Natural abundance carbon isotope composition of isoprene reflects incomplete coupling between isoprene synthesis and photosynthetic carbon flow

Isoprene emission from leaves is dynamically coupled to photosynthesis through the use of primary and recent photosynthate in the chloroplast. However, natural abundance carbon isotope composition (delta(13)C) measurements in myrtle (Myrtus communis), buckthorn (Rhamnus alaternus), and velvet bean (Mucuna pruriens) showed that only 72% to 91% of the variations in the delta(13)C values of fixed carbon were reflected in the delta(13)C values of concurrently emitted isoprene. The results indicated that 9% to 28% carbon was contributed from alternative, slow turnover, carbon source(s). This contribution increased when photosynthesis was inhibited by CO(2)-free air. The observed variations in the delta(13)C of isoprene under ambient and CO(2)-free air were consistent with contributions to isoprene synthesis in the chloroplast from pyruvate associated with cytosolic Glc metabolism. Irrespective of alternative carbon source(s), isoprene was depleted in (13)C relative to mean photosynthetically fixed carbon by 4 per thousand to 11 per thousand. Variable (13)C discrimination, its increase by partially inhibiting isoprene synthesis with fosmidomicin, and the associated accumulation of pyruvate suggested that the main isotopic discrimination step was the deoxyxylulose-5-phosphate synthase reaction.     

Characterization of juvenile and adult leaves of Eucalyptus globulus showing distinct heteroblastic development: photosynthesis and volatile isoprenoids

Heteroblastic Eucalyptus (Eucalyptus globulus L.) leaves were characterized for their functional diversity examining photosynthesis and photosynthesis limitations, transpiration, and the emission of isoprene and monoterpenes. In vivo and combined analyses of gas-exchange, chlorophyll fluorescence, and light absorbance at 830 nm were made on the adaxial and abaxial sides of juvenile and adult leaves. When adult leaves were reversed to illuminate the abaxial side, photosynthesis and isoprene emission were significantly lower than when the adaxial side was illuminated. Monoterpene emission, however, was independent on the side illuminated and similarly partitioned between the two leaf sides. The abaxial side of adult leaves showed less diffusive resistance to CO(2) acquisition by chloroplasts, but also lower ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity, than the adaxial leaf side. In juvenile leaves, photosynthesis, isoprene, and monoterpene emissions were similar when the adaxial or abaxial side was directly illuminated. In the abaxial side of juvenile leaves, photosynthesis did not match the rates attained by the other leaf types when exposed to elevated CO(2), which suggests the occurrence of a limitation of photosynthesis by ribulose bisphosphate (RuBP) regeneration. Accordingly, a reduced efficiency of both photosystems and a high non-radiative dissipation of energy was observed in the abaxial side of juvenile leaves. During light induction, the adaxial side of juvenile leaves also showed a reduced efficiency of photosystem II and a large non-radiative energy dissipation. Our report reveals distinct functional properties in Eucalyptus leaves. Juvenile leaves invest more carbon in isoprene, but not in monoterpenes, and have a lower water use efficiency than adult leaves. Under steady-state conditions, in adult leaves the isobilateral anatomy does not correspond to an equal functionality of the two sides, while in juvenile leaves the dorsiventral anatomy does not result in functional differences in primary or secondary metabolism in the two sides. However, photochemical limitations may reduce the efficiency of carbon fixation in the light, especially in the abaxial side of juvenile leaves.     

尾叶桉异戊二烯排放量初步研究

应用光离子化气相色谱仪(GC-PID)测定尾叶桉3月份的异戊二烯排放量,同时采用Li-6400光合作用测定仪同步测定叶片净光合速率、蒸腾速率、气孔导度和胞间CO2浓度,及其相关环境因子,并对截枝后尾叶桉异戊二烯排放量的变化进行分析。结果表明,春季尾叶桉异戊二烯白天的排放量变化呈单峰型曲线,最大值出现在14:00左右,截枝后尾叶桉的异戊二烯排放量下降。     

Stress-induced changes in carbon sources for isoprene production in Populus deltoides

