HORMONES IN PLANTS

Plant hormones(phytohormones)

• often triggered by external environmental factors.

• The best understood plant hormones are:– Auxins– Gibberellins– Cytokinins– Abscisic acid– Ethylene gas

AUXIN – Charles Darwin’s study, what does it tell us?

AUXINThe answer

• Auxin is a hormone that stimulates rapid cell growth and elongation

• Auxin is ‘photophobic’ it moves to the shaded side of the plant

• As the hormone is on the shaded side of the plant, it causes cell elongation on one side, causing bending toward the light

SOME KEY TERMS RELATING TO PLANT RESPONSE TO STIMULUS

• Taxis-movement of whole organism in response to stimulus.

• Tropism-growth movement in which the direction of stimulus determines the direction of response.

• Nastic movement- is not determined by the direction of the stimulus(eg.closing and opening of flowers)

Tropisms• Growth towards a stimulus is a

positive tropism.• Growth away from a stimulus is a

negative tropismE.g.• A shoot bending towards light is

showing positive phototropism!• A root responds to gravity by

growing downwards: a positive geotropism.

PHOTOPERIODISM• Photoperiodism is the physiological reaction of plants to

the length of day and night.

• ‘Day-neutral’ plants- is not effected by duration of day and night.

• ‘Short-day plants’ –needs long period of darkness to flower• ‘Long-day’ plants- needs long period of daylight to flower

The receptor that is stimulated by photoperiod appears to be a pigment in the leaves that responds to red light. If the dark period is the right duration, a hormone is produced in the leaves. The hormone passes to shoot tips and other parts of the plant where it stimulates flower development.

Animal & plant hormones

Similarities• Like animal hormones plant hormones are also triggered by

external environmental factors.• Plant and animal hormones are very effective at low

concentrations.Difference• The same plant hormone can produce different responses

in different parts of the plant. Animal hormones produce only one type of response.

• Animal hormone is produced by specific tissue or gland but plant hormones are produced by any growing tissue.

Pheromones• Chemical communication between

individuals of the same animal species– Pheromones are released and travel through

the air to provide chemical communication between individuals

– These chemical signals are highly volatile and are detected by receptors at extremely low concentrations.

– Animals, particularly insects use pheromones to communicate with each other about important things such as food, territory & sex.

– We can’t detect each others pheromones, but they have an effect (eg. Promote or retard ovulation).

Pheromone trail

• Since most ants live on the ground, they use the soil surface to leave pheromone trails that can be followed by other ants. In species that forage in groups, a forager that finds food marks a trail on the way back to the colony; this trail is followed by other ants, these ants then reinforce the trail when they head back with food to the colony.

Territorial marking

• Dogs are territorial animals; they mark their territories with poop and urine. A fight can ensue if a dog watched another pooping or urinating in the territory he has already marked.

• Also sometimes it's a way of attracting or tracking down mates for dogs.

What do we wear?

• Long ago musk perfume used to come exclusively from the animal known as the musk deer. Nowadays it can be made in a lab too, so the animals are safe from slaughter.

What does "VNO" mean?

• • VNO stands for Vomeronasal Organs. They are 2 small

sensory organs located in the nose that operate separately from the part of our nose that detect normal smells. The VNO is used by the body to detect some types of pheromones, but is not the exclusive pheromone sensing organ many believe it to be. The membranes of the VNO, along with other receptor cells on the main olfactory membrane, are adapted to detect pheromones at low concentrations and to transmit information to the brain/hypothalamus.