11During the past few decades it has been documented that the ultra-violet (UV) component of natural 12 sunlight alone or in combination with visible light can instantaneously stimulate aerobic plant production of 13 a range of important trace gases: CH 4 , CO 2 , CO, short-chain hydrocarbons/ non-methane volatile organic 14 compounds (NMVOC), NO x and N 2 O. This gas production, near or at the plant surface, is a new discovery 15 and is normally not included in emission budgets (e.g. by the Intergovernmental Panel on Climate Change, 16 IPCC) due to a lack of information with respect to validation and upscaling. For CH 4 it is known that the light 17 dose controls emission under ambient and artificial light conditions, but the atmospheric gas composition 18 and other environmental factors can influence gas production as well. Several plant components, including 19 pectin and leaf wax, have been suggested as a precursor for CH 4 , which equals 7-24% 28 of the natural terrestrial source strength accounted for ). In this review, we 29 summarize current knowledge, based on experimental work with sunlight and artificial light, and estimate 30 potential emission ranges and uncertainties, placing the available data into perspective. We discuss the 31 state of the art in proposed mechanisms, precursors, and environmental relationships, we consider the 32 relevance of measured emission rates, and we also suggest a range of future research topics. Furthermore 33 we propose and describe methods and techniques that can be used for future research. 34