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Chapter 39: Control Systems in Plants. Question. Do plants sense and respond to their environment ? Yes - By adjusting their pattern of growth and development. In Dark. In Light. Comment. Plants can’t “move” away from a stimulus, but can change their growth response.

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Chapter 39: Control Systems in Plants

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Chapter 39 control systems in plants

Chapter 39:Control Systems in Plants



  • Do plants sense and respond to their environment ?

  • Yes - By adjusting their pattern of growth and development.

In Dark

In Light



  • Plants can’t “move” away from a stimulus, but can change their growth response.

  • Result – plant bodies are more “flexible” in morphology than animals.

Classical example

Classical Example

  • Phototropism - plant growth response to unilateral light.

  • Observation – plants bend or grow towards the light.

Phototropism experimenters

Phototropism Experimenters

  • Darwins: late 1800's.

  • Boysen & Jenson: early 1900's.

  • F.W. Went: 1926

Went experiments

Went Experiments

Mechanism of phototropism

Mechanism of Phototropism

  • Cells on the dark side elongate faster than the cells on the light side.

  • The uneven growth rate causes the bending of the stem toward the light.



  • What is the adaptive value of phototropism?

  • It tilts the leaves toward the light source for more efficient photosynthesis.

Cause of phototropism

Cause of Phototropism

  • Chemical messenger from the tip caused the growth response in the stem.

  • The distribution of the chemical changes in the unequal light, resulting in unequal cell elongation.



  • Chemical signal produced in one location, transported, has effect in another location.

  • Phototropism is caused by a plant hormone.

Plant hormones

Plant Hormones

  • Are produced in small quantities.

  • Effects may reflect balance between several hormones.



Plant hormones1

Plant Hormones

1. Auxins

2. Cytokinins

3. Gibberellins

4. Abscisic Acid

5. Ethylene



  • Named by Went in 1926.

  • First plant hormone described.

  • Ex: IAA (natural) 2,4-D (synthetic)

Major functions

Major Functions

  • Stimulates cell elongation.

  • Fruit development.

  • Apical Dominance.

  • Tropism responses.

Apical dominance

Apical Dominance

Where produced

Where Produced

  • Apical Meristems.

  • Young leaves.

  • Embryos.



  • Isolated from coconut "milk" (endosperm) in the 1940’s.

  • Named because they stimulate cell division.

  • Ex: Zeatin

Major effects

Major Effects

  • Stimulates cell division.

  • Delays senescence.

  • Root growth and differentiation.

  • Where Produced - roots

Auxin cytokinin ratios

Control shoot or root differentiation in tissue cultures.

Auxin/Cytokinin Ratios



  • Found from the "Foolish Seedling" disease in rice.

  • Ex: GA3 70 types known

Foolish seedlings

Foolish Seedlings

Major effects1

Internode elongation.

Seed/Bud germination.

Flowering (some species).

Fruit development.

Major Effects

Extra GA3

No GA3

Chapter 39 control systems in plants

Have GA3

Lack GA3

Where produced1

Where Produced

  • Apical Meristems.

  • Young leaves.

  • Embryos.

Abscisic acid

Abscisic Acid

  • Slows or inhibits plant growth.

  • "Stress" hormone produced under unfavorable conditions.

Major effects2

Major Effects

  • Inhibits growth

  • Seed/Bud dormancy.

  • Stomata closure.

  • Leaf drop – produces abscission layer.

Chapter 39 control systems in plants

  • Abscission Layer

Where produced2

Where Produced

  • Leaves

  • Stems

  • Green fruit



  • Gaseous hormone (fast diffusion rates).

  • Often interacts with Auxin.

Major effects3

Major Effects

  • Fruit ripening.

  • Accelerates Senescence.

  • Stem/Root Elongation (+ or -).

Where produced3

Where Produced

  • Ripening fruits.

  • Senescent tissue.

  • Nodes.

New hormones

New Hormones

  • Oligosaccharins – short chains of sugars released from the cell wall.

  • Function:

    • Pathogen responses

    • Cell differentiation

    • Flowering

New hormones1

New Hormones

  • Brassinosteroids – steroid hormones similar to animal sex hormones.

  • Function:

    • Needed for normal growth and development.

Commercial applications of plant hormones

Commercial Applications of Plant Hormones

  • Weed killers

  • Seedless fruit

  • Rooting of cuttings

  • Tissue culture

Plant movements

Plant Movements

1. Tropisms

2. Circadian Rhythms



  • Growth responses in response to external stimuli.

