Relationships between carbonyl sulfide (COS) and CO2 during leaf gas exchange


Author for correspondence:
Dan Yakir
Tel: + 972 8 9342549


  • Carbonyl sulfide (COS) exchange in C3 leaves is linked to that of CO2, providing a basis for the use of COS as a powerful tracer of gross CO2 fluxes between plants and the atmosphere, a critical element in understanding the response of the land biosphere to global change.
  • Here, we carried out controlled leaf-scale gas-exchange measurements of COS and CO2 in representative C3 plants under a range of light intensities, relative humidities and temperatures, CO2 and COS concentrations, and following abscisic acid treatments.
  • No ‘respiration-like’ emission of COS or detectable compensation point, and no cross-inhibition effects between COS and CO2 were observed. The mean ratio of COS to CO2 assimilation flux rates, As/Ac, was c. 1.4 pmol μmol−1 and the leaf relative uptake (assimilation normalized to ambient concentrations, (As/Ac)(Cac/Cas)) was 1.6–1.7 across species and conditions, with significant deviations under certain conditions. Stomatal conductance was enhanced by increasing COS, which was possibly mediated by hydrogen sulfide (H2S) produced from COS hydrolysis, and a correlation was observed between As and leaf discrimination against C18OO.
  • The results provide systematic and quantitative information necessary for the use of COS in photosynthesis and carbon-cycle research on the physiological to global scales.


Carbonyl sulfide (COS), is the most abundant sulfur-containing gas in the troposphere with an average global mean mixing ratio of c.500 ± 100 pmol mol−1 (Montzka et al., 2007). Because of its long tropospheric lifetime, estimated at 2–4 yr (Montzka et al., 2007; Suntharalingam et al., 2008), it is transported into the stratosphere, and as a result of subsequent photolytic oxidation it is involved in the formation of stratospheric aerosol (Crutzen, 1983; Chin & Davis, 1993; Kjellstrom, 1998). Most of the main components of the global COS budget are known, but uncertainties remain regarding the precise fluxes and sink/source magnitudes involved. Emissions from the oceans, anthropogenic emissions and biomass burning are believed to be the major sources of atmospheric COS. Uptake by terrestrial vegetation and soils, and reaction with OH are considered the major sinks. Arguments for the lack of any significant secular trend in atmospheric COS concentration are consistent with a balance between sinks and sources (Chin & Davis, 1993; Watts, 2000; Kettle et al., 2002). However, recent improved measurements, such as in Antarctic firn air and air trapped in ice (Montzka et al., 2004), indicate contributions of anthropogenic sulfur emissions over time.

Notably, the new measurements of COS and COS/CO2 ratios during the seasonal cycle in background atmospheric measurements and modeling studies (Montzka et al., 2004, 2007; Blake et al., 2008; Campbell et al., 2008; Suntharalingam et al., 2008) show a large seasonal cycle, consistent with that of the land biosphere photosynthetic activity (GPP). This clearly indicates the existing potential of COS, beyond the sulfur cycle and aerosol chemistry, as a new tracer of photosynthesis, and the need for more research into processes associated with COS flux into plants, as performed in the present study.

Carbonyl sulfide uptake in plants and soil is likely dominated by its reversible hydration, resulting in the formation of H2S and CO2:

image(Eqn 1)

Under natural conditions, HCOOS and H2S are greatly diluted in plant and soil water and the formation of H2S is exergonic (Schenk et al., 2004), making the reverse reaction unfavorable. Hydration therefore represents an essentially one-way process for COS uptake. Note that any CO2 molecule taken up by leaves or wet soils undergoes similar, but fully reversible, hydration-dehydration. In leaves, the reaction in Eqn 1 is dependent on catalysis by carbonic anhydrase (CA), because of the short residence time of COS or CO2 inside leaves (usually < 1 s). This dependence on CA activity has been demonstrated for CO2 in leaves based on 18O measurements (Gillon & Yakir, 2001). It has also been shown for COS using CA inhibitors in soils (Kesselemier et al., 1999), but the dependence on CA activity is not well quantified. Enzymes other than CA, notably Rubisco (Lorimer & Pierce, 1989), can also consume COS, but their activities are very low compared with CA. Interestingly, it has been argued that CA’s affinity to COS is, in fact, greater than its affinity to CO2 (Protoschill-Krebs et al., 1996).

