How Light Affects Plant Growth:


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The Photosynthesis reaction that takes place in leaves

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An example of phototropism in a plant

Light is very important for plant growth. It is required for the photosynthetic reaction: 6CO2 + 6H2O -> C6H12O6 + 6O2 which provides the plant with its energy. Specifically, light is required for the first stage of photosynthesis, where the plant’s chloroplasts trap the light energy from the sun, so that it can be converted to chemical energy[1] . The main way that plants absorb this sunlight is through their leaves. Leaves absorb light emitted from the sun, which provides the complete range of wavelengths on the visible colour spectrum. The reason why leaves are green is because they don’t absorb green light, and reflect it instead. Although leaves absorb so many different colours, the two most important ones are blue and red. Blue light is very important for vegetative growth, such as leaves, while red light is responsible for flower growth when it is paired with blue light.[2] . Fun fact: another way that sunlight affects plant growth is ‘Phototropism’. Phototropism is the movement of plants based on the availability of the sun. Basically, plants will grow in the direction that will give them access to the most sunlight. This can sometimes result in plants having weird looking shapes [3] . Fun Fact: If plants are not exposed to any light then they will be able to germinate, but without being able to perform photosynthesis they won't develop properly.


Nutrients Plants Need to Grow:

Plants require a variety of different nutrients in order to be able to grow. In order for a plant to grow at its normal rate, there has to be enough nutrients available for it to use.
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A list of nutrients needed by plants to grow


Primary Nutrients

These nutrients are used extensively for plant growth and function
  1. Nitrogen: Nitrogen is used by the plant in a variety of ways. It’s needed for the creation of all the proteins and enzymes within the plant, it is a part of the chlorophyll molecule, it’s used in metabolic processes that convert energy to different forms and move it around the plant, it increases the amount of seeds and fruits produced by the plant, and it improves the quality of leaves. Most plant get their nitrogen from the air, or fertilizers.

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    Effects of different types of nutrient defficiencies

  2. Phosphorous: Phosphorous is another nutrient that is crucial for plant growth. It is required for photosynthesis, helps convert solar energy into chemical energy, and is needed for blooming and root growth. Plants get the phosphorous from sources such as bone meal, and fertilizer like superphosphate.
  3. Potassium: Potassium is the second most used mineral in most plants, next to nitrogen. It aids in photosynthesis and is part of the protein building process.
  4. Carbon, hydrogen and oxygen: All of these elements are needed for the formation of organic molecules within the plant. They are all used in photosynthesis and are required to form carbohydrates [4]. Plants get these nutrients from the CO2 and H2O that they are exposed to.

There are several other nutrients that plants use for growth, but are not required in as large quantities as these primary minerals/nutrients. These nutrients are: Calcium, magnesium, sulphur, boron, copper, chloride, iron, manganese, molybdenum, and zinc.[5]
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The pH ranges of different nutrients in soil



pH


The pH of soil dictates the availability of the nutrients that a plant needs to grow. The pH range that offers the optimal supply of nutrients is 5.5-7.0 [6] , this is because the availability of macronutrients decreases when the pH of the soil is too low, and the availability of micronutrients decreases when the pH is too high [7] . One reason why available nutrients decrease when the pH leaves this range is that the rage of 5.5 to 7.0 is the range that nitrogen producing bacteria thrive in. Another reason why this is the optimum range is that nutrients tend to leach out of soil much faster when the pH of the soil is below 5.0. Finally, this pH range also provides the proper conditions for clay in the soil. When in this range, the clay in the soil can be worked with easily by the plant and its roots, but when the soil is too acidic or basic, the soil becomes either too hard or too sticky [8] .

Some Effects of Climate Change on Plant Growth:

Global warming actually has the potential to cause an increase in the number of plants on Earth. One reason for this is that the increase of the greenhouse gas carbon dioxide in the atmosphere would cause the rate that plants can perform photosynthesis to increase, which allows for a greater supply of energy created by the plant. This increase in the occurrence of photosynthesis would also cause water use among pl
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Different climate regions on Earth. The pink and purple areas are the tropical and subtropical regions
ants to be more efficient [9] . Both of these changes would cause an increase in available organic matter for the soil, which is needed for growth. Another effect of global warming is that in tropical and subtropical regions, the soil's temperature will increase. This would create a longer period that microbes would be able to live in this soil for. These microbes can make different nutrients used by the plants. At the same time, global warming could cause a decrease in plant growth rates. In areas that could experience a significant increase in rainfall because of global warming, it's possible that the soil could become flooded and cause nutrients to leach out of the soil. The increase in events such as mud slides would also cause more plants to be damaged, which would hinder their growth rate as well . The decrease in permafrost could also create new areas for plants to grow [10] .

