Chapter 3: Plant responses to environmental stimuli.

Hey ! Where's your pictures ??

Introduction

Tropisms, nasties, taxes and more.

Auxins: plant growth hormones

A Table of plant tropisms

More plant hormones and responses

Movement of substances in & out of leaves

Gas exchange and saving water in leaves.

Guard cells

Summary of major plant adaptations

Plants may not be able to move in the way that animals do, but it is still possible for them to respond to environmental stimuli.

Sometimes the response will involve the movement of a part of the plant.

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Tropisms, nasties, taxes and other responses to the environment.

One type of movement seen in plants is called tropism (direction of stimulus determines direction of response). If the movement is towards the stimulus, then it is said to be positive tropism, but if the movement is away from the stimulus, then it is said to be a negative tropism.

Another type of plant movement is called nastic movement (plural nasties) (direction of response independent of direction of stimulus).

Small algae such as Euglena and Chlamydomonas can exhibit movements of the whole organism and such movements are called taxes.

Plants can also respond to environmental stimuli without moving, as is the case with the photo periodic response, flowering and vernalisation.

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Auxins: an important class of plant growth hormones

Plant growth, like animal growth is controlled by hormones.

A plant hormone is a chemical which is made by one part of the plant, but then travels to another part where it has some effect. (Is this different to the definition of an animal hormone?)

Auxins affect plant growth. The main auxin is indole acetic acid (IAA). Auxin controls phototropism (plant shoots bending towards light) and gravitropism - (roots growing down into soil). The stimulus/response sequence for auxin action is as follows:

Auxin is made by the growing tips, or meristem, of shoots and roots.
The stimulus for its production in the shoot is uneven light distribution.
In the root, slight variation in the force of gravity and also light seem to be the stimuli.
The auxin diffuses to the area of elongation just behind the tip.
Here newly formed cells are developing their vacuoles and becoming bigger.
Auxins change the flexibility of cell walls and this allows for expansion of the cells.
Because the distribution of the auxin is not uniform in the shoot or root, the response is not equal on each side of the tissue.
This leads to greater expansion of cells on one side than the other. The response depends on the amount of auxin received.
So the side with less expanded cells is smaller, the opposite side's more expanded cells are larger, and bending of the growing tissue results.
Biochemists have discovered some interesting relationships between auxin concentration and tissue response in shoots and roots. There is not a simple 'more auxin: more response' relationship in either tissue, and roots appear to be much more senstive to low levels of auxin than are shoots. ... The research continues!

Bending shoot

Here is a classic diagram of a shoot bending towards the light. Note how auxin released only on the dark side of the shoot is able to cause cell elongation on that side. It is the uneven cell growth which causes the shoot to bend - towards the light in this case to optimise the exposure to the light for photosynthesis.

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TABLE OF COMMON PLANT TROPISMS

TROPISM	STIMULUS	BIOLOGICAL CHANGE	RESPONSE
Positive phototropism	light	Cell elongation is stimulated on the shaded side of a stem but inhibited on the lighted side.	Growing shoot bends towards the light.
Positive gravitropism	gravity	Cell elongation is inhibited on the lower portion of the root but stimulated on the upper portion.	Growing root bends downwards towards the centre of the gravity.
Positive thigmotropism	touch or pressure	Cell elongation is inhibited on the stem side touching an object but stimulated on the non-touching side	Growing shoot coils around the object. For example, in some climbing vines.
Negative thigmotropism	touch or pressure	Cell elongation is stimulated on the stem side touching an object but inhibited on the non-touching side.	Shoot grows away from an obstacle.
Positive hydrotropism	water availability	Root tips closest to a source of water and soluble minerals simply grow faster than those further away. (Hormones not involved here.)	Growth of the root towards a water source.

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OTHER PLANT HORMONES AND RESPONSES TO ENVIRONMENTAL STIMULI

Cytokinins

Hormones which in combination with auxin stimulate cell division and influence tissue differentiation.

Gibberellins

Hormones that control stem elongation in most plants. Many dwarf mutants of plants will grow and develop normally if gibberellin is applied.

Ethylene

A simple hydrocarbon plant hormone. It controls abscission (=dropping) of leaves, flowering and fruiting, and hastens fruit ripening. It can be used commercially to cause fruit ripening or hasten fruit drop.

