar to that observed between sun and shade leaves collected within the mature tree crown. Moreover, mesophyll conductance was strongly correlated with maximal net assimilation and the relationships were not significantly different between the two experiments, despite marked differences in leaf anatomy. These results suggest that photosynthetic capacity is a valuable parameter for modelling within-canopies variations of mesophyll conductance due to leaf acclimation to light.     

Partitioning of the Leaf CO2 Exchange into Components Using CO2 Exchange and Fluorescence Measurements

Photorespiration was calculated from chlorophyll fluorescence and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) kinetics and compared with CO2 evolution rate in the light, measured by three gas-exchange methods in mature sunflower (Helianthus annuus L.) leaves. The gas-exchange methods were (a) postillumination CO2 burst at unchanged CO2 concentration, (b) postillumination CO2 burst with simultaneous transfer into CO2-free air, and (c) extrapolation of the CO2 uptake to zero CO2 concentration at Rubisco active sites. The steady-state CO2 compensation point was proportional to O2 concentration, revealing the Rubisco specificity coefficient (Ksp) of 86. Electron transport rate (ETR) was calculated from fluorescence, and photorespiration rate was calculated from ETR using CO2 and O2 concentrations, Ksp, and diffusion resistances. The values of the best-fit mesophyll diffusion resistance for CO2 ranged between 0.3 and 0.8 s cm-1. Comparison of the gas-exchange and fluorescence data showed that only ribulose-1,5-bisphosphate (RuBP) carboxylation and photorespiratory CO2 evolution were present at limiting CO2 concentrations. Carboxylation of a substrate other than RuBP, in addition to RuBP carboxylation, was detected at high CO2 concentrations. A simultaneous decarboxylation process not related to RuBP oxygenation was also detected at high CO2 concentrations in the light. We propose that these processes reflect carboxylation of phosphoenolpyruvate, formed from phosphoglyceric acid and the subsequent decarboxylation of malate.     

The quantum yield of CO2 fixation is reduced for several minutes after prior exposure to darkness. Exploration of the underlying causes

Previous work has shown that the apparent quantum yield of CO2 fixation can be reduced for up to several minutes after prior exposure to darkness. In the work reported here, we investigated this phenomenon more fully and have deduced information about the underlying processes. This was done mainly by concurrent measurements of O2 and CO2 exchange in an oxygen-free atmosphere. Measurements of O2 evolution indicated that photochemical efficiency was not lost through dark adaptation, and that O2 evolution could proceed immediately at high rates provided that there were reducible pools of photosynthetic intermediates. Part of the delay in reaching the full quantum yield of CO2 fixation could be attributed to the need to build up pools of photosynthetic intermediates to high enough levels to support steady rates of CO2 fixation. There was no evidence that Rubisco inactivation contributed towards delayed CO2 uptake (under measurement conditions of low light). However, we obtained evidence that an enzyme in the reaction path between triose phosphates and RuBP must become completely inactivated in the dark. As a consequence, in dark-adapted leaves, a large amount of triose phosphates were exported from the chloroplast over the first minute of light rather than being converted to RuBP for CO2 fixation. That pattern was not observed if the pre-incubation light level was increased to just 3-5 micromol quanta m(-2) s(-1). The findings from this work underscore that there are fundamental differences in enzyme activation between complete darkness and even a very low light level of only 3-5 micromol quanta m(-2) s(-1) which predispose leaves to different gas exchange patterns once leaves are transferred to higher light levels.     

Evidence for the Contribution of the Mehler-Peroxidase Reaction in Dissipating Excess Electrons in Drought-Stressed Wheat

Gross O2 evolution and uptake by attached, drought-stressed leaves of wheat (Triticum aestivum) were measured using a 16O2/ 18O2 isotope technique and mass spectrometry. The activity of photosystem II, determined from the rate of 16O2 evolution, is only slightly affected under drought conditions. During drought stress, net CO2 uptake decreases due to stomatal closure, whereas the uptake of 18O2 is stimulated. The main O2-consuming reactions in the light are the Mehler-peroxidase (MP) reaction and the photorespiratory pathway. From measurements of the rate of carbon flux through the photorespiratory pathway, estimated by the analysis of the specific radioactivities of glycolate, we conclude that the rate of photorespiration is decreased with drought stress. Therefore, the O2 taken up in the light appears to be preferentially used by the MP reaction. In stressed leaves, 29.1% of the photosynthetic electrons are consumed in the MP reaction and 18.4% drive the photorespiratory pathway. Thus, overreduction of the electron transport chain is avoided preferably by the MP reaction when drought stress restricts CO2 reduction.     

