Auxins

Auxins

Growth stimulants produced in plants

Where are auxins made?

Cells at tip of roots & shoots, also in meristems

How do auxins affect plasticity of cell wall?

1. Auxin molecules bind to specific receptor sites in plant cell membrane, causing pH to drop to 5 (approx.)

   1. Optimum pH for enzymes needed to keep walls flexible & plastic

2. As cells mature, auxin destroyed

3. As hormone level falls, pH rises, & enzymes maintaining plasticity become inactive

4. Wall becomes rigid, more fixed in shape, cell cannot expand & grow

What results in apical dominance?

High concentrations of auxins suppressing growth of lateral shoots

Auxin, and main shoot

Growth in main shoot stimulated by auxin produced at tip, so it grows quickly

What are lateral shoots inhibited by

Auxin that moved back down the stem, causing lateral shoots to not grow v. well

What occurs further down stem?

Auxin conc. lower than in main shoot

• Lateral shoots grow quicker

What happens to lateral shoots if apical shoot removed?

Grow faster

• Apical shoot removed → No auxin producing cells → No auxin

• Lateral shoots freed from apical shoot dominance

Auxin and root growth?

Low conc. of auxin promote root growth

• The more auxin ( up to a given conc.) that reaches roots, more they grow

NOTE: High auxin conc. INHIBIT root growth

Where is auxin produced in roots?

Root tips

• Reaches roots in low conc. from growing shoots

What happens to root growth if apical shoot removed?

Slows down & stops