Effect of Light on Growth 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. Friday, March 31, 2017 G Davidson

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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