12CO2 emission from different metabolic pathways measured in illuminated and darkened C3 and C4 leaves at low,atmospheric and elevated CO2 concentration

The detection of 12CO2 emission from leaves in air containing 13CO2 allows simple and fast determination of the CO2 emitted by different sources, which are separated on the basis of their labelling velocity. This technique was exploited to investigate the controversial effect of CO2 concentration on mitochondrial respiration. The 12CO2 emission was measured in illuminated and darkened leaves of one C4 plant and three C3 plants maintained at low (30-50 ppm), atmospheric (350-400 ppm) and elevated (700-800 ppm) CO2 concentration. In C3 leaves, the 12CO2 emission in the light (Rd) was low at ambient CO2 and was further quenched in elevated CO2, when it was often only 20-30% of the 12CO2 emission in the dark, interpreted as the mitochondrial respiration in the dark (Rn). Rn was also reduced in elevated CO2. At low CO2, Rd was often 70-80% of Rn, and a burst of 12CO2 was observed on darkening leaves of Mentha sativa and Phragmites australis after exposure for 4 min to 13CO2 in the light. The burst was partially removed at low oxygen and was never observed in C4 leaves, suggesting that it may be caused by incomplete labelling of the photorespiratory pool at low CO2. This pool may be low in sclerophyllous leaves, as in Quercus ilex where no burst was observed. Rd was inversely associated with photosynthesis, suggesting that the Rd/Rn ratio reflects the refixation of respiratory CO2 by photosynthesizing leaves rather than the inhibition of mitochondrial respiration in the light, and that CO2 produced by mitochondrial respiration in the light is mostly emitted at low CO2, and mostly refixed at elevated CO2. In the leaves of the C4 species Zea mays, the 12CO2 emission in the light also remained low at low CO2, suggesting efficient CO2 refixation associated with sustained photosynthesis in non-photorespiratory conditions. However, Rn was inhibited in CO2-free air, and the velocity of 12CO2 emission after darkening was inversely associated with the CO2 concentration. The emission may be modulated by the presence of post-illumination CO2 uptake deriving from temporary imbalance between C3 and C4 metabolism. These experiments suggest that this uptake lasts longer at low CO2 and that the imbalance is persistent once it has been generated by exposure to low CO2.     

空气CO2增高条件下荔枝叶片光合作用和超氧自由基产率

研究结果表明,生长在77±5PaCO2分压下30d的荔枝幼树,其光合速率较大气CO2分压（39.3Pa）下的低23％,光下线粒体呼吸速率和不包含光下呼吸的CO2补偿点亦略有降低。空气CO2增高使叶片最大羧化速率（Vcmax）和最大电子传递速率（Jmax）降低,表明大气增高CO2分压下叶片的光I(PSI)能量水平较低,呈片超氧自由基产率亦降低39％,叶片感染荔枝霜疫霉病率则从生长在大气CO2分压下的1.8％增至9.5％,可能较低光合和呼吸代谢诱致较低的超氧自由基产率,而使叶片易受病害侵染。叶片受病害侵染后表现为超氧自由基的激增。在全球大气CO2分压增高趋势下须加强对荔枝霜疫霉病的控制。     