In plants, the correlations of COS fluxes with those of CO2, as well as with water vapor, and photosynthetic active radiation (PAR), have been interpreted as evidence of stomatal control over COS-uptake rates, which is supported by the daily pattern of this flux (Bartell et al., 1993; Hofman, 1993; Kuhn et al., 1999; Xu et al., 2002). The deposition velocity (flux normalized to concentration) for COS is greater than that for CO2 in most species. The mean ratio of the deposition velocities at the leaf level is c. 3 (Sandoval-Soto et al., 2005), but at the ecosystem level, the observed ratio ranges between 1 and 10, which is not well constrained or understood (Xu et al., 2002; Geng & Mu, 2004; Sandoval-Soto et al., 2005; Montzka et al., 2007). Recently, Campbell et al. (2008) expressed these ratios as a leaf scale relative uptake (LRU), the ratio of the uptake of COS and CO2 normalized to their respective ambient concentrations. Their findings point to a LRU of 2.2, compared with an ecosystem scale relative uptake of 5.7 (which includes ecosystem respiration). The overall flux from the atmosphere to the leaves and soil is a diffusional flux that also depends on the ambient atmospheric COS concentrations. Some measurements have also shown the existence of a compensation point for COS exchange, which seems to indicate a balance between uptake and emission (Kesselemier et al., 1999; Xu et al., 2002). Little detail is available on these aspects, although vegetation can produce dimethyl sulfide, which can be rapidly oxidized to COS (Geng & Mu, 2004).

To date, quantitative aspects of the coupling of COS and CO2 exchange by plants are still uncertain and little is known about the effects of various environmental parameters on these relationships. Such information, together with new measurement technologies (Stimler et al., 2009), would help open the way for laboratory and field explorations of COS fluxes as a powerful new tracer for CO2 gross exchange in the land biosphere. The objective of this study was to provide systematic, high-precision data on the relationships between COS and CO2 exchange in C3 leaves under controlled but variable conditions.

Materials and Methods

Plant material

The C3 plant species, indigo spires sage (Salvia longispicata Martius Galeotti × Salvia farinacea Benth), hibiscus (Rosa sinensis Linn) and pepper (Capsicum annuum (L) Kuntze) were used in all experiments. The first two species were purchased in local nurseries and the last one was grown from seed in the glasshouse. All plants were kept in a glasshouse during the experimental period at c. 25°C under natural light.

Leaf gas-exchange measurements

The experimental system consisted of a flow-through leaf cuvette made of Teflon-coated stainless steel with a magnetically operated fan and a glass window at the top. A whole leaf or branch was sealed in the cuvette (O-ring seal except around the petiole, which was sealed with high-vacuum putty). If not otherwise indicated, measurements were performed under a relative humidity (RH) of c. 70% and an air temperature of c. 25°C. Light intensity was 55–1889 μmol photon m−2 s−1, regulated with layers of miracloth and filtered through 5 cm water. Airflow was passed through a magnesium perchlorate drying trap (Sigma-Aldrich) for drying before sampling in 180 ml flasks for the analysis of COS and isotopic composition of CO2. For abscisic acid (ABA) treatments, intact leaves were immersed in 10−6 M ABA solution (c. 98%, Sigma-Aldrich) and 0.01% (w/v) Tween 20 (Fluka Chemie, Steinheim, Germany) as the surfactant for 20 min before measurements. Synthetic air was purified by passing it (at 5 l min−1) through a hydrocarbon filter (Super-Clean; Restek, Bellefonte, PA, USA). COS and CO2 mixing ratios were adjusted to the desired values by mixing the purified synthetic air with known gas mixtures produced from a permeation device with a COS permeation tube (VICI Metronics, Poulsbo, WA, USA), and a 1% CO2-pressurized cylinder, followed by a three-stage dilution system. All flow rates were regulated and measured by mass-flow controllers (MKS, Andover, MA, USA).

