The responses of isoprene emission rate to temperature are characterized by complex time-dependent behaviors that are currently not entirely understood. To gain insight into the temperature ...dependencies of isoprene emission, we studied steady-state and transient responses of isoprene emission from hybrid aspen (Populus tremula x Populus tremuloides) leaves using a fast-response gas-exchange system coupled to a proton-transfer reaction mass spectrometer. A method based on postillumination isoprene release after rapid temperature transients was developed to determine the rate constant of isoprene synthase (IspS), the pool size of its substrate dimethylallyldiphosphate (DMADP), and to separate the component processes of the temperature dependence of isoprene emission. Temperature transients indicated that over the temperature range 25°C to 45°C, IspS was thermally stable and operated in the linear range of its substrate DMADP concentration. The in vivo rate constant of IspS obeyed the Arrhenius law, with an activation energy of 42.8 kJ mol⁻¹. In contrast, steady-state isoprene emission had a significantly lower temperature optimum than IspS and higher activation energy. The reversible temperature-dependent decrease in the rate of isoprene emission between 35°C and 44°C was caused by decreases in DMADP concentration, possibly reflecting reduced pools of energetic metabolites generated in photosynthesis, particularly of ATP. Strong control of isoprene temperature responses by the DMADP pool implies that transient temperature responses under fluctuating conditions in the field are driven by initial DMADP pool size as well as temperature-dependent modifications in DMADP pool size during temperature transients. These results have important implications for the development of process-based models of isoprene emission.
Acclimation of foliage to growth temperature involves both structural and physiological modifications, but the relative importance of these two mechanisms of acclimation is poorly known, especially ...for isoprene emission responses. We grew hybrid aspen (Populus tremula x P. tremuloides) under control (day/night temperature of 25/20 °C) and high temperature conditions (35/27 °C) to gain insight into the structural and physiological acclimation controls. Growth at high temperature resulted in larger and thinner leaves with smaller and more densely packed chloroplasts and with lower leaf dry mass per area (MA). High growth temperature also led to lower photosynthetic and respiration rates, isoprene emission rate and leaf pigment content and isoprene substrate dimethylallyl diphosphate pool size per unit area, but to greater stomatal conductance. However, all physiological characteristics were similar when expressed per unit dry mass, indicating that the area‐based differences were primarily driven by MA. Acclimation to high temperature further increased heat stability of photosynthesis and increased activation energies for isoprene emission and isoprene synthase rate constant. This study demonstrates that temperature acclimation of photosynthetic and isoprene emission characteristics per unit leaf area were primarily driven by structural modifications, and we argue that future studies investigating acclimation to growth temperature must consider structural modifications.
After darkening, isoprene emission continues for 20 to 30 min following biphasic kinetics. The initial dark release of isoprene (postillumination emission), for 200 to 300 s, occurs mainly at the ...expense of its immediate substrate, dimethylallyldiphosphate (DMADP), but the origin and controls of the secondary burst of isoprene release (dark-induced emission) between approximately 300 and 1,500 s, are not entirely understood. We used a fast-response gas-exchange system to characterize the controls of dark-induced isoprene emission by light, temperature, and CO₂ and oxygen concentrations preceding leaf darkening and the effects of short light pulses and changing gas concentrations during dark-induced isoprene release in hybrid aspen (Populus tremula × Populus tremuloides). The effect of the 2-C-methyl-D-erythritol-4-phosphate pathway inhibitor fosmidomycin was also investigated. The integral of postillumination isoprene release was considered to constitute the DMADP pool size, while the integral of dark-induced emission was defined as the "dark" pool. Overall, the steady-state emission rate in light and the maximum dark-induced emission rate responded similarly to variations in preceding environmental drivers and atmospheric composition, increasing with increasing light, having maxima at approximately 40°C and close to the CO₂ compensation point, and were suppressed by lack of oxygen. The DMADP and dark pool sizes were also similar through their environmental dependencies, except for high temperatures, where the dark pool significantly exceeded the DMADP pool. Isoprene release could be enhanced by short lightflecks early during dark-induced isoprene release, but not at later stages. Fosmidomycin strongly suppressed both the isoprene emission rates in light and in the dark, but the dark pool was only moderately affected. These results demonstrate a strong correspondence between the steady-state isoprene emission in light and the dark-induced emission and suggest that the dark pool reflects the total pool size of 2-C-methyl-D-erythritol-4-phosphate pathway metabolites upstream of DMADP. These metabolites are converted to isoprene as soon as ATP and NADPH become available, likely by dark activation of chloroplastic glycolysis and chlororespiration.