Abscisic acid

Abscisic acid suppresses cell growth. It also promotes leaf senescence (death) which results in the colour changes of leaves in autumn before they are dropped from deciduous plants. Abscissic acid also appears to be involved in stomatal opening and closing. It may have a role in root gravitropism.

Phytochromes

These chemicals change in concentration in plants in response to changes in length of dark (night)/light (day) periods. In turn, these changes in phytochromes stimulate or repress the flowering of plants. There are two categories of plants with respect to flowering. Short day plants flower in response to long nights. Long day plants flower in response to short nights. (The names now seem silly, as it is the night length rather than day length which is the stimulus for the response. But this is an example of many scientific discoveries; the response was observed, and named, long before the mechanism of its action was worked out.)

Vernalization

This is the observation that some seeds and bulbs will not germinate or flower unless they have had a suitable period of time at cold temperatures (usually -2^oC to +10^oC). (This discovery has implications in areas where there are mild winters; if a gardner in Melbourne wants to grow good tulips, she should put the bulbs in the refrigerator for several months before planting them in autumn, in spring beautiful tulips should flower!)

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MOVEMENT OF SUBSTANCES IN/OUT OF LEAVES

During daylight, plants are performing photosynthesis at a greater rate than cellular respiration if their stomata are open.

6CO₂ + 6 H₂O ----> C₆H₁₂O₆ + 6O₂

For this to occur, there must be a net input of carbon dioxide and water in the leaves.

Water enters the plant via osmosis in the roots. It travels up the xylem to the leaves. In the leaf, water enters the air spaces as water vapour. As photosynthetic cells use water more is drawn into them, again the process is osmosis.

Water can also leave the air spaces, where the atmosphere is at close to 100% humidity, to the outside of the leaf, where the atmosphere may be well below 100% humidity. The water passes out through the stomatal pores in this situation. In this case the process is diffusion, as water is passing down its concentration gradient.

Diffusion is also the process by which gases move in and out of the leaf. In the case of photosynthesis dominating, CO₂ is used by the cells. The concentration gradient of CO₂ is therefore high in the air spaces, lower in the cell, so the CO₂ diffuses into the cell. This means that the leaf air spaces become deficient in CO₂ when compared with the external environment. Thus CO₂ diffuses into the leaf via the stomatal pores.

Using similar reasoning, it can be expalined why O₂ diffuses out of the leaves of photosynthesising plants.

Now write the equation for cellular respiration and think through the inputs and outputs when that is the dominant process (ie at night)!

Remember that even when photosynthesis is occurring, so too is cellular respiration, so in the day, some of the O₂ produced in photosynthesis is immediately used for respiration. Similarly, some of the CO₂produced in respiration is immediately used in photosynthesis. There is, of course, a point where the oxygen output is just balanced by its rate of use, and the same is true for carbon dioxide. This is the compensation point. This is often illustrated graphically:

Graph

Graph

Graph of rates of cellular respiration and photosynthesis at varying light intensities. Note the following:

At A. Cellular respiration continues at a constant rate regardless of light intensity.

At B. Photosynthesis is either unable to occur, or occurs at a rate limited by the amount of light. This is true at low/moderate light intensities.

At C. The Compenstaion point. Here the rates of photosynthesis and respiration are equal.

At D. Here the light intensity is high. The enzymes of photosynthesis are working at their maximum level, so the rate of photosynthesis is maximum. The enzymes (or perhaps the rate of supply of substrate) are controlling the rate of photosynthesis.

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GAS EXCHANGE OR WATER CONSERVATION? A delicate balance during hot, dry Australian summers.

If stomata were constantly open, gasses could diffuse in/out as concentration gradients dictated. Respiration could go on at a maximum rate constantly, and photosynthesis would only be regulated by light intensity. But what about the water?

In a well watered habitat this would not be a problem for the plant. Soil water would be freely available and the atmosphere would be relatively humid from evaporation of soil water. The need to conserve water would not be great in these conditions. In fact, plants adapted for life in or near permanent water have either unusual positioning of stomata, or very little change in stomatal aperture each day.

However, in a hot, dry habitat, water constantly evaporates from the leaves (good for cooling) and a plant is in danger of dehydration if this water is not replaced sufficiently quickly. If the soil around the roots is dry, failure to replace water lost is inevitable. Water diffuses out of the plant cell vacuoles, the cell become flaccid, and the plant wilts. (Remember the plants in your garden during the long hot summer in January and February? Did you take pity on them and provide their roots with some water? Why was it the non-Australian plants which wilted first?)