CO2 and COS analysis

Carbon dioxide and H2O concentrations in the air entering and leaving the leaf cuvette were measured by infrared gas analyzer (Li-6262; Li-Cor, Lincoln, NE, USA), at a precision better than 1 μmol mol−1 for CO2 and 0.1 mmol mol−1 for water vapor. CO2 concentrations were based on periodical intercomparison of flask air samples with the NOAA Earth System Research Laboratory (Boulder, CO, USA). COS concentrations were determined by a GC-MS (Varian model SATURN3) following an automated cryogenic trap (normally c. 40 s trapping time, longer for low concentrations). The precision of the COS analysis was 2.5% (< 14 pmol mol−1) at ambient atmospheric concentrations. Leaf area was estimated by tracing leaves on paper and weighing the traces at the end of each experiment. Photosynthetic parameters were calculated according to Von Caemmerer & Farquhar (1981). As for CO2, COS-uptake rates were calculated based on the concentration difference between the inlet and outlet of the leaf cuvette, the flow rate and the leaf area.

Conductance to CO2 was estimated from conventional gas-exchange measurements (Von Caemmerer & Farquhar, 1981) and ‘online’ stable isotope measurements (Evans et al., 1986; Gillon & Yakir, 2000). Similarly, estimated leaf conductance to COS (gls) was based on the leaf conductance to water vapor:

image(Eqn 2)

where gsw and gbl (mol m−2 s−1) are the stomatal and boundary layer conductance to water vapor, respectively, 1.94 is the COS/H2O binary gas phase diffusivity ratio according to Fuller et al. (1966), yielding a COS/CO2 diffusivity ratio of 0.827, and 1.56 is the ratio for the leaf boundary layer based on the two-thirds power effect in going from stagnant air to the laminar boundary layer, as done for CO2 (Farquhar & Lloyd, 1993). Cis could then be calculated from As and the estimated leaf conductance, corrected for the effects of the evaporation flux, also as conventionally done for CO2.

Isotopic analysis of CO2

Carbon and oxygen isotopic analysis of CO2 was based on sampling CO2 in the air entering and exiting the leaf cuvette by passing it through a 180 ml glass flask. The CO2 in the flask was measured as described in Klein et al. (2005). Briefly, an aliquot of 1.5 ml was removed from each flask into a sampling loop and the CO2 was cryogenically trapped using helium as the carrier gas. It was then passed through a Carbosieve G packed column at 70°C to separate N2O, and the eluted CO2 was analyzed on a Europa 20-20 continuous-flow isotope ratio MS (Europa Scientific Crewe, UK). Batches of 15 flasks at a time were measured from an automated manifold system, with five flasks of a standard gas being measured for every 10 samples. Precision of the measurements was ± 0.1 and ± 0.2‰ for 13C and 18O, respectively. Results are expressed in the small delta notation (δ‰) vs VPDB and VPDB-CO2 for 13C and 18O, respectively, where δ% = (R/Rs – 1) × 1000, and R and Rs are the isotope ratios of the sample and the appropriate standard, respectively. Instantaneous leaf discriminations, Δ, were calculated as described by Evans et al. (1986) for both 13C and 18O (Gillon & Yakir, 2000). The isotopic composition of CO2 in air supplied to the leaves during the experiments was c. δ13C = −25% and δ18O = −27%.

Results and Discussion

COS uptake

The potential use of COS as a tracer for gross CO2 uptake by leaves is associated with three basic assumptions: COS co-diffuses with CO2 from the atmosphere through stomata to the chloroplast along the same pathway; COS is not associated with any respiration-like emission; and the two co-diffusing species, CO2 and COS, do not interact (directly or indirectly). All three assumptions are well supported by the results presented here, but some exceptions were also noted.

