Short-term dynamics of isotopic composition of leaf-respired CO2 upon darkening: measurements and implications

RAPID COMMUNICATIONS IN MASS SPECTROMETRY

Rapid Commun. Mass Spectrom. 2009; 23: 2428–2438

) DOI: 10.1002/rcm.4036
Published online in Wiley InterScience (www.interscience.wiley.com
Short-term dynamics of isotopic composition of

leaf-respired CO2 upon darkening: measurements and

implicationsy

Christiane Werner1*, Frederik Wegener1, Stephan Unger1, Salvador Nogues2

and Pierrick Priault1,2,3

1Experimental and Systems Ecology, University of Bielefeld, Universitatsstr. 25, D-33615 Bielefeld, Germany2Departament de Biologia Vegetal, Universitat de Barcelona, 645 Diagonal Av, 08028 Barcelona, Spain3Universite Henri Poincare Nancy I, Faculte des Sciences, UMR UHP/INRA 1137, BP 239, 54506 Vandoeuvre-les-nancy cedex, France

Received 27 January 2009; Revised 18 March 2009; Accepted 20 March 2009

*CorrespoOkologieD-33615 BE-mail: c.yPresente(JESIUM)2008.Contract/FoundatiContract/Education

Recent advances in understanding the metabolic origin and the temporal dynamics in d13C of dark-

respired CO2 (d13Cres) have led to an increasing awareness of the importance of plant isotopic

fractionation in respiratory processes. Pronounced dynamics in d13Cres have been observed in a

number of species and three main hypotheses have been proposed: first, diurnal changes in d13C of

respiratory substrates; second, post-photosynthetic discrimination in respiratory pathways; and

third, dynamic decarboxylation of enriched carbon pools during the post-illumination respiration

period. Since different functional groups exhibit distinct diurnal patterns in d13Cres (ranging from 0 to

10% diurnal increase), we explored these hypotheses for different ecotypes and environmental (i.e.

growth light) conditions. Mass balance calculations revealed that the effect of respiratory substrates

on diurnal changes in d13Cres was negligible in all investigated species. Further, rapid post-illumina-

tion changes in d13Cres (30min), which increased from 2.6% to 5% over the course of the day, were

examined by positional 13C-labelling to quantify changes in pyruvate dehydrogenase (PDH) and

Krebs cycle (KC) activity. We investigated the origin of these dynamics with Rayleigh mass balance

calculations based on theoretical assumptions on fractionation processes. Neither the estimated

changes of PDH and KC, nor decarboxylation of a malate pool entirely explained the observed

pattern in d13Cres. However, a Rayleigh fractionation of 12C-discriminating enzymes and/or a rapid

decline in the decarboxylation rate of an enriched substrate pool may explain the post-illumination

peak in d13Cres. These results are highly relevant since d13Cres is used in large-scale carbon cycle

studies. Copyright # 2009 John Wiley & Sons, Ltd.

The metabolic origin and temporal dynamics of the carbon

isotope composition of dark-respired CO2 by plants and their

implications for large-scale carbon dynamics are of major

importance for ecological studies, as reflected by recent

publications.1–7 The isotopic composition of respired CO2

(d13Cres) is widely used as a sensitive tool to partition

ecosystem respiration, tracing carbon flow through ecosys-

tems and disentangling key physiological processes.

Recently, the isotopic composition of respiratory CO2 has

been shown to exhibit high spatial and temporal variability

(i.e. changing with plant organs, environmental factors and

ndence to: C. Werner, Lehrstuhl fur Experimentelleund Okosystembiologie, Universitatsstr. 25 (W4-111),ielefeld, [email protected] at the 2nd Joint European Stable Isotope User Meeting, Presqu’ıle de Giens, France, 31 August–5 September,

grant sponsor: ISOFLUXProject of theGerman Scienceon (DFG); contract/grant number: WE 2681/2-2.grant sponsor: The Spanish Ministry of Science and; contract/grant number: PR2008-0247.

species),2,4,8,9 whose origins are not yet fully understood.

Some years ago, d13Cres was found to be 13C-depleted

compared with total organic matter in Pinus radiata and Zea

mays;10 however, recent studies have clearly shown that

d13Cres is generally 13C-enriched compared with leaf major

metabolites.4,5,8,9,11–19 This was explained by ‘apparent’

carbon isotope fractionation during dark respiration in

leaves,mainly attributed to the heterogeneous distribution of13C-atoms within hexose sugar molecules20,21 and their

incomplete oxidation in the respiratory pathways:5,9 the first

decarboxylation step of the pyruvate dehydrogenase (PDH)

releases 13C-enriched CO2, while light carbon atoms enter the

Krebs cycle (KC) as acetyl-CoA molecules. Increased

deviation of light acetyl-CoA to biosynthetic pathways will

lead to an overall respiratory signature that is 13C-enriched

relative to the hexose molecules.11

Variation in d13Cres have been related to changes in the

respiratory quotient (CO2 produced/O2 consumed) and,

thus, changes in the respiratory substrates, in response to

temperature and long darkening periods.9 However, the

observed gap between the d13Cres and d13C of the putative

Copyright # 2009 John Wiley & Sons, Ltd.

Short-term dynamics of leaf-respired d13CO2 2429

respiratory substrates was attributed to ‘apparent’ fraction-

ation.9 Changes in carbon allocation between respiratory and

biosynthetic pathways were also implicated in observed

diurnal changes of d13Cres.4,15,19 Indeed, d13Cres is not

constant on a diurnal time scale, but reveals a pronounced

increase during the light period of up to 10% relative to pre-

dawn values; this has been observed in both field and

laboratory studies in a variety of species.4,15,18,22 Distinct

diurnal dynamics were identified among species, with a

significant increase in d13Cres during the light period

followed by a continuous decrease in d13Cres during the

dark period in slow-growing evergreen or aromatic species,

while in fast-growing herbs no significant changes in d13Cres

occurred throughout the light and dark periods.4,22 Using13C-labelled pyruvate in position C-1 or in both the C-2 and

