U1T3 - Coordination & Control (1)

Question 1

What controls flowering?

Answer 1

The photoperiod. More specifically, phytochrome.

Question 2

What does red light do to phytochrome?

Answer 2

Absorbed by P660 + rapidly converts it into P730. Daylight has more red light so P730 accumulates.

Question 3

What does far red light do to phytochrome?

Answer 3

Absorbed by P730, rapidly converts it to P660.

Question 4

What happens to P730 during darkness?

Answer 4

P730 is unstable so slowly converts back into P660.

Question 5

What happens overall to phytochrome during the day and night?

Answer 5

During day, P730 rapidly produced so less P660. During night, P660 slowly produced so less P730.

Question 6

How can we control when plants flower?

Answer 6

Control light. e.g. chrysanthemum flowers in Autumn as an SDP. Can provide long day lighting scheme and then block light just before Christmas to stimulate flowering. (Artificial lighting in greenhouse + blackout screens)

Question 7

What are the 3 classes of plant hormone which control plant growth?

Answer 7

Auxins, gibberellins + cytokinins.

Question 8

What do auxins do?

Answer 8

Cause positive phototropism.

Question 9

What are the main stages in the response of auxin in positive phototropism?

Answer 9

Auxins produced in cells of apical meristems. Diffuse down shoot to elongation zone. Bind to receptors in cell membranes of new cells in zone (as cell walls thinner + more flexible). Pump H+ ions into cellulose cell walls. This acidification loosens cross links in cellulose microfibrils so wall more flexible. As cells absorb water by osmosis, cell walls stretch more easily due to hydrostatic pressure exerted. More auxin, more flexible walls.

Question 10

What happens if there’s differential illumination?

Answer 10

Auxin conc on on shaded side higher, more cell elongation on shaded side results in curvature of shoot towards light source.

Question 11

Darwin:

Why would a coleoptile bend towards light?

Answer 11

It’s positively phototropic. Bending occurs behind tip in region of cell elongation.

Question 12

Darwin:

Coleoptile tip removed + discard, why would there be no response?

Answer 12

Tip must perceive stimulus or produce messenger (or both) as its removal prevents response.

Question 13

Darwin:

Lightproof cover placed over intact tip of coleoptile, why is there no response?

Answer 13

Stimulus must be perceived by tip.

Question 14

Darwin:

Fine black sand placed around base + body of coleoptile, but not tip. Why does it bend towards the light?

Answer 14

Stimulus is perceived by tip rather than regions behind it + messenger comes from tip.

Question 15

Boysen-Jenson:
Thin, impermeable barrier of mica placed just below tip on illuminated side of coleoptile. Why does it bend towards the light?

Answer 15

Mica on illuminated side allows hormone to pass down dark side only where it increases growth rate + causes bending.

Question 16

Boysen-Jenson: Mica inserted on shaded side of coleoptile. Why is there no response?

Answer 16

As mica conducts electricity, it wouldn’t prevent electrical messages passing down from tip but does prevent chemical messages. As no response, message must be chemical and must pass down shaded side.

Question 17

Boysen-Jenson: Tip of coleoptile removed + gelatin blocked inserted + tip replaced. Why does it bend towards the light?

Answer 17

Gelatin allows chemicals through but not electrical messages so bending must be due to chemical passing from tip.

Question 18

Went: Coleoptile in darkness. Tip removed + placed on agar block. Block placed on one side of decapitated coleoptile. Why does it bend towards side where agar is not present?

Answer 18

Hormone from tip collected in agar. Hormone moves down agar side, increasing growth + causing bending. Degree of curvature proportional to amount of auxin.

Question 19

Went: Coleoptile in darkness. Tip removed + placed on agar block. Block placed on one side of upside down decapitated coleoptile. Why does it bend towards side where agar is not present?

Answer 19

Hormone from tip collected in agar. Hormone moves down agar side, increasing growth + causing bending. Degree of curvature proportional to amount of auxin. Movement of hormone in upward direction so doesn’t require gravity to move.

Question 20

Briggs: Thin glass plate separates 2 sides of coleoptile, why is there elongation but no bending?

Answer 20

Plate prevents lateral movement so equal auxin on both sides, hence it grows straight.

Question 21

Paal: Coleoptile in darkness. Tip removed + replaced displaced to one side. Why do they bend to side without tip?

Answer 21

Auxin only passes down one side where elongation occurs, resulting in bending.

Question 22

Why would you place a decapitated plant in darkness during an experiment with agar + plant hormones?

Answer 22

Prevents differential movement of auxin due to light.

Question 23

Why does auxin inhibit growth in high concentrations?

Answer 23

It’s toxic. Causes cell wall to become too flexible and so bursts as water floods in.

Question 24

Why does gibberellin produce greatest growth when cytokinin is not limiting?

Answer 24

Cytokinins increase cell division rate so they have more cells to act on.