The importance of stomata in COS uptake could be clearly seen in our experiments, as has been shown previously (Kluczewski et al., 1985; Goldan et al., 1988). First, in the dark when stomatal conductance, gs, is minimal, the rates of COS uptake were low (Table 1). Even under high ambient COS concentrations of c. 3000 pmol mol−1, COS uptake in the dark by sage, pepper and hibiscus was 1–17% as large as the rates measured when light intensity was 500 μmol−2 s−1. The residual uptake in the dark likely reflects incomplete stomatal closure. Conductance in the dark ranged between 6.4 × 10−6 and 1.8 × 10−5 mol m−2 s−1. Second, a significant co-reduction in gs and COS-assimilation rate, As, was observed in the presence of light following leaf fumigation with ABA. Reductions in gs were 91.4 ± 0.02 and 67.5 ± 0.03% in sage and hibiscus, respectively, leading to a proportional reduction in As from 80 ± 22 to 16 ± 0.7 pmol m−2 s−1 and from 93 ± 3 to 24 ± 0.7 pmol m−2 s−1 in these plants, respectively, consistent with previous results (Mawson et al., 1981; Sandoval-Soto et al., 2005).

Table 1.   Mean rates of carbonyl sulfide (COS) assimilation (As) and stomatal conductance (gsw) in three C3 species under dark and light (500 μmol photon m−2 s−1) conditions
As (pmol m−2 s−1)gsw (mol m−2 s−1)As (pmol m−2 s−1)gsw (mol m−2 s−1)
  1. Values for As are means ± SD.

  2. Experiments were conducted under CO2 and COS ambient concentrations of c. 380 μmol mol−1 and c. 3000 pmol mol−1, respectively.

Rosa sinensis0.5 ± 5.91.8E–0571.3 ± 3.70.2380.01
Capsicum annuum15.9 ± 10.41.1E–0593.7 ± 8.60.3250.17
Salvia indigo spires0.9 ± 4.16.4E–06113.0 ± 13.40.3950.01

For leaves not treated with ABA, a linear increase in As, as a function of ambient COS concentration, Cas, was observed across species and across a wide range of concentrations (Fig. 1a). The apparent compensation point (ambient COS concentration at zero uptake) estimated from the best-fit line to data compiled from different experiments (r2 = 0.99, < 0.0001) indicated an overall low mean of 60.7 pmol mol−1 (statistically not different from zero), and an average CO2 compensation point for the individual experiments of 38.2 ± 1.6 μmol mol−1. Note that possible effects of soil COS transported via the xylem, as observed for CO2 (Aubrey & Teskey, 2009) are likely negligible because of the essentially one-way hydration of COS to H2S (see the Introduction section).

Figure 1.

 Carbonyl sulfide (COS) uptake rates (As) (a), ratio of COS intercellular and ambient concentrations, expressed as Cis/Cas (b), and stomatal conductance (gs) (c), as a function of COS ambient concentration (Cas). Data compiled from light response experiments in (b) and from different COS response experiments, under an ambient concentration of c. 380 μmol mol−1 CO2 and 1179 μmol photon m−2 s−1 light intensity, in (c). Inset: data from experiments conducted across a wider range of COS mixing ratios in C3 plants. C3 species used were Salvia indigo spires, Capsicum annuum and Rosa sinesis. (panel a is S. indigo spires only).

While these values are associated with the regression statistics, the more direct approach of exposing the different plant species to COS-free air under illumination of 1170 μmol photon m−2 s−1 and gs values ranging between 0.42 and 0.64 mol m−2 s−1 showed no detectable COS emissions from any of the studied species. Previous reports noted COS compensation points of 50–320 pmol mol−1 based on somewhat different methodology (Kesselemier & Merk, 1993,, but see also reanalysis in Seibt et al. (2009). The results presented here therefore appear to confirm the hypothesis that COS uptake in leaves is not associated with any respiration-like emission.

