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Tropisms, hormones and transport systems
Animals grow throughout their lives. But, there are many influences that trigger and hinder their growth such as availability of food and water, oxygen used in metabolism and many other factors. But, what about plants? What makes them grow the way they do, tall or short? Some plants seem to be leaning onto one side. What are the reasons for this? The answer to these questions is tropism. Tropism is a biological phenomenon which creates a growth response to certain environmental stimuli in plants. This growth can be either towards or away from the stimuli.
There are several types of tropisms, each trigger a response in plants in a different way and each is in response to different stimuli in the environment. Some common examples of tropisms are Phototropism, Geotropism, Thigmotropism, Heliotropism, Hydrotropism and Photoperiodism. Phototropism is the response in plants to or away from light or colour while Geotropism is the response towards or away from gravity. Similarly, Thigmotropism is the movement of plants due to touch or contact with something else, Heliotropism to the direction of sunlight, Hydrotropism to water, and Photoperiodism to the lengths of darkness in a photoperiod (length of day or night within a 24-hour period).
Phototropism and Geotropism act on plant parts in a certain way. Leaves, stems and shoots show positive phototropism (response towards light) to be able to gain sunlight for photosynthesis, while roots show negative phototropism (away from light) as they do not have a need for it but also to stabilize the plant. On the contrary, roots show positive geotropism (towards gravity) to gain stability, while the leaves, stems and shoots show negative geotropism (away from gravity). Roots and leaves both show positive hydrotropism (towards water) to obtain as much water as they could as it is useful for photosynthesis. Not every plant shows the responses to all of the tropisms. For example, sunflowers turn towards the direction of sunlight as a positive response called Heliotropism. Thigmotropism (reaction to touch or contact) can be seen positively in the stems of bean plants. These plants take the support of a pillar or a stick to grow in the upwards direction, so that they can gain some sunlight for photosynthesis.
Photoperiodism (reaction to the length of darkness) influences the plant’s ability to flower. Plants generally flower during the night (the periods of darkness). But, it is the length of darkness that helps the plants to flower. This also depends on the type of plant. There are three basic types of plants: short-day plants, long-day plants, and day-neutral plants. As the name itself suggests, short-day plants flower during the long periods of darkness, most likely during winter months. Examples of these types of plants are Chrysanthemums and strawberries. Long-day plants are the plants that usually flower during less periods of darkness such as in summer months. Mangoes, spinach and lettuce are some examples of these plants. Day-neutral plants do not depend on the photoperiod at all. These plants, such as Rice, Corn and Tomatoes grow all year round.
Although all of these tropisms act on a plant in a certain way, it is not possible for them to act on it without the help of hormones. Hormones are the chemical messengers made of protein that are produced in one part of plant and act on another. Some of these hormones include Auxins, Cytokinins, Gibberellins, Ethylene Gas and Abscisic acid. These hormones are the reason the plant responds to the tropisms the way it does.
For example, one type of hormones is the auxins, which are responsible for the cell elongation and phototropic response in plants. They are present in the coleoptile (tip) of the meristematic tissue (growing tissue). The parts of plant containing auxins have rapid cell growth, resulting in cell elongation. Auxins also trigger positive phototropism and negative geotropism in shoots. Light interacts with receptors that control membrane permeability to auxin (for auxin to be able to pass through the cell membrane), so that the Auxin moves away from the light. This results in a higher concentration of auxin on the dark side of the shoot, causing that region to grow at a faster rate than the side receiving the light. This causes the tip to bend towards the light. Another important function of Auxins is that they stimulate apical dominance in plants. Apical dominance is the phenomenon whereby the main central stem of the plant is dominant meaning it grows more strongly than the side stems in order to obtain as much sunlight as possible for photosynthesis. Using this phenomenon, in gardening, the coleoptile of a plant is often cut after it has grown to a certain extent, so that the plant would grow sideways and the fruit or vegetable would be easily reached.
Cytokinins are another type of hormones produced in actively growing tissues such as roots, shoots, seeds and fruits. Cytokinins stimulate cell division, cell differentiation (separating cells according to their functions) and apical dominance. They increase the rate of mitosis (division of cells), thus forming more cells. If there is an increase in the number if cells, there is an increase in plant growth. These hormones are used in industries to help certain plants grow faster and in cloning. A segment is taken from the selected plant, placed in a nutrient-enriched environment, and exposed to auxins and Cytokinins which cause it to divide and grow. The grown segment is then moved to a different medium and exposed to different ratios of plant hormones, at which point it can develop roots and then become a new plant.