Isoprene is emitted from leaves of numerous plant species and has important implications for plant metabolism and atmospheric chemistry. The ability to use stored carbon (alternative carbon sources), as opposed to recently assimilated photosynthate, for isoprene production may be important as plants routinely experience photosynthetic depression in response to environmental stress. A CO₂‐labelling study was performed and stable isotopes of carbon were used to examine the role of alternative carbon sources in isoprene production in Populus deltoides during conditions of water stress and high leaf temperature. Isotopic fractionation during isoprene production was higher in heat‐ and water‐stressed leaves (?8.5 and ?9.3‰, respectively) than in unstressed controls (?2.5 to ?3.2‰). In unstressed plants, 84–88% of the carbon in isoprene was derived from recently assimilated photosynthate. A significant shift in the isoprene carbon composition from photosynthate to alternative carbon sources was observed only under severe photosynthetic limitation (stomatal conductance < 0.05 mol m^?2 s^?1). The contribution of photosynthate to isoprene production decreased to 77 and 61% in heat‐ and water‐stressed leaves, respectively. Across water‐ and heat‐stress experiments, allocation of photosynthate was negatively correlated to the ratio of isoprene emission to photosynthesis. In water‐stressed plants, the use of alternative carbon was also related to stomatal conductance. It has been proposed that isoprene emission may be regulated by substrate availability. Thus, understanding carbon partitioning to isoprene production from multiple sources is essential for building predictive models of isoprene emission.     

Influence of Environmental Factors and Air Composition on the Emission of [alpha]-Pinene from Quercus ilex Leaves

We studied the emission of [alpha]-pinene from Quercus ilex leaves. Only the abaxial side of the hypostomatous Q. ilex leaf emits [alpha]-pinene. Light induced photosynthesis and [alpha]-pinene emission. However, the response of photosynthesis to dark-to-light transitions was faster than that of [alpha]-pinene, suggesting that ATP controls the emission. The emission was higher at 30 than at 20[deg]C, whereas photosynthesis did not change. Therefore, the relationship between photosynthesis and [alpha]-pinene emission does not always hold. When CO2 was removed from the air, transpiration was stimulated but photosynthesis and [alpha]-pinene emission were inhibited. [alpha]-Pinene inhibition was more rapid under low O2. When CO2 in the air was increased, photosynthesis was stimulated and transpiration was reduced, but [alpha]-pinene emission was unaffected. Therefore, the emission depends on the availability of photosynthetic carbon, is not saturated at ambient CO2, and is not dependent on stomatal opening. The pattern of [alpha]-pinene emission from Q. ilex is different from that of plants having specialized structures for storage and emission of terpenes. We suggest that [alpha]-pinene emitted by Q. ilex leaves is synthesized in the chloroplasts and shares the same biochemical pathway with isoprene emitted by isoprene-emitting oak species.     

Isoprene emission from aspen leaves : influence of environment and relation to photosynthesis and photorespiration

Isoprene emission rates from quaking aspen (Populus tremuloides Michx.) leaves were measured simultaneously with photosynthesis rate, stomatal conductance, and intercellular CO₂ partial pressure. Isoprene emission required the presence of CO₂ or O₂, but not both. The light response of isoprene emission rate paralleled that of photosynthesis. Isoprene emission was inhibited by decreasing ambient O₂ from 21% to 2%, only when there was oxygen insensitive photosynthesis. Mannose (10 millimolar) fed through cut stems resulted in strong inhibition of isoprene emission rate and is interpreted as evidence that isoprene biosynthesis requires either the export of triose phosphates from the chloroplast, or the continued synthesis of ATP. Light response experiments suggest that photosynthetically generated reductant or ATP is required for isoprene biosynthesis. Isoprene biosynthesis and emission are not directly linked to glycolate production through photorespiration, contrary to previous reports. Isoprene emission rate was inhibited by above-ambient CO₂ partial pressures (640 microbar outside and 425 microbar inside the leaf). The inhibition was not due to stomatal closure. This was established by varying ambient humidity at normal and elevated CO₂ partial pressures to measure isoprene emission rates over a range of stomatal conductances. Isoprene emission rates were inhibited at elevated CO₂ despite no change in stomatal conductance. Addition of abscisic acid to the transpiration stream dramatically inhibited stomatal conductance and photosynthesis rate, with a slight increase in isoprene emission rate. Thus, isoprene emission is independent of stomatal conductance, and may occur through the cuticle. Temperature had an influence on isoprene emission rate, with the Q₁₀ being 1.8 to 2.4 between 35 and 45°C. At these high temperatures the amount of carbon lost through isoprene emission was between 2.5 and 8% of that assimilated through photosynthesis. This represents a significant carbon cost that should be taken into account in determining midsummer carbon budgets for plants that are isoprene emitters.     

