Form 5 · Revision notes

Responses in Plants, revision notes

Complete revision notes for Responses in Plants: every content standard, the key definitions, comparison tables and a recall checklist for SPM Biology Form 5.

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Overview

Plants respond to their surroundings mainly by growing towards or away from a stimulus, since they cannot move from place to place like animals. This chapter covers the types of plant responses called tropisms, the phytohormones such as auxin that control them, and how these hormones are applied in agriculture to control rooting, fruiting and weeds.

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.

20.1 Tropisms: types and direction of growth

A tropism is a directional growth response of a plant organ to a stimulus coming from one particular direction, and each type of tropism takes its name from the stimulus that triggers it: phototropism responds to light, geotropism to gravity, hydrotropism to water, and thigmotropism to touch. Because a tropism is a growth response, it develops gradually as cells on one side of the organ elongate faster than cells on the other side, causing the organ to curve.

Growth towards the stimulus is called a positive tropism, and growth away from it a negative tropism. A shoot growing towards light is positively phototropic, while a root growing away from light and down into the soil is negatively phototropic and positively geotropic at the same time.

This opposite pairing in shoots and roots means the same stimulus can produce different directions of growth depending on which organ is responding, and depending on the organ's role in keeping the whole plant supplied with light, water and anchorage.

20.1 Tropisms compared with nastic movements

A nastic movement, such as the rapid folding of Mimosa pudica leaflets when touched or the daily opening and closing of stomata, is also a plant's response to a stimulus, which makes it easy to confuse with a tropism in an exam answer. The key difference is that a nastic movement is non-directional: the plant gives the same response no matter which direction the stimulus comes from, because it is driven by a rapid change in turgor pressure in specific motor cells rather than by unequal growth.

Because it does not depend on growth, a nastic movement can happen within seconds, while a tropism typically takes hours or days to become visible. This speed makes a nastic response useful for defence, such as folding away from a feeding insect, in a way that slow tropic growth cannot achieve.

Feature	Tropism	Nastic movement
Direction of response	Depends on the direction of the stimulus	Same regardless of the stimulus's direction
Mechanism	Unequal cell elongation on two sides of the organ	Rapid change in turgor pressure in motor cells
Speed	Hours to days	Seconds to minutes
Example	Shoot bending towards light (phototropism)	Leaf-folding of Mimosa pudica (seismonasty)

20.2 Phytohormones and the mechanism of phototropism

A phytohormone is a chemical messenger made in one part of a plant, usually the growing tip, that travels to another part in very small amounts to control growth and responses. Auxin is the phytohormone most closely linked to tropisms, while other phytohormones separately control processes such as fruit ripening, dormancy and ageing.

When light shines on a shoot from one side, auxin produced at the shoot tip moves laterally and accumulates on the shaded side rather than the lit side. Because auxin promotes cell elongation in shoot cells, the cells on the shaded side elongate more than those on the lit side, and this unequal growth bends the shoot towards the light, a positive phototropic response that helps the plant capture more light for photosynthesis.

20.2 Geotropism, the statolith hypothesis, and other tropisms

In a root laid on its side, gravity causes dense, starch-filled organelles called statoliths inside cells of the root cap to settle towards the lower side. This settling is believed to trigger a redistribution of auxin so that more auxin builds up on the lower side of the root, an explanation known as the statolith hypothesis.

Root cells respond to auxin differently from shoot cells: at the concentration reached on the lower side, auxin inhibits rather than promotes elongation in root cells, so the lower side grows more slowly than the upper side and the root curves downward, a positive geotropic response. A shoot laid on its side shows the opposite pattern, curving upward, because the same higher auxin concentration on its lower side promotes extra elongation there instead.

Two further tropisms are examined less often. Hydrotropism is a root's growth response towards a source of moisture, and it can override geotropism in very dry soil so that a 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 contact stimulates one side of the tendril.

20.3 Commercial application of phytohormones in agriculture

Because synthetic phytohormones based on auxin reproduce the growth effects of the natural hormone, they are applied commercially at carefully controlled concentrations for four main purposes in farming and horticulture, summarised below.

Agricultural use	How the hormone acts	Example
Promoting rooting	Stimulates root initiation at the cut surface of a stem cutting before it is planted	Rooting powder applied to a hibiscus cutting
Producing seedless fruit	Triggers ovary development into fruit without fertilisation taking place	Seedless varieties of certain fruit crops
Controlling ripening	Regulates the timing of fruit ripening so produce reaches the market at the right stage	Ripening control before long-distance transport
Selective weed control	At a much higher concentration, causes uncontrolled growth in certain broad-leaved weeds until they die, while leaving narrow-leaved crops largely unaffected	Herbicide used in a cereal crop field

Key concepts to master

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.

Quick recall checklist

Can you define and explain Types of responses?
Can you define and explain Positive and negative tropism?
Can you define and explain Phytohormones?
Can you define and explain Auxin and phototropism?
Can you define and explain Application of phytohormones?
Can you define and explain Importance of responses?
Can you define and explain Geotropism mechanism?
Can you define and explain Hydrotropism and thigmotropism?
Can you define and explain Nastic movements compared with tropisms?

Frequently asked questions

How does a shoot bend towards light?

When light shines on one side of a shoot, the hormone auxin produced at the tip moves to and accumulates on the shaded side rather than the lit side. Auxin makes cells elongate, so the cells on the shaded side grow longer than those on the lit side. This unequal growth causes the shoot to bend towards the light, a positive phototropic response that helps the plant capture more light for photosynthesis.

What is the difference between phototropism and geotropism?

Phototropism is a plant's growth response to the direction of light: shoots grow towards light (positive) and roots grow away from it (negative). Geotropism is a growth response to gravity: roots grow downwards towards gravity (positive) and shoots grow upwards away from it (negative). Both responses are controlled by the same hormone, auxin, redistributed to different effect in shoots and roots.

How are plant hormones used in farming?

Plant hormones are applied to promote root growth in cuttings before planting, to produce seedless fruits by triggering ovary growth without fertilisation, to control the timing of fruit ripening for transport and sale, and, at higher concentrations, as selective weedkillers that make certain broad-leaved weeds grow uncontrollably until they die. These uses all come from understanding how auxin controls plant growth. Because the effective dose is so small, commercial products specify exact concentrations to avoid damaging the crop being treated.