PHOTOTROPISM -MECHANISM

=Phototropism -Mechanism= [image:http://i.imgur.com/1roWyMz.png?1] '''Auxin''' (IAA - Indole-3-acetic acid) is a water soluble hormone produced in the tips of coleoptiles (emerging plant shoots). In the presence of light, auxin is redistributed to the dark / shaded side of the tip. Auxin then diffuses down the dark / shaded side of the shoot. [image:http://i.imgur.com/VJq5Rbj.png?1] Auxin stimulates the shoot cells to elongate (lengthen) through the uptake of water. Because there is a greater concentration of auxin on the dark / shaded size, cells elongation is more pronounced on this side too. This causes the shoot to curve towards the light. ''You can model this by placing your hand together as though you were praying. Pretend one hand represents the illuminated side and the other represents the shaded side of the plant shoot. Slowly move the shaded hand upwards while keeping your finger tips together. You should notice that your finger start to curve towards the illuminated side.'' ==Phototropism in Roots== The uneven distribution of auxin is responsible for the negative phototropism of roots. The underlying mechanism is exactly the same with one noticeable difference; '''auxin inhibits cell elongation and growth in roots'''. Thus the cells on the lighter side of the root grow faster causing the roots to curve away from the light. [image:http://i.imgur.com/08fQzVz.jpg?2v] This graph shows the affect of Auxin on both plant stems and roots. Notice that a small amount of auxin actually promotes growth in both but that a higher concentration only promotes elongation and growth in stems / inhibits elongation and growth in roots. However, even stems have a certain tolerance for auxin and an excessive amount will eventually start to inhibit stem elongation / growth too.