*Chapter 26: Plant Physiology

Plant Growth Requirement

• Plants are autotrophic: able to survive in an inorganic environment

• 17 essential nutrients are required for plant to complete its life cycle

  • From atmosphere: carbon dioxide

  • From soil: water, fertilizer

Nutrient Deficiency Symptons on Plants

• Nitrogen deficiency: Uniform yellowing on older leaves

• Iron deficiency: interveinal yellowing on younger leaves

• Potassium deficiency: Purplish to reddish-purple leaves

Plant Response to External Signals

• Tropism: Directional growth in a plant

  • Phototropism

  • Gravitropism

  • Hydrotropism

  • Thigmotropism

• Nastic Movement: Non-directional response to stimuli

  • Nyctinasty

  • Seismonasty

Phototropism

• Plants bend towards the light to maximize the light intercept for photosynthesis

Gravitropism

• Plants are orientated to the directionality of gravity

  • Positive in the root

  • Negative in the shoot

Hydrotropism

• Movement of a plant towards or away from water

  • Toward water (drought)

  • Away from water (flooding)

Thigmotropism

• Curling of climbing plants that responds to the stimuli of contact

Nyctinasty

• Associated with diurnal light and changes in temperature

Seismonasty

• Occur in response to touch, wounding, and intense heat

• Essential characteristics

  • Rapid

  • “All or none” response— not proportional to stimulus

  • Excitation is propagated from the point of stimulation

Plant Growth Controlled by Internal Signals

• Plant growth and development are regulated by plant hormones

• Signal molecules produced at specific locations

• Alter processes in target cells at local or other locations

• Act in very low concentrations

Auxin

• 1st discovered

• Produced in the shoot apical meristem

• Associated with apical dominance

Gibberellin

• Enhance stem elongation

• Reducing its level would suppress vertical shoot growth

Cytokinin

• Produced in roots

• Stimulate cell division

• Delay leaf yellowing under environmental stress

Abscisic Acid (ABA)

• Produced in mature leaves

• Close the leaf stomata under water stress to preserve water

Ethylene

• Gas form

• Emitted from yellowing leaves and ripening fruits

• Stimulate leaf and fruit absorption

• Ethylene remover