As could be expected, the calculated average of intercellular-to-ambient concentration (Cis/Cas) ratios for the C3 species used here under different conditions also showed a linear correlation with Cas (Fig. 1b). In turn, these relationships indicated that under atmospheric concentrations of COS, its intercellular concentrations (Cis) in the leaf are rather low and the mean Cis/Cas ratio in this case was 0.22 ± 0.12. Note that under COS concentrations three times higher than typical atmospheric values, Ci/Ca for COS reached 0.63 ± 0.13, indicating a significant retrodiffusion flux out of the leaves proportional to internal concentrations and leaf conductance. The highly linear response shown in Fig. 1(b) could therefore be useful in permitting the use of high-Cas conditions for calculating leaf internal conductance (gm) to COS (see the section `Co-diffusion of COS and CO2').

Stomatal response

While stomatal control of COS uptake seems unequivocal, an unexpected effect was observed: gs was found to be enhanced at high COS mixing ratios. As shown in Fig. 1(c), gs was enhanced by a factor of c. 3 by increasing Cas to four times typical ambient atmospheric concentrations. The response was consistent and linear for all three species studies (gsw = 0.0003[COS] + 0.07; r2 = 0.9, see Fig. 1c). These observations are in fact consistent with an early report by Goldan et al. (1988), in which exposing leaves to low and high COS concentrations (100 and 700 pmol mol−1) induced reduced leaf resistance by a factor of 2.8–5.5 at the higher COS concentration.

A possible mechanism for the stomatal response to COS is a H2S effect. H2S is produced from COS in the hydration reaction (Eqn 1) and while part of this product is converted to organic sulfur and used for protein synthesis (e.g. cysteine; Rennenberg, 1984), the rest could accumulate in the leaves and eventually be emitted to the atmosphere (constituting a significant global source of H2S; Watts, 2000). Some early studies reported a possible stomatal response to H2S (Coyne & Bingham, 1978; Unsworth & Black, 1981). According to those studies, at low concentrations of H2S, plant membranes may weaken, causing subsequent loss of turgor in some epidermal cells and leading to stomatal opening, similar to the effects of SO2 (Coyne & Bingham, 1978).

The observed gs response to COS clearly needs to be further researched to establish the underlying mechanism and its relevance under ambient conditions. We note, however, that previous studies have indicated an c. 25% increase in gs in response to exposure to H2S at a concentration of 740 nmol mol−1 (and reduced gs at concentrations of 3250 nmol mol−1; Coyne & Bingham, 1978), while atmospheric concentrations of H2S are only c. 7 pmol mol−1 (Watts, 2000). Considering that the COS in the chloroplasts is converted to H2S and most of it diffuses back to the substomatal air spaces, H2S concentrations of c. 10–20 pmol mol−1 (based on observed As values and estimated internal conductance, as discussed later) can be produced, potentially raising the concentration somewhat above ambient values. Nevertheless, if most of the continuous flux of COS taken up by the leaves is counteracted by a similar emission flux of H2S, its concentrations near the leaf surface could build up under certain conditions (e.g. low turbulence, low OH) and potentially affect gs to a significant extent. Note that while COS effect on gs can help explain increasing Cis/Cas, as in Fig. 1(b), this should not affect LRU or As/Ac since, irrespective of cause, any changes in gs influence both CO2 and COS (see the section `Leaf relative uptake of COS vs CO2').

Co-diffusion of COS and CO2

Carbonyl sulfide and CO2 co-diffuse from the atmosphere into the chloroplasts of leaves, where both gas species undergo hydration catalyzed by CA. This is the terminal step for COS, which is converted to H2S with a strongly unfavorable reverse reaction, but it is a fully reversible step for CO2, allowing it to then be fixed by Rubisco. While the pathway for the two gases from the atmosphere to the site of CA is identical, there are chemophysical differences between the two gas species that influence their diffusion and reaction rates (see the Materials and Methods section).

