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 CO(2) 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 CO(2) 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.
Summary
Effects of elevated atmospheric
CO
2
on plant isoprene emissions are controversial. Relying on leaf‐scale measurements, most models simulating isoprene emissions in future higher
CO
2
...atmospheres suggest reduced emission fluxes. However, combined effects of elevated
CO
2
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
CO
2
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 (
P
opulus tremula
×
P
opulus
tremuloides
) grown under 380 and 780 μmol mol
−1
CO
2
. A theoretical framework based on the
C
hapman–
R
ichards function to model canopy growth and numerically compare the growth dynamics among ambient and elevated atmospheric
CO
2
‐grown plants was developed.
Plants grown under elevated
CO
2
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
CO
2
. 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
CO
2
as a result of enhanced leaf area production.
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 × 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.
ABSTRACT
Photosynthesis rate (
A
n
) becomes unstable above a threshold temperature, and the recovery upon return to low temperature varies because of reasons not fully understood. We investigated ...responses of
A
n
, dark respiration and chlorophyll fluorescence to supraoptimal temperatures of varying duration and kinetics in
Phaseolus vulgaris
asking whether the instability of photosynthesis under severe heat stress is associated with cellular damage. Cellular damage was assessed by Evans blue penetration (enhanced membrane permeability) and by H
2
O
2
generation 3,3′‐diaminobenzidine 4HCl (DAB)‐staining. Critical temperature for dark fluorescence (
F
0
) rise (
T
F
) was at 46–48 °C, and a burst of respiration was observed near
T
F
. However,
A
n
was strongly inhibited already before
T
F
was reached. Membrane permeability increased with temperature according to a switch‐type response, with enhanced permeability observed above 48 °C. Experiments with varying heat pulse lengths and intensities underscored the threshold‐type loss of photosynthetic function, and indicated that the degree of photosynthetic deterioration and cellular damage depended on accumulated heat‐dose. Beyond the ‘point of no return’, propagation of cellular damage and reduction of photosynthesis continued upon transfer to lower temperatures and photosynthetic recovery was slow or absent. We conclude that instability of photosynthesis under severe heat stress is associated with time‐dependent propagation of cellular lesions.
Leaf isoprene emission scales positively with light intensity, is inhibited by high carbon dioxide (CO(2)) concentrations, and may be enhanced or inhibited by low oxygen (O(2)) 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 CO(2) and O(2) concentrations. Both net assimilation and isoprene emission rates increased hyperbolically with light intensity. The photosynthetic response to CO(2) concentration was also hyperbolic, while the CO(2) response curve of isoprene emission exhibited a maximum at close to CO(2) compensation point. Low O(2) 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 CO(2) and O(2) concentrations and that the pool size is determined by the level of energetic metabolites generated in photosynthesis.
Background and aims Only the carbon (C) isotope pulse labeling approach can provide time-resolved data concerning the input and turnover of plant-derived C in the soil, which are urgently needed to ...improve the performance of terrestrial C cycle models. However, there is currently very limited information about the point in time after pulse labeling at which the distribution of tracer C accurately represents the usage of photosynthates in different components of the plant-soil system. This should be the case as soon as the tracer has disappeared from the mobile C pool due to respiration, incorporation into the structural C pool of shoot and root tissue and exudation into the soil (rhizodeposition). Methods Following pulse labeling in laboratory and outdoor experiments with spring rye, the .sup.14C dilution rates of soluble fractions and different substances from the structural C pool of the shoot (molecular level), the release of labeled CO.sub.2 by belowground respiration (component level), and the .sup.14C kinetics of shoot respiration and .sup.14C remaining in the plant-soil-soil gas continuum (system level) were analyzed during different stages of plant development. Results At all three levels investigated, .sup.14C kinetics indicated that the C tracer levels changed very little between 15 and 21 days after labeling. Results also showed increasing tracer depletion in the mobile C pool. Consequently, only 0.42 % and 0.06 % of all .sup.14C was still available for shoot respiration 15 and 21 days after labeling, respectively. Conclusions The similarities between .sup.14C tracer kinetics at the three investigated levels indicate that tracer disappearance from the mobile pool and distribution throughout the plant-soil system was nearly complete between 15 and 21 days after labeling. Therefore, this appears to be the point at which the pulse labeling approach provides sufficiently precise data concerning the use of C (assimilated during labeling) for root growth, rhizodeposition, root respiration and the microbial turnover of rhizodeposits.
