1. Effect of Light on Growth
Control & Regulation
Effect of Light on Growth
Mr G Davidson

2. Effect of light on shoot growth and development
Light is essential to all green plants.
Germination of seeds usually occurs underground in darkness.
Friday, March 31, 2017
G Davidson

3. Effect of light on shoot growth and development
The young plant uses its stored food supply to grow.
At this point, chlorophyll is not required to make food and this pigment is only produced when the plant is exposed to light which ensures that newly formed shoots reach the light quickly.
Friday, March 31, 2017
G Davidson

4. Effect of light on shoot growth and development
Plants which have germinated in darkness do not show expanded leaves.
It would be a problem if plants had to drag expanded leaves through the soil.
These effects are known as etiolation.
Friday, March 31, 2017
G Davidson

5. Effect of light on shoot growth and development
Symptom of Etiolation
Advantage to the plant
Lack of chlorophyll
Not required in darkness and therefore energy is not wasted producing it
Elongated shoots
If the plant is underground, it will quickly reach the surface
Folded leaves
Leaves will not be dragged through the soil when the plant germinates in the darkness
Friday, March 31, 2017
G Davidson

6. The effect of light on flowering
Most plants flower at a particular time of year when the day length is just right.
Photoperiodism is the influence of relative lengths of day and night on the activities of an organism.
Friday, March 31, 2017
G Davidson

7. The effect of light on flowering
In many plants the length of the photo period determines flowering.
Roses require a long photo period in order to flower.
They are very expensive to by at Christmas as they must be kept in artificial light to flower.
Friday, March 31, 2017
G Davidson

8. The effect of light on flowering
In order for plants to respond to light, they must be able to perceive it.
They have a photoreceptor substance called phytochrome, and it occurs in the shoot tips in concentrations of about 1 part per 10 million.
Friday, March 31, 2017
G Davidson

9. The effect of light on flowering
Phytochrome exists in two forms;
Phytochrome 660 (P660) which absorbs red light
Phytochrome 730 (P730) which absorbs far red light (non-visible light)
Friday, March 31, 2017
G Davidson

10. The effect of light on flowering
660 and 730 refer to wavelengths of the light absorbed measured in nano-metres.
When phytochrome absorbs light of the proper wavelength, it is converted into the other form.
e.g. if P660 absorbs light 660nm it becomes P730, and vice versa.
Friday, March 31, 2017
G Davidson

11. The effect of light on flowering
Sunlight contains a lot more light of 660nm than 730nm therefore there is a build up of P730 due to P660 being converted.
This P730 revert back to P660 during the night.
Flowering is thought to be started by a hormone which has yet to be isolated and is referred to as Florigen.
Friday, March 31, 2017
G Davidson

12. The effect of light on flowering
In short day plants, high concentrations of P660 are required for florigen to be released.
In long day plants, high concentrations of P730 are required from florigen.
The florigen then travels through the phloem to the meristems where flowering is induced by switching on certain genes.
Friday, March 31, 2017
G Davidson

13. Phytochrome & Flowering
Daylight
Darkness
Hormone Florigen
Travels through phloem
Flowering induced
P660
P730
Long days
High conc. P730
Short days
High conc. P660
In short
day plants
In long
Friday, March 31, 2017
G Davidson

14. The effect of light on Gonad activity in birds and mammals
The reproductive behaviour of organisms other than plants can be affected by the photo period.
In birds and mammals, sexual activity is triggered by day length.
As in plants, in order for the photo period to affect growth and development, it must be perceived.
Friday, March 31, 2017
G Davidson

15. The effect of light on Gonad activity in birds and mammals
In mammals, a gland called the pineal gland found attached to the brain perceived light and is sometimes called the “third eye”.
This gland releases a hormone melatonin which is produced at night only.
Friday, March 31, 2017
G Davidson

16. The effect of light on Gonad activity in birds and mammals
The pineal gland is a kind of biological time keeper but exactly how it works is not fully understood.
The hormone enables the brain to sense changes in the photo period.
Friday, March 31, 2017
G Davidson

17. The effect of light on Gonad activity in birds and mammals
Since most mammals are seasonal breeders, the hormone has an effect on the gonads to enable them to become active at the right time of year.
In birds, reproductive activity is brought about by longer days.
The photo period is detected by the hypothalamus in the brain.
Friday, March 31, 2017
G Davidson

18. The effect of light on Gonad activity in birds and mammals
Light receptor
Hypothalamus
Pineal Gland
Hormone producing gland
Pituitary
Pineal gland
Hormone secreted
Gonadotrophins
Melatonin
Effect
Gonadal activity
Friday, March 31, 2017
G Davidson