The diffusion steps from the atmosphere through a series of restricted conductance steps include conductance through the leaf boundary layer, gbl, the stomata, gs, and the mesophyll, gm (Fig. 2). The latter can be further divided into three additional steps associated with dissolution: gds, liquid-phase diffusion, gdf, and biochemical ‘conductance’, gbc. The drop in COS concentration from the intercellular spaces to the site of hydration is influenced by the internal ‘conductance’gms (where gm = (1/gds+1/gdf+1/gbc)−1). The rate of CA’s reaction with COS at the very low concentrations used in this study is likely to be approximately proportional to the COS concentration at the site of the reaction (i.e. its partial pressure). We do not have sufficient information at this time to separate this from other limitations resulting from dissolution and diffusion of COS from the intercellular air spaces to its site of reaction. For the purpose of this study, we combine all of these steps into a single apparent conductance, gm, which can be evaluated as a residual from our gas-exchange analysis:

image(Eqn 3)
Figure 2.

 Schematic representation of the resistance path to diffusion of both carbonyl sulfide (COS; pmol mol−1) and CO2 (μmol mol−1) in C3 leaves. Values are estimated from mean values in light response measurements in Salvia indigo spires. Rates of COS and CO2 net assimilation were 20 pmol m−2 s−1 and 12 μmol m−2 s−1, respectively. Squares indicate concentrations; parentheses indicate conductance (mol m−2 s−1; Eqn 2) and biochemical and dissolution limitations (Eqn 3) in the gas and liquid phase for COS. The partitioning of gm to wall conductance (gw), and residual concentration within the chloroplast (gc) are based on stable isotope measurements of 18O and 13C of CO2.

Using data from the four highest light intensities in the light-response experiments, we obtained gm values (mol m−2 s−1) of 0.26 ± 0.21 (= 27, range 0.11–0.70) for sage and 0.30 ± 0.21 (= 10, range 0.02–0.60) for hibiscus, and an overall average for all of our experiments of 0.27 ± 0.21. This estimate is similar to the more empirical one in which we estimated As (Eqn 3 rearranged) by simply ‘tuning’ for a mean gm value that provides the best fit to the observed As values (Fig. 3). In this case, the mean gm value was 0.30 mol m−2 s−1.

Figure 3.

 Carbonyl sulfide (COS) uptake rates (As) derived from Eqn 3 vs COS uptake rates observed during light response gas-exchange measurements with different C3 species. As values were estimated based on a single estimated value of COS conductance (gm) that combines the effects of dissolution and diffusion from the intercellular air spaces to the reaction site. The best fit between model and observed As was obtained with a gm of 0.3 mol m−2 s−1. Data were taken from light-response experiments with leaf temperature at c. 24°C, relative humidity of c. 70%, and CO2 and COS ambient concentrations of c. 380 μmol mol−1 and c. 500 pmol mol−1, respectively.

For CO2, the concentration at the site of the enzymes CA and Rubisco in the chloroplasts, Cc, was estimated from online stable isotope analysis of the CO2 during gas exchange (see the Materials and Methods section; Evans et al., 1986; Farquhar et al., 1989). The total internal conductance (gm) was then estimated from the measured flux, Ac, and the concentration gradient CiCc.

As proposed by Gillon & Yakir (2000), the CO2 concentrations at the boundary of CA activity (Ccs, e.g. at the chloroplast surface) can be estimated by applying the same isotopic approach used for 13C to analyze 18O in CO2. The additional gradients, CiCcs and CcsCc, are used to estimate the internal conductance likely to be dominated by the cell wall and membranes, gw, and within the chloroplast, gch (see also Evans et al., 2009 for a recent review). Using the data from the light-response experiments for sage and hibiscus, we obtain gw = 0.15 ± 0.05 and 0.11 ± 0.04 and gch = 0.12 ± 0.003 and 0.09 ± 0.02 mol m−2 s−1 for the two species, respectively.

The entire pathway, showing both the similarities and distinctions in the CO2/COS diffusion steps as well as possible concentration gradients for a typical C3 leaf based on our measurements, is summarized in Fig. 2.

