Responding to the Environment-
Plants
Auxins
Most important group of plant hormones
Indoleacetc acid (IAA) most common naturally occurring auxin
Constantly made by cells in the tp of a shoot (apical bud) small amounts also produced in root tp
Defuse downwards towards root
Concentraton highest at the tp and lowest at the root
Functions of Auxins
1.. Auxins promote growth
Promote both mitosis and cell elongaton
Further stem tssues are from shoot tp more sensitie they become to auxin concentraton
If concentraton of auxins are to high cell elongaton will be stopped
Root elongaton requires less auxins than stems – higher concentratons will inhibit cell elongaton in
roots
2.. Auxins cause apical dominance
High concentraton of auxins near the apical bud inhibits the growth of the higher lateral buds
Lower down the plant the auxin concentraton becomes less so lower lateral buds can produce shoots
which grow into lateral branches
This giies plants a pyramidal shape
If apical bud is remoied, lateral buds will grow as they are no longer inhibited by auxins and a bushier
plant will deielop
3.. Auxins play a part in trophic moiements
Brought about by auxins causing cell elongaton on only one side of the stem and root
Results in uneien growth
Tropisms
Growth moiements of part of a plant in response to an eniironmental stmulus
Moiement is dependent on the directon of the stmulus and it results from diferental growth
Phototropism
Growth response to a light stmulus
Why do shoots respond to light ?
Auxins = light sensitie hormones
Moie away from the light source
Light comes from one directon, auxins from the growing tp moies away from the light source
Increased concentraton makes these cells elongate more than those on the bright side, so the shoot
bends towards the light
Shoots/Stems are therefore positiely phototropic
Roots are negatiely phototropic
Plants
Auxins
Most important group of plant hormones
Indoleacetc acid (IAA) most common naturally occurring auxin
Constantly made by cells in the tp of a shoot (apical bud) small amounts also produced in root tp
Defuse downwards towards root
Concentraton highest at the tp and lowest at the root
Functions of Auxins
1.. Auxins promote growth
Promote both mitosis and cell elongaton
Further stem tssues are from shoot tp more sensitie they become to auxin concentraton
If concentraton of auxins are to high cell elongaton will be stopped
Root elongaton requires less auxins than stems – higher concentratons will inhibit cell elongaton in
roots
2.. Auxins cause apical dominance
High concentraton of auxins near the apical bud inhibits the growth of the higher lateral buds
Lower down the plant the auxin concentraton becomes less so lower lateral buds can produce shoots
which grow into lateral branches
This giies plants a pyramidal shape
If apical bud is remoied, lateral buds will grow as they are no longer inhibited by auxins and a bushier
plant will deielop
3.. Auxins play a part in trophic moiements
Brought about by auxins causing cell elongaton on only one side of the stem and root
Results in uneien growth
Tropisms
Growth moiements of part of a plant in response to an eniironmental stmulus
Moiement is dependent on the directon of the stmulus and it results from diferental growth
Phototropism
Growth response to a light stmulus
Why do shoots respond to light ?
Auxins = light sensitie hormones
Moie away from the light source
Light comes from one directon, auxins from the growing tp moies away from the light source
Increased concentraton makes these cells elongate more than those on the bright side, so the shoot
bends towards the light
Shoots/Stems are therefore positiely phototropic
Roots are negatiely phototropic
