Aerobic methane emissions from stinkweed (Thlaspi arvense) capsules

Abstract

Aerobic methane (CH₄) emission from plant vegetative parts has been confirmed by many studies. However, the origin of aerobic CH₄ from plants and its emission from reproductive parts have not been well documented. We determined the effects of developmental stages (early, mid, late) and incubation conditions (darkness, dim light, bright light) on CH₄ emissions from stinkweed (Thlaspi arvense) capsules. We found that CH₄ emissions from capsules varied with developmental stage and incubation light. Methane emission was highest for the late harvested capsules and for those incubated under lower (dim) light condition. Our results also showed a significant negative correlation between CH₄ emission and capsule moisture content. We conclude that CH₄ emissions vary with capsule age and diurnal light environment.

Keywords:

• aerobic methane emission
• capsule developmental stage
• light condition
• moisture
• plant
• reproductive organ
• stinkweed
• Thlaspi arvense

Abbreviations

AWOVA	=	analysis of variance
dw	=	dry weight
LSD	=	least significant difference
PPFD	=	photosynthetically active photon flux density

Methane is the second most important greenhouse gas, producing an enhanced ‘greenhouse effect’ and contributing to global warming. It is approximately 25 times more potent than CO₂ in trapping heat.¹⁰ Atmospheric concentrations of CH₄ have more than doubled since pre-industrial times.¹ Its natural and anthropogenic sources have contributed to increased air temperature in recent years.^2,17 Keppler et al.¹¹ have shown that, besides anaerobic sources, plants can also emit CH₄ under aerobic conditions and this new source may have implications to global methane budget. There has been discussion on the methods of estimation,^6,8,9 emission rates,^7,13,21 and origin of aerobic CH₄ from plants.^4,5,12,14 In these studies, CH₄ emissions were measured mostly from plant vegetative parts,⁵ but rarely from reproductive parts.³ It has been shown that stressed plants emit more methane than non-stressed ones,^{5, 18-21} and emission rates vary with plant species. The objective of this study was to measure CH₄ emissions from the stinkweed capsules at 3 developmental stages that were incubated under 3 light conditions. We hypothesized that aerobic CH₄ emissions increase with capsule maturity, which can be influenced by light conditions and capsule moisture content.

We used stinkweed (Thlaspi arvense) to measure CH₄ emissions from its capsules. Stinkweed was selected because of its prolific nature of seed production, its potential for aggressively competing with crops under future climate conditions, its distinct and spread out seed pods along the stem, and its wide distribution in Canada. In September 2007, mature seeds of stinkweed were collected from a population located near the fence at the corner of Shaganappi Trail NW and 40 Avenue NW in Calgary, Canada. Plants were grown from seeds in 1 L pots, containing a mixture of peat moss, Perlite, Vermiculite and Terragreen (2:1:1:0.5, by volume) and a few pellets of slow-releasing fertilizer (N-P-K, 14–14–14) in a controlled-environment walk-in growth chamber (model GRW36, Econair Technologies, Inc.., Winnipeg, MB, Canada) under a temperature regime of 22/18°C (16 h day/8 h night) and a relative humidity of 60%. Light was provided by a mix of cool white fluorescent tubes (24 of 2.44 m Philips F96T12/T12/CW/VHO 1500, 6 of 1.22 m Philips F48T12/CW/VHO 1500, Philips Lighting Company, Somerset, NJ) and incandescent lamps (36 of Philips 60 W, Philips Electronics Ltd., Markham, ON, Canada) on a 16-h photoperiod. The photosynthetically active photon flux density (PPFD), measured with a quantum LI-185B radiometer/photometer (LI-COR, Inc.., Lincoln, NE), was 600 μmol m⁻² s⁻¹ at the shoot apex (Fig. 1A). Plants were watered daily to field capacity until the conclusion of the experiment – seed maturity. We measured CH₄ emission from capsules that were harvested at 3 developmental stages (early, mid, late; Fig. 1B) and incubated in clear glass vials, sealed with gas sampling septa, under 3 light conditions (PPFD of 0, 5 and 160 μmol m⁻² s⁻¹) at room temperature (∼22°C) for 2 hours. A gas chromatograph with a flame ionization detector was used to measure CH₄ emission rates¹⁷ (ng g dw⁻¹ h⁻¹) from freshly detached capsules. Moisture content of capsules was also measured, as described previously.¹⁶ Data were analyzed by a 2-way analysis of variance (ANOVA) and differences among treatments were determined by Fisher's least significant difference (LSD) test at the 5% level.¹⁵ Pearson's correlation analysis was used to show the relationship between each of the parameters.¹⁵

Figure 1. Mature plants of stinkweed (Thlaspi arvense) and their capsules that were used to measure methane emissions. (A) Plants that were grown in a walk-in growth chamber under a temperature regime of 22/18°C (16 h day/8 h night), and (B) capsules and seeds harvested from plants at 3 developmental stages – early, mid and late.