C-3 positions,23 Priault et al.4 provided direct evidence that

differences in carbon allocation between respiratory path-

ways in different species may be involved in the d13Cres

patterns: a diurnal increase in C-flux through PDH combined

with a consistently low KC activity resulted in a pronounced

concomitant d13Cres enrichment in slow-growing species,

reaching 8% in the evergreen tree Quercus ilex and the

Mediterranean shrub Halimium halimifolium.4 In contrast,

deciduous trees such as Quercus petraea and herbs such as

Tolpis barbata, with a higher carbon demand for growth

respiration and lower investment into secondary metab-

olism, exhibited only minor diurnal changes in d13Cres,

indicating that available sugar molecules are fully respired.4

Furthermore, d13Cres has been shown to be markedly 13C-

enriched during the first minutes of darkening after

illumination, in light-acclimated leaves, compared with the

more depleted steady state of dark-adapted leaves.16,19,22,24

For Ricinus communis, this transient post-illumination

d13Cres-peak was not associated with concomitant similar

changes in d13C values of respiratory substrates.19 The

metabolic origin of this pattern is currently not well

understood, but it has been proposed that light-enhanced

dark respiration (LEDR) may be involved.24 LEDR originates

from the use of organic acids as respiratory substrates such

as the rapid decarboxylation of malate representing about

20% of the CO2 evolved during the LEDR-peak.19,24,25 After

the LEDR-peak in light-acclimated leaves, d13Cres still

generally remains 13C-enriched compared with possible

respiratory substrates, with the degree of enrichment

varying by species. The extent to which rapid post-

illumination changes in the respiratory pathways are

involved in diurnal dynamics in d13Cres is still unclear as

these two phenomena have not been investigated concomi-

tantly. Similarly, there are contradicting results on the effect

of the putative respiratory substrates for temporal dynamics

of d13Cres12,15,26–28 for different species and conditions,

whereas systematic differences in d13Cres between different

functional plant types4,22 have been disregarded in most

studies.15–18,22 Hence, a consistent explanation of the effect of

respiratory substrates in plant functional types with different

variations in d13Cres is lacking.

Therefore, the main objective for this study was the

evaluation of the processes causing diurnal and post-

illumination dynamics in d13Cres of different functional plant

groups by (i) quantifying the influence of changes in the


putative substrates; (ii) identifying potential changes in

respiratory pathways by positional labelling; and (iii) estimat-

ing the potential effect of different regulatory mechanisms on

rapid changes in d13Cres using theoretical assumptions and

mass balance calculations.

EXPERIMENTAL

Plant material

Controlled growth conditionsThe Mediterranean oak Quercus ilex L. (3-year-old trees), the

woody shrub Halimium halimifolium L. and the herbaceous

Tolpis barbata (L.) Gaertn. were grown under controlled

environmental conditions. Artificial light in a growth

chamber was provided with 200–300mmolm�2 s�1 during

12 hday�1 for all species, and up to 350–450mmolm�2 s�1 for

oak leaves. The air temperature was 258C/158C during the

light and dark periods, respectively. The relative air

humidity was 60%. Plants received 150mL water twice a

week and were fertilised once a week with a modified 1/8th

strength Hoagland fertiliser solution.29 The d13CO2 in the

climate chamber was approximately �11.5� 1.5%.

Natural growth conditionsFully developed leaves from the south-facing canopy of a

deciduous oak (Quercus petraea L.) were sampled near the

University of Bielefeld in the last week of October and the

first week of November 2006, during two periods of the day:

at the end of the dark period (between 6 and 6:30 a.m.) and

1 h before the end of the light period (from 4 to 5 p.m.). At the

time of collection, the mean temperature was 78C in the

morning and 158C with a maximum light level around

700mmolm�2 s�1 in the afternoon.

Stable isotope measurements

d13C of respired CO2

Sampling and analysis were performed by the rapid in-tube

incubation method of Werner et al.22 Briefly, collected leaf

segments or entire leaves were inserted into 12mL

borosilicate glass vials capped with pierceable self-sealing

rubber septa (exetainer, LABCO, High Wycombe, UK) and

flushed for 1min with CO2-free air, provided by a 10 Lmin�1

membrane pump pushing atmospheric air through two

Plexiglass columns of soda lime. Afterwards, the leaves were

left to respire in the dark for precisely 3min (5min in the case

of Fig. 3), the time required to reach a CO2 concentration

allowing for mass spectrometry analysis (>280ppm).

Samples were processed by an automatic sampler (Microgas,

GV,Manchester, UK) connected to a continuous-flow isotope

ratiomass spectrometer (IsoPrime, GV), which allowed high-

precision gas separation. We used the large sample loop

(volume 200mL) for atmospheric air samples. Nitrogen,

oxygen and N2O were separated from CO2 in the mGas gas

chromatography (GC) column, and eluted prior to the CO2

peak. The mass spectrometer was tuned to yield peak

amplitudes of approximately 1.6 to 1.8 nA at 300ppm, which

resulted in high linearity and low noise in the data. The

precision obtained for repeated measurements of standard

laboratory gas was 0.05% (standard deviation (SD)) for


DOI: 10.1002/rcm

2430 C. Werner et al.

d13C. The isotopic ratios are expressed relative to a known

reference as d-notations (%):

d13C 0=00ð Þ ¼Rsample

Rstandard� 1

� �� 1000 (1)

where R is themeasured 13C/12C ratio in the samples and the

standard. d13C is reported relative to VPDBee (Vienna Pee

Dee Belemnite). Samples were measured against calibrated

reference gas (ISOTOP, Messer, Griesheim, Germany), and a

laboratory reference bottle (CO2, Linde, Hollriegelskreuth,

Germany), which were both cross-calibrated to IAEA-C-4

and IAEA-CH-6 (International Atomic Energy Agency,

Vienna, Austria).

External standard gas samples from laboratory standard

gases of 303 ppm CO2 (�1ppm, CO2, Messer, Griesheim,

Germany) weremeasured every 10 samples to evaluate drifts

during the measurement cycles.

d13C of sucrose, glucose and fructoseFor sugar extraction, the plant samples were lyophilized and

then ground to a fine powder (<10mm). About 50mg of the

powder was suspended with 1mL of distilled water in an

Eppendorf tube (Eppendorf Scientific, Hamburg, Germany).

