A method of improving the growth and/or pathogen resistance of a plant, comprising the step of exposing at least part of the plant to a transient period of high intensity illumination providing a photon flux at the plant surface having at least one of the following characteristics: (a) a red photon flux comprising at least 100 micromoles photons per square metre per second, and having a wavelength of between 600 and 700 nm； (b) a blue photon flux comprising at least 100 micromoles photons per square metre per second, having a wavelength of between 420 and 480 nm. The Invention also provides apparatus for providing such conditions to growing plants.
