Plant Responses to Internal and External Signals

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The term thigmomorphogenesis refers to changes in form that result from mechanical disturbance. Rubbing stems of young plants a couple of times daily results in plants that are shorter than controls.

Thigmotropism is growth in response to touch. It occurs in vines and other climbing plants.

Rapid leaf movements in response to mechanical stimulation are examples of transmission of electrical impulses called action potentials.

Slide 63

Rapid turgor movements by the sensitive plant (Mimosa pudica)

(a) Unstimulated state

Leaflets

after

stimulation

Pulvinus

(motor

organ)

(c) Cross section of a leaflet pair in the stimulated state (LM)

(b) Stimulated state

Side of pulvinus with

flaccid cells

Side of pulvinus with

turgid cells

Vein

0.5 µm

Slide 64

Environmental Stresses

Environmental stresses have a potentially adverse effect on survival, growth, and reproduction.

Stresses can be abiotic (nonliving) or biotic (living).

Abiotic stresses include drought, flooding, salt stress, heat stress, and cold stress.

Slide 65

Drought and Flooding

During drought, plants reduce transpiration by closing stomata, slowing leaf growth, and reducing exposed surface area.

Growth of shallow roots is inhibited, while deeper roots continue to grow.

Enzymatic destruction of root cortex cells creates air tubes that help plants survive oxygen deprivation during flooding.

Slide 66

A developmental response of maize roots to flooding and oxygen deprivation

(a) Control root (aerated)

Vascular

cylinder

Air tubes

Epidermis

(b) Experimental root (nonaerated)

100 µm

100 µm

Slide 67

Salt Stress

Salt can lower the water potential of the soil solution and reduce water uptake.

Plants respond to salt stress by producing solutes tolerated at high concentrations.

This process keeps the water potential of cells more negative than that of the soil solution.

Slide 68

Heat Stress and Cold Stress

Excessive heat can denature a plant’s enzymes. Heat-shock proteins help protect other proteins from heat stress.

Cold temperatures decrease membrane fluidity.

Altering lipid composition of membranes is a response to cold stress.

Freezing causes ice to form in a plant’s cell walls and intercellular spaces.

Slide 69

Plants respond to attacks by herbivores and pathogens