  • + toward a stimulus

  • - away from stimulus



1. Phototropism

2. Gravitropism



  • Response to light (blue).





  • Response to gravity.

  • Stems are – gravitropic and roots are + gravitropic.

Gravitropism mechanism

Statolith movement may be the receptor for the stimulus.

Gravitropism - mechanism



  • Response to touch.

  • A series of 5 genes are involved.

  • Ex: Tendrils Climbing stems Wind direction response of stems.

Turgor movements

Turgor Movements

  • Movement caused by turgor pressure differences in certain cells.


1. Rapid Leaf Movement

Ex: Mimosa

2. Sleep Movements

Ex: Bean Leaves Prayer Plant


Sleep Movements



Chapter 39 control systems in plants

Mimosa Rapid Leaf Movement

Circadian rhythms

Circadian Rhythms

  • A physiological cycle about 24 hours long.

  • Ex: Stomata opening Sleep movements



  • Synthesis of a transcription factor protein that regulates is own manufacturing through feedback control.

  • Gene is believed to be common in most eukaryotic organisms.



  • A physiological response to changing day lengths.

  • Used to detect and direct growth responses to seasonal changes.



  • Match growth responses to proper season.

  • Ex: Leaf drop in fall Flowering

Flowering types

Flowering Types

1. Short - Day Plants

2. Long - Day Plants

3. Day - Neutral Plants

Short day plants

Short-Day Plants

  • Flower when days are shorter than a critical period (long nights).

  • Ex: Mums Poinsettias

Long day plants

Long-Day Plants

  • Flower when days are longer than a critical period (short nights).

  • Ex: Spinach Iris Lettuce

Day neutral plants

Day-Neutral Plants

  • Flower whenever they have enough energy.

  • Ex: Roses African Violets

Night length

Night Length

  • Actually controls flowering response, not day length.

  • Proof – experiments show that if you interrupt the dark period, you reset the “clock”.



  • Length of night not absolute, but relative for the response to be triggered.



  • What detects day/night length changes?

  • Phytochrome - plant pigment involved with photoperiodism.

Phytochrome forms

Pr- responds to 660nm (red light).

Pfr - responds to 730nm (far red).

Phytochrome Forms



  • Changes between the two forms.

  • Ratio or accumulation of enough Pfr triggers the responses

Chapter 39 control systems in plants

  • In Red light: Pr Pfr

  • Far-red light or darkness: Pfr Pr


Very sensitive (1 minute difference).

Sets clocks for plant responses.


Other effects

Other Effects

  • Seed Germination

  • Stomatal Opening

  • Leaf Drop

Lettuce germination

Lettuce Germination

Responses to stress

Responses to Stress

  • Stress – an environmental condition that can have an adverse effect on a plant’s growth, reproduction and survival.

Plant response

Plant Response

1. Developmental changes

2. Physiological changes

Water deficit

Water Deficit

  • During high Ts, guard cells may close.

  • Young leaves may slow expansion.

  • Leaves may roll to reduce surface area.

Oxygen deprivation

Oxygen Deprivation

  • Common in roots in water-logged soils.

  • Air tubes in roots may bring oxygen to the cells.

Salt stress

Salt Stress

  • Damages the plant through unfavorable soil water potentials and toxic ions.

  • Some plants can concentrate and excrete salt through salt glands (ex. halophytes).

Heat and cold stress

Heat and Cold Stress

  • Heat - use heat-shock proteins to protect other proteins from denaturing.

  • Cold – lipid shifts to keep lipid bilayers “liquid”.

  • Cold – solute changes to lower freezing point.



  • Plants have many physical and chemical defenses against herbivores.

  • Physical – thorns

  • Chemical – crystals, tannins and other toxic compounds.



  • Often trigger a plant to release chemicals to attract predators or to warn other plants to increase their production of toxins.



  • First Defense – epidermis

  • Second Defense – chemical events to restrict or kill the invader.

Chapter 39 control systems in plants


  • Systemic Acquired Resistance: chemicals that spread the “alarm” of an infection to other parts of the plant.

  • Possible Candidate: salicylic acid



  • Know the general plant hormones and their effects.

  • Know tropisms.

  • Know photoperiodism.

  • Know general ideas about how plants respond to stress.

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