The solution was mixed and centrifuged at 12000 g for 5min

at 58C. After centrifugation, the supernatant was removed

and placed in an Eppendorf tube. The Eppendorf tube was

then heated at 1008C for 3min and put on ice to denature and

precipitate proteins. After centrifugation at 12000 g for 5min

at 58C the protein-free supernatantwas removed and kept for

sugar content analysis.27

After lyophilization soluble sugar samples were purified

with a solid-phase extraction column (Oasis MCX 3cc,

Waters Millipore Corp., Milford, MA, USA). The sugar

contents were then analysed using high-performance liquid

chromatography (HPLC) (Waters 600). The HPLC refractive

index detector (Waters 2414) was set at 378C. Samples were

eluted from the columns at 858C (connected in series

Aminex HPX-87P and Aminex HPX-87C, 300mm� 7.8mm;

BioRad, Hercules, CA, USA) with water at a flow rate of

0.6mLmin�1 and run up to 45min retention time. Sucrose,

glucose and fructose were collected and transferred into tin

capsules for isotope analysis. The d13C values of the

individual sugars were measured by isotope ratio mass

spectrometry (Delta C, Finnigan MAT, Bremen, Germany)

as described previously.30 The precision obtained for

repeated measurements of standard laboratory sugars

was 0.1% (SD) for d13C.

Quantifying substrate effects on d13Cres by mass balancecalculationsThe possible contribution of diurnal changes in the pools of

soluble sugars and their isotopic signatures was evaluated

using an isotopic mass balance:

ð12 � cSuc � d13CSucÞ þ ð6 � cGlu � d13CGluÞ þ ð6 � cFru � d13CFruÞ¼ ðcSC � d13CSCÞ

(2)

where c is the molar concentration (mmol gDM�1) of sucrose

(Suc), glucose (Glu), fructose (Fru) and total soluble carbon

(SC) and d13C is the isotopic composition of the respective


sugars. Thus, assuming that the fractions of soluble sugars

produced during the day are respired during night time, we

calculated the maximal possible influence of diurnal

variation in d13C of sucrose, glucose and fructose as well

as the total soluble sugar fraction on the observed d13Cres. It

should be noted that, due to the lack of detectable sucrose, in

H. halimifolium only the fractions of fructose and glucose

were used to calculate d13CSC in this species.

Pyruvate labelling and gas exchangemeasurementsThe carbon flux rate and the isotopic signature of

respired CO2 were measured simultaneously on mature

H. halimifolium leaves. The leaves were cut at the petiole,

immediately recut under water and enclosed in a gas

exchange chamber (GFS-3000 with standard measuring head

3010-S, Walz, Effeltrich, Germany). The leaves were fed with13C-labelled pyruvate solution (45mM) through the tran-

spiration stream for approx. 2 h. The pyruvate was 99% 13C-

labelled (Cambridge Isotope Laboratories, Andover, MA,

USA) either at the C-1 (which is decarboxylated by PDH) or

at both the C-2 and C-3 carbon positions (which are

decarboxylated in the KC) to investigate changes in the

relative activity of the PDH reaction and Krebs cycle.23

The d13C of dark-respired CO2 (d13Cres) was determined

during gas exchange measurements by analysing the

difference in isotopic composition of the reference (RG)

and measuring gas (MG) as described by Werner et al.22 The

outlet (RG or MG) of the infrared gas analyzer (GFS-3000,

Walz) was interfaced via an open split to the inlet of a

Microgas autosampler (GV) interfaced to the Isoprime

continuous-flow isotope ratio mass spectrometer (GV). The

leaf chamber temperature and the relative humidity were

258C and 60%, respectively. The photon flux density at leaf

level wasmaintained at 300mmolm�2 s�1. Ingoing air passed

through the chamber at a rate of 600mmol s�1. The d13Cres

was calculated using a two-source mixing model, where the

respired CO2 is mixed with the background air entering the

leaf chamber:14

d13Cres ¼dM½CO2�M�dR½CO2�R

½CO2�M�½CO2�R(3)

where d is the isotope composition and [CO2] the CO2

concentration. The indices R and M denote the reference and

measuring gas at the chamber inlet and outlet, respectively.

The d13Cres values and dark respiration rates of leaves fed

with 13C-1 pyruvate and leaves fed with 13C-2-3 pyruvate

were used to determine the relative contributions of PDH

and KC to respiration, as the first labelled compound yields

the rate of the first decarboxylation step by PDH, while the

second yields the rate of the two decarboxylation steps in the

KC. Assuming that all respired CO2 evolved from PDH and

KC reactions, we calculated theoretical d13Cres changes

(d13Cres_calc) by the following mass balance equation:

d13Cres calc ¼PDHrated

13CPDH þ KCrated13CKC

PDHrate þ KCrate(4)

with d13CPDH and d13CKC representing the C-isotope

composition of CO2 originating from PDH and KC,

respectively, and the subscript ‘rate’ denotes the decarboxy-


DOI: 10.1002/rcm

Figure 1. Diurnal variation in carbon isotopic composition of

the dark-respired CO2 (d13Cres, %), measured immediately

upon darkening (<5 min) from fully mature leaves of different

species grown either under controlled (a) or natural (b) con-

ditions. Dark period is indicated by shaded areas. (a) Quercus

ilex, Halimium halimifolium (*) and Tolpis barbata (*) grown

undercontrolledconditions(lightperiod from09:00to21:00with

a growth light intensity between 300 and 350mmol m�2 s�1).

Measurements on Quercus ilex were performed on plants

grown under 350mmol m�2 s�1 (Quercus ilex 350: ) or

after 3 weeks of acclimation under 180mmol m�2 s�1 (Quercus

ilex 180: ). (b) Q. petraea leaves (&) collected in the last

weekof October and thefirst weekof November2006 at theend

of the dark period between 6 and 6:30 a.m. and just before

sunset from 4 to 5 p.m. At least three independent replicates

were analysed, �SE.


lation rate of each process. Subsequently, d13Cres_calc was

estimated incorporating the influence of malate decarboxy-

lation:

d13Cres calc¼PDHrated

13CPDHþKCrated13CKCþMalErated

13CMal

PDHrateþKCrate þMalErate

(5)

where the decarboxylation rate of the malic enzyme

(MalErate) was calculated as 22% of the total respiration rate

(based on data from Gessler et al.19 and Barbour et al.24). For

the d13C of malate (d13CMal) we either used a fixed value of

5.1%19,24 or considered a fractionation of the NADþ-

dependent malic enzyme of 14%,31 as described by Gessler

et al.19 Following a classical Rayleigh process we estimated

the change in the remaining substrate, with a stepwise

decrease in the substrate pool to 50% as in McNevin et al.:32

lnR

R0

� �¼ �D0 ln

12C� �12C½ �0

� �¼ �D0 ln f 0 (6)

where f0 is the fraction of substrate not consumed andD0 is the

fractionation factor defined by McNevin et al.32 This version

of the Rayleigh fractionation equation depends on an initial

start point (�ln[12C]0, R0) with which to compare all

subsequent points (�ln[12C], R).

