Many plant species need to be able to time their flowering period so that they produce flowers at the same time as other members of the population and at a time when they are able to produce seeds and then disperse them. The one feature which varies in a regular way throughout the year in temperate regions is the day length. Plants use the day length changes during the annual cycle to regulate their flowering times. To enable them to detect changes in day length they use a pigment which can occur in two isomeric forms, we call this pigment Phytochrome and define the isomers of it as P₆₆₀ and P₇₃₀, the numbers referring to the wavelengths of light which are able to affect its transition to the other isomer. P₆₆₀ absorbs red light which is common in sunlight and is transformed by it into P₇₃₀. The P₇₃₀ absorbs far red light of which there is less in sunlight and is turned back into P₆₆₀. The isomer P₇₃₀ will slowly transform into P₆₆₀ in complete darkness.

 

The important part about this process shown in the diagram opposite is that it is the length of the dark period which is important in determining the relative amounts of P₆₆₀ and P₇₃₀ in the plant tissue. P₇₃₀ will quickly be built up when the plant is exposed to sunlight, but will then slowly break down into P₆₆₀ during the dark period. If the dark period is very long then the P₇₃₀ will be completely converted into P₆₆₀. If the dark period is short then the P₇₃₀ does not have time to break down completely before the light converts it back to P₇₃₀. During the spring as days get longer and nights get shorter the level of P₆₆₀ decreases as P₇₃₀ increases then in autumn the level of P₇₃₀ falls while P₆₆₀ rises. This change in balance is used by the plant to cause the release of growth regulating substances such as giberellin which in combination with other regulators causes the rapid growth of flowering stems and the production of flowers rather than leafy stems.

There are four groups of plants with respect to phytochrome responses:

1. Increasing day length types flower in the spring, bluebells etc.
2. Decreasing day length types such as chrysanthemums which flower in the autumn.
3. Long day plants which flower at the height of summer.
4. Day neutral plants which flower at any time throughout the year. Plants such as these are generally from tropical regions where day length is relatively constant throughout the annual cycle.

You may also see P₆₆₀ referred to as P_R or Phytochrome_red, while P₇₃₀ is referred to as P_FR or Phytochrome_{far red}. The phytochrome molecule consists of a pigment portion which is combined with a protein and it is a blue-green colour. It is very dilute throughout the plant and is therefore not noticed. In older text books a hormone called florigen was often quoted as being responsible for flowering of plants but years of searching have failed to discover such a substance. Phytochromes are also important in other plant responses to light such as germination of some light sensitive seeds such as certain varieties of lettuce, the expansion of leaves in response to light, the restriction of internode growth and the conversion of protochlorophyll into active chlorophyll within developing chloroplasts. All of these show a phytochrome influence since the wavelengths of light causing them exactly match the absorption spectrum of phytochrome.