To assess protection of the mesophyll cell plasmalemma against O3 by apoplasmic reduced ascorbate (AA), its concentration in the leaf cell wall of common bean (Phaseolus vulgaris L.) was lowered from ...0.6 mM to 0.1 mM by pre-exposing plants to continuous darkness for up to 48 h. Subsequent ozonization of ascorbate-deficient leaves with 350-450 nmol O3 mol-1 resulted in a rapid rise of apoplasmic AA within the second hour of the treatment, the concomitant appearance of cytoplasmic marker enzymes in cell wall solute extracts and the development of water-logged spots on leaves. Prior to these events, stomatal conductances had just reached values close to those observed in AA- nondeficient leaves, whereas AA concentration in the cell wall was still 24 times lower than in leaves pre-exposed to the normal 10-h dark period. In AA-nondeficient leaves the initial apoplasmic AA level of 0.6 mM was maintained under O3 for 2.5 h; thereafter, it increased moderately. There appeared to be no signs of injury even 2 d after the whole 4.5-h treatment. During the period of equal stomatal conductances, the O3 decay rate in direct reaction with AA in AA-deficient cell walls was estimated to be 50-70% of that occurring in AA-nondeficient leaves. It is suggested that under AA deficiency some threshold for the stability of the plasmalemma was surpassed owing to the more "O3-permeable" cell wall. The mesophyll conductance was found to be stable throughout O3 exposure, indicating that the cytoplasmic O3 defense barrier was not exceeded. Possible changes in oxyradical reactions and in cell wall phenolics are discussed. It is suggested that after prolonged darkness the flow rate of reactive oxygen intermediates to the plasmalemma may also be higher because they are less trapped in direct and peroxidase-catalyzed reactions.
Leaves of several plant species were illuminated either in air or in a gas atmosphere with reduced oxygen content, and photosynthesis and transpiration were recorded after brief exposure to high ...concentrations of SO
2. Inhibition of photosynthesis by SO
2 was similar in air and at reduced oxygen concentrations or, in some experiments, even larger under low oxygen. Recovery from inhibition was always faster in high than in low oxygen. In leaves of
Pelargonium zonale, stomata remained open or closed only slowly after fumigation, whereas in spinach and potato leaves stomata closed rapidly under the influence of high SO
2. After cessation of fumigation, stomata always reopened.
pH-indicating fluorescent dyes were employed to monitor SO
2-dependent pH changes in the apoplast and the cytosol of leaf tissue. In the cytosol, acidification was weaker in the dark than in the light. In the light, it was stronger in 21 % oxygen than in 1% oxygen. Cytosolic acidification could not fully be attributed to the hydration and oxidation of SO
2 but was also caused by a transient breakdown of transmembrane proton gradients that resulted in the influx of acid from acidic leaf compartments into the cytosol. This was concluded from opposite pH changes in the cytosol and the apoplast under the influence of SO
2. Even chloroplasts were transiently acidified by SO
2 as shown by the kinetics of 505 nm absorption, which reflect the pH-sensitive interconversion of violaxanthin to zeaxanthin.
Differences in the kinetics of SO
2-dependent inhibition of photosynthesis and of cytosolic acidification in high and low oxygen suggest combined nucleophilic attack of sulfite, and of radicals generated in the light, on sensitive cellular constituents. However, the reversibility of photosynthesis inhibition and of acidification, particularly in the presence of air levels of oxygen, demonstrates fast repair.
The observations suggest that detrimental effects on plants caused by long-term exposure to ambient concentrations of SO
2 which are several hundredfold lower than used in the present investigation, can neither be explained by radical damage nor by immediate effects of cellular acidification. Both radical detoxification and pH regulation are highly effective in healthy plants.
PDF
