5.1.6 Plant responses- Summary Notes

Plant responses

• Tropisms are directional growth responses of plants

• Phototropism is the growth response to light, shoots exhibit positive phototropism as they grow towards light whereas roots exhibit negative phototropism and grow away from light

• Geotropism is a growth response to gravity, roots grow with gravity thus they exhibit positive geotropism whereas shoots exhibit negative geotropism and oppose the force of gravity

• Chemotropism is a growth response to chemicals, thigmotropism is touch, hydro is water

Plant growth responses can also be triggered by plant growth hormones.

Examples include auxins which promote cell elongation

Gibberellins which promote seed germination and stem growth

Abscisic acid which inhibits seeds germination and causes closing of stomata and ethene which is a gas that promotes ripening of fruit.

Auxins

Auxin is a plant hormone which causes cell elongation

Auxins cause cell elongation via the transport of hydrogen ions into the cell walls, which as a result lowers the pH of the walls making them stretch to accommodate more water.

When light is illuminated from all sides, auxin is evenly distributed and moves down the shoot tip causing elongation of cells across the zone of elongation. When a shoot is only illuminated from one side auxin moves towards the shaded side causing elongated as the shoot bends towards the light

Leaf abscission

Leaf abscission means leaf loss and is controlled by auxin, ethene and cytokinins. Both auxin and cytokinins are produced by young leaves and inhibit leaf loss but once leaves get older these concentrations reduce as ethene rises as its produced by older leaves, enzymes are produced to break down the cell wall in the abscission layer and weaker the leaves suberin is produced to prevent enter of pathogen from the leaf

Apical dominance

Where Side shoots are inhibited by the apical bud due to the concentration of auxin, once the apical bud is removed the side shoots begin to grow as there is more auxin concertation at the shoot tip than lower in the plant

Commercial uses of plant hormones

• Auxins – used in rooting powder, for growth of seedless fruit, in herbicides, to prevent leaf and fruit growth in small concentrations and in high concentrations to promote fruit drop. Auxins are also used as commercial weed killers making weeds produce long stems so the weeds grow to fast and cannot get enough water or nutrients to so they die

• Gibberellins- used to delay senescence in citrus fruits, elongation of apples in combination with cytokinins, for elongation of grape stalks, in brewing of beer for production of malt, to increase yield of sugar cane, to speed up seed formation in young conifer trees, to prevent lodging, seed germination, Lateral bud growth by inhibiting apical dominance

• Cytokinins – to prevent yellowing of lettuce leaves, to promote shoot growth

• Ethene – speeds up ripening, to promote lateral growth, to promote fruit drop