Soil Composition and Plant Growth


There are many different physical properties of soil that can determine whether or not a plant will be able to grow in it. One property is the wetness of the soil, which is basically how much water, and how little air is in the soil. If there's too much water, then nutrients could possibly leach out, and if there's too little air then the air supply can be used up quickly by the plant and other microbes, making the soil anaerobic, which can hinder plant growth (an example of this is that in anaerobic conditions, iron becomes soluble and migrates out of the soil). If there's too little water in the soil, then the plant also won't be able to grow, because it wouldn't have enough water needed for important plant functions like photosynthesis. The number of stones in the soil can also affect a plant’s growth. Stones are able to trap water for plants, so if there’s too few of them, then it could be difficult for plants to access water. At the same time, if there are too many stones then th
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Very Dry Soil
ere isn’t enough room for the plant’s roots to grow properly. The age of the soil also has an impact on the amount of nutrients that a plant can access. Young soil tends to not be very acidic, and have a good supply of phosphorous, potassium, calcium and magnesium. The only issue is that due to their age, there may not have been enough time for organic matter to form healthy topsoil, which can cause a nitrogen deficiency, which is not good, because plants use nitrogen for growth and require large quantities of it. Mature soils usually have similar nutrient levels as young soil, except they also contain a lot more organic matter, giving it a rich topsoil. This topsoil can provide the nitrogen that is often lacking in young soil. Ancient soils have very little potassium, magnesium, calcium, and phosphorous, but make up for this by having a very rich topsoil that can slowly provide nutrients to plants. Depending on the type of plant, different types of soil will be beneficial for its growth. For example, plants that can fix their own nitrogen can grow very easily in young soil[11] . Another soil component that can affect plant growth is the presence of earthworms in the soil. Earthworms perform many tasks that help plants grow, such as making tunnels that are lined with nutrients that roots can grow in very easily. Another key task they perform in the excretion of bacteria that are able to convert nutrients in to forms that can be used by plants. [12]
Fun fact: Given the right soil conditions, the length of the roots of a rye plant can reach up to 613 km in length.

Article Summary

In one experiment carried out in 2008, the impact of six antibiotics (chlortetracycline, tetracycline, tylosin, sulfamethoxazole, sulfamethazine and trimethoprim) on plant growth and seed germination was tested. The number of microbes remaining in the soil afterwards was also tested for. The plants that were being used in these experiments were sweet oat (not used to test plant growth), rice and cucumber. This test is pretty important for this day and age, as most antibiotics leave the body intact, and still active. This means they can enter terrestrial ecosystems through both wastewater and through manure produced by animals that had been given drugs. The effect of the drugs on the plants was measured by using root lengths as an endpoint for their seed germination tests, while roots and shoots lengths were compared for their plant growth tests. After the tests had been performed, one pattern that was observed was that the antibiotics had a negative impact on seed germination (sweet oat seeds were the most sensitive to the antibiotics), but these effects varied from plant to plant. It was also found that sulfamethoxazole and sulfamethazine caused the growth of rice to be hindered, but the other 4 drugs had no visible effects. Cucumber was also affected by sulfamethoxazole and sulfamethazine, but not as significantly. The test also found that sulfamethoxazole, sulfamethazine and trimethoprim initially eliminated a lot of the microbes living in the soil samples used. It ended up being determined that based on the concentrations of these antibiotics that nature would be exposed to, very little damage should be done to the plants, and any harmful effects would only be temporary since the soil would eventually absorb and bind these antibiotic onto soil components. These tests provided information that related well to the topic of plant growth, as they show how changing the composition of the soil does not always have a significant effect on their growth, it also shows that temporary damage to populations of bacteria in the soil doesn’t always have a large impact on growth either. It also showed how all plants' growths rely on different factors, this was proven by how not all of these 3 plants shared the same reaction when exposed to these drugs.[13]

Additional Ressources



http://school.discoveryeducation.com/schooladventures/soil/down_dirty.html
http://www.ngfl-cymru.org.uk/vtc/factors_plant_growth/eng/Introduction/MainSessionPart1.htm
http://www.ngfl-cymru.org.uk/vtc/factors_plant_growth/eng/Introduction/MainSessionPart2.htm