In the case of the malate decarboxylation, Eqn. (6) can be

converted into:19

ln1þ d

1� 5:1=1000ð Þ

� �¼ � 14

1000lnðf 0Þ (7)

with a consumption up to 50% of the initial malate content ( f0

ranging between 1–0.5) and an initial d13C value of �5.1% in

C-4 of malate. The d13CO2 from the decarboxylation of the

malate pool was estimated from the isotopic signature (d) of

the remaining substrate minus the enzymatic fractionation

factor (14%) in the case of the malic enzyme.

The relative changes in d13Cres_calc during the post-

illumination period were calculated under different theor-

etical assumptions for fractionation of KC, PDH and malic

enzyme as described in detail in the Results and Discussion

sections.

If not indicated otherwise, all experiments were repeated

independently at least three times and the standard error is

given. Analyses of variance and LSD post-hoc tests were

performed using Statistica software (Statsoft Inc., Tulsa, OK,

USA) at p< 0.05.

RESULTS

Large differences in the diurnal dynamics ofleaf-respired CO2

The four studied species varied widely in the d13C of leaf-

respired CO2, with d13Cres values ranging from�30 to�25%at the end of the dark period and from �28 to �19% at the

end of the light period (Fig. 1). d13Cres exhibited two distinct

diurnal patterns: (1) a significant diurnal d13Cres increase of

8.1 and 9.1% occurred in the Mediterranean oak Quercus ilex

and in the semi-deciduous shrub Halimium halimifolium,

respectively, whereas (2) no significant d13Cres variations

during the light and dark period were observed for the fast

growing herb Tolpis barbata and the deciduous oak Quercus


petraea (Figs. 1(a) and 1(b)). The diurnal increase in d13Cres

was dependent on the light level received by the leaves,

which was visible inQ. ilex grown under 350mmolm�2 s�1 or

acclimated to 180mmolm�2 s�1. The diurnal increase in

d13Cres was less than half (3.7%) in the latter and, thus,

proportional to the intercepted light.

Diurnal variation in d13C of soluble sugarsThe carbon isotopic composition and concentration of leaf

sucrose, glucose and fructose were measured in all species

1 h before either sunrise or sunset (Fig. 2 and Table 1).

Highest concentrations were found for sucrose (p< 0.05),

being the main sugar present in leaves (between 5 and

20mmol gDW�1), except for low-light-acclimated Q. ilex

(p¼ 0.05) and for H. halimifolium, where no sucrose was

detected (Table 1). Q. petraea showed the highest accumu-

lated sugar concentrations with approx. 25–30mmol gDW�1,

while H. halimifolium exhibited very low sugar concen-

trations, around 1mmol gDW�1 (Table 1). Sugar concen-

trations were intermediate in T. barbata andQ. ilex leaves (ca.

7–10 and 10–15mmol gDW�1, respectively, with slightly

lower concentrations in low-light-acclimated Quercus leaves

(Table 1). No significant differences in sugar content were

found between day and night in any of the species.


DOI: 10.1002/rcm

Figure 2. Sucrose, glucose and fructose carbon isotopic signatures measured at the end of the dark and light periods (black and

grey bars, respectively) on (a) Quercus petraea, (b,c) Quercus ilex grown either under (b) 350mmol m�2 s�1 or after 3 weeks of

acclimation under (c) 180mmol m�2 s�1, (d) Tolpis barbata and (e) Halimium halimifolium. No sucrose was detected in

H. halimifolium leaf samples (n.d.: not detected). N¼ 2–3�SD.


For the carbon isotopic signature of the soluble sugars,

fructose exhibited the most d13C enrichment compared with

glucose and sucrose (which was significant for both Quercus

species, p< 0.05, Fig. 2). No marked differences in isotopic

signature occurred between the light and the dark period,

apart from a slight diurnal decrease in sucrose d13C in Q. ilex

180 and in Q. petraea (ca. 1 and 0.5%, respectively; p< 0.05;

Figs. 2(a) and 2(c)) and ca. 2% increase in the d13C of fructose

in H. halimifolium (p< 0.05; Fig. 2(e)).

Based on these data we quantified the maximal possible

influence of diurnal variations in the soluble sugar fraction

(d13CSC, Table 2) on d13Cres. As sucrose was the main

component of the soluble carbon fraction d13CSC was similar

to d13CSuc. Using Eqn. (2) we calculated the fraction of soluble

carbon produced during daytime as the difference between

night and day of the soluble carbon fractions and d13CSC.

Specieswith strong diurnal variation in d13Cres, such asQ. ilex

350 and H. halimifolium, showed rather enriched values of

d13C in the assimilated C-fraction of �22 and �16%,

Table 1. Sucrose, glucose and fructose contents and total soluble

light periods (night and day, respectively) on Quercus petraea, Quer

of acclimation under 180mmol m�2 s�1, Tolpis barbata and Halimium

samples (n.d.: not detected). At least two independent replicates

Species

Sucrose [mg gDW�1] Glucose [mg

day night day

Q. petraea 21.9 (�4.6) 17.1 (�1.9) 3.2 (�0.6)Q. ilex 350 7.3 (�1.9) 5.0 (�0.1) 1.4 (�0.5)Q. ilex 180 5.5 (�1.9) 2.7 (�0.3) 2.3 (�0.1)T. barbata 8.7 (�1.9) 7.0 (�0.9) 0.3 (�0.1)H. halimifolium n.d. n.d. 0.8 (�0.2)


respectively (Table 2). All species without a marked diurnal

increase in d13Cres, such as T. barbata, Q. petraea and Q. ilex

180, exhibited more depleted values (ca. �31%) in this

fraction. Considering this pattern, we tested whether or not

the carbon pool produced during the day had a strong

influence on the diurnal variation in d13Cres. Under the

assumption that the net carbon fraction produced during

the day is a possible source for respiration at night, we were

able to compare diurnal variations in d13CSC directly with

diurnal variations in d13Cres. Diurnal changes in the soluble

sugar pools only explained 0.3% and 1.1% of the diurnal

increase in d13Cres in Q. ilex 350 and H. halimifolium,

respectively (Table 2). In Q. petraea and T. barbata diurnal

variations in both d13CSC and d13Cres were not significant and

inQ. ilex 180 the pattern in d13CSCwas even opposite to that in

d13Cres and, therefore, did not explain the observed changes

in d13Cres.

