The present invention provides: a method for processing a plant which includes irradiating the plant with an ultraviolet ray predominantly including one of either a right-circularly polarized light component or a left-circularly polarized light component a method for producing a plant including the above processing method a sheet for use in the above processing method including a circularly polarized light selectively reflective layer that selectively reflects or transmits right-circularly polarized light or left-circularly polarized light in any wavelength region of an ultraviolet region and an illumination device for use in the above processing method including a light source capable of radiating an ultraviolet ray and the above sheet. By the above processing method, in storage of plants or the like, the amount of a specific component in plants can be adjusted, which is useful for the production of plants.