Diffusive and metabolic limitations to photosynthesis under drought and salinity in C(3) plants

Drought and salinity are two widespread environmental conditions leading to low water availability for plants. Low water availability is considered the main environmental factor limiting photosynthesis and, consequently, plant growth and yield worldwide. There has been a long-standing controversy as to whether drought and salt stresses mainly limit photosynthesis through diffusive resistances or by metabolic impairment. Reviewing in vitro and in vivo measurements, it is concluded that salt and drought stress predominantly affect diffusion of CO(2) in the leaves through a decrease of stomatal and mesophyll conductances, but not the biochemical capacity to assimilate CO(2), at mild to rather severe stress levels. The general failure of metabolism observed at more severe stress suggests the occurrence of secondary oxidative stresses, particularly under high-light conditions. Estimates of photosynthetic limitations based on the photosynthetic response to intercellular CO(2) may lead to artefactual conclusions, even if patchy stomatal closure and the relative increase of cuticular conductance are taken into account, as decreasing mesophyll conductance can cause the CO(2) concentration in chloroplasts of stressed leaves to be considerably lower than the intercellular CO(2) concentration. Measurements based on the photosynthetic response to chloroplast CO(2) often confirm that the photosynthetic capacity is preserved but photosynthesis is limited by diffusive resistances in drought and salt-stressed leaves.     

Isoprene emission rate and intercellular isoprene concentration as influenced by stomatal distribution and conductance

Isoprene emission in relation to stomatal distribution and conductance was determined for the hypostomatous species, aspen and white oak, and the amphistomatous species, cottonwood. For aspen and oak, isoprene emission from the adaxial (nonstomatal) surface was <2% of that from the abaxial (stomatal) surface, even when stomata were closed by addition of abscisic acid (ABA). When treated with ABA, the total flux rate of isoprene emission from leaves of these two hypostomatous species was unchanged, despite decreases in stomatal conductance of over 90%. The lack of control over isoprene emission rate by stomatal conductance, despite the apparent movement of isoprene through the stomatal pores, was due to increases in the intercellular isoprene concentration that compensated for the decreased stomatal conductance and restored the equilibrium between the isoprene synthesis rate and emission rate. This relationship was demonstrated by (a) an experiment in which the decrease in the internal isoprene pool following the imposition of darkness took longer in the presence of ABA than in its absence, and (b) direct measurements of the internal isoprene concentration through vacuum extraction, which revealed substantially higher values in the presence of ABA than in its absence. In the amphistomatous species, cottonwood, isoprene was emitted from both surfaces and addition of ABA caused an increase in isoprene emission from one surface coupled with a decrease from the other surface. The specific surface exhibiting an increase varied among leaves, with some leaves exhibiting an increase from the adaxial surface and other leaves from the abaxial surface. We interpret this as indicating nonuniform stomatal closure with concomitant emission of isoprene at the greatest rate from the surface with the highest stomatal conductance. We also observed an increase in the total isoprene emission rate from cottonwood leaves following treatment with ABA. We interpret this as indicating a stimulation of isoprene synthesis in response to ABA or stomatal closure, with unknown cause.     

The relationship between isoprene emission rate and dark respiration rate in white poplar (Populus alba L.) leaves

In past studies, it was hypothesized that reductions in chloroplast isoprene emissions at high atmospheric CO(2) concentrations were caused by competition between cytosolic and mitochondrial processes for the same substrate, possibly phosphoenolpyruvate (PEP). We conducted field and laboratory experiments using leaves of white poplar (Populus alba L.) to identify whether an inverse relationship occurs between the dark respiration rate (a mitochondrial process) and the isoprene emission rate. Field experiments that were carried out in a free-air CO(2)-enriched (FACE) facility showed no clear effect of elevated CO(2) on either isoprene emission rate or respiration rate by leaves. In young, not yet fully expanded leaves, low isoprene emission and high dark respiration rates were measured in both ambient and elevated CO(2). In these leaves, isoprene emission was inversely correlated with dark respiration. It is possible to interpret from these results that, in young leaves, high rates of growth respiration compete with isoprene biosynthesis for the same substrate. However, it is also possible that the negative correlation reflects the contrasting reductions in growth respiration and increases in expression of the enzyme isoprene synthase at this final stage of leaf maturation. In contrast to our observations on young leaves, respiration rate and isoprene emission rate were positively correlated in older, fully expanded leaves (8 and 11 from apex). A positive correlation was also found between respiration rate and isoprene emission rate when these parameters were modulated using different ozone exposure, growth light intensity, growth temperature and exposure to different leaf temperatures in laboratory experiments. These data show that competition for substrate between isoprene biosynthesis and leaf respiration does not determine the rate of isoprene emission in most circumstances that affect both processes. A negative correlation was observed across all experiments between isoprene emission rate and the activity of phosphoenolpyruvate carboxylase (PEPc), a cytosolic enzyme that competes with isoprene biosynthesis for substrate. The cytosolic metabolite, PEP, occurs at a metabolic branch point from which substrate flows into three processes: (1) the production of pyruvate for mitochondrial respiration, (2) the production of oxaloacetate (OAA) by PEPc for anabolic support of mitochondrial respiration and (3) transport into the chloroplast to support chloroplastic demands for pyruvate, including isoprenoid biosynthesis. The results of our observations suggest that only the second process competes for substrate with isoprenoid synthesis, while the partitioning of PEP between mitochondrial respiration and chloroplast isoprenoid biosynthesis is controlled in a way that retains balance in substrate demand.     

