Chapter 16

PLants are multicellular organisms that live in complex changing enviroments, their key limitation being that they cannot move and do not have a rapidly responding nervous system. However they are are coordinated organisms that show clear responses to their environment. The timescale of plant responses are slower, they have evolved a system of hormones that are produced in one region of the plant and transported to another. Important ones are:

• Auxins: Control cell elongation, prevent abscission, maintain apical dominance, involved in tropisms, stimulate the production of ethene.
• Gibberllins: Cause stem elongation, trigger mobilisation of food stores in seed germination, stimulate pollen tube growth in fertilisation.
• Ethene: Causes ripening of fruit, promotes abscission in deciduous trees.
• ABA: Maintains dormancy of seeds and buds, stimulates cold protective responses such as production of antifreeze. 

Plants produce chemicals which signal to other species, eg to protect against insects, scientists are still unsure about some details, as plant hormones are found in very small amounts, and it is hard to isolate certain chemicals due to multiple interactions. 

For a plant to start growing, the seed must germinate, when the seed absorbs water, the embryo is activated and it begins to produce gibberelins. They in turn stimulate the production of enzymes that break down the food stores found in the seed. These stores are used to produce ATP for building materials sp it anf grow and break out of the seed coat. Evidence suggests that gibberellins switch on genes which code for amylases and proteases, the digestive enzymes. Also evidence to suggest that ABA acts against gibberellins, and it is the relative levels of these two hormones which will result in germination or continued dormancy.

Experimental evidence:

• Mutuant varieties of seeds that have been bred with lack of genes to produce gibberellins do not germinate, if it applied externally, they germinate normally.
• If gibberellin biosynthesis inhibitors are applied to seeds, they do not germinate as they cannot produce the gibberellin needed to break seed dormancy. If the inhibition is removed, the seeds germinate.

Auxins such as indoleacetic acid (IAA) are growth stiumulants. Small quantities have powerful effects. They are made in cells at the tips of roots and shoots and in meristematic tissue, auxins can move down the stem and up the root in the transport tissue from cell to cell. The effect of the auxin depends on the concentration and any interactions with other hormones.

• They stimulate the growth of the main, apical shoot. Evidence suggests that auxins effect the plasticity of the cell wall - prescence causes the cell wall to stretch more easily. Auxin molecules bind to specific receptors site on plant cell membranes, causing a fall in pH to 5, which is the optimum for the enzymes needed to keep the cell walls flexible and plastic. As the cell matures, auxin is destroyed and the pH rises, as a result the cell walls become fixed.
• High concentration of auxins suppress the growth of lateral shoots, which results in apical dominance, stimulates growth of tip of main shoot. The lateral shoots are inhibited by the hormones that move back down the stem, further down the stem the auxin concentration is lower so more gorwth of lateral shoots. If apical shoot is removed, lateral shoots grow faster.
• Low concentrations of auxins promote root growth. Up to a given concentration, the more auxin, the more growth, and it is produced in the root tip. If apical root tip is removed root growth decreases rapidly.

Gibberellins are also important in the elongation of plant stems during growth, they effect the length of the internodes, which are the regions between the leaves on a stem. Plants that have short stems produce few or no gibberellins, without them plants would be much shorter, reduces waste and lowers risk of damage from weather.

There are many ways to investigate the effect of plant hormones on growth, most involve serial dilutions to observe the effects of different concentrations of hormones and a number of plants.

Most plant hormones do not work on their own but by interacting with other substances so that fine control over responses is achieved. If different hormones work together, complementing each other and giving a greater response, the interaction is known as synergism. If the substances have opposite effects, eg one promotes growth and one inhibits, the balance between them will determine the response of the plant. This is known as antagonism.

When enviromental conditions around plants change, they have to cope or die, abiotic stresses include changes in day length, cold/heat, lack/excss of water,wind and changes in salinity.

Leaf loss:Plants that grow in temperate climates experience great changes during the year. For example the range of daylight hours in Scotland ranges massively. As light and temperature affect the rate of photosynthesis, seasonal changes impact this massively. The point comes around where the amount of glucose required for photosynthesis to maintain the leaaves and to produce chemicals to maintain them is greater than the amount of glucose produced by respiration, particularily in tall trees due to srong wimter winds. At this point deciduous trees lose all of their leaves in winter and remain dormant until the days lenghen and the temperature rises in spring. 

Daylight sensitivity: Scientists have discovered that plants are snesitive to a lack of light in their environment, known as photoperiodism. New research suggests that is a lack of light that is the trigger for change in plants. Many different plant responses are affected by photoperiodism includig the breaking of dormancy of the leaf bud so they open, timing it for when tuburs are formed. The sensitivty of plants to day length results from a light sensitive pigment called phytochrome that has two forms that absorb different types of light which changes as the levels of light do.

When enviromental conditions around plants change, they have to cope or die, abiotic stresses include changes in day length, cold/heat, lack/excss of water,wind and changes in salinity.

