Unit 1 Lesson 5 - Plant Hormones and Growth

auxins

Group of plant hormones primarily consisting of indole-3-acetic acid that promote cell elongation through cell wall acidification, establish apical dominance by inhibiting lateral buds, and drive directional growth responses by redistributing to shaded or lower sides.

indole-3-acetic acid (IAA)

The primary natural auxin produced in shoot apical meristems and young leaves that is transported downward in a polar manner to regulate elongation, root initiation, and tropisms.

polar transport

Directed movement of auxins always from the tip (source) toward the base of stems or roots using specific membrane carrier proteins, ensuring one-way signaling for growth coordination.

acid growth hypothesis

Mechanism by which auxins lower cell wall pH through proton pumps, activating enzymes that loosen cellulose microfibrils and allow water uptake to drive irreversible cell elongation.

apical dominance

Phenomenon where the shoot apical meristem produces auxins that suppress growth of lateral buds below it, maintaining a single main stem until the tip is removed.

phototropism

Directional growth of shoots toward light caused by auxin migration to the shaded side, leading to greater cell elongation on that side and bending of the stem.

gravitropism

Response to gravity in which auxins settle to the lower side of organs, causing shoots to bend upward (negative gravitropism) and roots to bend downward (positive gravitropism).

statoliths

Dense starch-filled organelles in root cap or shoot endodermis cells that settle under gravity and trigger auxin redistribution to initiate gravitropic curvature.

thigmotropism

Growth response to touch or mechanical stimulation, often mediated by ethylene and auxins, causing vines or tendrils to coil around supports.

hydrotropism

Directional growth of roots toward a water source, regulated by abscisic acid signaling and auxin gradients to optimize water uptake.

gibberellins

Class of hormones produced in young leaves, roots, and developing seeds that stimulate stem elongation, seed germination by inducing enzyme release in the aleurone layer, and fruit development.

aleurone layer

Protein-rich outer layer of cereal endosperm that responds to gibberellins by synthesizing and secreting hydrolytic enzymes to break down stored starch during germination.

cytokinins

Hormones produced mainly in root apical meristems that promote cell division when auxin levels are balanced, delay leaf senescence, and stimulate lateral bud outgrowth.

zeatin

A naturally occurring cytokinin first isolated from maize that is highly active in promoting cell division and nutrient mobilization to prevent aging in leaves and tissues.

senescence

Programmed aging process in leaves or organs where chlorophyll breaks down and nutrients are remobilized, accelerated by ethylene and abscisic acid but delayed by cytokinins.

abscisic acid (ABA)

Stress hormone produced in leaves, roots, and seeds under drought or cold that closes stomata by reducing guard cell turgor, maintains seed dormancy, and promotes hardening for winter survival.

ethylene

Gaseous hormone produced in ripening fruits and stressed tissues that triggers fruit softening and sugar conversion, promotes senescence and abscission, and induces adventitious root formation.

synergism

Cooperative interaction between hormones such as auxin and gibberellins that together produce a greater stem elongation response than either alone.

antagonism

Opposing interaction between hormones such as gibberellins promoting germination while abscisic acid inhibits it, allowing precise control based on environmental conditions.

dormancy

Temporary suspension of growth or germination maintained by abscisic acid until favorable conditions (water, temperature) reduce its levels and allow gibberellins to activate.