Chemical control systems in plants

why do plants respond to stimuli?

respond to changes in their environment; this maximises their survival chances e.g.

• Growing towards light maximises the rate of photosynthesis and therefore glucose production

• Producing harmful or foul-tasting chemicals in response to being eaten by a herbivore reduces the likelihood of being eaten

• Producing flowers at the right time of year increases the chances of reproducing successfully

what stimuli can plants respond to

• Light

• Gravity

• Physical objects

• Herbivory

• Water

• Physical touch

how do plants respond to stimuli?

• Unlike many animals, plants do not possess a nervous system; the responses of plants rely on chemical substances that are released or altered in response to a stimulus

what are plant hormones

• The chemicals which cotnrol growth of plants. tehse are growth factors.

• Growth factors are produced in the growing parts of a plant before moving from the growing regions to other tissues where they regulate cell growth in response to a directional stimulus

what is tropism

The directional growth responses to specific environmental cues

what is positive tropism

• Positive tropisms involve growth towards a stimulus

  • E.g. positive phototropism is a growth response towards light

what is negative tropism

• Negative tropisms involve growth away from a stimulus

  • E.g. negative geotropism is a growth response away from gravity i.e. upwards

exampels of plant hormones

• Gibberellins 

  • Stem elongation

  • Flowering

  • Seed germination

• Cytokinins 

  • Cell growth and division

• Abscisic acid (ABA) 

  • Leaf loss

  • Seed dormancy

• Ethene 

  • Fruit ripening

  • Flowering

what is growth

Growth is a permanent increase in the size of an organism or of some part of it.

• It is brought about by cell division and the assimilation of new material within the cells that result from the division, followed by cell expansion

where does growth typically occur

• The maina reas of cell division in plants are knwon as the meristems and they occur just behind the tip of root or shoot. They are particualrly sensitive to plant growth susbtances

• Some plant growth susbtances make it easier for cellulose walls to be stretched and this makes it easier for cells to expand and grow. Some increase the number of divisions that occur

what are auxins

Auxins are a group of plant growth factors that influence many aspects of plant growth for example IAA

where is auxin produced?

• auzins are produced in young shoots and always move down the plant from the shoots to the roots

• This mvoement involves some active transport and calcium ions

Job of auxin

• Apical dominance; the suppression of the growth of side shoots by auxins in the growing shoot tip

• Promoting the growth of roots at low concentrations and inhibiting the growth of roots at high concentrations.

• Phototropism in shoots. The more auxin that is transported down the stem, the more roots grow. If the tips of stems are removed, removing the source of auxins, the stimualiton of root growth removed and root growth stops.

Job of IAA

• brings about phototropism by altering the transcription of genes inside plant cells

where is IAA founs

• IAA is made in the tip of the shoot and diffuses back towards zone fo elongation

• IAA can be transported from cell to cell by diffusion and active transport 

• Transport of IAA over longer distances occurs in the phloem

what brings about uneven plant growth

• The redistribution of IAA is affected by environmental stimuli such as light and gravity, leading to an uneven distribution of IAA in different parts of the plant

  • This brings about uneven plant growth

how IAA works

• The molecules of IAA bind to specific receptor sites on the cell surface membrane, activating the active