Isoprene increases thermotolerance of fosmidomycin-fed leaves

Isoprene is synthesized and emitted in large amounts by a number of plant species, especially oak (Quercus sp.) and aspen (Populus sp.) trees. It has been suggested that isoprene improves thermotolerance by helping photosynthesis cope with high temperature. However, the evidence for the thermotolerance hypothesis is indirect and one of three methods used to support this hypothesis has recently been called into question. More direct evidence required new methods of controlling endogenous isoprene. An inhibitor of the deoxyxylulose 5-phosphate pathway, the alternative pathway to the mevalonic acid pathway and the pathway by which isoprene is made, is now available. Fosmidomycin eliminates isoprene emission without affecting photosynthesis for several hours after feeding to detached leaves. Photosynthesis of fosmidomycin-fed leaves recovered less following a 2-min high-temperature treatment at 46 degrees C than did photosynthesis of leaves fed water or fosmidomycin-fed leaves in air supplemented with isoprene. Photosynthesis of Phaseolus vulgaris leaves, which do not make isoprene, exhibited increased thermotolerance when isoprene was supplied in the airstream flowing over the leaf. Other short-chain alkenes also improved thermotolerance, whereas alkanes reduced thermotolerance. It is concluded that thermotolerance of photosynthesis is a substantial benefit to plants that make isoprene and that this benefit explains why plants make isoprene. The effect may be a general hydrocarbon effect and related to the double bonds in the isoprene molecule.     

Delayed Onset of Isoprene Emission in Developing Velvet Bean (Mucuna sp.) Leaves

Isoprene (2-methyl-1,3-butadiene) is one of the major volatile hydrocarbons emitted by plants, but its biosynthetic pathway and role in plant metabolism are unknown. Mucuna sp. (velvet bean) is an isoprene emitter, and leaf isoprene emission rate increased as much as 125-fold as leaves developed, and declined in older leaves. Net CO₂ assimilation and stomatal conductance, under different growth and environmental conditions, increased 3 to 5 days prior to an increase in isoprene emission rate, indicating that photosynthetic competence develops before significant isoprene emission occurs.     

The relationship between the methyl-erythritol phosphate pathway leading to emission of volatile isoprenoids and abscisic acid content in leaves

It was investigated whether the methyl-erythritol phosphate (MEP) pathway that generates volatile isoprenoids and carotenoids also produces foliar abscisic acid (ABA) and controls stomatal opening. When the MEP pathway was blocked by fosmidomycin and volatile isoprenoid emission was largely suppressed, leaf ABA content decreased to about 50% and leaf stomatal conductance increased significantly. No effect of fosmidomycin was seen in leaves with constitutively high rates of stomatal conductance and in plant species with low foliar ABA concentration. In all other cases, isoprene emission was directly associated with foliar ABA, but ABA reduction upon MEP pathway inhibition was also observed in plant species that do not emit isoprenoids. Stomatal closure causing a midday depression of photosynthesis was also associated with a concurrent increase of isoprene emission and ABA content. It is suggested that the MEP pathway generates a labile pool of ABA that responds rapidly to environmental changes. This pool also regulates stomatal conductance, possibly when coping with frequent changes of water availability. MEP pathway inhibition by leaf darkening, and its down-regulation by exposure to elevated CO2, was also associated with a reduction of foliar ABA content. However, stomatal conductance was reduced, indicating that stomatal aperture is not regulated by the MEP-dependent foliar ABA pool, under these specific cases.     

