14.4 Plant Responses to Environmental Stimuli

Plants are nature's masters of adaptation. They respond to environmental cues like light, gravity, and touch through growth patterns called tropisms. These responses help plants optimize their positioning for survival and reproduction.

Beyond daily adaptations, plants also sync with seasonal changes. They use light-sensing pigments and internal clocks to time crucial processes like flowering and dormancy. Plant hormones orchestrate these responses, fine-tuning growth and development.

Plant Growth Responses

Tropisms: Plant Growth Responses to Directional Stimuli

Top images from around the web for Tropisms: Plant Growth Responses to Directional Stimuli

• 

  Frontiers | Genome-Wide Association Study Reveals Natural Variations Contributing to Drought ... View original

  Is this image relevant?

• 

  Frontiers | TCP Transcription Factors at the Interface between Environmental Challenges and the ... View original

  Is this image relevant?

• 

  What chemicals and structures control the *direction* of plant growth in leaves, stems and roots ... View original

  Is this image relevant?

• 

  Frontiers | Genome-Wide Association Study Reveals Natural Variations Contributing to Drought ... View original

  Is this image relevant?

• 

  Frontiers | TCP Transcription Factors at the Interface between Environmental Challenges and the ... View original

  Is this image relevant?

1 of 3

Top images from around the web for Tropisms: Plant Growth Responses to Directional Stimuli

• 

  Frontiers | Genome-Wide Association Study Reveals Natural Variations Contributing to Drought ... View original

  Is this image relevant?

• 

  Frontiers | TCP Transcription Factors at the Interface between Environmental Challenges and the ... View original

  Is this image relevant?

• 

  What chemicals and structures control the *direction* of plant growth in leaves, stems and roots ... View original

  Is this image relevant?

• 

  Frontiers | Genome-Wide Association Study Reveals Natural Variations Contributing to Drought ... View original

  Is this image relevant?

• 

  Frontiers | TCP Transcription Factors at the Interface between Environmental Challenges and the ... View original

  Is this image relevant?

1 of 3

• Tropisms are growth responses that occur in response to directional environmental stimuli
• Cause the plant to grow toward or away from the stimulus depending on the specific type of tropism
• Positive tropisms involve growth toward the stimulus while negative tropisms involve growth away from it
• Three main types of tropisms in plants are phototropism, gravitropism, and thigmotropism

Phototropism: Growth Response to Light

• Phototropism is a plant's growth response to light
• Shoots exhibit positive phototropism and grow toward light to maximize photosynthesis (sunflowers tracking the sun across the sky)
• Roots often display negative phototropism and grow away from light
• Auxins, a type of plant hormone, accumulate on the shaded side of a shoot causing the cells there to elongate more resulting in the shoot bending towards the light

Gravitropism and Thigmotropism: Responses to Gravity and Touch

• Gravitropism is a plant's growth response to gravity
• Roots show positive gravitropism growing downward while shoots demonstrate negative gravitropism growing upward against gravity (corn stalks growing straight up)
• Thigmotropism is a growth response to touch or contact
• Occurs when plants grow around a surface after coming in contact with it
• Commonly seen in vines and climbing plants that use tendrils to wrap around supports (pea plants winding around trellises)

Plant Rhythms and Cycles

Photoperiodism: Plant Responses to Day Length

• Photoperiodism refers to a plant's physiological responses to day length or the relative lengths of light and dark periods
• Plants use photoperiodism to coordinate functions like flowering and dormancy with the seasons
• Three main categories of photoperiodism are short-day plants, long-day plants, and day-neutral plants
• Short-day plants flower when the night exceeds a critical length (poinsettias) while long-day plants flower only when the day length exceeds a critical duration (spinach)
• Day-neutral plants flower regardless of day length (tomatoes, corn)

Phytochromes: Light Receptors Controlling Plant Responses

• Phytochromes are pigments that function as light receptors in plants
• Absorb red and far-red light which controls germination, flowering, and other developmental processes
• Two interconvertible forms: Pr (inactive form that absorbs red light) and Pfr (active form made by exposure to red light that absorbs far-red light)
• Pr is converted to Pfr with red light while far-red light converts it back
• The relative amounts of Pr and Pfr control many plant responses like seed germination and flowering

Circadian Rhythms: Internal Biological Clocks

• Circadian rhythms are internal biological clocks with a period of about 24 hours
• Control processes like leaf movements, growth, germination, and flowering
• Synchronized to the natural cycles of day and night but can persist even under constant environmental conditions
• Help plants anticipate and prepare for regular changes in the environment (flowers opening in the morning, leaves folding up at night)
• Controlled by genes that regulate the production of specific proteins on a 24-hour cycle

Plant Hormones and Regulation

Types and Functions of Major Plant Hormones

• Plant hormones are chemical messengers that are produced in one part of the plant and cause responses in target cells and tissues
• Five major types of plant hormones are auxins, cytokinins, gibberellins, abscisic acid, and ethylene
• Each hormone has specific functions in regulating plant growth, development, and responses to the environment
• Auxins promote stem elongation, root growth, and fruit development
• Cytokinins stimulate cell division, delay aging, and promote bud growth
• Gibberellins stimulate stem elongation, seed germination, and fruit growth

Auxins, Cytokinins, and Gibberellins: Growth-Promoting Hormones

• Auxins, cytokinins, and gibberellins are plant hormones that promote various aspects of growth and development
• Auxins are produced in shoot and root tips and stimulate cell elongation (phototropism)
• Cytokinins are produced in roots and promote cell division, leaf expansion, and delay leaf aging
• Gibberellins stimulate stem elongation, break seed dormancy, and promote fruit growth (spraying gibberellin on grapes increases fruit size)
• These hormones work together and in opposition to regulate plant growth

Abscisic Acid and Ethylene: Stress Response and Ripening Hormones

• Abscisic acid (ABA) and ethylene are plant hormones involved in stress responses, dormancy, and fruit ripening
• ABA is produced in response to stresses like drought and stimulates stomatal closure to reduce water loss
• Also inhibits growth and promotes seed dormancy under unfavorable conditions (high ABA levels keep seeds dormant)
• Ethylene is a gaseous hormone associated with fruit ripening, leaf abscission, and plant defense
• Stimulates the ripening of climacteric fruits and promotes leaf and fruit abscission (ripening of bananas)
• Also involved in plant stress responses and defense against pathogens