The role of ultraviolet radiation, photosensitizers, reactive oxygen species and ester groups in mechanisms of methane formation from pectin

Abstract

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. Certain ROS scavengers [mannitol, a hydroxyl radical (•OH) scavenger; 1,4-diazabicyclo[2.2.2] octane; and iodide] strongly inhibited UV-induced CH4 production from dry pectin. Furthermore, pectin solutions emitted CH4 in darkness upon the addition of •OH, but not superoxide or H2O2. Model carbohydrates reacted similarly if they possessed - CH3 groups [e.g. methyl esters or (more weakly) acetyl esters but not rhamnose]. We conclude that UV evokes CH4 production from pectic methyl groups by interacting with UV photosensitizers to generate •OH. We suggest that diverse processes generating •OH could contribute to CH4 emissions independently of UV irradiation, and that environmental stresses and constitutive physiological processes generating ROS require careful evaluation in studies of CH4 formation from foliage. © 2008 The Authors.