plant responses

• A tropism is a plant growth response to a stimulus, can be negative or positive. 

  • Phototropism 
  • Gravitropism 
  • Hydrotropism 
  • Thigmotropism 
  • Chemotropism 
  • Thermotropic
•  two stages in plant growth:

  1. Cell division - mitosis near the tip 
  2. Elongation - vacuoles of new cells expanding as water enters by osmosis 
• IAA is indoleacetic acid. It was the first specific growth factor to be discovered and it works in the elongating region and is made in the root and shoot tips. 
• With no difference in stimuli IAA is transported evenly throughout all regions. 
• Unilateral light exposure results in shoots bending to the right - positively phototrophic. 
• Shoot tip removal causes no response - tip must detect stimulus or produce the messenger.
• Opaque cap placed over intact tip creates no response - light stimulus must be detected by the tip. 
• Mica placed on the side of light causes the shoot to bend to the light - movement of chemical down shaded side. 
• Mica placed in the shaded side results in no response because the movement of chemical down the shaded side is prevented. 
• Gelatin block inserted under tip results in a bend to the light - movement of chemicals not electrical impulses down the shaded side. 
• A possible explanation is that extra IAA is found on the shaded side after being transported there. 
• Roots show a negative phototropism. 
• As IAA moves along the elongating region it attaches to protein receptors (target cells) on the membranes. 
• IAA seems to lower the pH by releasing hydrogen ions that break some of the bonds between microfibrils in the cellulose walls making them easier to stretch as water increases the turgor of the cells. 
• In gravitropism IAA collects on the lower side (bottom). IAA is actively transported to the lower side of a root, a higher concentration of IAA inhibits elongation.