16.1 Plant hormones and growth in plants

Question 1

role of auxins in plants

Answer 1

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

Question 2

role of gibberellin in plants

Answer 2

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

Question 3

role of ethene in plants

Answer 3

• causes fruit ripening

- promotes abscission in deciduous trees

Question 4

role of ABA (abscisic acid) in plants

Answer 4

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

Question 5

why are scientists unsure about the details of many plant responses

Answer 5

• plant hormones work at very low concentrations

- multiple interactions between different chemical control systems so hard to isolate the role of a single chemical

Question 6

what experimental evidence is there to support the role of gibberellins in the germination of seeds

Answer 6

• mutant seeds have been bred which don’t have the gene to make gibberellins (don’t germinate unless gibberellins are applied externally)
• if gibberellin biosynthesis inhibitors are applied to seeds, they don’t germinate as they can’t break dormancy until its is removed or gibberellins are applied

Question 7

what are auxins and where are they made

Answer 7

• are growth stimulants
• made in cells at the tip of the roots and shoots, and in the meristems
• can move in the transport tissue and from cell to cell
• present on the side away from light to cause cells on that side to elongate so the stem bends towards light

Question 8

why do plant hormones differ from animal hormones

Answer 8

• produced from specific organ in animals, but from specific region in plants
• in plants, they are produced by unspecialised cells
• in plants, the effect varies depending on the circumstances

Question 9

describe the phototropic mechanism

Answer 9

• coordinated by IAA (type of auxin)
• IAA is synthesised in the meristem and passes down the stem (down a conc gradient)
• switches genes on and off
• stimulates extension growth
• stimulates proteins in cell wall called expansins (make cell walls more flexible by loosening cellulose fibres)

Question 10

what does auxin do in the roots

Answer 10

inhibits cell elongation and therefore the upper side continues to grow causing the root to bend downwards

Question 11

describe geotropism in shoots

Answer 11

• negative tropism in plant stems
• results from accumulation of auxin in the lower side
• H.E Dolk decapitated coleoptiles tips and grew them horizontally on an agar block divided by a razor blade
• —> more auxin in block B —> concluded that gravity modifies distribution of auxin so it accumulates on the lower side

Question 12

describe apical dominance

Answer 12

• inhibition of lateral buds by chemicals produced by the apical bud
• auxins growing at the apex of the plant stem cause it to grow upwards but also inhibit the growth of lateral buds
• plants need to grow upwards to gain sunlight
• however, if growing point at apex is removed then the lateral buds will begin to grow

Question 13

what do cytokinins do

Answer 13

• promote lateral bud growth if the tip is removed
• produced in the roots
• can override apical dominance

Question 14

define synergism

Answer 14

• when 2 factors actions are greater than their effective actions added together
• e.g. auxin and gibberellins work synergistically

Question 15

what stimuli do plants have

Answer 15

• no sense organs
  1. abiotic: non-living
  2. biotic: living (i.e organisms)