plant hormones & growth hormones (b,c,d,e)

Question 1

What is a tropism?

Answer 1

A growth response by a plant in response to a unidirectional stimulus

Question 2

Known roles of auxins

Answer 2

• control cell elongation
• prevent abscission
• maintain apical dominance
• involved in tropisms
• stimulate the release of ethene
• involved in fruit ripening

Question 3

Known roles of gibberellin

Answer 3

• cause stem elongation
• trigger the mobilisation of food stores in a seed at germination
• stimulate pollen tube growth in fertilisation

Question 4

Known roles of ethene

Answer 4

• causes fruit ripening

- promotes abscission in deciduous trees

Question 5

Known roles of ABA (abscisic acid)

Answer 5

• maintains dormancy of seeds and buds
• stimulates cold protective responses e.g. antifreeze production
• stimulates stomatal closing

Question 6

The role of hormones in seed germination

Answer 6

• seed absorbs water, activating the embryo to begin producing gibberellins
• they stimulate the production of enzymes that break down the food stores found in the seed (cotyledons in dicots and endosperm in monocots)
• embryo plant uses food stores to produce ATP for building materials so it can grow and break out through the seed coat
• gibberellins switch on genes that code for amylases and proteases (digestive enzymes required for germination)
• ABA acts as an antagonist to gibberellins

Question 7

Experimental evidence for the role of gibberellin in the control of seed germination

Answer 7

• mutant varieties of seeds have been bred, which lack the gene that enables them to make gibberellins, and do not germinate. if the gibberellins are applied externally then the seed with germinate normally
• if gibberellin biosynthesis inhibitors are applied to seeds, they do not germinate as they cannot make the gibberellins needed for them to break dormancy - if inhibition is removed or gibberellins applied then seeds germinate

Question 8

Describe auxins

Answer 8

• growth stimulants produced in plants
• made in cells at the tip of the roots and shoots and in meristems
• can move down the stem and up the root in transport tissue and from cell to cell
• e.g. indoleacetic acid (IAA)

Question 9

Evidence for the role of auxins in cell-wall plasticity

Answer 9

• they stimulate the growth of the mina apical shoot
• auxins affect the plasticity of the cell-wall (it stretches more easily)
• bind to specific receptor sites in the plant cell membrane - causes pH fall to 5 = optimum pH for enzymes to keep the walls very flexible + plastic
• as the cells mature, auxin is destroyed
• as hormone levels fall, pH rises, so enzymes maintaining plasticity become inactive
• thus the wall becomes rigid and more fixed in shape and size and the cells can no longer expand and grow

Question 10

Effect of high auxin concentrations on apical dominance

Answer 10

• ## high concentrations of auxins suppress the growth of lateral shoots resulting in apical dominance