Abstract.The recently reported finding that plant matter and living plants produce significant amounts of the important greenhouse gas methane under aerobic conditions has led to an intense scientific and public controversy. Whereas some studies question the up-scaling method that was used to estimate the global source strength, others have suggested that experimental artifacts could have caused the reported signals, and two studies, one based on isotope labeling, have recently reported the absence of CH 4 emissions from plants. Here we show -using several independent experimental analysis techniques -that dry and detached fresh plant matter, as well as several structural plant components, emit significant amounts of methane upon irradiation with UV light and/or heating. Emissions from UV irradiation are almost instantaneous, indicating a direct photochemical process. Longtime irradiation experiments demonstrate that the size of the CH 4 producing reservoir is large, exceeding potential interferences from degassing or desorption processes by several orders of magnitude. A dry leaf of a pure 13 C plant produces 13 CH 4 at a similar rate as dry leaves of non-labeled plants produce non-labeled methane.

Summary• Recent studies demonstrating an in situ formation of methane (CH 4 ) within foliage and separate observations that soil-derived CH 4 can be released from the stems of trees have continued the debate about the role of vegetation in CH 4 emissions to the atmosphere. Here, a study of the role of ultraviolet (UV) radiation in the formation of CH 4 and other trace gases from plant pectins in vitro and from leaves of tobacco (Nicotiana tabacum) in planta is reported.• Plant pectins were investigated for CH 4 production under UV irradiation before and after de-methylesterification and with and without the singlet oxygen scavenger 1,4-diazabicyclo[2.2.2]octane (DABCO). Leaves of tobacco were also investigated under UV irradiation and following leaf infiltration with the singlet oxygen generator rose bengal or the bacterial pathogen Pseudomonas syringae.• Results demonstrated production of CH 4 , ethane and ethylene from pectins and from tobacco leaves following all treatments, that methyl-ester groups of pectin are a source of CH 4 , and that reactive oxygen species (ROS) arising from environmental stresses have a potential role in mechanisms of CH 4 formation.• Rates of CH 4 production were lower than those previously reported for intact plants in sunlight but the results clearly show that foliage can emit CH 4 under aerobic conditions.

Ultraviolet (UV) radiation has recently been demonstrated to drive an aerobic production of methane (CH4) from plant tissues and pectins, as do agents that generate reactive oxygen species (ROS) in vivo independently of UV. As the major building-blocks of pectin do not absorb solar UV found at the earth's surface (i.e. >280 nm), we explored the hypothesis that UV radiation affects pectin indirectly via generation of ROS which themselves release CH4 from pectin. Decreasing the UV absorbance of commercial pectin by ethanol washing diminished UV-dependent CH4 production, and this was restored by the addition of the UV photosensitizer tryptophan.

Almost a decade after methane was first reported in the atmosphere of Mars there is an intensive discussion about both the reliability of the observations--particularly the suggested seasonal and latitudinal variations--and the sources of methane on Mars. Given that the lifetime of methane in the Martian atmosphere is limited, a process on or below the planet's surface would need to be continuously producing methane. A biological source would provide support for the potential existence of life on Mars, whereas a chemical origin would imply that there are unexpected geological processes. Methane release from carbonaceous meteorites associated with ablation during atmospheric entry is considered negligible. Here we show that methane is produced in much larger quantities from the Murchison meteorite (a type CM2 carbonaceous chondrite) when exposed to ultraviolet radiation under conditions similar to those expected at the Martian surface. Meteorites containing several per cent of intact organic matter reach the Martian surface at high rates, and our experiments suggest that a significant fraction of the organic matter accessible to ultraviolet radiation is converted to methane. Ultraviolet-radiation-induced methane formation from meteorites could explain a substantial fraction of the most recently estimated atmospheric methane mixing ratios. Stable hydrogen isotope analysis unambiguously confirms that the methane released from Murchison is of extraterrestrial origin. The stable carbon isotope composition, in contrast, is similar to that of terrestrial microbial origin; hence, measurements of this signature in future Mars missions may not enable an unambiguous identification of biogenic methane.

Summary• Several studies have reported in situ methane (CH 4 ) emissions from vegetation foliage, but there remains considerable debate about its significance as a global source. Here, we report a study that evaluates the role of ultraviolet (UV) radiation-driven CH 4 emissions from foliar pectin as a global CH 4 source.• We combine a relationship for spectrally weighted CH 4 production from pectin with a global UV irradiation climatology model, satellite-derived leaf area index (LAI) and air temperature data to estimate the potential global CH 4 emissions from vegetation foliage.• Our results suggest that global foliar CH 4 emissions from UV-irradiated pectin could account for 0.2-1.0 Tg yr )1 , of which 60% is from tropical latitudes, corresponding to < 0.2% of total CH 4 sources.• Our estimate is one to two orders of magnitude lower than previous estimates of global foliar CH 4 emissions. Recent studies have reported that pectin is not the only molecular source of UV-driven CH 4 emissions and that other environmental stresses may also generate CH 4 . Consequently, further evaluation of such mechanisms of CH 4 generation is needed to confirm the contribution of foliage to the global CH 4 budget.

The controversial claim that attached leaves of terrestrial plants emit CH 4 aerobically remains to be corroborated. Here, we report CH 4 fluxes and CO 2 exchange rates for leaves of the C 4 species Zea mays using a high-accuracy traceable online analytical system. In contrast to earlier results for Z. mays, our measurements provide no evidence for substantial aerobic CH 4 emissions from photosynthesizing leaves illuminated with photosynthetically active radiation (k 5 400-700 nm), or from dark-respiring leaves. Preliminary measurements with the same system indicated a similar lack of aerobic CH 4 emissions in the light or dark from leaves of the C 3 species Nicotiana tabacum. These findings are supported by independent high-precision 13 C-labeling studies that also failed to confirm substantial aerobic CH 4 emissions from plants. Nevertheless, we are not able to exclude the possibility that CH 4 emissions from plants may be linked to nonenzymatic processes with an action spectrum lying outside the wavelength range for photosynthesis.

The original report that plants emit methane (CH 4) under aerobic conditions caused much debate and controversy. Critics questioned experimental techniques, possible mechanisms for CH 4 production and the nature of estimating global emissions. Several studies have now confirmed that aerobic CH 4 emissions can be detected from plant foliage but the extent of the phenomenon in plants and the precise mechanisms and precursors involved remain uncertain. In this study, we investigated the role of environmentally realistic levels of ultraviolet (UV) radiation in causing the emission of CH 4 and other gases from foliage obtained from a wide variety of plant types. We related our measured emissions to the foliar content of methyl esters and lignin and to the epidermal UV absorbance of the species investigated. Our data demonstrate that the terrestrial vegetation foliage sampled did emit CH 4, with a range in emissions of 0.6–31.8 ng CH 4 g−1 leaf DW h−1, which compares favourably with the original reports of experimental work. In addition to CH 4 emissions, our data show that carbon monoxide, ethene and propane are also emitted under UV stress but we detected no significant emissions of carbon dioxide or ethane.

No abstract