1. Plant growth factors and control of tropisms by IAA
Plants need to respond to a variety of factors to survive:
• Light – stems grow towards the light (positively phototrophic)
• Gravity – roots grow towards the pull of gravity to firmly anchor the plant in the soil
(positively geotrophic)
• Water – almost all plants grow towards water (positively hydrotrophic)
Plants don't have a nervous system so respond to these stimuli by means of plant growth
factors. These are called plant growth factors because:
• They exert their influence by affecting growth
• They are made by cells located throughout the plant not in particular organs
• They may affect the tissue that releases them rather than acting on a distant organ
Plant growth factors
Plant growth factors are produced in small quantities and have their effects close to the
tissue that produces them. An example of a plant growth factor is IAA which has many
effects, one of them is to cause plant cells to elongate
Control of tropisms by IAA
A tropism is a growth movement of a plant in response to a directional stimulus. In a young
shoot this can be observed as it will bend towards the light that is directed at it from one
side. This is due to IAA and the following sequence of events:
1. Cells in the tip of the shoot produce IAA which is transported down the shoot
2. IAA is initially transported down all sides of the shoot
3. Light causes the movement of IAA from the light side to the shaded side of the
shoot
4. A greater concentration of IAA builds up in the shaded side of the shoot
5. IAA causes the elongation of cells and there is a greater concentration of it on the
shaded side so the cells on the shaded side elongate more
6. The shaded side of the shoot grows faster so the shoot bends towards the light