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Control & Regulation

Control & Regulation. Effect of Light on Growth. Mr G Davidson. Effect of light on shoot growth and development. Light is essential to all green plants. Germination of seeds usually occurs underground in darkness. Effect of light on shoot growth and development.

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Control & Regulation

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  1. 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. 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. 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. G Davidson

  5. Effect of light on shoot growth and development 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. 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. 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. 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) 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. 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. 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. G Davidson

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

  18. The effect of light on Gonad activity in birds and mammals G Davidson

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