Plant responses

• factors that regulate plant growth

  light intensity

  day length

  gravity

  temperature
• a growth regulator is a chemical that controls the growth of plants
• growth regulators are made in the meristematic region of the plant
• growth regulators are transported in the vascular system
• growth promotors increase the rate of growth of plants (auxins)
• growth inhibitors slow down growth (ethylene)
• commercial use of plant regulators

  artificial auxins are used in rooting powders and to produce seedless fruits

  ethylene is used to unripe bananas
• plants protect themselves anatomically and chemically
• anatomical


  nettle leaves have sharp hairs that contain methanoic acid
• chemical

  oak produce tannins in their leaves to protect them against caterpillars
• a tropism is the response of a plant to a stimulus such as light or gravity
• phototropism is a response to light

  benefit: leaves are held up to more light for photosynthesis
• geotropism is a response to gravity

  benefit: roots grow down into soil for minerals and water
• thigmotropism is a response to touch

  benefit: stems and tendrils wrap around objects to keep leaves upright
• hydrotropism is a response to water

  benefit: roots grow towards water in the soil
• chemotropism is a response to chemicals

  benefit: roots grow towards minerals, pollen tube towards ovules
• tropisms ensure that a plant is positioned most favourably for its growth
• Auxin example


  IAA (indole-acetic-acid) is made in the meristem of shoots, roots and buds and made in the tips of coleoptiles (young shoots).

  it travels backwards down the stem by active transport in the phloem & promotes growth in cells

  auxin loosens the cellulose fibres in plant cell walls allowing the cells to elongate
• auxins effect growth apical dominance and tropisms

  growth: zone of elongation grows extending the shoot/root

  apical dominance: auxin produced in the tip of plant moves down and inhibits growth of the side branches

  tropisms: auxins involved with phototropism and geotropism
• phototropism


  IAA is produced in the meristem of the shoot

  if the shoot is exposed to light from one side, auxins will diffuse down the shaded side

  the higher conc of auxin in the shaded side cause the cells of the shaded side to elongate more than the cells of the unshaded side resulting in the stem to grows towards the light
• function of auxins


  stimulates stem elongation

  stimulates root growth

  assists with phototropism and geotropism
• functions (horticulture)

  ripens fruit

  weed killer

  seedless fruits

  tissue culture
• stimulus is something that causes a reaction in an organism
• response is the activity of an organism as a result of a stimulus
• positive tropism: plant growth response towards the stimulus (phototropism)

  negative tropism: plant growth response away from the stimulus (geotropism)
• an auxin is a plant growth regulator
• apical dominance is an inhibitory function
• how are auxins considered to be similar to the action of a hormone in the human body

  chemicals

  produced in one place acts in another
• what is the benefit of a plant that the shoot is positively phototropic

  more photosynthesis
• how does an unequal conc of auxin in the elongation zone affect the growth of a shoot

  higher rate of cell division
• how does an unequal conc of auxin in the elongation zone affect the growth of a root

  lower rate of cell division
• suggest what could case an unequal concentration of auxin in a shoot


  difference in light direction
• suggest what could case an unequal concentration of auxin in a root

  gravity