Leaf loss:Plants that grow in temperate climates experience great changes during the year. For example the range of daylight hours in Scotland ranges massively. As light and temperature affect the rate of photosynthesis, seasonal changes impact this massively. The point comes around where the amount of glucose required for photosynthesis to maintain the leaaves and to produce chemicals to maintain them is greater than the amount of glucose produced by respiration, particularily in tall trees due to srong wimter winds. At this point deciduous trees lose all of their leaves in winter and remain dormant until the days lenghen and the temperature rises in spring. 

Daylight sensitivity: Scientists have discovered that plants are snesitive to a lack of light in their environment, known as photoperiodism. New research suggests that is a lack of light that is the trigger for change in plants. Many different plant responses are affected by photoperiodism includig the breaking of dormancy of the leaf bud so they open, timing it for when tuburs are formed. The sensitivty of plants to day length results from a light sensitive pigment called phytochrome that has two forms that absorb different types of light which changes as the levels of light do.

After a summer of long days, short nights and warm temperatures, the nights lengthen, the days shorten and temperatures fall as autumn progresses. The lenghteneing of the dark period triggers a number of changes, including abscission and a period of dormancy during the winter months. The falling light levels result in falling concentrations of auxin, the leaves respond to this by producing the gaseous plant hormone ethene. At the base of the leaf stalk is a region called the absicssion zone, made up of two layers of cells sensitive to ethene. Ethene appears to initiate genes switching in these cells resulting in the production of new enzymes that digest and weakn the cell walls in the outer layer of the abscission zone, known as the seperation layer.

The vascular bundles which carry materials in and out of the leaf are sealed off. At the same time fatty material is deposited in the cells on the stem side of the seperation layer. This layer forms a protective scar when the leaf falls, preventing the entry of pathogens. Cells in the seperation zone respond to hormonal cues by retaining water ans swelling, putting strain on the already weakened outer layer. Finally further abiotic factors such low temperatures or Autumn winds cause the leaves to detach, leaving a neat waterproof scar.

Another major abiotic factor which affects plants is the decreases in temperature, if cells freeze, their membranes are disrupted and will die. Many plants therefore have evolved mehcanisms to prevent this. The cytoplasm of the plant and the sap in the vacuoles contains solutes which lower the freezing point point. Some plants produce sugars or polysaccarides or amino acids that act as an antifreeze to prevent the cytoplasm of the cell from freezing. Most species only produce chemicals that make them hardy and frost resistant during the winter months. It appears that different genes are surpressed or activated in response to a sustained fall in temperatures along with a reduction in day length, effectively preparing plants to withstand frosty conditions. A sustained spell of warmth and extended day length reverse these changes in spring. 

Stomatal control: One of the ways plants respond to water availibility as an abiotic stress is to open the stomata to cool down the plant, or close them to reduce transpiration. The opening and closing of the stomata is under the control of ABA as it is released when the plant is under abiotic stress. Reserch suggests roots also act as an early water system, as if the levels of soil water fall, the roots produce ABA which is transported to the leaves where it binds to receptors on the plasma membrane of the guard cells, activating changes in ionic concentration reducing their water potential, resulting in turgor of the guard cells, causing them to swell and close, greatly reducing the rate of transpiration. 

Plants cannot escape the animals that want to eat them, so have evolved a wide range of chemical defences to minimise damage done by herbivores. Physical defences include thorns and barbs along with fibrous, indedible tissue, hairy leaves to discourage herbivores from eating them. Plants have also evolved a wide range of chemical defences that include:

• Tannins: Part of a group of compounds called phenoles produced by many plants, theyhave a very bitter taste and are present in the leaves, and are also toxic to insects.
• Alkaloids: These are a large group of bitter tasting, nitrogenous compounds that often affect the metaboism of animals, poisoning them, for example caffeine is toxic to fungi and insects, and when spread in the soil can prevent the germentation of other plants as it is produced by the seedings of the coffee bush. Nicotine is a toxin produced in the roots of tobacco plants and is then transported to the leaves to be released when the leaf is eaten.
• Terpenoids: A group of compounds that often form essential oils but also act as toxins to insects and fungi, and can at as neurotoxins or repellents to insects, for example chrysanthmums or citronella.

 A pheremone is a chemical made by an organism which affects the social behaviour of other members of the same species.

• If a maple tree is attacked by the insects, it releases a pheremone which is absorbed by leaves on other branches. These leaves then make chemicals such as callose to protect themselves if they're attacked, and the leaves of other branches also prepare for attack.
• There is some evidence that plants communicate by chemicals produced in the root systems, and that they can tell other plants if they are under water stress.

However plants do produce chemicals called volatile organic compounds (VOCs) which act as pheremones between themselves and other organisms like insects, and diffuse into the air around the plant.

• When cabbages are attacked by caterpillars, they produce a chemical which attracts a type of parasitic wasps, which lays its eggs in the caterpillars and then kills them, preventing the cabbage from being eaten.
• When apple trees are attacked by spider mites, they produce VOCs that attract predatory mites. VOCs may also signal neighbouring plants to also begin producing them.