Thus, temporal variation in concentrations as well as in the

isotopic compositions of soluble sugars are of minor

sugars (in mg�gDW�1) measured at the end of the dark and

cus ilex grown either under 350mmol m�2 s�1 or after 3 weeks

halimifolium. No sucrose was detected in H. halimifolium leaf

were analysed (n¼ 2–3, �SD)

gDW�1] Fructose [mg gDW�1]

Total solublesugars [mggDW�1]

night day night day night

2.9 (�0.2) 7.3 (�0.2) 6.2 (�0.1) 32.3 26.21.5 (�0.9) 3.9 (�1.2) 3.7 (�0.1) 12.6 10.21.9 (�0.3) 4.8 (�0.6) 4.2 (�0.1) 12.7 8.90.1 (�0,1) 0.4 (�0.1) 0.1 (�0.1) 9.4 7.10.8 (�0.2) 0.4 (�0.1) 0.3 (�0.1) 1.2 1.1


DOI: 10.1002/rcm

Table 2. Mean isotopic compositions of respired CO2 (d13Cres), total soluble leaf carbon (d13CSC) and the d13C fraction of soluble

carbon (SC) produced during the day, as calculated by Eqn. (2). Values shown are at the beginning (night) and at the end (day) of

the light period and their difference (Dday-night, %) for leaves of Q. petraea, Q. ilex grown at full and reduced light levels (Q. ilex 350

and Q. ilex 180, respectively), T. barbata and H. halimifolium. At least three independent replicates were analysed

Species

d13Cres [%] d13CSC [%]

d13C of SC-pool producedduring day [%]day night Dday-night day night Dday-night

Q. petraea �28.1 �28.5 0.4 �30.4 �30 �0.4 �32.2Q. ilex 350 �19.4 �27.5 8.1 �23.6 �23.8 0.2 �22.5Q. ilex 180 �26.4 �30.1 3.7 �26.6 �25 �1.6 �30.3T. barbata �24.4 �25 0.6 �30.9 �30.4 �0.5 �32.3H. halimifolium �20.7 �29.6 8.9 �28.7 �29.7 1 �16


importance for the diurnal increase in d13Cres. Given this

result, we refrained from disentangling the possible

influences of variation in the pools of sucrose, glucose and

fructose on d13Cres.

Short-term dynamics in d13C of respired CO2

In addition to the diurnal pattern of leaf-respired d13Cres

measured immediately upon darkening (<5min, Fig. 1), we

repeatedly measured the short-term post-illumination

changes within the first 30min of darkening over the diurnal

cycle. In contrast to the strong diurnal increase in d13Cres of

approximately 10% (Q. ilex, Fig. 3), d13Cres rapidly decreased

by up to 5% during the dark periods. The magnitude of this

post-illumination dynamics in d13Cres was smallest in the

morning, when d13Cres was most depleted, and increased

over the photoperiod (Fig. 3). The pattern of an overall

diurnal increase in d13Cres was also maintained after 30min

of darkening.

To determine if changes in the relative contributions of

different respiratory pathways occur during this post-

illumination period, 13C-1- and 13C-2-3-labelled pyruvate

as indicators of the PDH and KC activities, respectively,

were fed into the transpiration stream during on-line gas

Figure 3. Short-term post-illumination changes of leaf dark-

respired d13CO2 (d13Cres) during 25 min dark phases over the

diurnal course (grey areas indicate the dark period). Black

bars at the bottom of the figure represent the time during

which the measured leaf was darkened, while the rest of the

plant remained under the growth light conditions. Data are

mean values of Q. ilex leaves (n¼ 3, � SE).


exchange measurements. The measurements of dark-

respired CO2 during the light-dark transition showed a

larger increase of d13C in the measuring gas for leaves fed

with 13C-1 pyruvate than with 13C-2-3 pyruvate

(H. halimifolium, Fig. 4(a)), thus indicating a higher PDH

than KC activity. The relative decarboxylation rates were

calculated based on the simplified assumption that all

respired CO2 evolved from these two pathways (Fig. 4(b)).

Given that we utilized pyruvate labelled either at the C-1 or

at both the C-2 and C-3 positions, the rate of the

decarboxylation of the PDH (releasing one CO2 molecule)

Figure 4. Short-term dynamics measured by on-line gas

exchange onH. halimifolium leaves fed with labelled pyruvate:

(a) Respiration rate (diamonds) and increase in d13C in

measuring gas in the dark respired from 13C-1 (closed

triangles) or 13C-2-3-labelled pyruvate (open triangles);

(b) calculated respiration rates of PDH (closed circles) and

KC (open circles), based on the assumption that no other

processes contributed to total leaf respiration. The grey fields

denote the dark period for respiration measurements. Data

are mean values (n¼ 3–6, � SE).


DOI: 10.1002/rcm

Figure 6. Simplified metabolic scheme showing major fluxes

of respiratory substrates (black arrows), isotopic compo-

sitions (%) and fractionation factors (a) used to estimate

changes in dark respired CO2 (Fig. 5) during the post-illumi-

nation period (adapted from9,19,24): C-1 of pyruvate which is

decarboxylated during pyruvate dehydrogenase (PDH) reac-

tion is 13C-enriched (�20.9%9), while relatively depleted C-2


and KC (decarboxylation of two CO2 molecules) could be

assessed directly. The KC ratewas nearly stable over the dark

period and accounted for around 0.60mmolm�2 s�1, whereas

the PDH rate decreased considerably from 0.99 to

0.81mmolm�2 s�1 within the first 25min.