Ethylene gas is the only plant hormone in a gaseous state. It is produced in all tissues such as stems, roots, flowers, seeds etc. Auxin and Abscisic acid trigger the production of ethylene gas. The main function of ethylene gas is the ageing the plant parts. Ethylene gas participates in the ripening and softening of fruits. It also causes the cellular respiration to rise, and deepens the fruit color. These color changes are useful in wild plants to attract animals that disperse fruit and make it easy for the seeds to be released. It also triggers the abscission of leaves. Abscission is the process of dropping the leaves when the plants are unable to produce food and growth regulators. It is a way of shutting-down and the sealing-off begins. Trees shed many parts besides leaves, including fruit, flowers, twigs and bark. There are two types of fruits. Climatoric fruits such as apples, bananas and avocados produce ethylene gas while non-climatoric fruits such as berries and pineapples do not. In farming, ripened climatoric fruits are often packed in plastic bags along with unripened non-climatoric ones so that the ethylene gas given off by the climatoric fruit acts on the non-climatoric fruit by ripening it. The ethylene gas released by the climatoric fruits help ripen the non-climatoric ones.
Abscisic acid is produced in the chloroplasts of leaves. This acid acts particularly acts on mature leaves, fruits and root caps. Abscisic acid or ABA kicks in when there is lack of water and the plant is dehydrated. It sends the guard cells a message to close the stomata to ensure that any more water does not escape. On the other hand, during the high water availability, ABA instructs the guard cells to open the stoma to let the water escape. Abscisic acid also increases the seed sizes and enduce dormancy. Seed dormancy is the nature’s way of setting a time clock to allow the seed to germinate when the conditions are most favorable.
Gibberellins are found in growing tissues such as shoots, roots and flowers, apical region and meristematic (growing) tissues. Their main function is to increase the overall cell growth by increasing the rate of mitosis. They also trigger stem elongation and bolting (elongation of cells rapidly). Furthermore, gibberellins enable seeds to germinate and also determine the sex of the plant. Farmers use gibberellins at times when conditions are not right to delay or reduce the flowering of plants until the conditions are favorable. In recent years, researchers have wondered if Abscisic acid could be beneficial to humans as well. A study completed in 2010 at Virginia Tech suggests that ABA might help people fight various diseases causing inflammation, including diabetes, inflammatory bowel disease, atherosclerosis and inflammatory conditions related to obesity.
As mentioned previously, hormones are produced in one part of the plant, but act on another. There is no use of them if there is no way of transporting them to the place they are required. Therefore, to enable the transportation, there are various transport systems incorporated in plants. The above described hormones are all transported through vascular bundles which consist of xylem and phloem tissue. These vessels not only transport the hormones around the plant, but also transport various substances produced as a product to a plant response. For example, with the help of Auxins, plant shoots show positive response towards sunlight and produce glucose by performing photosynthesis. However, to perform photosynthesis, water has to be transported from the roots to shoots. After photosynthesis, food produced has to be transported to all parts of the plant. Therefore, both water and food have to be transported around the plant.
Xylem tissue is made up of dead cells. These cells act as pipes to transport water and dissolved minerals to flow through them. The transportation is unidirectional meaning the water cannot be transported back to roots. Adhesion, cohesion and transpirational pull help in their transportation. Cohesion is the ‘sticking together’ of water molecules so that they form a continuous stream extending from leaves to roots. The water molecules also attach to the cellulose molecules in the walls of xylem. This process is called adhesion. Transpiration is the process that initiates the water going up. The water in the leaves evaporate due to the action of sunlight and the resulting surface tension pulls the water molecules up. The water molecules attached to the cellulose molecules in the xylem travels upwards and as the water molecules ‘stick together’, a chain of water molecules is formed called the transpirational stream. In this way, water is carried up the xylem out via leaves and keeps the pull going.
Phloem tissue consists of live cells. The purpose of phloem is to transport the food formed during photosynthesis, which is glucose to all the parts of the plant. This is two-way transportation. The transport of food molecules occurs due to osmosis (movement of water molecules from low solute concentration to high solute concentration) and turgor (pressure) in the phloem. The transportation of sugars throughout the plant is called translocation. Phloem loading is the process through which carbohydrates enter the source. Leaves and shoots are considered as source and the roots as sink. As a result of phloem loading, a high concentration of sugar develops in phloem cells near the source. This result in lowered water level compared with adjacent xylem cells, causing the water to move from xylem to phloem through osmosis. This influx of water creates a high turgor pressure near the source and low turgor pressure near the sink, causing the movement of water and sugar to the sink, which are the roots. In this way, glucose is carried to all the plant parts.
In summary, plants require hormones such as Auxins, Cytokinins, Gibberellins, Ethylene Gas and Abscisic Acid to respond to external stimuli. External stimuli also enable the plants to respond to them by growing towards or away from them. These stimuli and hormones together help the plant survive by helping it to produce certain products that are useful for them or to utilize certain things such as stability in the case of root growth. Also, the plant transport systems help in the transportation of hormones and other useful materials to different parts of the plant. Overall, the hormones, different kinds of tropisms and the transport systems are closely associated.


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