tropisms in plants (a)

Question 1

What are phototropisms?

Answer 1

• phototropisms are the result of movement of auxins across the shoot or root if it is exposed to light that is stronger on one side than the other

Question 2

What effects does light have on a phototropism?

Answer 2

• the brightness of the light has no effect
• unilateral light will cause the shoot to grow towards the light, roots away from the light
• shoots = positively phototropic, roots = negatively phototropic

Question 3

Describe the effect of unilateral light

Answer 3

• side of the shoot exposed to light contains less auxin than the unilluminated side
• light causes auxins to move laterally across the shoot
• this stimulates cell elongation on the shaded side, thus growth occurs towards the light
• once the shoot is growing directly towards the light, the unilateral stimulus is removed - the transport of auxin stops and the shoot then grows straight towards the light

Question 4

Describe geotropisms

Answer 4

• in normal conditions, plants receive a unilateral gravitational stimulus (acting downwards)
• shoots = negatively geotropic, roots = positively geotropic
• ensures roots grow down into the soil and shoots grow up to the light

Question 5

Practical investigations into phototropisms

Answer 5

• e.g. germinate and grow seedlings in different conditions, observe measure and record the patterns of growth
• germinate and grow seedings in unilateral light with different colour filters to see which wavelengths of light trigger the phototropic response

Question 6

Practical investigations into geotropisms

Answer 6

• grow plants slowly on a clinostat so that the gravitational stimulus is applied evenly to all sides of the plant - so the root and shoot grow straight (in the dark)
• place seeds in petri dishes stuck to the walls of the lab and rotate dishes at 90 degree intervals as the seedlings grow.