Sensory System of Plants

• Stimulus

  • defined as any external/internal changes in the environment that causes a response
  • in plants, a stimulus causes a sensitive cells produce a particular hormone
• What are the plant responses?

  • plant responses to light
  • plant responses to gravity
  • plant responses to mechanical stimuli
  • plant responses to water and temperature
• plant responses to light

  • plants produce food through photosynthesis process where light is indispensable
  • light also influences many physiological processes in plants such as seed germination, flowering, and other critical developmental processes
• What is phytochrome?

  A pigment-containing protein, present in plant as well as in green algae
• phytochrome have two interconvertible forms:

  • pr (pythochrome red)- which absorbs red light rays at 660 nm
  • pfr (phytochrome far red)- which absorbs far red-light rays at 730 nm
• examples of plant responses to light are:

  • seed germination
  • shoot elongation
  • plant spacing
  • phototropism
• seed germination

  • inhibited by far red light and stimulated by red light in many plants
  • this happens in the presence of light when pfr is connected to its inactive pr
• shoot elongation

  • results in etiolation is explains by the absence of red light
  • these plants only become normal once they're exposed to red light, which increases the amount of pfr
  • etiolated plants show long, weak stems, and smaller leaves due to longer internodes
• plant spacing

  • both red light and far red light are involved
  • happens when shading in plants results in an increase far red light compared to red light
  • when plants are closer to one another there is more far red light which causes plants to grow taller, allowing them to get more sunlight than the rest
• phototropism

  • an example of a directional response due to a directional stimuli like light
  • this includes the bonding of young stems and other plant parts towards the light source, particularly the blue wavelength
  • growing shoot that responds positively to sunlight and that the roots of plants do not respond positively to light
• pr is inactive and its converted to pfr, the active form, when red photons of light are available

  • means that biological reactions that are affected by phytochrome happens when pfr is present
  • some of these responses are: seed germination, shoot elongation, and detection of spacing in plants
• plant responses to gravity

  shoots always tends to bend upward and the roots downward as the plant struggle to recover
• there are two responses to gravity:

  • gravitropism
  • amyloplasts
• gravitropism

  • response of plants to gravitational force
  • a plant's shoot show negative gravitropic response, its roots show positive gravitropic response.
  • stems bend away from the center of gravity while roots move toward the center of gravity
• amyloplast (plastids that contain starch)

  • sink towards the center of gravity and maybe involved in sensing gravity in plants
• plant responses to mechanical stimuli

  example of growth responses to contact or touch:

  • tigmotropic responses
  • tigmonastic responses
• thigmotropic responses

  • thigmotropism- directional growth response of a plant or a plant part in response to a contact with an object like fence, wire, plant, or animal.
• Thigmonastic responses

  • Thigmonasty- show and the response of makahiya(Mimosa pudica) to touch. when touch there are changes happens in the turgor pressure at the base of each pulvinus cell. this change turgor pressure causes the leaflets to close or fold
• the plant responses to water and temperature are:

  • seed dormancy
  • abscission
• seed dormancy

  • adaptive measure in plants when environmental conditions are not favorable
  • seeds in dormant can stay for hundreds of years
  • water break the dormancy of seeds
• abscission

  • the process by which leaves or shedding
  • shedding of leaves help plants conserve water and energy
  • when temperature drops, plants needs to shed their leaves to withstand the cold temperature
• abscission layer

  • formed between the stem and base of the petiole of the leaf
  • hormones such as the abscisic acid, is responsible for the formation of the abscission layer