Most plants move slowly and it cannot be seen by the naked eye, however plants such as the Mimosa pudica that folds and contracts as a response to touch, that is designed to frighten larger herbivores and dislodge insects and happens in a few seconds. the leaf folding comes as the result of chemical changes in thin walled cells found at the bases of the leaves and induvidual leaflets. These cells have elastic walls and surround a vascular bundle to form a thickened region called a pulvinus which acts as a joint. When the leaf is touched, there is an electrochemical changeb that results in potassium ions moving into the upper, flexor region of the pulvinus from the lower extensor side. Water follows the ions by osmosis so turgor in the top cells increases and decreases in the lower cells, as a result the whole leaf bends down with the help of the elastic cell walls. The reverse occurs when the plant recovers from the situation.

The growth in a specific direction as a result of environmental factors is called tropism, and involves differential plant growth cells by chemical messengers triggered by specific stimuli. To be able to maximise the use of environmental conditions, plants must respond to variations in those conditions. For example once a seed has germinated, the shoot and root need to grow in the right direction to ensure the survival of the plant, here the direction of the response is stimulated by the the direction it comes from. Much work researching tropism uses seedlings as they are easy to work with and the changes tend to affect the whole plant making the tropisms much easier to observe and measure.

Phototropism is the growth of plants in response to the direction of light, and happens as a result of auxins across the shoot or root if it is exposed to more intense light on one side or another. If plants are grown in bright, all round light in standard conditions then the plant will grow straight upwards. However if plants aare exposed to light which is stronger on one side than other, then the plant will grow towards that light and roots will grow in the opposite direction. Therefore shoots are said to be positively phototropic and roots are said to be negatively phototropic. This is a survival response as it alows the maxiumum amount of photosynthesis and so roots remain in the soil.

Effect of unilateral light: Examples of responses to unilateral light can be seen in any garden or woodland, as where plants are partially shaded the shoots grow towards the light, this response appears to be the result of the way auxin moves within the plant under the influence of light. The side of a shoot which is exposed to light has less auxin, and it appears that light causes auxin to move laterally across the shoot. This in turn stimulates cell elongation and growth on the dark side, resulting in observed growth towards the light, once the shoot is growing towards the light the unilateral stimulus is removed, and the transport of auxin is removed.

Practical investigations into phototropism: You can germinate and grow seedlings in different conditions of dark, all-round light, and unilateral light, and then observe patterns in growth. You can germinate and grow seedlings in unilateral light with different colour filters to see which wavelenghts of light trigger a phototropic response.

The fact that plants can grow more rapidly in the dark at first seems illogical. However if a plant is in the dark the biological imperative is to grow upwards rapidly to reach light in order to photosynthesise. The seedlings that break through the soil first will not have to compete with other seedlings for light. Evidence suggests that it is gibberellins that are responsible for the extreme elongation of the internodes when the plant is grown in the dark, resources can be used for synthesising leaves, strengthening stems and overall growth. Gardeners sometimes use this to force growth in plants. 

Plants are also sensitive to gravity, and the different responses of roots and shoots are very important in the control of plant growth. In normal conditions, plants recieve a unilateral gravitational stimulus, where gravity acts downwards. The response of plants to gravity can be seen in the laboratory using seedlings that have been placed on their sides, shoots are usually negatively geotropic and roots are usually positively geotropic (grow towards gravitational pull). This adaptation ensures that roots grow down into the soil and the shoots grow upwards. 

The gaseous plant hormone ethene is involved in the ripening of climacteric fruits. These fruits that continue to ripen after they have been harvested, their ripening is linked to a peak of ethene production triggering a series of chemical reactions including greatly increases respiration rate. Examples include bannanas, tomaoes and mangos. Ethene is widely used commercially in the production of perfectly ripe fruit, which are cooled and transported after being harvested long before they are ripe, as unripe fruit is much easier to transport as it gets a lot less damaged, and when the plants are needed they are exposed to ethene gas under controlled conditions to ensure that all the fruit ripen at the same rate, and this prevents wastage of over ripe fruit.

Auxin affects the the growth of both shoots and roots, and the application to cuttings stimulates growth and makes propagation from cuttings much easier, and rooting powders make it a lot easier for horticulturists to sell their cuttings. Synthetic auxins can act as weedkillers as they can cause unsustainable growth in weeds that results in their death, whilst it doesn't affect the narrow leaved crop plants, they are cheap to produce and have a low toxcicity to mammals, and so removes the weeds that compete with crops for space, light, water and minerals. 

Auxins can be used to produce seedless fruit, etehen can promote fruit dropping in plants like cotton and cherries, cytokinins can be used to prevent the ageing of ripened fruit and in micropropagation to control tissue development. Gibberellins can be used to delay ripening in fruit, and to improve the shape and size of fruits.