Glossary

  1. Chloroplasts: Organelles that are found in plant cells, as well as other eukaryotes. They contain cholorophyll.
  2. Light Energy: The energy of electromagnetic waves.
  3. Chemical Energy: The potential for a chemical to react with other substances, or transform into a new substance.
  4. Visible Colour Spectrum: Colours that can be seen by humans.
  5. Germinate: The process of a plant emerging from its seed and beginning to grow.
  6. Macronutrient: Nutrients needed by plants in the largest quantities.
  7. Micronutrients: Nutrients neede by plants in very small quantities.
  8. Leach: The process of minerals leaving soil by being dissolved in a liquid.
  9. Permafrost: Soil that has a temperature below 0 degrees Celcius
  10. Soluble: The property of a substance to dissolve in a liquid to form a homogeneous solution.
  11. Superphosphate: A type of fertilizer.
  12. pH: Short for 'Power of Hydrogen'. It's used as a measurement of how acidic something is. It is measured on a scale from 0 t o14, with 0 being the most acidic, and 14 being the least acidic.

  1. ^ Rogier M. How do Plants Use Light?[Internet].[; cited 2012 Jan 15]. Available from: http://www.gardenguides.com/74755-plants-use-light.html.
  2. ^ Environmental Factors Affecting Growth – light, temperature, water, nutrition: Light [Internet]. Corvallis (Or); [;cited 2012 Jan 15]. Available from: http://extension.oregonstate.edu/mg/botany/light.html.
  3. ^ Rogier M. How do Plants Use Light?[Internet].[; cited 2012 Jan 15]. Available from: http://www.gardenguides.com/74755-plants-use-light.html.
  4. ^ Carbon (C), Hydrogen (H), and Oxygen (O) [Internet]. Pullman (WA), Washington State University; [updated 2004 July 13; cited 2012 Jan 15]. Available from:
  5. ^ Plant Nutrients [Internet]. [;cited 2012 Jan 15]. Available from: http://www.ncagr.gov/cyber/kidswrld/plant/nutrient.htm.
  6. ^ Leonard P. pH for the Garden [Internet]. Burlington (Ve): University of Vermont; [updated 2003; cited 2012 Jan 15]. Available from: http://pss.uvm.edu/ppp/pubs/oh34.htm.
  7. ^ Plant Nutrients [Internet]. [;cited 2012 Jan 15]. Available from: http://www.ncagr.gov/cyber/kidswrld/plant/nutrient.htm.
  8. ^ Leonard P. pH for the Garden [Internet]. Burlington (Ve): University of Vermont; [updated 2003; cited 2012 Jan 15]. Available from: http://pss.uvm.edu/ppp/pubs/oh34.htm.
  9. ^ Brinkman R, Sombroek W. The effects of global change on soil conditions in relation to plant growth and food production [Internet]. Rome (Italy): Food and Agriculture Organization of the United Nations, Land and Water Development Division. 1996 [; cited 2012 Jan 15]. Available from: http://www.fao.org/docrep/w5183e/w5183e05.htm.
  10. ^ Brinkman R, Sombroek W. The effects of global change on soil conditions in relation to plant growth and food production [Internet]. Rome (Italy): Food and Agriculture Organization of the United Nations, Land and Water Development Division. 1996 [; cited 2012 Jan 15]. Available from: http://www.fao.org/docrep/w5183e/w5183e05.htm.
  11. ^ Hewitt A. Soil properties for plant growth: A guide to recognising soil attributes relevant to plant growth and plant selection. Landcare Research Science [Internet]. 2004 [cited 2012 Jan 15]; 26 (1): 3-18. Available from: http://www.landcareresearch.co.nz/publications/scienceseries/downloads/lrsciseries26_4web.pdf.
  12. ^ Edwards C. The Living Soil: Earthworms [Internet]. Ohio State University, Department of Agriculture; [; cited 2012 Jan 15]. Available from:http://soils.usda.gov/sqi/concepts/soil_biology/earthworms.html
  13. ^ Liu F, Ying GG, Tao R, Zhao JL, Yang JF, Zhao LF. Effects of six selected antibiotics on plant growth and soil microbial and enzymatic activities. Environmental Pollution [Internet].2009 [cited 2012 Jan 15]; 157(1): 1636-1642. Available from: http://it03.net/com/oxymatrine/down/1248583097.pdf.