Leaf relative uptake of COS vs CO2

While testing the three basic assumptions noted earlier is a prerequisite for the implementation of COS as a tracer for gross CO2 uptake by leaves, our ability to predict the As/Ac ratio is required in practice. A priori, we could expect the ratio to be relatively constant as long as gs is the primary control. But it would be equally expected that when factors other than gs are involved (e.g. humidity and temperature effects on dissolution, diffusion and enzyme reactions), the ratio will vary significantly. Fig. 4 shows that, as expected, when compiling the gas-exchange data, an overall linear relationship between As and Ac was observed, with a mean As/Ac ratio of c. 1.4 pmol μmol−1, which seems quite robust across a wide range of assimilation rates.

Figure 4.

 Relationship between assimilation rates of carbonyl sulfide (COS) (As) and CO2 (Ac) compiled from various gas-exchange measurements conducted with different C3 species measured under different environmental parameters (light and temperature, closed circles; relative humidity (RH), open circles). The linear best-fit line, excluding RH data, indicates a mean As/Ac ratio of 1.4 pmol μmol−1. Much of the scatter was contributed by data from the RH response measurements that used older plants and lower light intensities, with more limited photosynthetic response but no effect on slope (linear-fit line including RH data: = 1.41x + 8.17, r2 = 0.37, < 0.0001).

Note, however, that as the atmospheric concentrations of both COS and CO2 are not constant, under either experimental or natural conditions, the ratios of the normalized assimilation rates are often used. The LRU ratio (see the Introduction section) is estimated from (As/Ac)(Cac/Cas). LRU as a function of light intensity, ambient COS and intercellular CO2 concentrations, RH and temperature is presented in Fig. 5. The average ratios for sage, hibiscus and pepper were 1.58 ± 0.12, 1.55 ± 0.20 and 1.74 ± 0.12, respectively, during the CO2, COS and light-response measurements, and were relatively constant across the experimental range (Fig. 5). This is probably because of the linear response of Ac to [CO2] across this range. That is, Ac/Cac is constant and As is insensitive to CO2, similar to the constant As/Cas across much of the COS range and insensitivity of Ac to Cas. At low Cac, the linear relationships break down, approaching the compensation point for CO2. In the light-response measurements (Fig. 5e), the stable ratios under high light probably reflect the dominance of the control of stomatal conductance on both CO2 and COS fluxes, but under low light, greater sensitivity of CO2 assimilation (e.g. Rubisco, but not CA, sensitivity to light) results in a sharp increase in ratios. The temperature response (Fig. 5d) provides another example where the LRU varies significantly, because of the differential temperature response of the two fluxes. While Ac peaks at c. 38°C, and As at c. 25°C the normalized ratio, LRU, peaks at 15°C, for both sage and hibiscus. The results presented in Fig. 5 also represent a range of experiments with different plants of different ages and somewhat different growing conditions (even for the same species, reflecting standard glasshouse practices). This provides some perspective on the natural variability in LRU, irrespective of the response to experimental environmental parameters, for example, from a LRU of c. 1 in the plants used in the light-response measurements, to > 3 in the plants used for the RH-response measurements (Fig. 5c; the latter were older plants measured in lower light intensities). Both the natural variability and the specific responses to environmental variables clearly show that LRU is far from constant. In most cases, however, it may be predicted if the responses are well characterized. These LRU ratios are consistent with previous estimates in the range of 0.95–3.84 (Kesselemier et al., 1993; Sandoval-Soto et al., 2005; cf. reanalysis of Seibt et al., 2009).

Figure 5.

 Leaf relative uptake (LRU; (As/Ac)(Cac/Cas)) of carbonyl sulfide (COS) over CO2 under different experimental conditions in different C3 species in response to variations in ambient CO2 concentrations (a), ambient COS concentrations (b), relative humidity (RH) (c), air temperature (d) and light intensity (e). Plant species uses were: Capsicum annuum (closed circles), Salvia indigo spires (open circles) and Rosa sinesis (triangles). Two or three measurements were made for each point (SD indicated). Symbols without error bars have SD < 2%. Unless otherwise indicated, standard conditions maintained during experiments were as follows: temperature, c. 22°C; RH, c. 70%; [COS], c. 550 pmol mol−1; [CO2], c. 380 μmol mol−1; light, 1180 (a, b, d) or 500 (c) μmol photon m−2 s−1.

