Chap. 13 : Plants Response to Stimuli

• Phototropism


  Movement of part of a plant in response to light

  
• Lesson Outcomes


  • Plant respond in taxis movement
  • Plant respond in nastic movement
  • Plant tropism in reaction to different stimuli
  • The plant photomorphogenesis
  • The plant photoperiodism and flowering
  • Plants response to environmental stresses

  
• Plants, being rooted to the ground, must respond to whatever environmental change comes their way
• Phototaxis


  Taxis movement that is caused by light
• Phototaxis


  • Movement of chlorophyll to surface/nearer surface of the leave to get light

  
• Chemotaxis


  Movement in response to chemical stimuli
• Nastic movement


  Movements of plant organs in response to external stimuli that is independent of the direction of the stimuli
• Nastic movement


  • Not regulated by hormones
  • Temporary and reversible
  • Involves flowers and leaves
• Photonasty


  Nastic movements towards or away from light stimuli
• Photonasty


  • Oxalis triangularis (also known as the love plant or purple shamrock) goes to "sleep" every night as the leaves are light sensitive and fold away as the light levels drop towards the end of a day
• Thermonasty


  Nastic movements in response to temperature stimuli
• Tropism


  Any growth response that results in curvatures of whole plant organs toward or away from a stimulus
• Tropisms


  • Often caused by hormones
  • Growth toward a stimulus is a positive tropism
  • Growth away from a stimulus is negative tropism
  • Tropisms are named for the stimulus that causes the response
• Phototropism experiment by Charles Darwin and Francis Darwin


  1. Determined that only the tip of the coleoptile senses light

  2. Boysen-Jensen experiment showed the signal for phototropism is a light-activated mobile chemical

  
• Geotropism


  Movement of part of plant in response to the earth's gravity
• Geotropism


  • Shoots & coleoptiles → -ve geotropic
  • Roots → +ve geotropic
  • Auxin plays a major role in gravitropic responses
• Statoliths


  Specialized plastids containing dense starch grains that plants may use to detect gravity
• Thigmotropism


  Movement of part of a plant in response to solid surface touch
• Thigmomorphogenesis


  Changes in form that result from mechanical perturbation
• Chemotropism


  Movement in response to chemical stimuli
• Hydrotropism


  Movement in response to water
• Aerotropism


  Movement in response to air (oxygen)
• Photomorphogenesis


  Effects of light on plant morphology

  
• Phytochrome


  Photoreceptor responsible for the opposing effects of red and far-red light
• Phytochrome structure and function

  1. Consists of two identical proteins joined to form one functional molecule

  2. One domain functions as the photoreceptor, covalently bonded to a chromophore

  3. The other domain has protein kinase activity to link light reception to cellular responses
• Biological clock and circadian rhythms

  Physiological processes that oscillate with a roughly 24-hour period
• Photoperiod


  The relative lengths of night and day
• Photoperiodism


  Physiological response to photoperiod
• Types of plants based on photoperiod response

  • Short-day plants
  • Long-day plants
  • Intermediate day plants
  • Day-neutral plants

  
• Critical night length


  The minimum length of uninterrupted darkness required for flowering in some plants
• Experiments on photoperiodism and critical night length


  1. Showed that flowering is controlled by night length, not day length

  2. Phytochrome is the pigment that measures interruption of dark periods
• Response to environmental stress


  Physiological changes plants undergo in response to environmental stresses like drought and flooding
• Response to drought


  • Reducing transpiration
  • Deeper root growth
• Response to flooding


  • Producing ethylene to stimulate formation of air tubes ("snorkels") to provide oxygen to submerged roots