Isoprene Emission from Velvet Bean Leaves (Interactions among Nitrogen Availability,Growth Photon Flux Density,and Leaf Development)

Although isoprene synthesis is closely coupled to photosynthesis, both via ATP requirements and carbon substrate availability, control of isoprene emission is not always closely linked to photosynthetic processes. In this study we grew velvet bean (Mucuna sp.) under different levels of photon flux density (PFD) and nitrogen availability in an effort to understand better the degree to which these two processes are linked. As has been observed in past studies, we found that during early leaf ontogeny the onset of positive rates of net photosynthesis precedes that of isoprene emission by 3 to 4 d. Other studies have shown that this lag is correlated with the induction of isoprene synthase activity, indicating that overall control of the process is under control of that enzyme. During leaf senescence, photosynthesis rate and isoprene emission rate declined in parallel, suggesting similar controls over the two processes. This coordinated decline was accelerated when plants were grown with high PFD and high nitrogen availability. The latter effect included declines in the photon yield of photosynthesis, suggesting that an unexplained stress arose during growth under these conditions, triggering a premature decline in photosynthesis and isoprene emission rate. In mature leaves, growth PFD and nitrogen nutrition affected photosynthesis and isoprene emission in qualitatively similar, but quantitatively different, ways. This resulted in a significant shift in the percentage of fixed carbon that was re-emitted as isoprene. In the case of increasing growth PFD, isoprene emission rate was more strongly affected than photosynthesis rate, and more carbon was lost as isoprene. In the case of increasing nitrogen, photosynthesis rate increased more than isoprene emission rate, and leaves containing high amounts of nitrogen lost a lower percentage of their assimilated carbon as isoprene. Taken together, our results demonstrate that, although the general correlation between isoprene emission rate and photosynthesis rate is consistently expressed, there is evidence that both processes are capable of independent responses to plant growth environment.     

Isoprene and nitric oxide reduce damages in leaves exposed to oxidative stress

Isoprene and nitric oxide (NO) are two volatile molecules that are produced in leaves. Both compounds were suggested to have an important protective role against stresses. We tested, in two isoprene-emitting species, Populus nigra and Phragmites australis, whether: (1) NO emission outside leaves is measurable and is affected by oxidative stresses; and (2) isoprene and NO protect leaves against oxidative stresses, both singularly and in combination. The emission of NO was undetectable, and the compensation point was very low in control poplar leaves. Both emission and compensation point increased dramatically in stressed leaves. NO emission was inversely associated with stomatal conductance. More NO was emitted in leaves that were isoprene-inhibited, and more isoprene was emitted when NO was reduced by NO scavenger c-PTIO. Both isoprene and NO reduced oxidative damages. Isoprene-emitting leaves which were also fumigated with NO, or treated with NO donor, showed low damage to photosynthesis, a reduced accumulation of H(2)O(2) and a reduced membrane denaturation. We conclude that measurable amounts of NO are only produced and emitted by stressed leaves, that both isoprene and NO are effective antioxidant molecules and that an additional protection is achieved when both molecules are released.     

Isoprene emission and primary metabolism in Phragmites australis grown under different phosphorus levels

Aquatic plants are generally used for wastewater purification and phytoremediation, but some of them also emit large amounts of isoprene, the most abundant biogenic volatile organic compound. Since isoprenoid biosynthesis requires high amounts of phosphorylated intermediates, the emission may also be controlled by inorganic phosphorus concentration (Pi) in leaves. We carried out experiments to determine the emission of isoprene from Phragmites australis plants used in reconstructed wetlands to phytoremediate elevated levels of phosphorus contributed by urban wastes. Four groups of plants were grown hydroponically in water containing different levels of KH(2)PO(4). High levels of phosphorus in the water resulted in high Pi in the leaves. High Pi stimulated photosynthesis at intercellular CO(2) concentrations lower and higher than ambient, implying higher ribulose 1,5-bisphosphate carboxylase (Rubisco) activity and higher ribulose 1,5-bisphosphate regeneration rates, respectively. However, isoprene emission was substantially lower at high Pi than at low Pi, and was not associated to photosynthesis rates at high Pi. This surprising result suggests that isoprene is limited by processes other than photosynthetic intermediate availability or by energetic (ATP) requirements under high Pi levels. Irrespective of the mechanism responsible for the observed reduction of isoprene emission, our results show that Phragmites plants may effectively remove phosphorus from water without concurrently increase isoprene emission, at least on a leaf area basis. Thus, Phragmites used in reconstructed wetlands for phytoremediation of urban wastes rich of phosphates will not contribute high loads of hydrocarbons which may influence air quality over urban and peri-urban areas.     