Changes in leaf sugar concentrations are a possible mechanism of short‐term adaptation to temperature changes, with natural fluctuations in sugar concentrations in the field expected to modify the ...heat sensitivity of respiration. We studied temperature‐response curves of leaf dark respiration in the temperate tree Populus tremula (L.) in relation to leaf sugar concentration (1) under natural conditions or (2) leaves with artificially enhanced sugar concentration. Temperature‐response curves were obtained by increasing the leaf temperature at a rate of 1°C min−1. We demonstrate that respiration, similarly to chlorophyll fluorescence, has a break‐point at high temperature, where respiration starts to increase with a faster rate. The average break‐point temperature (TRD) was 48.6 ± 0.7°C at natural sugar concentration. Pulse‐chase experiments with 14CO2 demonstrated that substrates of respiration were derived mainly from the products of starch degradation. Starch degradation exhibited a similar temperature‐response curve as respiration with a break‐point at high temperatures. Acceleration of starch breakdown may be one of the reasons for the observed high‐temperature rise in respiration. We also demonstrate that enhanced leaf sugar concentrations or enhanced osmotic potential may protect leaf cells from heat stress, i.e. higher sugar concentrations significantly modify the temperature‐response curve of respiration, abolishing the fast increase of respiration. Sugars or enhanced osmotic potential may non‐specifically protect respiratory membranes or may block the high‐temperature increase in starch degradation and consumption in respiratory processes, thus eliminating the break‐points in temperature curves of respiration in sugar‐fed leaves.
Effects of elevated atmospheric CO2 on plant isoprene emissions are controversial. Relying on leaf-scale measurements, most models simulating isoprene emissions in future higher CO2 atmospheres ...suggest reduced emission fluxes. However, combined effects of elevated CO2 on leaf area growth, net assimilation and isoprene emission rates have rarely been studied on the canopy scale, but stimulation of leaf area growth may largely compensate for possible CO2 inhibition reported at the leaf scale. This study tests the hypothesis that stimulated leaf area growth leads to increased canopy isoprene emission rates.
We studied the dynamics of canopy growth, and net assimilation and isoprene emission rates in hybrid aspen (Populus tremula × Populus tremuloides) grown under 380 and 780 μmol mol−1 CO2. A theoretical framework based on the Chapman–Richards function to model canopy growth and numerically compare the growth dynamics among ambient and elevated atmospheric CO2-grown plants was developed.
Plants grown under elevated CO2 had higher C:N ratio, and greater total leaf area, and canopy net assimilation and isoprene emission rates. During ontogeny, these key canopy characteristics developed faster and stabilized earlier under elevated CO2. However, on a leaf area basis, foliage physiological traits remained in a transient state over the whole experiment.
These results demonstrate that canopy-scale dynamics importantly complements the leaf-scale processes, and that isoprene emissions may actually increase under higher CO2 as a result of enhanced leaf area production.
Isoprene emission potential (ES) varies in tree canopies, and such variations have potentially major implications for predicting canopy level emissions. So far, quantitative relationships of ES with ...irradiance are missing, and interspecific variation in ES plasticity and potential effects on canopy level emissions have not been characterized. ES, foliage structural, chemical, and photosynthetic characteristics were studied relative to integrated within‐canopy daily quantum flux density (Qint) in temperate deciduous tree species Quercus robur, Populus tremula, Salix alba, and Salix caprea, and canopy isoprene emissions were calculated considering observed variation in ES and under different simplifying assumptions. Strong positive curvilinear relationships between nitrogen and dry mass per unit area, photosynthetic potentials and ES per area with Qint were observed. Structural, chemical, and photosynthetic traits varied 1.5‐fold to 4‐fold and ES per area 3‐fold to 27‐fold within the canopy. ES variation reflected accumulation of mesophyll cell layers and greater emission capacity of average cells. Species with largest structural and photosynthetic plasticity had greatest plasticity in ES. Relative to the simulation considering within‐canopy variation in ES, the bias from assuming a constant ES varied between −8% and +68%, and it scaled positively with ES plasticity. The bias of big‐leaf simulations varied between −22% and −35%, and it scaled negatively with ES plasticity. A generalized canopy response function of ES developed for all species resulted in the lowest bias between −11% and 6% and can be recommended for practical applications. The results highlight huge within‐canopy and interspecific variation in ES and demonstrate that ignoring these variations strongly biases canopy emission predictions.
Controversial evidence of CO2‐responsiveness of isoprene emission has been reported in the literature with the response ranging from inhibition to enhancement, but the reasons for such differences ...are not understood. We studied isoprene emission characteristics of hybrid aspen (Populus tremula x P. tremuloides) grown under ambient (380 μmol mol−1) and elevated (780 μmol mol−1) CO2 to test the hypothesis that growth CO2 effects on isoprene emission are driven by modifications in substrate pool size, reflecting altered light use efficiency for isoprene synthesis. A novel in vivo method for estimation of the pool size of the immediate isoprene precursor, dimethylallyldiphosphate (DMADP) and the activity of isoprene synthase was used. Growth at elevated CO2 resulted in greater leaf thickness, more advanced development of mesophyll and moderately increased photosynthetic capacity due to morphological “upregulation”, but isoprene emission rate under growth light and temperature was not significantly different among ambient‐ and elevated‐CO2‐grown plants independent of whether measured at 380 μmol mol−1 or 780 μmol mol−1 CO2. However, DMADP pool size was significantly less in elevated‐CO2‐grown plants, but this was compensated by increased isoprene synthase activity. Analysis of CO2 and light response curves of isoprene emission demonstrated that the CO2 for maximum isoprene emission was shifted to lower CO2 in elevated‐CO2‐grown plants. The light‐saturated isoprene emission rate (Imax,Q) was greater, but the quantum efficiency at given Imax,Q was less in elevated‐CO2‐grown plants, especially at higher CO2 measurement concentration, reflecting stronger DMADP limitation at lower light and higher CO2. These results collectively demonstrate important shifts in light and CO2‐responsiveness of isoprene emission in elevated‐CO2‐acclimated plants that need consideration in modeling isoprene emissions in future climates.
