Tropisms

Nota:

• phototropism-response to light

1. Positive phototropism

   Nota:

   • Most leaves are diaphototropic (orientate at right angles to light). Most roots are insensitive to light, but those of root-climbers such as ivy are negatively phototropic.
   1. Auxin Transport

      Nota:

      • The absorption of light by riboflavin causes auxin to be transported. It requires energy, since it is inhibited by low temperatures and lack of oxygen, It is much slower than sugar transported by the phloem, it only moves in a tip -to-base direction, since it cannot pass through a section of inverted coleoptile.
      1. Auxin Action

         Nota:

         • Auxin makes a cell wall more 'stretchable', making it easier for the osmotic uptake of water and thus the enlargement of the cell. Because the cell wall stretches more readily in the lengthwise direction, auxin causes plant cells to elongate.
      2. Concentration

         Nota:

         • The more brightly lit leaf exports more auxin, causing it to bend towards the light

Nota:

• Division-increases the number of cells and in plants occur in the apical meristem., Enlargement - occurs further away from the root of shoot tip; involves the active uptake of ions and the osmotic uptake of water with the formation of a large vacuole, Differentiation - specialisation of cells for different fuctions

Nota:

• Shoots is strongly negative gravitropic and the roots tend to be positive gravitropic.

1. Site of gravity detection

   Nota:

   • If a radicle is decapitated before being changed from a vertical to a horizontal position gravitropic bending doesn't occur
2. Detection

   Nota:

   • In plants, the function of statoliths is preformed by starch-containing organelles called amyloplasts. These sink onto cytoplasmic membranes, which in some way triggers the gravitropic response.