Isotope fractionation effects on short-termdynamics in d13C of respired CO2 upondarkeningThe estimated KC and PDH rates were used to reproduce the

observed rapid post-illumination changes in d13Cres based on

different theoretical assumptions. In a first simplified

approach, assuming that the respired CO2 evolves from no

other respiratory sources than PDH and KC, changes in the

respired d13Cres were calculated by a simple mass balance

(Eqn. (4)) assuming a value of �20.9% and �27.2% for

the CO2 released by the PDH and KC, respectively (see Fig. 6

and the Experimental section for details and references). The

effect on d13Cres was negligible (�0.2%), given that neither

the differences in the isotopic signatures nor the rate changes

were large. We further considered the effect of enzyme

fractionations during Krebs cycle reactions (see Fig. 6),

including a potential isotope effect of the enzyme citrate

synthase using a discrimination of 25% as an upper limit.33

Even assuming this relatively large, constant isotope effect,

Figure 5. Relative changes in d13Cres after darkening, calcu-

lated upon different theoretical considerations about the

involved mechanisms. Calculations are based on changes

in: (I) the decarboxylation rate of pyruvate dehydrogenase

(PDH) and Krebs cycle considering a constant fractionation

factor for the Krebs cycle enzymes (KC); (II) decarboxylation

of a malate pool by the malic enzyme (MalE); (III) a Rayleigh

fractionation for malic enzyme (MalEfrac); (IV) a rate change in

the activity of the MalEfrac with and without considering a

constant fractionation factor for the Krebs cycle enzyme

(IVa and IVb, respectively) as well as (V) a hypothetical

Rayleigh fractionation of any respiratory enzyme (Enzfrac)

discriminating against 12C. Based on Tcherkez et al.9

�20.9% and �27.2% were used for the CO2 released by

the PDH and KC, respectively. For a detailed description,

see Discussion; utilized fractionation factors are given in

Fig. 6.

and C-3 (�27.2%9) which form acetyl-CoA enter the Krebs

cycle (KC). Fractionation processes in the KC are exemplified

by Citrate synthase (a¼ 1.02538,39). Further, the potential

involvement of an enriched malate pool (�5.1%19) which is

produced during phosphoenolpyruvate carboxylase (PEPc)

reaction with small kinetic enzyme fractionation against13C (a¼ 1.00219) and equilibrium fractionation against12C (a¼ 0.99119) during HCO�

3 equilibration is indicated

(dashed lines). Decarboxylation of this malate pool by Malic

enzyme reaction with fractionation in favour of12C (a¼ 1.01419,44) will produce CO2 with a signature from

�19.1% to �9.4% with a decarboxylation rate of 22% during

the initial 50% turnover of the malate pool.9


the calculated dynamics in the respired d13Cres remained

negligible (0.9%, Fig. 5, I).

We further tested the hypothesis of the decarboxylation of

an enriched malate pool during the post-illumination

respiration pulse (please refer to the Discussion section for

a thorough explanation of this hypothesis). We calculated

d13Cres with Eqn. (5) considering a decarboxylation rate of

malate of 22%.19,24 Adding this third source does indeed

produce more enriched d13Cres (ca. �19%, data not shown)

consistent with the measured d13Cres, which can be highly

enriched compared with the putative substrate (Table 2).

However, it did not reproduce the rapid dynamics in d13Cres

upon darkening (Fig. 5, II).

Furthermore, we tested the effect of fractionation during

the decarboxylation by the malic enzyme.24 We used

Rayleigh mass balance calculations (based on Eqns. (5)

and (6), see Experimental section), based on data from

Gessler et al.19 with a 50% decline in the malate pool in the

first 25min of darkness. Assuming 14% fractionation of the

NADþ-dependent malic enzyme,19,34 we calculated the effect


DOI: 10.1002/rcm


of a transient change in carbon release from malate on the

overall respired d13Cres: the discrimination of the enzyme

does produce more depleted CO2 (i.e.�5.1 – 14%¼�19.1%)

at the beginning of the process and more enriched d13Cres

(�9.4%) after 50% decarboxylation, i.e. the fractionation

process results in the inverse pattern to what is actually

observed (increasing d13Cres over time, Fig. 5, III). We further

tested the hypothesis of an additional change in the malate

decarboxylation rate and its relative contribution to the

overall respired CO2. Assuming a hypothetical decrease

from 40 to 15% in the decarboxylation rate, a marked

decrease in d13Cres could be reproduced (Fig. 5, IVa). This did

indeed cover an approximately 3% decrease in d13Cres, a

value which approaches themeasured pattern. However, the

produced d13Cres would be rather depleted (�27.8 to�30.7%,

data not shown). Furthermore, the marked decline in d13Cres

could only be reproduced when assuming a large fraction-

ation by the KC enzymes (see above); otherwise, no marked

changes in d13Cres occurred (Fig. 5, IVb). These were the only

circumstances under which a fractionation factor in the KC

had a significant effect on the calculated d13Cres.

Finally, we estimated the fractionation factor which would

be required to reproduce the observed dynamics in d13Cres.

Assuming a Rayleigh process with an initial substrate signal

of �25% revealed that an enzyme fractionation of 10%against 12C would be sufficient to produce a decline of about

4–5% and would, thus, fit the measured d13Cres post-

illumination changes (Fig. 5, V).

DISCUSSION

Our results clearly show large variations in leaf-respired

d13Cres (both on a diurnal and a short-term scale during the

post-illumination CO2 peak), which is in agreement with an

increasing number of recent reports indicating marked

dynamics in d13Cres.2,3,15,18,19,22,24 The underlying causes for

these dynamics in d13Cres are still a matter of debate. Here, we

will evaluate three current hypotheses: (i) changes in

respiratory substrates signatures; (ii) post-photosynthetic

discrimination processes due to the interplay of different

metabolic pathways; and (iii) dynamic decarboxylation of

enriched carbon pools during the post-illumination period.

Mechanisms at the diurnal scaleTwo diurnal d13Cres patterns were observed: some species

