(5) Plant Responses

Question 1

Types of plant responses

Answer 1

Important to avoid abiotic stress and herbivory, allows plants to maximise photosynthesis and ensure germination.

Chemical defences protect against herbivory - alkaloids (bitter tasting, nicotine in roots of tobacco plants which are toxic to insects), pheromones (affect the behaviour of other species, corn plants being eaten by caterpillars = chemicals that attract parasitic wasps), tannins - bitter.

Tropisms - directional growth response determined by external stimulus.
Phototropism - growth towards light (positive)
Geotropism - grows away from gravity (negative)
Thigmotropism - growth in response to touch, causing leaf drop or folding.

Question 2

Cytokinins

Answer 2

Promotes cell division in roots and shoots.

Question 3

Giberellins

Answer 3

Promotes seed germination - seed soaks up water and GA is synthesised in embryo. Also causes growth at the internodes by stimulated cell elongation and cell division.

Question 4

Abscisic acid

Answer 4

Causes the stomata to close when low water availability (released from roots and bind to plasma membrane of guard cell causing water potential to decrease).

Question 5

Auxin

Answer 5

Control of cell elongation - synthesised in meristem, diffuse away from tip and bind to receptor sites, forms vacuoles = low pH = cell wall is flexible and expands as they absorb water).

Plants grow towards the light because auxin is broken down due to the light, this causes cell elongation on one side. Gravity causes more auxin to be on lower side. This maintains apical dominance because there is a high concentration so it suppresses the growth of later shoots - saves energy.

Question 6

Commercial use of plant hormones

Answer 6

Rooting powder and micropropagation - auxin stimulates shoot growth, used in hormone rooting powders to stimulate root growth. Ideal for seedless plants.

Ethene - promotes leaf fall, during the winter months so less auxin is made, cells in abscission zone digest and weaken the leaf petiole. Promotes fruit ripening which preserves fruit and veg that has been cut.

Weed killers - synthetic auxins, promote growth in broad leaved weed, cannot keep up and dies. Narrowed leaved plants can still grow.

Giberellins - produce malt for beer making.

Cytokinins - prevent ageing