Plant Hormones and Their Effect 24.3

Hormones A hormone is a chemical signal that affects growth, activity or development in a living organism A plant hormones control development, but also coordinate responses to environment.

How do hormones work? Specific target cell that have a receptor for the hormone Response depends on the receptors

Auxin Auxin Apical Dominance Also play a role in phototropism Stimulate cell elongation Promote growth of roots Regulate division in meristem Apical Dominance Prevent growth of buds closer to the stem’s tip Also play a role in phototropism Bending towards a light source

Cytokinins Produced in growing roots and developing seeds and fruits Stimulate cell division Regeneration of damaged tissue Delay aging of leaves Usually have the opposite affect of Auxin Auxin: made in shoots, sent to roots Cytokinins: made in roots, sent to shoots Helps to restore lost organs

More Hormones Gibberellins Abscisic Acid Stimulate growth Dramatic increase is size, specially in stems and fruits Abscisic Acid Controls seed dormancy until a shift in hormones brought on by environmental conditions Inhibits the cellular division There opposite affect helps to control homeostasis

Ethylene In the form of a gas Stimulates fruits to ripen Helps to “seal off” and drop organs that are no longer useful for the plant

Tropisms Tropism – a response to the environment Phototropism Gravitropism Thigmotoropism

Phototropism Tendency to grow towards light Auxin in responsible Migrate towards shaded area Seedlings can quickly change to grow in opposite directions

Gravitropism Shoots grow upward, roots grow downward Auxin is also responsible for this Still not understood why auxin migrates to the areas that cause this

Thigmotoropism Response to touch Plant will wrap around an object to help support its growth Vines and climbing plants Tomato plants and wind

Rapid Response Some responses are extremely quick Ex: Mimosa pudica “sensitive plant” – fold together in seconds when touched Ex: Venus flytrap Close when an organism lands inside Triggers sensory to close the leaves

Response to Seasons Plants flower according to photoperiod Length of daylight Photoperiodism Major factors in the timing of seasonal activities Flowering and growth Phytochrome – absorb red light and different signaling pathways within the plant Not completely undestood

Photoperiodism Page: 713

Winter Dormancy Deciduous trees stop photosynthesis, transport important material to roots and seal leaves off from rest of plant This is why leaves drop Phytochrome absorbs less due to day length  less auxin produced  ethylene production increases

Winter Dormancy Because of the change in hormone levels Chlorophyll breaks down and pigments that were there but were not displayed become more visible Yellow and red pigments This is why leaves change color. What hormone helps them fall off? Ethylene

Winter Dormancy More effects: Meristems produce protecting scale over buds Waxy coating Xylem and Phloem are occupied by many ions and organic compounds Anti-freeze Prevents sap from freezing