exhibited a pronounced d13Cres diurnal enrichment of up to

10% while others showed no variation in d13Cres (Figs. 1

and 3).4,22 Previous studies found that ‘apparent’ fraction-

ation processes occurring along respiratory pathways5,9 can

explain the marked diurnal enrichment due to changes in the

partitioning of depleted acetyl-CoA molecules between

respiration in the Krebs cycle and biosynthetic pathways,

where the latter results in the release of enriched d13CO2 from

the PDH (see Fig. 6).12,15,35 Using positional labelled

pyruvate, Priault et al.4 have shown recently that these

processes were involved in diurnal changes in d13Cres among

functional groups: fast-growing herbal species showed a

stable low activity of KC and PDH reaction, while the diurnal

increase in d13Cres observed in slow-growing and evergreen

species was related to a marked increase in the C-flux


through PDH relative to a low and constant KC activity. A

species survey led to the definition of two functional groups

differing in d13Cres patterns and their C-allocation between

respiratory pathways owing to different metabolic

demands:4 slow growing, evergreen and/or aromatic species

probably deviate more carbon into secondary metabolism

such as the synthesis of isoprenoids and aromatic com-

pounds, while actively growing mesophytic plants show a

higher carbon demand for growth respiration and, thus, may

have a larger acetyl-CoA deviation to the KC (cf. Fig. 6).4

Further support to this concept is given by the result that the

deciduous Q. petraea showed a slight diurnal d13Cres

enrichment of 2.9% in summer but none in autumn

samplings (see Fig. 1 and Priault et al.4). This might be

explained by greater investment of respiratory substrates

into isoprene emission during summer, which is strongly

related to leaf temperature and photosynthetic radiation36

and, thus, would play a smaller role in autumn.4 A recent

comparison of slow-growing with fast-growing plants led to

the conclusion that there might be two distinct ‘respiratory

physiotypes’ depending on the use of newly assimilated

carbon, favouring (i) the investment of carbon into secondary

metabolism or (ii) an investment of nearly 50% of recently

assimilated carbon into the growth respiration of fast-

growing plants.17,37

However, the isotopic signature of the organic substrate

for respiration is imprinted on plant-respired CO2 and,

thus, diurnal changes in available respiratory substrates

may influence the patterns in d13Cres. A linear relationship

between d13Cres and the respiratory quotient (CO2 pro-

duced/O2 consumed) and, thus, changes in the respiratory

substrates has been observed in response to temperature

and long darkening periods.9 On a diurnal scale some

studies even found a slight enrichment in d13C of water-

soluble leaf organic matter between day and night (e.g.11,38),

although the observed variations were generally below 3%and, thus, did not explain the large shifts in d13Cres. Our

mass balance calculations showed that the soluble sugar

fraction of fresh assimilates was enriched in the species that

showed a distinct diurnal cycle in d13Cres such as Q. ilex and

H. halimifolium and depleted in the other species; this might

be explained by decreased photosynthetic discrimination

through higher stomatal control among the Mediterranean

plants (see Table 2). Increasing accumulation and, thus,

respiration of enriched fresh assimilates could therefore

potentially alter d13Cres. However, the pool of these fresh

assimilates, as well as the diurnal changes in soluble sugar

contents and their isotopic signatures, was very small and,

thus, of only minor influence. Mass balance calculations

indicated that changes in d13C of individual components or

the entire soluble sugar fractions did only account for 1.1%(of 8% d13Cres increase) in H. halimifolium (Table 2). Hence,

our results clearly show that the diurnal variation of d13Cres

does not arise from concomitant changes in either the

soluble sugar contents or their d13C values (Table 2). These

results are in accordance with previous studies,12,15,35

lending further support to the argument that changes in

d13C of the respiratory substrates (leaf soluble sugars,

starch, proteins, lipids) are not sufficient to explain the

observed changes in d13Cres.


DOI: 10.1002/rcm


Mechanisms at the short-term(post-illumination) scaleIn addition to the diurnal increase in d13Cres a second type of

rapid dynamics was observed during the post-illumination

period: a very rapid decline of 2–5% occurred within the first

30min of darkening after illumination with an increasing

amplitude over the day (Fig. 3). Such a transient d13Cres peak

during the first minutes of darkening has been observed

previously,16,19,22,24 but the underlying processes have not

yet been identified.

To our knowledge this is the first report using positional

labelling during short-term measurements and, thus,

demonstrating dynamic changes in the relative contribution

of PDH and KC activity upon darkening (Fig. 4). Both KC

and PDH activity are down-regulated in the light, though the

latter to a smaller extent (Fig. 4).23,24,39 The rapid increase in

PDH activity upon darkening was always followed by a

subsequent decline and exceeded the activity of the KC,

which reached a stable state (Fig. 4(b) and unpublished data).

Such changes in the relative activity of PDH and KC did

explain differences between functional plant groups in the

diurnal dynamics of d13Cres, as pointed out above.4 Could the

same mechanism explain the rapid changes in d13Cres upon

darkening, although the measured differences in PDH and

KC on the shorter time scale (Fig. 4) were much smaller?

To evaluate the potential mechanisms of rapid post-

illumination changes we predicted d13Cres based on different

theoretical assumptions: (I) effect of changes in the

decarboxylation rates from PDH and KC upon darkening

and isotopic fractionation in the KC; (II) additional effects

through decarboxylation of an enriched malate pool;

(III) considering fractionation associated with the malic

enzyme; and (IV) changes in the rate of malate decarboxyla-

tion as the pool declines.

The determined shifts between the two respiratory path-

ways (PDH and KC activity) had a negligible effect on d13Cres

upon darkening, due to the small differences in the isotopic

signatures of the released CO2 and the minor rate changes of

PDH and KC (Figs. 4 and 6), and, thus, failed to account for

themuch larger observed dynamics (Fig. 3). There are several

equilibrium and kinetic isotope fractionations in the

enzymatic reactions of the KC (see Fig. 6 and for a thorough

discussion33,40). Nevertheless, even assuming a large poten-

tial effect of discrimination from citrate synthase (utilising a

discrimination rate of 25% as an upper limit), and hence, a

markedly depleted CO2 source from decarboxylation in the

KC, the calculated dynamics in the respired d13Cres remained

negligible (0.9%, Fig. 5, I).

Hence, other processes must be involved to explain the

observed pattern. Barbour et al.24 suggested that the transient

peak in d13Cres is related to the light-enhanced dark

respiration (LEDR), which can be observed as a post-

illumination respiration pulse.24,41 Although the metabolic

origin is not well known,41 organic acids, like malate, might

be used as respiratory substrates during this respiration

peak,25 with an enriched d13C-signature as compared to

glucose. Malate may be produced fromHCO�3 , initially fixed

by phosphoenolpyruvate decarboxylase (PEPc) which is

known to discriminate against 13C by ca. 2.2%, while the

hydration equilibrium favours 13C by 9%, resulting in an


overall discrimination of 6.8% against 12C.42,43 Thus, the

enrichment in the C4 position of malate would produce

enriched CO2 during the decarboxylation by the NADþ-

malic enzyme during the LEDR peak.19,24

We tested this hypothesis by adding the decarboxylation

of a transient malate pool based on data from Gessler et al.19

and Barbour et al.24,44 Adding this third source did indeed

produce more enriched d13Cres but failed to reproduce the

rapid decline in d13Cres upon darkening (Fig. 5, II).

