Photoperiodism

Plants do not have a nervous system;growth is coordinated by plant growth substance, and hormones. This makes coordination slow.

One response is to light and dark and is called photoperiodism.

e.g. FLOWERING IS INFLUENCED BY DAY LIGHT.

Phytochrome

This is the photoreceptor responisble for absorbing light, and is a blue-green pigment in very small quantitites.

It exists in two inter-convertible forms:

red light PR (660)<><><><> far-red light PFR ( 730)

• When absorbing light, the pigment form converts from PR to PFR.
• Sunlight contains more 630nm light, so during the day, PR is converted to PFR which accumulates.
• PFR is unstable and  breaks down during the night, back to PR which accumulates.
• …