Abstract: A composition for application to a plant is provided. The composition includes a photosensitizer that generates reactive oxygen species in the presence of light and oxygen, the photosensitizer being selected from the group consisting of a porphyrin, a reduced porphyrin and a combination thereof; a film-forming agent, the film-forming agent forming a film that is substantially impermeable to oxygen when in a non-hydrated state; an antioxidant agent; and an aqueous carrier in which the photosensitizer, the film-forming agent and the antioxidant agent are solubilized and/or dispersed. The composition is used for improving the health of a plant.