Furthermore, the malic enzyme is probably associated with

an isotope effect and decarboxylation must follow a classical

Rayleigh fractionation process;24 hence the isotope compo-

sition of the CO2 evolved by this enzyme reaction would

change with the decrease of the substrate pool. We used

Rayleigh mass balance calculations based on data from

Gessler et al.,19 assuming a 50% decline in the malate pool

during the first 25min of darkness after which the pool size

remained stable, and 14% fractionation of the NADþ-

dependent malic enzyme.19,34 In accordance with Gessler

et al.,19 the overall mean isotopic signature of the

respired CO2 during 50% of decarboxylation of the malate

pool is�14.8%. However, discrimination of the enzyme does

produce more depleted d13CO2 (�19.1%) at the beginning of

the process and more enriched d13CO2 (�9.4%) after 50%

decarboxylation, i.e. the fractionation process results in the

inverse pattern to what is actually observed (increasing

d13Cres over time, Fig. 5, III). Hence, as already pointed out by

Barbour et al.,24 the 13C-discrimination of the enzyme can

eliminate the expected effect of an enrichment in the

respired CO2. We further tested the hypothesis that the

enzyme activity would decline as the substrate pool

diminishes. Assuming a rather large hypothetical decrease

in the rate of malate decarboxylation of 40–15% did indeed

produce a 3% decrease in d13Cres (Fig. 5, IVa), a value which

approaches the measured pattern. However, the produced

d13Cres would be rather depleted (�27.8 to �30.7%) and it

should be noted that this pattern was only obtained when

assuming a large fractionation by the KC enzymes (25%;33 in

all other calculated processes, the effect of fractionation in the

KC was of minor relevance, data not shown). Hence, the

involvement of malate decarboxylation on post-illumination

d13Cres dynamics is not straightforward and further infor-

mation on the rate of metabolic fluxes and associated

fractionation processes is needed.

Our calculations indicate that an enzyme discriminating

against 12C could potentially explain themeasured dynamics

in d13Cres as the Rayleigh fractionation process would

produce highly enriched d13Cres followed by a subsequent

decline. Hence, based on theoretical considerations we

estimated that implying a Rayleigh process with an enzyme

fractionation of 10% against 12C would be sufficient to

produce the observed d13Cres dynamics upon darkening and,

thus, fit the measured post-illumination changes (Fig. 5, V).

There are several kinetic and isotopic disequilibrium effects

in the KC which discriminate against 12C;33,40 however, the

described effects are small (�4%) and the overall fraction-

ation of the KC is expected to result in a 13C-depletion.

Nevertheless, it should be considered that the light-dark

transition period does not present steady-state conditions.

Many respiratory enzymes are light-inhibited and rapidly


DOI: 10.1002/rcm


up-regulated in the dark33,44 (see, e.g., Fig. 4) and transient

fractionation processes could occur during the initiation of

these enzyme reactions. Further research is needed to assess

which enzymes or substrate pools could be involved in these

processes.

Our calculations indicate that eithermarked changes in the

decarboxylation rate of a highly enriched substrate pool or

Rayleigh fractionation processes of other enzymes discrimi-

nating against 12C provide themost plausible explanation for

the large transient changes in d13Cres immediately upon

darkening.

Impacts of post-illumination mechanisms ondiurnal dynamicsThe question then arises: to what extent may short-term post-

illumination processes be involved in the measured diurnal

increase in d13Cres? The limited amount of available data

suggest that these might be independent processes: we have

found a post-illumination d13Cres-peak in fast-growing

species such as T. barbata, which did not exhibit a diurnal

increase in d13Cres,22 indicating that this short-term response

does occur independently from diurnal changes in d13Cres.

Further indication for two independent processes is gained

from the fact that malate accumulation has been observed in

species which do not exhibit a distinct diurnal increase in

d13Cres.19 On the other hand, functional groups with a

marked diurnal increase in d13Cres show a pronounced

increase in PDH activity and a constant, low KC rate

probably as a result of an increased investment into

secondary metabolisms (Priault et al.4 and unpublished

results). In these species at least two processes are probably

co-occurring which would be consistent with the pattern

shown in Fig. 3: the short-term post-illumination dynamics

are probably superimposed on the diurnal increase in d13Cres.

These results highlight the importance of a precise timing

of the dark adaptation before measurement and of rapid and

standardised sample collection to capture the post-illumina-

tion dynamics in d13Cres. This knowledge is not only relevant

for physiological studies, but has implications at larger scales

as significant diurnal dynamics in d13Cres have also been

observed in many other respiratory sources such as trunk,45

soil,3,46,47 roots,46 and ecosystems.2,3,16,45–48 These short-term

variations in d13Cres of different ecosystem compounds have

marked implications, as they affect isotope-based partition-

ing studies of ecosystem carbon fluxes46 andmay, thus, affect

our predictions on ecosystem carbon exchange.

CONCLUSIONS

The results show marked dynamics in leaf-respired d13Cres

both during short-term post-illumination periods and on a

diurnal scale, which were independent from changes in the

signature and pool size of the putative respiratory substrates.

Two differentmechanismsmay drive the observed pattern in

this study. First, the marked diurnal increase in d13Cres in

functional plant groups with a high investment into

secondary metabolism may be attributed to changes in the

allocation pattern in the different respiratory pathways.

Second, additional short-term changes probably result from

the transient decarboxylation of an enriched substrate pool


and/or Rayleigh fractionation processes of enzymatic

reactions in the respiratory pathways which are effective

during the rapid light-dark transition and may be linked to

light enhanced dark respiration (time frame 5–30min).

Further research on the rapid dynamics in d13Cres at different

time scales and the associated metabolic changes in different

functional plant types is required. These results indicate the

importance of timing of measurements and are of major

relevance for the application of isotope studies exploring

carbon cycle processes at larger scales.

AcknowledgementsThis work was financed by the ISOFLUX Project of the

German Science Foundation (DFG, WE 2681/2-2) to CW

and by the Spanish Ministry of Science and Education

(PR2008-0247) to SN. We gratefully acknowledge valuable

comments from K. Rascher, the help of N. Hasenbein on

Q. ilex measurements, as well as skilfull technical assistance

from B. Teichner and E. Furlkroger.

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DOI: 10.1002/rcm

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