As the name would suggest this type of response is triggered by very low levels of light and is thought to be mediated by phytochrome A. It can be initiated by fluences as low as 0.0001μmol/m up to about 0.05μmol/m. Germination of Arabidopsis can be induced with very low levels of red light, as can oat seedlings. Such low levels of light are sufficient for inducing this response since they only convert 0.02% of the phytochrome to its active form. The backward reaction by far red light is only 98% efficient making the conversion non-photoreversible and allowing the response to proceed. VLFRs can also be induced by making up the required fluence by brief flashes of light. Since this depends on light levels and time it is known as the law of reciprocity.