Plant Responses

In a phototropism investigation, the tip of a coleoptile is removed and replaced with an opaque cap, then the coleoptile is illuminated from one side. The most likely outcome is that the coleoptile:

• Abends sharply towards the light
• Bbends away from the light
• Cgrows straight up with no bending✓
• Dstops growing completely and dies

Answer: The tip is the region that detects light and produces and redistributes auxin (IAA). Capping the tip with an opaque cover prevents light detection, so no asymmetric auxin distribution occurs and the coleoptile grows straight up rather than bending.

Indoleacetic acid (IAA) causes a positive phototropic response in a shoot because it:

• Aaccumulates on the light side, where it inhibits cell elongation
• Baccumulates on the shaded side, where it promotes cell elongation so that side grows faster✓
• Cis destroyed on the shaded side, slowing growth there
• Daccumulates evenly on both sides, causing equal growth

Answer: IAA is redistributed laterally so it becomes more concentrated on the shaded side of the shoot. There it stimulates cell elongation, so the shaded side grows faster than the illuminated side and the shoot bends towards the light.

When a young root is placed horizontally, it shows positive gravitropism. The accepted explanation is that IAA accumulates on the lower side of the root, where it:

• Apromotes cell elongation, so the upper side grows faster and the root bends upward
• Binhibits cell elongation, so the upper side grows faster and the root bends downward✓
• Cpromotes cell division only, with no effect on elongation
• Dhas no effect because roots do not respond to IAA

Answer: IAA collects on the lower side of a horizontal root. In roots, relatively high IAA concentrations inhibit cell elongation, so the lower side elongates less than the upper side. The root therefore curves downward, towards gravity (positive gravitropism).

Apical dominance, in which the growth of lateral (side) buds is suppressed, is primarily maintained by:

• Agibberellins produced in the lateral buds
• Bauxin (IAA) produced by the apical bud✓
• Cabscisic acid produced in the roots
• Dethene produced by the leaves

Answer: Auxin produced by the apical (terminal) bud is transported down the stem and inhibits the growth of lateral buds, maintaining apical dominance. Removing the apical bud lowers auxin levels and allows the side buds to grow, which is why pruning encourages bushier growth.

Gibberellins promote the germination of cereal seeds such as barley by:

• Ainhibiting the production of amylase so that starch is conserved
• Bstimulating the aleurone layer to produce amylase, which hydrolyses stored starch to maltose✓
• Cclosing the stomata to reduce water loss during germination
• Ddirectly providing glucose to the embryo

Answer: The embryo releases gibberellins that diffuse to the aleurone layer surrounding the endosperm. This stimulates synthesis of amylase, which hydrolyses stored starch into maltose. The soluble sugar is then used by the growing embryo as a respiratory substrate.

The phytochrome pigment exists in two interconvertible forms, Pr and Pfr. Which statement is correct?

• APr absorbs far-red light and is rapidly converted to Pfr
• BPr absorbs red light and is converted to Pfr, which is the physiologically active form✓
• CPfr absorbs red light and is converted to Pr in daylight
• DPr is the active form that accumulates during the day

Answer: Pr absorbs red light (around 660 nm) and is rapidly converted to Pfr. Pfr is the active form that influences responses such as flowering; it absorbs far-red light and is slowly converted back to Pr in darkness, allowing plants to measure night length.

In a long-day plant, flowering is promoted when the night length is short. The form of phytochrome that accumulates by the end of a short night and promotes flowering in such plants is:

• APr
• BPfr✓
• Cchlorophyll a
• Dabscisic acid

Answer: During daylight Pr is converted to Pfr. With short nights there is little time for Pfr to revert to Pr, so a high level of Pfr remains at dawn. In long-day plants this high Pfr level promotes flowering; long-day plants are effectively short-night plants.

When a plant experiences water stress, abscisic acid (ABA) helps reduce water loss by:

• Astimulating guard cells to take up K⁺ and water so that stomata open
• Bcausing loss of solutes from guard cells so that they lose turgor and the stomata close✓
• Cincreasing the rate of transpiration through the stomata
• Dpromoting cell elongation in the leaf mesophyll

Answer: ABA accumulates in leaves during water stress and acts on guard cells, triggering the loss of K⁺ and other solutes. Water then leaves the guard cells by osmosis, they become flaccid, and the stomata close, reducing transpirational water loss.

Which of the following is an example of a chemical plant defence against herbivores or pathogens?

• Athe waxy cuticle on the leaf surface
• Bproduction of tannins that make leaf tissue unpalatable or toxic to insects✓
• Cthe presence of thorns on a stem
• Dfolding of leaves in response to touch

Answer: Tannins are chemical defences: they are bitter-tasting and can be toxic or reduce digestibility, deterring herbivores from eating the plant. Thorns and a waxy cuticle are physical defences, and leaf folding is a rapid movement response, not a chemical defence.

A motile single-celled alga swims directly towards a region of higher light intensity. This directional movement of a whole organism in response to a stimulus is best described as a:

• Apositive kinesis
• Bpositive taxis✓
• Ctropism
• Dnastic response

Answer: A taxis is a directional movement of a whole motile organism towards (positive) or away from (negative) a directional stimulus. Movement towards light is positive phototaxis. A kinesis is non-directional, and a tropism is a directional growth response of a fixed plant.

Woodlice placed in a choice chamber move more rapidly and turn more frequently in dry conditions, but slow down and turn less in humid conditions, so they tend to aggregate in damp areas. This behaviour is an example of:

• Aa taxis, because movement is directed towards humidity
• Ba kinesis, because the rate of movement and turning changes with conditions but is non-directional✓
• Ca tropism, because it is a growth response
• Da reflex arc involving the spinal cord

Answer: In a kinesis the organism changes its speed and rate of turning in response to the intensity of a non-directional stimulus, rather than moving straight towards or away from it. Faster movement and more turning in dry air mean woodlice spend more time, and thus accumulate, in damp regions.