Overall, we found significantly higher CH₄ emissions from those capsules that were incubated under dim light than those incubated under bright light. Methane emission was 1.22 times lower in the latter incubation conditions than that in the earlier ones (Table 1). Capsules that were incubated in darkness did not differ in CH₄ emission than those incubated under dim or bright light (Table 1). In general, there were no differences in moisture content within each category of capsules that were incubated under 3 light conditions (Table 1). Thus, differences in CH₄ emissions among incubation conditions were strongly related to incubation light than to capsule moisture. In contrast, Keppler et al.¹¹ have reported that the CH₄ emission rate was higher from tissues exposed to sunlight than those measured in darkness. However, they did not discuss the age of plant tissues, which also play a role in the processes, as discussed below.

Table 1. Methane emissions and moisture contents from stinkweed (Thlaspi arvense) capsules harvested at 3 developmental stages and incubated under 3 light conditions for 2 hours

Treatment	Methane (ng g dw^−1 h^−1)	Moisture (%)
Light condition
Dark	99.97 ± 5.98 ab	5.05 ± 0.75 a
Dim	101.33 ± 7.04 a	4.38 ± 0.97 a
Bright	83.30 ± 4.78 b	5.29 ± 0.46 a
Capsule developmental stage
Early	98.71 ± 4.75 ab	6.77 ± 0.39 a
Mid	83.54 ± 3.95 b	5.77 ± 0.53 a
Late	102.35 ± 8.38 a	2.18 ± 0.51 b

Means (± SE) followed by different letters within columns and treatments for each parameter are significantly different (P < 0.05) according to Fisher's LSD test.

Our study revealed that capsules at late developmental stage emit 1.23 times more CH₄ than those at mid developmental stage (Table 1). However, capsules from the early developmental stage did not have significant difference in CH₄ emission than those from the other 2 developmental stages (Table 1). Capsules from the late developmental stage had significantly less moisture content (3.10 and 2.65 times, respectively) than those from the early and mid developmental stages (Table 1). A significant negative correlation (r = –0.6, P < 0.001) was found between CH₄ emission and the moisture content of capsule (see Fig. 1). We previously have shown that water-stressed plants emit more CH₄ than well-watered plants.^17,18

We also analyzed data on CH₄ emissions from capsules at 3 developmental stages for each incubation condition separately (Fig. 2). In darkness, capsules from the late developmental stage had significantly higher CH₄ emission than capsules from the mid developmental stage (P < 0.001), but they did not differ than those from the early stage (Fig. 2). Capsules from the late developmental stage had significantly lower moisture than those from the other 2 stages (P < 0.001, Fig. 2). At dim light, capsules from the late developmental stage had significantly higher CH₄ emission, but lower moisture, than capsules from the early or mid developmental stage (P < 0.001, Fig. 2). At bright light, capsules from the early developmental stage emitted more CH₄ than those from the later stages and there were no differences in moisture content among them (P < 0.001, Fig. 2). These findings indicate the importance of incubation conditions on methane emission from plants, in general, and plant parts, in particular. The results suggest that CH₄ emissions vary with capsule age and light condition.

Figure 2. Methane emissions and moisture contents from stinkweed (Thlaspi arvense) capsules harvested at 3 developmental stages and incubated under 3 light conditions for 2 hours. Capsules incubated under darkness (0 μmol m⁻² s⁻¹), dim light (5 μmol m⁻² s⁻¹), and bright light (160 μmol m⁻² s⁻¹). Letters that follow the symbols (mean ± SE) indicate capsule developmental stages (E, early; M, mid; L, late).

Not only did incubation condition and capsule developmental stage individually affect CH₄ emissions, but their interactions also significantly (P < 0.001) changed emission rates. Lower (dim) light intensity led to the highest rate of CH₄ emission from capsules at the late developmental stage (130.06 ± 7.61 SE), but higher (bright) light intensity caused the lowest emission rate from capsules at the same developmental stage (68.75 ± 5.56 SE) (see Fig. 2).

In summary, CH₄ emission increases with capsule age, the lower incubation light condition accelerates methane emission, and an inverse relationship between methane emission and capsule moisture content exists.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

Acknowledgments

We appreciate useful comments on the manuscript from 2 anonymous referees.

Funding

We thank the Natural Sciences and Engineering Research Council (NSERC) of Canada for financial support through Discovery grants to MM Qaderi and DM Reid. The financial support from Mount Saint Vincent University through an Internal Research grant to MMQ is greatly appreciated.