Responses in Plants

Because plants cannot escape unfavourable conditions by moving away, these growth responses are one of their main tools for reaching light, water and support over a lifetime.

Phototropism, the response to light, and geotropism, the response to gravity, are the responses most often examined, usually through a labelled diagram or a description of an experiment using seedlings grown in a box with a single light source or laid on their side.

All tropisms share the same underlying mechanism: an unequal distribution of auxin across the two sides of a shoot or root causes unequal cell elongation, so one side grows faster than the other and the organ bends. In roots, dense starch-containing organelles called statoliths settle towards the lower side of cells in the root cap, which is thought to help the root sense the direction of gravity.

This explanation, based on differential auxin distribution and gravity-sensing cell contents, is the standard model used at this level, even though research into the exact signalling pathway continues.

Tropisms are easy to confuse with nastic movements, such as the rapid leaf-folding of Mimosa pudica when touched, because both are plant responses to a stimulus. The key difference is that a tropism is a directional growth response, so its direction depends on the direction of the stimulus, while a nastic movement is a non-directional response driven by a rapid change in turgor pressure rather than growth.

Content standards in this chapter

1. 20.1 Types of Responses in Plants
2. 20.2 Phytohormones
3. 20.3 Application of Phytohormones in Agriculture

Key concepts

Types of responses

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.

Positive and negative tropism

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

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.

Auxin and phototropism

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.

Application of phytohormones

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.

Importance of responses

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.

Geotropism mechanism

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 and thigmotropism

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.

Nastic movements compared with tropisms

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.

How this chapter is examined

SPM Biology is assessed over three papers. Paper 1 has 40 objective questions (40 marks) in 1 hour 15 minutes; Paper 2 carries 100 marks across Sections A, B and C in 2 hours 30 minutes; Paper 3 is the practical, testing science process skills. Content from this chapter can appear in any of them, so lessons drill recall for Paper 1 and structured answers for Paper 2.

Common exam angles

• Explaining phototropism in terms of the lateral movement and accumulation of auxin.
• Classifying a described plant response as a positive or negative tropism.
• Giving named agricultural or horticultural uses of synthetic plant hormones.
• Explaining how statoliths in the root cap are thought to help a root sense gravity.
• Distinguishing a tropism from a nastic movement by direction and mechanism.
• Describing the design and expected result of an experiment demonstrating phototropism or geotropism in seedlings.

Common mistakes

Study this chapter

Processes in this chapter

• Phototropism
• Geotropism
• Nastic movement

Experiments in this chapter

• Effect of the Phytohormone Ethylene on Fruit Ripening

Key terms

TropismPhototropismGeotropismAuxin

Frequently asked questions

Source:SRC-DSKP-EN, SRC-FORMAT