BOT Exam 2: Hormones and Stimuli

Light

One of the most important factors affecting the form, growth rate and reproduction of plants

Ultra-violet B light, Blue/near UV, & Red/far red

The 3 colors of light that plants can sense.

Phytochromes

Photoreceptor proteins that can exist in 2 different states based on photochromicity (wavelength it absorbs). Plays a key role in helping plants detect changes in day/night length.

Pr (Ground State) - Phytochromes

Absorbs red light (common in sunlight)

Pfr (Active State) - Phytochromes

Absorbs far-red light (common in shade). Inhibits activity of Gibberellin. Plants in full sunlight have more Pfr and undergo less internode expansion; plants in shade have less Pfr and undergo more internode expansion.

Fast Process with Pr and Pfr

Exposure to red light changes Pr into Pfr; exposure to far-red light changes Pfr into Pr

Slow Process with Pr and Pfr

Darkness changes Pfr into Pr

Phytochrome - Germination

Some seeds require light to germinate; red light promotes germination, reversed by far-red light. Seedlings germinated in the dark form long, weak stems with a hook below the apical meristem (etiolation).

Etiolation

Plant morphological adaptations for growing in darkness.

De-etiolation

The changes in a plant that occur upon exposure to red light (sunlight)

Photoperiodism

Ability of organisms to control life processes based on changes in light (day/night length). Regulates timing of germination, dormancy-induction/breaking, and flowering

Long-Day Plants

Plants that begin flowering as days get longer (January-June in Northern Hemisphere)

Short-Day Plants

Begin flowering as days get shorter (July-December in Northern Hemisphere)

Photoperiod

The day length, or length of light in a 24-hour cycle, regardless of quantity

Nastic Movements

Non-directional responses to stimuli

Nyctinastic

Refers to circadian rhythm-based responses controlled by daily changes in light. Leaf movement is caused by ion transport that leads to changes in cell turgor pressure.

Tropism

A directional growth in response to stimuli from the environment

Positive Tropism

Growth toward the stimulus

Negative Tropism

Growth away from the stimulus

Geotropism/Gravitropism

A growth response to gravity. Involves unequal accumulation of auxin. Geotropism overrides hydrotropism.

Negative Geotropism

A plant's shoot grows away from gravity

Positive Geotropism

A plant's roots grow down toward gravity

Statoliths

Specialized amyloplasts in cells of the stem cortex and root cap that settle to the bottom of the cell under gravity, contacting the ER and releasing calcium ions.

Calcium Ions

Promote an accumulation of PIN (auxin transport) proteins

Hydrotropism

A growth response to moisture. Mechanisms remain largely unknown.

Thigmotropism

Growth resulting from constant directional touch/pressure. Tendrils exhibit positive thigmotropism (coiling); roots exhibit negative thigmotropism (bending around obstructions).

Thigmomorphogenesis

A slow developmental change in plant shape subjected to continuous mechanical stress. Likely mediated by ethylene and jasmonic acid

Thigmonastic Movement

Touch response independent of the direction of the stimulus

Jasmonic Acid

An important molecule in plant defense against herbivores; mediates thigmomorphogenesis responses

Photoreceptors

Plant features that detect characteristics of light; consist of a protein portion and a light-absorbing chromophore

Phototropins

Mediate phototropic responses (located in apical meristem); absorb blue light

Positive Phototropism

Shown in shoots (growth toward light)

Negative Phototropism

Shown in roots (growth away from light)

Phototropism

A growth response to light occurring as a result of unequal auxin accumulation

Acid Growth Hypothesis

Auxin increases proton pumping, lowering cell wall pH and activating expansins/enzymes that loosen polysaccharides to allow cell expansion