水稻叶片光合作用对开放式空气CO2浓度增高(FACE)的响应与适应

利用便携式光合气体分析系统 (LI 6 4 0 0 ) ,比较测定了高CO2 浓度 (FACE ,free airCO2 enrich ment)和普通空气CO2 浓度下生长的水稻叶片的净光合速率、水分利用率、表观量子效率和RuBP羧化效率等光合参数 .在各自生长CO2 浓度 (380vs 5 80 μmol·mol-1)下测定时 ,高CO2 浓度 (5 80 μmol·mol-1)下生长的水稻叶片的净光合速率、碳同化的表观量子效率和水分利用率明显高于普通空气 (380 μmol·mol-1)下生长的水稻叶片 .但是 ,随着FACE处理时间的延长 ,高CO2 浓度对净光合速率的促进作用逐渐减小 .在相同CO2 浓度下测定时 ,FACE条件下生长的水稻叶片净光合速率和羧化效率明显比普通空气下生长的对照低 .尽管高CO2 浓度下生长的水稻叶片的气孔导度明显低于普通空气中生长的水稻叶片 ,但两者胞间CO2 浓度差异不显著 ,因此高CO2 浓度下生长的水稻叶片光合下调似乎不是由气孔导度降低造成的 .     

A new approach to measure gross CO2 fluxes in leaves. Gross CO2 assimilation, photorespiration, and mitochondrial respiration in the light in tomato under drought stress

We developed a new method using 13CO2 and mass spectrometry to elucidate the role of photorespiration as an alternative electron dissipating pathway under drought stress. This was achieved by experimentally distinguishing between the CO2 fluxes into and out of the leaf. The method allows us to determine the rates of gross CO2 assimilation and gross CO2 evolution in addition to net CO2 uptake by attached leaves during steady-state photosynthesis. Furthermore, a comparison between measurements under photorespiratory and non-photorespiratory conditions may give information about the contribution of photorespiration and mitochondrial respiration to the rate of gross CO2 evolution at photosynthetic steady state. In tomato (Lycopersicon esculentum Mill. cv Moneymaker) leaves, drought stress decreases the rates of net and gross CO2 uptake as well as CO2 release from photorespiration and mitochondrial respiration in the light. However, the ratio of photorespiratory CO2 evolution to gross CO2 assimilation rises with water deficit. Also the contribution of re-assimilation of (photo) respiratory CO2 to gross CO2 assimilation increases under drought.     

Growth and photosynthetic responses of wheat plants grown in space.

Growth and photosynthesis of wheat (Triticum aestivum L. cv Super Dwarf) plants grown onboard the space shuttle Discovery for 10 d were examined. Compared to ground control plants, the shoot fresh weight of space-grown seedlings decreased by 25%. Postflight measurements of the O2 evolution/photosynthetic photon flux density response curves of leaf samples revealed that the CO2-saturated photosynthetic rate at saturating light intensities in space-grown plants declined 25% relative to the rate in ground control plants. The relative quantum yield of CO2-saturated photosynthetic O2 evolution measured at limiting light intensities was not significantly affected. In space-grown plants, the light compensation point of the leaves increased by 33%, which likely was due to an increase (27%) in leaf dark-respiration rates. Related experiments with thylakoids isolated from space-grown plants showed that the light-saturated photosynthetic electron transport rate from H2O through photosystems II and I was reduced by 28%. These results demonstrate that photosynthetic functions are affected by the microgravity environment.     

Emission of isoprene from salt-stressed Eucalyptus globulus leaves

Eucalyptus spp. are among the highest isoprene emitting plants. In the Mediterranean area these plants are often cultivated along the seashore and cope with recurrent salt stress. Transient salinity may severely but reversibly reduce photosynthesis and stomatal conductance of Eucalyptus globulus leaves but the effect on isoprene emission is not significant. When the stress is relieved, a burst of isoprene emission occurs, simultaneously with the recovery of photosynthetic performance. Later on, photosynthesis, stomatal conductance, and isoprene emission decay, probably because of the onset of leaf senescence. Isoprene emission is not remarkably affected by the stress at different light intensities, CO(2) concentrations, and leaf temperatures. When CO(2) was removed and O(2) was lowered to inhibit both photosynthesis and photorespiration, we found that the residual emission is actually higher in salt-stressed leaves than in controls. This stimulation is particularly evident at high-light intensities and high temperatures. The maximum emission occurs at 40 degrees C in both salt-stressed and control leaves sampled in ambient air and in control leaves sampled in CO(2)-free and low-O(2) air. However, the maximum emission occurs at 45 degrees C in salt-stressed leaves sampled in CO(2)-free and low-O(2) air. Our results suggest the activation of alternative non-photosynthetic pathways of isoprene synthesis in salt-stressed leaves and perhaps in general in leaves exposed to stress conditions. The temperature dependence indicates that this alternative synthesis is also under enzymatic control. If this alternative synthesis still occurs in the chloroplasts, it may involve a thylakoid-bound isoprene synthase.     

