A21.3 Plant co-ordination

What are phytochromes?

A pigmented molecule which detect light from the red spectrum. It is a photoreceptor which is sensitive to photoperiods (duration of light). It controls the flowering of plants and are found in the leaves of plants. They consist of two interchangeable forms: phytochrome P660 and P730.

What is P660 and P730?

P660-absorbs red light , P730-absorbs far red light. P730 is the active form and must be referenced to when talking about flowering. P660 is rapidly converted to P730 during the day and P730 slowly converts to P660 at night.

What are long day, short day and day neutral plants?

Long day plants- plants that only flower when the duration of light (day) exceeds a min value. Here P730 stimulates flowering as there is a short night so P730 is slowly converted to P660 creating high levels of P730.

Short day plants- plants that only flower when the duration of darkness (night) exceeds the max value or when the duration of light is below the min value. Here P660 inhibits flowering as there is a long night so most of the P730 is slowly converted to P660 creating low levels of P730.

Day neutral plants- plants the flower regardless of the duration of light and darkness

What is the chemical messenger called?

As the flowering response takes place in the flower buds, a chemical messenger is needed to transport information from leaves to flower buds, this is called florigen which is transported in the phloem tissue.

What happens if there is a flash of light or darkness during the day or night?

If there is a long day and short night, and a flash of darkness flashes during the day, short day plants will not flower as although the long day is being interrupted by darkness, there is still only a short night, and as P730 is slowly converted to P660, there is high levels of P730 inhibiting flowering. Long day plants will flower as there is high levels of P730 which stimulates flowering. If there is a short day and long night, and a flash of light flashes during the night, short day plants will not flower as although there is a long night meaning P730 is converted to P660, creating high levels of P660. That flash of light rapidly converts P660-P730, causing there to be high levels of P730, inhibiting flowering and long day plants will therefore flower.

What are the three main hormones in response to plant growth?

Auxin- promotes cell elongation- found in shoot root tips

Cytokinin- promotes cell division- found in zone of division

Gibererellin- promotes cell elongation of internodal regions (regions between leaves)- found in leaves

How does auxin cause cell elongation?

Auxin is produced in the shoot tips

The auxin diffuses away from the shoot cell to cell

This causes the cell to become flexible and elongate

If the light is unidirectional the auxin travels to the shaded side causing more cell elongation on that side causing the stem to travel towards the unshaded area.