Leaf isoprene emission scales positively with light intensity, is inhibited by high carbon dioxide (CO₂) concentrations, and may be enhanced or inhibited by low oxygen (O₂) concentrations, but the ...mechanisms of environmental regulation of isoprene emission are still not fully understood. Emission controls by isoprene synthase, availability of carbon intermediates, or energetic cofactors have been suggested previously. In this study, we asked whether the short-term (tens of minutes) environmental control of isoprene synthesis results from alterations in the immediate isoprene precursor dimethylallyldiphosphate (DMADP) pool size, and to what extent DMADP concentrations are affected by the supply of carbon and energetic metabolites. A novel in vivo method based on postillumination isoprene release was employed to measure the pool size of DMADP simultaneously with the rates of isoprene emission and net assimilation at different light intensities and CO₂ and O₂ concentrations. Both net assimilation and isoprene emission rates increased hyperbolically with light intensity. The photosynthetic response to CO₂ concentration was also hyperbolic, while the CO₂ response curve of isoprene emission exhibited a maximum at close to CO₂ compensation point. Low O₂ positively affected both net assimilation and isoprene emission. In all cases, the variation in isoprene emission was matched with changes in DMADP pool size. The results of these experiments suggest that DMADP pool size controls the response of isoprene emission to light intensity and to CO₂ and O₂ concentrations and that the pool size is determined by the level of energetic metabolites generated in photosynthesis.
To follow stomatal responses to ozone (O₃) in different Arabidopsis lines, we constructed a rapid-response O₃ exposure/gas-exchange measurement device consisting of eight through-flow whole-rosette ...cuvettes. To separate rosette from roots and growth substrate, plant is grown through an agar-filled hole in a polished glass plate fixed on the pot. Following insertion of the plant, the plate forms air-tight bottom surface of the cuvette; thus the rosette is enclosed without touching it during any phase of the insertion of the plant to the cuvette. The device allows monitoring rapid responses in the stomatal function. For example, an acute exposure to 150 ppb O₃ decreased stomatal conductance to 60-70% of its initial value within 9-12 min. Thereafter, the conductance regained its preexposure value within further 30-40 min in spite of the continuing O₃ exposure. The transient decrease was absent in the abscisic acid-insensitive mutant abi2 defective in a class 2C protein phosphatase. This provides an in vivo confirmation that the early transient decrease in stomatal conductance is not a result of physical damage by the reactive oxygen species (ROS) formed from O₃ breakdown but reflects the biological action of ROS, transduced through a signalling cascade. Thus, the apparatus will be helpful in specifying complex molecular and genetic interactions in rapid responses in guard cells in vivo.
ABSTRACT
In water‐stressed leaves, accumulation of neutral osmotica enhances the heat tolerance of photosynthetic electron transport. There are large diurnal and day‐to‐day changes in leaf sugar ...content because of variations in net photosynthetic production, respiration and retranslocation. To test the hypothesis that diurnal and day‐to‐day variations in leaf sugar content and osmotic potential significantly modify the responses to temperature of photosynthetic electron transport rate, we studied chlorophyll fluorescence rise temperatures (i.e. critical temperatures at break‐points in fluorescence versus temperature response curves, corresponding to enhanced damage of PSII centers and detachment of pigment‐binding complexes) in the dark at a background of weak far‐red light (TFR) and under actinic light (TL), and responses of foliar photosynthetic electron transport rate to temperature using gas‐exchange and chlorophyll fluorescence techniques in the temperate tree Populus tremula L. Sucrose and sorbitol feeding experiments demonstrated strong increases of fluorescence rise temperatures TFR and TL with decreasing leaf osmotic potential and increasing internal sugar concentration. Similar TFR and TL changes were observed in response to natural variation in leaf sugar concentration throughout the day. Increases in leaf sugar concentration led to an overall down‐regulation of the rate of photosynthetic electron transport (J), but increases in the optimum temperature (Topt) of J. For the entire dataset, Topt varied from 33.8 °C to 43 °C due to natural variation in sugars and from 33.8 °C to 52.6 °C in the sugar feeding experiments, underscoring the importance of sugars in modifying the response of J to temperature. However, the correlations between the sugar concentration and fluorescence rise temperature varied between the days. This variation in fluorescence rise temperature was best explained by the average temperature of the preceding 5 or 6 days. In addition, there was a significant year‐to‐year variation in heat sensitivity of photosynthetic electron transport that was associated with year‐to‐year differences in endogenous sugar content. Our data demonstrate a diurnal variation in leaf heat tolerance due to changes in sugar concentration, but they also show that this short‐term modification in heat tolerance is superimposed by long‐term changes in heat resistance driven by average temperature of preceding days.
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