Continuous monitoring, by mass spectrometry, of H2 production and recycling in Rhodopseudomonas capsulata.

Hydrogen evolution and consumption by cell and chromatophore suspensions of the photosynthetic bacterium Rhodopseudomonas capsulata was measured with a sensitive and specific mass spectrometric technique which directly monitors dissolved gases. H2 production by nitrogenase was inhibited by acetylene and restored by carbon monoxide. An H2 evolution activity coupled with HD formation and D2 uptake (H-D exchange) was unaffected by C2H2 and CO. Cultures lacking nitrogenase activity also exhibited H-D exchange activity, which was catalyzed by a membrane-bound hydrogenase present in the chromatophores of R. capsulata. A net hydrogen uptake, mediated by hydrogenase, was observed when electron acceptors such as CO2, O2, or ferricyanide were present in the medium.     

Photosynthetic oxygen reduction in isolated intact chloroplasts and cells in spinach

The time course of light-induced O(2) exchange by isolated intact chloroplasts and cells from spinach was determined under various conditions using isotopically labeled O(2) and a mass spectrometer. In dark-adapted chloroplasts and cells supplemented with saturating amounts of bicarbonate, O(2) evolution began immediately upon illumination. However, this initial rate of O(2) evolution was counterbalanced by a simultaneous increase in the rate of O(2) uptake, so that little net O(2) was evolved or consumed during the first approximately 1 minute of illumination. After this induction (lag) phase, the rate of O(2) evolution increased 3- to 4-fold while the rate of O(2) uptake diminished to a very low level. Inhibition of the Calvin cycle, e.g. with dl-glyceraldehyde or iodoacetamide, had negligible effects on the initial rate of O(2) evolution or O(2) uptake; both rates were sutained for several minutes, and about balanced so that no net O(2) was produced. Uncouplers had an effect similar to that observed with Calvin cycle inhibitors, except that rates of O(2) evolution and photoreduction were stimulated 40 to 50%.These results suggest that higher plant phostosynthetic preparations which retain the ability to reduce CO(2) also have a significant capacity to photoreduce O(2). With near-saturating light and sufficient CO(2), O(2) reduction appears to take place primarily via a direct interaction between O(2) and reduced electron transport carriers, and occurs principally when CO(2)-fixation reactions are suboptimal, e.g. during induction or in the presence of Calvin cycle inhibitors. The inherent maximum endogenous rate of O(2) reduction is approximately 25 to 50% of the maximum rate of noncyclic electron transport coupled to CO(2) fixation. Although the photoreduction of O(2) is coupled to ion transport and/or phosphorylation, this process does not appear to supply significant amounts of ATP directly during steady-state CO(2) fixation in strong light.     

Comparison of Methods to Estimate Dark Respiration in the Light in Leaves of Two Woody Species

Dark respiration in the light was estimated in leaves of two woody species (Heteromeles arbutifolia Ait. and Lepechinia fragans Greene) using two different approaches based on gas-exchange techniques: the Kok method and the Laisk method. In all cases, dark respiration in the light was lower (P < 0.05) than respiration in darkness, indicating that dark respiration was inhibited in the light. Rates of dark respiration in the light estimated by the Laisk method were 52% higher (P < 0.05) than those estimated by the Kok method. Differences between the methods could be explained by the low ambient CO2 concentrations required by the Laisk approach. The mean value of the inhibition of respiration by light for the two species, corrected for the ambient CO2 concentration effect, was 55%. Despite the differences in leaf characteristics between the species, values of the CO2 photocompensation point, at which the rate of photosynthetic CO2 uptake equaled that of photorespiratory CO2 evolution, were very constant, suggesting an excellent consistency in the results obtained with the Laisk approach.     

