Tropic Responses

Objectives:*Define geotropism and phototropism**Explain role of auxin in plant growth***Explain the use of synthetic plant hormones as weedkillers

• What is controlled by hormones in plants?

Growth – roots and shoots, seed germination, leaf fall, disease resistance, fruit formation and ripening, flowering time, bud formation.

Sensitivity – Responding to the environment

The way Plants grow is controlled by Hormones. They are sensitive to their environment and can make adjustments to survive.

Copy and CompleteTropism is how plants respond to their environment.When sunlight is the stimulus this is known as ______tropism.Positive phototropism is where the plant grows _______the light. This happens at the _____tip.________ phototropism is where the plant grows away from the______, this happens at the_____.When gravity is the stimulus this is known as ______tropism.________Geotropism is where the plant grows in the direction of_______. This happens at the roots.Negative ___tropism is where the plant grows apposing gravity, this happens at the _____tips..

Auxins• Auxins are one of the hormones used by plants for

growth.• They have different effects depending where it is.• In the shoots any concentration promotes growth.• At the root, high concentration inhibits growth, small

concentration promotes growth.• The Sunlight breaks down auxins, therefore the shoot

tip which has direct sunlight will have the least amount of auxins. This means the area of the plant which is shaded has a higher concentration of auxins, and so more growth occurs, this leads to lop sided growth and the plant bends towards the light

• diagram

Auxin in actionCells become longer when they absorb water by osmosis. The vacuole swells increasing the turgor pressure inside the cell which causes the cell wall to stretch. Auxins help by making it easier for the walls to stretch.

Uses of Plant hormones

• Developed by scientists• Plant cultivation and food production

• Rooting powder• Selective weedkillers• Fruit ripening Ethene• Dormancy in seeds• Neater Hedges. Cutless hormone.

Match the correct scenario to the tropism.

Roots Growing Downwards into Dark SoilRoots Growing Upwards towards light.

Shoots Growing Upwards

Roots Growing Downwards.

Positive Phototropism

Negative Geotropism

Negative Phototropism

Positive Geotropism

Synthetic Plant HormoneSynthetic auxins are very effective as weedkillers.Crops like barley and wheat has narrow leaves whereas the weeds competing with them for sunlight water and minerals have broad leaves. 2,4-D are sprayed which runs off the narrow leaves because they are thin and pointed upwards but absorbed by the broad leaves.The auxins increase the growth rate of affected weeds by increasing the rate of cell division. The weeds cannot provide enough food from photosynthesis to maintain the growth rate and die.

• Homework – Revise Coordination and response topic for test on 26/05/11