plant responses

Just as we respond to our environment, plants do too. In this topic we will study the way in which plants respond to stimuli in their surroundings in order to increase their chances of survival.
Plants respond to 3 main stimuli:
  • water
  • gravity
  • light

The responses of plant roots and shoots to light, gravity and water are known as tropisms.
When a plant grows towards a stimulus, the tropism is positive
When a plant grows away from a stimulus, the tropism is negative
STIMULUS


Light




Gravity





Water
NAME OF TROPISM

Phototropism




Gravitropism/
geotropism




Hydrotropism
RESPONSE OF PLANT


Shoots grow towards the light (positive phototropism)
Roots grow away from light (negative phototropism)

Shoots grow away from gravity (negative geotropism)
Roots grow towards gravity (positive geotropism)


Roots grow towards water
HOW TROPISM CONTRIBUTES TO SURVIVAL

Light is necessary for photosynthesis



Shoots are growing towards the light - see above
Roots grow down to anchor the plant in the soil, and also this is where water is likely to be

Water is essential for survival of the plant
HYDROTROPISM IS DOMINANT OVER GEOTROPISM. Plants will grow towards water even if it means growing upwards

Auxins

Auxins are plant hormones and they are responsible for causing the responses of plants to light and gravity (it is still unclear how plants respond to water!). The response of the plant is the result of an uneven distribution of auxin in the growing shoot or root. This causes an unequal growth rate, and as a result the shoot or root bends.
Picture
Phototropism








  • In the shoot, auxin is produced at the tip
  • It diffused down the shoot, away from the tip
  • If light is shone on the tip from one side, the auxin moves to the shaded side, away from the light
  • In the shoot, the cells grow more where there is more auxin
  • This means that the cells on the shaded side of the shoot grow more, and so the shoot bends over towards the light (positive phototropism)
IF YOU ARE UNSURE ABOUT THE ACTION OF AUXIN IN SHOOTS AND ROOTS, JUST SAY THAT IT CAUSES UNEQUAL GROWTH OF CELLS ON ONE SIDE WHICH CAUSES THE SHOOT OR ROOT TO BEND
 Picture
Gravitropism












When a plant is laid on its side, the auxin that was evenly distributed settles on the lower side of the shoot and root due to gravity
In the shoot, the cells grow more where there is more auxin 
This means that the cells on the lower side of the shoot will grow more, and the shoot will bend upwards, growing away from gravity (negative gravitropism)
In the root, the cells grow more where there is less auxin
This means that the cells on the upper side of the root will grow more, and the root will bend downwards, growing towards gravity (positive gravitropism)

This is why roots and shoots respond differently to gravity

Uses of plant hormones

Auxin isn't the only plant hormone. There are a number of others, including gibberellins, ethene and abscisic acid. Plant hormones are incredibly useful, both in the garden (horticulture) and on commercial farms which provide food (agriculture).
We can use plant hormones to help plants grow and also to get rid of plants that we don't want.
Picture
Rooting powders
These contain auxin. Cuttings are taken from plants, and the cut end is dipped in the power. This encourages roots to form, making it more likely that the plant will grow successfully.
Very important for commercial horticulturists
Weed killers
These contain synthetic auxins which encourage weeds to grow rapidly, and therefore uncontrollably. This kills them. Since most weeds are broad-leaved plants, they absorb a lot more of the weed killer than the desirable narrow-leaved cereal or grass plants. As a result, the weeds are selectively killed, leaving the crops unaffected. Result for the farmers!

Be sure you know WHY farmers want to kill off weeds.
 Picture
Ethene
This is a plant hormone produced by ripe fruit. It is commonly given off by bananas. It can be used to ripen fruit. This is a great advantage to us in the UK who want tropical fruit year-round. It means that the unripe fruit can be picked where it is grown, and shipped over without going off during the trip. Once here ethene can be used to ripen it
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Agent Orange
Agent Orange is a chemical containing synthetic plant hormones that was used during the Vietnam War to strip the forests of leaves, effectively removing all hiding places. It caused terrible damage to the forests, and hundreds of thousands of people were badly affected by the chemicals. It continues to cause birth defects, even today