The water-water cycle in leaves is not a major alternative electron sink for dissipation of excess excitation energy when CO(2) assimilation is restricted

Electron flux from water via photosystem II (PSII) and PSI to oxygen (water-water cycle) may provide a mechanism for dissipation of excess excitation energy in leaves when CO(2) assimilation is restricted. Mass spectrometry was used to measure O(2) uptake and evolution together with CO(2) uptake in leaves of French bean and maize at CO(2) concentrations saturating for photosynthesis and the CO(2) compensation point. In French bean at high CO(2) and low O(2) concentrations no significant water-water cycle activity was observed. At the CO(2) compensation point and 3% O(2) a low rate of water-water cycle activity was observed, which accounted for 30% of the linear electron flux from water. In maize leaves negligible water-water cycle activity was detected at the compensation point. During induction of photosynthesis in maize linear electron flux was considerably greater than CO(2) assimilation, but no significant water-water cycle activity was detected. Miscanthus × giganteus grown at chilling temperature also exhibited rates of linear electron transport considerably in excess of CO(2) assimilation; however, no significant water-water cycle activity was detected. Clearly the water-water cycle can operate in leaves under some conditions, but it does not act as a major sink for excess excitation energy when CO(2) assimilation is restricted.     

Induction of reduced photorespiratory activity in submersed and amphibious aquatic macrophytes

Incubation under water in a 30 C/14-hour or 12 C/10-hour photoperiod caused the CO₂ compensation points of 10 aquatic macrophytes to decrease below 25 or increase above 50 microliters CO₂ per liter, respectively. Submerged and aerial leaves of two amphibious angiosperms (Myriophyllum brasiliense and Proserpinaca palustris) maintained high compensation points when incubated in air but, when the submerged or aerial leaves of Proserpinaca were incubated under water, the compensation points dropped as low as 10. This suggests that, in addition to temperature and photoperiod, some factor associated with submergence regulates the compensation point of aquatic plants. In the high-compensation point plants, photorespiration, as a percentage of net photosynthesis, was equivalent to that in terrestrial C₃ plants. For Hydrilla verticillata, the decreasing CO₂ compensation points (110, 40, and 10) were associated with reduced photorespiration, as indicated by decreased O₂ inhibition, decreased rates of CO₂ evolution into CO₂-free air, and increased net photosynthetic rates.     

Does Long-Term Elevation of CO2 Concentration Increase Photosynthesis in Forest Floor Vegetation? (Indiana Strawberry in a Maryland Forest)

As the partial pressure of CO2 (pCO2) in the atmosphere rises, photorespiratory loss of carbon in C3 photosynthesis will diminish and the net efficiency of light-limited photosynthetic carbon uptake should rise. We tested this expectation for Indiana strawberry (Duchesnea indica) growing on a Maryland forest floor. Open-top chambers were used to elevate the pCO2 of a forest floor habitat to 67 Pa and were paired with control chambers providing an ambient pCO2 of 38 Pa. After 3.5 years, D. indica leaves grown and measured in the elevated pCO2 showed a significantly greater maximum quantum efficiency of net photosynthesis (by 22%) and a lower light compensation point (by 42%) than leaves grown and measured in the control chambers. The quantum efficiency to minimize photorespiration, measured in 1% O2, was the same for controls and plants grown at elevated pCO2. This showed that the maximum efficiency of light-energy transduction into assimilated carbon was not altered by acclimation and that the increase in light-limited photosynthesis at elevated pCO2 was simply a function of the decrease in photorespiration. Acclimation did decrease the ribulose-1,5-bisphosphate carboxylase/oxygenase and light-harvesting chlorophyll protein content of the leaf by more than 30%. These changes were associated with a decreased capacity for light-saturated, but not light-limited, photosynthesis. Even so, leaves of D. indica grown and measured at elevated pCO2 showed greater light-saturated photosynthetic rates than leaves grown and measured at the current atmospheric pCO2. In situ measurements under natural forest floor lighting showed large increases in leaf photosynthesis at elevated pCO2, relative to controls, in both summer and fall. The increase in efficiency of light-limited photosynthesis with elevated pCO2 allowed positive net photosynthetic carbon uptake on days and at locations on the forest floor that light fluxes were insufficient for positive net photosynthesis in the current atmospheric pCO2.