Passage I

Scientists once thought that on Earth the methane (CH₄) from natural sources was produced only by methanogens (bacteria that cannot survive in the presence of O₂). It has been recently discovered that some plants emit CH₄ that is generated by an unknown process operating under aerobic conditions (O₂ present). Three studies examined CH₄ emission by plants.

Study 1

Beech tree leaves were collected, air-dried, and sterilized. A small quantity (1-6 g) of the air-dried, sterilized Ieaves was placed in each of 20 identical glass tubes. Each tube was then capped. Using a needle inserted through the cap, the air in each tube was replaced with CH₄-free air. The tubes were then separated into 5 groups of 4 tubes each. Each group of tubes was incubated in the dark for 16 hr at 30°C, 40°C, 50°C, 60°C, or 70°C. The average CH₄ emission rate was determined for each group at the end of the incubation period. This procedure was repeated using ash tree leaves (see Figure 1).

Study 2

The procedure of Study 1 was repeated except that air dried, unsterilized beech and ash leaves were tested (see Figure 2) .

Study 3

Several sweetgrass plants were placed in a large Plexiglas chamber. This chamber and an identical chamber with no sweetgrass inside were then sealed. Using a pump, air in each chamber was replaced with CH₄-free air, after which the chambers were incubated in the dark at 30°C. At 102 min after incubation began, the chambers were placed in direct sunlight for a short period, then returned to the dark to continue incubation at 30°C. Beginning when the chambers were first placed in the dark, the CH₄ concentration in the air inside the chambers was measured every 25 or 50 min for 250 min (see Figure 3).

Figures adapted from Frank Keppler et al., "Methane Emissions from Terrestrial Plants under Aerobic Conditions." @2006 by Nature Publishing Group.