Auxins: Plant growth Hormones.

Authors Avatar

Biology Revision Notes

Auxins: Plant growth Hormones

Auxins are hormones and are “chemical messengers”

The have many roles in plants including:

  1. Growth of cells by cell elongation
  2. Prevention of side shoot development

Growth of cells

The auxin in the cells makes the cells stretchy and this is called cell elongation without it the cell would just continue to multiply

Auxins allow the cellulose cell walls of young plant cells to become stretchy

Because the cells contain sugars and salts, they will take up water (by osmosis) and expand.

Light influences the movement of Auxin within a plant i.e. the Auxin MOVES AWAY FROM THE LIGHT. This causes the pants to grow towards the light

The reason that it does this is because the auxin elongates the cells on the side that it is on (the side opposite the light) and elongates the cells making the plant longer on one side so naturally the plants bends towards the light

Summary

  • Auxin causes cell elongation and hence growth
  • Auxin is produced in the tip of a shoot
  • Auxin is a water soluble chemical which can be absorbed into agar

Prevention of side shoot development

The apical bud is the source of Auxin .If the bud is present it will prevent side shoot development =Apical dominance

This means that it is a tall plant to out compete all other plants for light.

If the bud is removed it will allow side branches to grow into a bushy plant

Summary

Plant hormones co-ordinate + control growth in a plant

 

Commercial applications of hormones

  1. Produce large numbers of plants quickly by simulating growth of roots from plant cuttings.
  2. Regulate the ripening of fruit on the plant and during transport to consumers e.g. bananas
  3. Kills weeds by disrupting normal growth patterns(weedkillers)

Stomata

Stomata- open to allow CO2 into leaf for p/s

-Close to reduce water loss by transpiration

On a very hot day or if in dry soil, a plant may close stomata during hottest time to avoid wilting.

Wilt = more water lost than taken in which means loss of turgidity which means the loss of support which means the collapse of the plant.

Growth movements by plants

Plant shoots respond to gravity and Light

A plant growth response is called tropism

When shoots grow towards light it is called positive phototropism

When shoots grow in the opposite direction to gravity it is called negative geotropism

When roots grow in the same direction as gravity it is called positive geotropism

Obvious advantages to these responses
  1. Plant sheets grow vertically up through soil following germination (negative geotropism
  2. Plant shoots grow towards the brightest source of light for maximum/s(positive phototropism)
  3. Roots live in dark environment will grow downwards tap water stores in the soil and to anchor the plant more firmly in the soil.(positive geotropism)
Join now!

Transpiration

The process by which weather loss from a lead draws water and dissolved minerals up the xylem inside of the plant from the roots. This creates the transpiration stream

  • Plant loses about 90% of all water taken up by roots to transpiration
  • Provides a transport mechanism for minerals within the plant
  • Evaporation at leaf helps to cool leaf down. P/s is not very efficient at high temperature
  • Water evaporates on the surface of the spongy mesophyll cells in a leaf
  • Water vapour passes out of the leaf via stomata high concentration of water vapour in leaf, lower ...

This is a preview of the whole essay