Responses in Plants, practice questions

B, Phototropism is named after light, the stimulus that triggers it; the other tropisms respond to gravity, touch and water respectively.

D, Growth towards a stimulus is a positive tropism, so a shoot bending towards light shows positive phototropism.

C, Auxin moves to the shaded side and promotes elongation there, so that side grows faster and the shoot bends towards the light.

C, Statoliths settle inside root cap cells and are thought to help the root sense the direction of gravity.

B, Statoliths are starch-filled organelles, which makes them dense enough to settle towards the lower side of a cell under gravity.

C, Auxin inhibits elongation in root cells at this concentration, so the lower side grows less and the root curves downward.

C, Mimosa pudica's leaf-folding is a non-directional response driven by a rapid turgor change, unlike the directional growth of a tropism.

B, A synthetic auxin, often as a rooting powder, stimulates root initiation at the cut surface of a cutting.

C, At a much higher concentration, the same class of hormone causes uncontrolled growth in certain broad-leaved weeds until they die.

B, A tropism's direction always depends on the direction of the stimulus; a nastic movement's response is the same regardless of that direction.

• (a) The shoot grew and curved towards the hole, that is, towards the light source.
• (b) The response is phototropism, and it is positive because the shoot grows towards the light.
• (c) Auxin produced at the shoot tip moved to and accumulated on the shaded side of the shoot.
• (c) Auxin promotes elongation, so cells on the shaded side elongated more than cells on the lit side, causing the shoot to bend towards the light.

• (a) The root tip curved and grew downward, showing a positive geotropic response.
• (a) This happens because auxin builds up on the lower side of the root and inhibits elongation there, so the upper side elongates more and the root bends downward.
• (b) The organelle is the statolith, a dense, starch-filled structure inside root cap cells.
• (b) When the root is not growing straight down, statoliths settle towards the lower side of the cell under gravity, which is thought to trigger the redistribution of auxin.

• (a)(i) A synthetic auxin can be applied to the flower to trigger ovary development into a fruit without fertilisation taking place, so no seeds form.
• (a)(ii) A plant hormone treatment can be used to control the timing of fruit ripening, keeping the fruit unripe during transport and letting it ripen closer to sale.
• (b) The grower must apply the hormone at the concentration recommended for that specific use, because too high a concentration can damage or kill the crop instead of helping it.

Tropisms are directional growth responses of a plant organ to a stimulus that comes from one particular direction: phototropism responds to light, geotropism to gravity, hydrotropism to water, and thigmotropism to touch, and each is named after the stimulus that causes it.

Growth towards a stimulus is called a positive tropism and growth away from it a negative tropism; for example, shoots are positively phototropic and negatively geotropic, while roots are negatively phototropic and positively geotropic, so each organ grows in the direction that best helps the whole plant survive.

Phytohormones are chemical messengers made in one part of a plant that travel to another part, usually in very small amounts, to control growth and responses; auxin is the phytohormone most closely linked to tropisms, while other phytohormones control processes such as fruit ripening, dormancy and ageing.

When light shines from one direction, auxin produced at the shoot tip moves towards and accumulates on the shaded side rather than the lit side, making the cells on that shaded side elongate more than those on the lit side, so the shoot bends towards the light source.

Synthetic phytohormones based on auxin are used commercially to promote root growth in stem cuttings before they are planted, to produce seedless fruit by triggering ovary development without fertilisation, to control the timing of fruit ripening for transport and sale, and, at a much higher concentration, as selective weedkillers. Because these hormones act at very low concentrations, farmers must follow recommended dosages carefully, since too much can damage or kill the crop instead of helping it.

Tropic responses help a plant position its leaves and shoots to capture more light for photosynthesis, direct its roots towards water and mineral-rich soil, and anchor itself firmly, all of which improve the plant's chances of survival and reproduction in a fixed location. Without these responses, a seedling germinating in shade or planted at an angle would have little chance of reaching open light or anchoring properly.

In a root laid horizontally, gravity causes dense statoliths inside cells of the root cap to settle towards the lower side, and this is believed to trigger a redistribution of auxin so that more auxin accumulates on the lower side of the root; because root cells are inhibited rather than promoted by high auxin concentration, the lower side elongates less and the root curves downward. This explanation is sometimes called the statolith hypothesis, and it is the model most commonly taught and examined at this level.

Hydrotropism is a root's growth response towards a source of moisture, which can override geotropism in very dry soil so the root grows sideways towards water instead of straight down; thigmotropism is a growth response to touch, seen when a climbing plant's tendril coils around a support after one side of the tendril is stimulated by contact.

A nastic movement, such as the rapid folding of Mimosa pudica leaves when touched or the opening and closing of stomata, is a plant's response to a stimulus that does not depend on the direction the stimulus comes from and is caused by a fast change in turgor pressure in specific cells rather than by unequal growth, unlike a tropism. Because the response is so fast, a nastic movement is useful for defence, such as deterring a feeding insect, in a way that slow tropic growth cannot achieve.