Cross-interactions of COS and CO2

At low [CO2], both Ac and As were well correlated with the intercellular CO2 concentration (Cic), as shown for all three species in Fig. 6. By contrast, at high [CO2], Ac continued to increase while As became saturated, in synchronization with stomatal conductance. No inhibitory effect of CO2 on As was apparent, and As was essentially constant at high [CO2]. Similarly, in response to increasing Cas, Ac remained essentially constant (light-saturated), while As increased nearly linearly in all species (to > 2000 pmol mol−1; Fig. 6).

Figure 6.

 Changes in the rates of CO2 (Ac) and carbonyl sulfide (COS, As) assimilation, and of stomatal conductance (gs) in response to changes in CO2 internal concentrations (Ci), and ambient COS concentrations (under light intensity of 1180 μmol m−2 s−1, air temperature of c. 24°C and relative humidity (RH) of c. 70%). Plant species use were Salvia officinalis (a, d), Capsicum annuum (b, e) and Rosa sinesis (c, f). Symbols indicate CO2 (closed circles), COS (open circles), and gs (triangles). Symbols without error bars have SD < 2%. CO2 and COS ambient concentrations were c. 380 μmol mol−1 and c. 500 pmol mol−1, respectively, where not otherwise indicated.

Integrating the results of this experiment across all three C3 species showed no apparent cross-interactions or toxicity effects between CO2 and COS in the concentration ranges used, probably since both were well below their Km values. This is significant because both CO2 and COS react with CA and could act as competitive inhibitors, or become toxic at very high concentrations (Miller et al., 1989; Goyal et al., 1991).

Link to 18O-CO2

Finally, we report on a preliminary exploration of the link between As and isotopic discrimination against 18O in CO2, 18Δ. Such a link is expected, as both processes critically depend on CA activity. Hydration of CO2 in the leaves is catalyzed by CA, and has been previously shown to control the exchange of 18O between evaporatively 18O-enriched leaf water and CO2 (Gillon & Yakir, 2001). As a result, 18Δ reflects both 18O enrichment in the leaf water and the turnover rate of the reversible CA-catalyzed hydration of CO2. CA is also responsible for the consumption of COS, and increased CA activity could therefore be expected to show both greater 18Δ and greater As, and vice versa. Initial examination indeed showed the expected relationships: As correlated well with 18Δ in the species examined (= 0.31x–2.05, r2 = 0.77; Fig. 7). These are preliminary results that will be further explored in our future research, but they clearly indicate the expected link between two powerful atmospheric tracers associated with photosynthesis of land plants (Francey & Tans, 1987; Ciais et al., 1997; Gillon & Yakir, 2001; Montzka et al., 2007). Such links can ultimately help constrain estimates made with these tracers, improving the use of both.

Figure 7.

 Rates of carbonyl sulfide (COS) assimilation (As) plotted vs leaf discrimination against C18OO (Δ18O) in leaves used for light-response measurements. Solid lines indicate linear best fit.


The results support the basic assumptions made in developing the use of COS as a tracer for gross CO2 fluxes and indicate that the COS/CO2 LRU should be predictable under known environmental conditions and plant species. The data, across species and conditions, show no indication of any COS emission, no detectable compensation points, and no cross-inhibition or competitive effects between COS and CO2 in measurements at near-ambient concentrations. The shared diffusion path of COS and CO2 from the atmosphere to the chloroplasts was constructed, allowing the estimation of a mean As/Ac of c. 1.4 pmol μmol−1 and LRU of 1.6–1.7, with significant deviations when nonstomatal effects on As or Ac are in play. The results also indicated a strong stomatal response to COS, possibly mediated by H2S, which requires further research. More specific information is also needed on plant CA activities with respect to COS. The results provide quantitative basis for the application of COS/CO2 as a tracer in the terrestrial carbon cycle.