Auxins and gibberellins

Auxins (from Greek auxein, to grow) were first isolated from human urine. Natural auxins are IAA and IBA; synthetic ones include NAA and 2,4-D. They are made at the growing tips and move to where they act.

Uses of auxins:

• Help stem cuttings form roots, so plants can be multiplied.
• Make pineapples flower.
• Prevent early dropping of fruits and leaves (but they also help old leaves and fruits fall off when the time comes).
• Make seedless fruits without fertilisation (parthenocarpy) in tomato.
• 2,4-D kills dicot weeds but not mature monocots like grass, so it is used for weed-free lawns.
• Control the formation of xylem and help cell division.

Apical dominance: the bud at the tip of the shoot (apical bud) stops the side (lateral) buds below it from growing. Cut off the tip (decapitation) and the side buds start growing, making the plant bushy. This is used in tea plantations and for trimming hedges.

Gibberellins: more than 100 kinds are known (GA₁, GA₂, GA₃...), all acidic; GA₃ is the best studied. They make things longer:

• Lengthen the stalks of grapes, and make apples longer and better shaped.
• Delay ageing (senescence), so fruits can stay on the tree longer and keep in the market.
• Speed up malting in the brewing industry.
• Lengthen the stem of sugarcane, raising the yield by up to 20 tonnes per acre.
• Make young conifers mature faster, so seeds are produced early.
• Cause bolting: in beet, cabbage and other plants with leaves in a low rosette, the internodes suddenly lengthen just before flowering.