On the relationship between isoprene emission and thermotolerance in Phragmites australis leaves exposed to high temperatures and during the recovery from a heat stress

Thermotolerance induced by isoprene has been assessed during heat bursts but there is little information on the ability of endogenous isoprene to confer thermotolerance under naturally elevated temperature, on the interaction between isoprene-induced thermotolerance and light stress, and on the persistence of this protection in leaves recovering at lower temperatures. Moderately high temperature treatment (38 °C for 1.5 h) reduced photosynthesis, stomatal conductance, and photochemical efficiency of photosystem II in isoprene-emitting, but to a significantly lower extent than in isoprene-inhibited Phragmites australis leaves. Isoprene inhibition and high temperature independently, as well as together, induced lipid peroxidation, increased level of H₂O₂, and increased catalase and peroxidase activities. However, leaves in which isoprene emission was previously inhibited developed stronger oxidative stress under high temperature with respect to isoprene-emitting leaves. The heaviest photosynthetic stress was observed in isoprene-inhibited leaves exposed to the brightest illumination (1500 µmol m⁻² s⁻¹) and, in general, there was also a clear additive effect of light excess on the formation of reactive oxygen species, antioxidant enzymes, and membrane damage. The increased thermotolerance capability of isoprene-emitting leaves may be due to isoprene ability to stabilize membranes or to scavenge reactive oxygen species. Irrespective of the mechanism by which isoprene reduces thermal stress, isoprene-emitting leaves are able to quickly recover after the stress. This may be an important feature for plants coping with frequent and transient temperature changes in nature.     

Photosynthesis and isoprene emission from trees along an urban–rural gradient in Texas

Isoprene emission is an important mechanism for improving the thermotolerance of plant photosystems as temperatures increase. In this study, we measured photosynthesis and isoprene emission in trees along an urban–rural gradient that serves as a proxy for climate change, to understand daily and seasonal responses to changes in temperature and other environmental variables. Leaf‐level gas exchange and basal isoprene emission of post oak (Quercus stellata) and sweet gum (Liquidambar styraciflua) were recorded at regular intervals over an entire growing season at urban, suburban, and rural sites in eastern Texas. In addition, the temperature and atmospheric carbon dioxide concentration experienced by leaves were experimentally manipulated in spring, early summer, and late summer. We found that trees experienced lower stomatal conductance and photosynthesis and higher isoprene emission, at the urban and suburban sites compared to the rural site. Path analysis indicated a daily positive effect of isoprene emission on photosynthesis, but unexpectedly, higher isoprene emission from urban trees was not associated with improved photosynthesis as temperatures increased during the growing season. Furthermore, urban trees experienced relatively higher isoprene emission at high CO₂ concentrations, while isoprene emission was suppressed at the other sites. These results suggest that isoprene emission may be less beneficial in urban, and potentially future, environmental conditions, particularly if higher temperatures override the suppressive effects of high CO₂ on isoprene emission. These are important considerations for modeling future biosphere–atmosphere interactions and for understanding tree physiological responses to climate change.     

On the relationship between isoprene emission and photosynthetic metabolites under different environmental conditions

Isoprene emission is related to photosynthesis but the nature of the relationship is not yet known. To explore this relationship we have examined the rate of isoprene emission, photosynthesis, and the contents of photosynthetic metabolites in leaves of velvet bean (Mucuna deeringeniana L.) and red oak (Quercus rubra L.) in response to a light-to-dark transition and to changes in air composition. Isoprene emission fell when darkness was imposed and the drop was associated with reduced amounts of ribulose-1,5-bisphosphate and ATP. The rate of isoprene emission and ATP content were reduced to the same extent by exposure to low O₂ or high CO₂ partial pressures. Only when O₂ and CO₂ were simultaneously removed from the air did the rate of isoprene emission drop without a corresponding change in ATP. The results demonstrate that when carbon is not limiting, isoprene emission is highly correlated with ATP content. When synthesis of phosphoglyceric acid is inhibited, however, carbon availability may control isoprene production. Mr. Peter Vanderveer assisted with the measurements of enzymatic metabolites. Mr. Xavier Socias is gratefully acknowledged for Rubisco preparation. This research was supported by the U.S. National Science Foundation, grant no. IBN 9105274.