Many Australian plants have adaptations for hot dry summers, such as that of 1996-97. These plants are able to survive because of the compromise their guard cells achieve. Frequently other adaptations such as thick waxy cuticles and leaves which hang down, or are narrow and short, and have reduced exposure to the direst sun are also seen in Australian plants from arid regions.

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Guard cells

Guard cells/stomata open/closed

Guard cells are specialised cells in the leaf epidermis (usually mostly on the lower leaf surface). There is a space of variable size between pairs of guard cells, the stomatal pore or stoma (pl. stomata). Guard cells, unlike other epidermal cells, possess chloroplasts and are therefore able to perform photosynthesis. (This is the 'secret weapon' against dehydration!) Here's how one of nature's cleverest survival tricks works ...

In the presence of light, photosynthesis begins in the guard cells and the other photosynthetic cells of the leaf.
Glucose accumulates.
Water is drawn into the guard cells by osmosis because of the osmotic gradient which develops as the glucose increases.
Guard cells become fully turgid, and because of their specialised structure, they 'bend' apart.
The stomata are open - gas diffusion is possible.
That's all very well, if it's early morning and the air is still relatively humid. But what about later in the day, when the temperature rises, humidity falls, and water vapour would be pouring out of the plant? Here's the clever bit ...
Water diffuses out of the guard cells in response to the decreasing water content of the air adjacent to them.
Guard cells become flaccid.
Stomatal pores close.
Gas exchange, including water loss, through stomata is shut down.
Photosynthesis can still provide the inputs for cellular respiration to continue in the cells inside the sealed leaf whilst the stomata are closed.
At dusk and again in the early morning, when temperatures are cooler, humidity higher and stomata are open again photosynthesis can again accumulate stores of glucose for the heat of the day and at night.

Now isn't that clever? It's actually a bit more complicated than this according to the plant biochemists There are also some plant hormones and movement of K⁺ ions involved, and of course wind makes the water loss even worse, but the basics here are correct.

From this it can be seen that plants have specialised adaptations, either structural (such as cacti' or ferns' specialised leaves), or functional (hormones or stomatal rhythms) which enable them to survive in their own particular environments.

In preparing for this topic in your CAT 1 exam, you should ensure that you have read widely about different plant adaptations to environmental stress. Questions have appeared in the past where this topic is merged with a question testing your interpretation of experimental data given in the introduction to the question. If that happens this year, remember to read the data carefully, it will probably contain several marks worth of information. Some more marks are likely to be awarded for your ability to demonstrate that you understand how plants survive in difficult environments. (I, like you, have no idea of what's on the exam - don't take this as a 'hint', it's just advice based on experience.)

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A SUMMARY OF COMMON ADAPTATIONS OF PLANTS IN PARTICULAR ENVIRONMENTS

ENVIRONMENT (and plant type)	ADAPTATIONS
HOT AND DRY (xerophytes)	Stomata closed during heat of day Reduced or absent leaves (cacti) Leaves aligned away from direct sunlight Stoma almost all on lower leaf surface Stoma protected in pits or surrounded by hairs
HOT AND HUMID (tropical)	Guttation (drops of water released from leaf surface) removes excess water which enters plant due to root pressure.
VERY COLD WINTERS - temps below 0^oC	Deciduous - plants lose leaves and reduce metabolism to ensure survival. Leaf oils - act as 'anti freeze' to protect, as in conifers. Vernalization ensures that reproduction occurs during warm weather.
FRESH WATER (aquatic plants = hydrophytes)	Large vacuoles collect and expel excess water. Large air spaces in leaves allow leaves to float on or near surface and obtain light. Reduced stomata - gas exchange is by diffusion in/out of water.
MARINE (algae) seaweed	Non chlorophyll pigments (so they are not always green) since penetration of light is different under water. Flotation bladders, full of air, hold fronds near surface where light and gas levels are highest. Holdfasts anchor plants in presence of strong tides and currents (not true roots, since water is plentiful and minerals diffuse from environment).
HIGH SOIL [SALT] (halophytes) eg mangroves	Salt excreted through leaves Very thick leaf epidermis protects from salt.
LOW SOIL [MINERAL] (carnivorous plants)	Special structures attract and trap insects. Enzymes digest the insects, releasing minerals to diffuse into plant.

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Last update :17 March 97