chapter 16

• tropism


  directional growth responses of plants
• phototropism


  growth response to light, shoots exhibit positive phototropism as they grow towards light and roots exhibit negative phototropism and grow away from light
• geotropism


  growth response to gravity, roots grow with gravity so have positive geotropism, and shoots exhibit negative geotropism and oppose the force of gravity
• chemotropism


  growth response to chemicals
• plant growth can be triggered by plant growth regulators, e.g auxins, gibberellins, abscisic acid and ethene
• auxins


  cell elongation, prevent lead fall, maintain apical dominance, produce tropic responses and stimulate the use of ethene in fruit ripening
• how auxins work


  • cause cell elongation via transport of hydrogen ions into the cell walls which lowers the pH of the walls, required for expansins
  • expansins are an enzyme that loosen cellulose so it makes cell wall stretch to accomodate more water
• when shoot is illuminated from all sides, auxins are distributed evenly and move down the shoot tip thus causing elongation of cells across the zone of elongation
• if shoot is only illuminated from one side, auxins move towards the shaded part of the shoot causing elongation of the shaded side only, which results in bending of the shoot towards the light
• leaf abscission


  dropping of leaves controlled by auxin, ethene and cytokinins
• cytokinins


  produced by young leaves to make it a suitable sink for transport in the phloem
• as a leaf ages, levels of auxin and cytokinins decrease and ethene level increases, this causes cellulose enzyme production which breakdown the abscission layer causing weakening of leaves, they break down from the branch and a layer of suberin forms to prevent pathogen entry