Light Affecting Plant Growth

Aside from water and nutrients, light is most essential when growing and flourishing a plant. The amount of light received for the plant can ulitimately determine the length and time alive consisting for the plant itself. Light is necessary for all plants because they use this energy source to photosynthesize, and can easily die off if absent for an extended amount of time from light.^[1] Photosynthesis mainly permits plants to combine glucose from water and carbon dioxide while releasing oxygen. A plants chloroplasts simply trap light energy where the water can then enter into the leaf and then become chemical energy. ^[2] Plants require light energy for the process of Photosynthesis by which they convert this light energy to chemical energy.With the presence of chlorophyll in plants, they have the ability to absorb light emitted from the sun. Chlorophyll appears green in color and absorbs red and blue light. The red light has the important role of flower growth when combined with blue light, where blue light is responsible for vegetative growth.

FUN FACT: Chlorophyll is unable to absorb green light, therefore with the inability makes the leaves of plants and chlorophyll appear green to the human eye!

Sunlight also comes into play with plant growth with 'Phototropisms'. Phototropism is a plants reaction to light. The cause of phototropism is an auxin. Auxin is a hormone which explains the side of the plant with the most shade to grow longer than the other side, which then results in a plant to bend toward the light source.^[3] Without light, plants reduce their ability to develope and reproduce as much as they naturally can. Without light, photosynthesis would not take place.      





 

Photosynthesis and light.

http://amrita.olabs.co.in/?sub=79&brch=16&sim=126&cnt=1

Photosynthesis is the process in which light energy is converted into chemical energy. Using the energy of light, carbohydrates such as sugars are synthesised from carbon dioxide and water.

The name photosynthesis is derived from the Greek words, photo for ‘light’ and synthesis meaning ‘putting together’. Oxygen is also released, as a waste product. Light is the major factor for photosynthesis to take place and by doing this experiment we need to prove that light is necessary for photosynthesis.

The Process of Photosynthesis

The process of photosynthesis occurs when green plants use the energy of light to convert carbon dioxide (CO₂) and water (H₂O) into carbohydrates. Light energy is absorbed by chlorophyll, a photosynthetic pigment of the plant, while air containing carbon dioxide and oxygen enters the plant through the leaf stomata. An extremely important by-product of photosynthesis is oxygen, on which most organisms depend.

Glucose, a carbohydrate processed during photosynthesis, is mostly used by plants as an energy source to build leaves, flowers, fruits, and seeds. Molecules of glucose later combine with each other to form more complex carbohydrates such as starch and cellulose. The cellulose is the structural material used in plant cell walls. Photosynthesis provides the basic energy source for virtually all organisms.

We can express the overall reaction of photosynthesis as:

Where does Photosynthesis occur?

Photosynthesis takes place primarily in leaves and little to none occurs in stems. It takes place within specialised cell structures called chloroplasts. A leaf has a petiole or the stalk and a lamina, the flat portion of the leaf. As its area is broad, the lamina helps in the absorption of sunlight and carbon dioxide during photosynthesis. Photosynthesis takes place in the chloroplasts that have chlorophyll present in them. It is the chlorophyll that absorbs light energy from the sun. There are tiny pores called stomata that function as roadways for carbon dioxide to enter and oxygen to leave the plant.

Role of the colour of light during Photosynthesis

Did you know that the colour of light plays an important role during photosynthesis? Yes, it does. Plants use only certain colours from light for the process of photosynthesis. The chlorophyll absorbs blue, red and violet light rays. Photosynthesis occurs more in blue and red light rays and less, or not at all, in green light rays.

The light that is absorbed the best is blue, so this shows the highest rate of photosynthesis, after which comes red light. Green light cannot be absorbed by the plant, and thus cannot be used for photosynthesis. Chlorophyll looks green because it absorbs red and blue light, making these colours unavailable to be seen by our eyes. It is the green light which is not absorbed that finally reaches our eyes, making the chlorophyll appear green

The spectrum

http://www.botany.uwc.ac.za/ecotree/photosynthesis/spectrum.htm#middle 

The nature of light

The energy produced by the sun reaches the earth as electromagnetic radiation. Light and other forms of electromagnetic radiation are considered to have both a wave nature and a particle nature. Particles or packets of light (its particle nature) are known as photons - the smallest divisible units of light. The brightness of light depends on the number of photons absorbed per unit time. Each photon carries a fixed amount of energy which determines the amount that the photon vibrates. The distance moved by a photon during one of it vibrations is referred to as its wavelength and is measured in nanometres.

Electromagnetic radiation  spans a broad range of wavelengths. At the one end of the spectrum of electromagnetic radiation there are gamma rays which have a wavelength of 10^-5 nm and at the other end,  radio waves which have a wavelength of 10¹²nm. A very small part of this spectrum can be seen by the human eye i.e. between the wavelengths 380 and 750 nm. This part of the electromagnetic spectrum is called visible light. Almost all life depends ultimately on this part of the spectrum for its energy. Humans perceive the different wavelengths of visible light as different colours.
Within the spectrum the longer the wavelength of the radiation, the slower the vibration of the photons and the less energy each photon contains. Thus photons of ultraviolet light, at the blue end of the visible spectrum, have shorter wavelengths and contain more energy than red light and infrared radiation.
Sunlight contains 4% ultraviolet radiation, 52% infrared radiation and 44% visible light.
Why is only visible light used by plants ?

Light and photosynthesis.

Chlorophyll does not absorb all the wavelengths of visible light equally. Chlorophyll a, the most important light-absorbing pigment in plants, does not absorb light in the green part of the spectrum.  Light in this range of wavelengths is reflected. This is the reason why chlorophyll is green and also why plants (which contain a lot of chlorophyll) are also green. Note in the graph above that the absorption of light by chlorophyll a is at a maximum at two points on the graph 430 and 662 nm. The rate of photosynthesis at the different wavelengths of visible light also show two peaks which roughly correspond to the absorption peaks of chlorophyll a. Plants do not depend only on chlorophyll a in their light harvesting machinery but also have other pigments (accessory pigments) which absorb light of  different wavelengths.

Lettuce seeds germination

 

(..)La siguiente clave importante en la comprensión de la respuesta de las plantas a las proporciones relativas de luz y oscuridad la aportó el trabajo de los investigadores de la Estación de Beltsville, en Maryland, perteneciente al U.S.D.A. La clave se encontró en el informe de un estudio previo realizado con semillas de lechuga (Lactuca sativa). 

	Las semillas de lechuga germinan solamente si se han expuesto a la luz. Muchas semillas pequeñas tienen este requerimiento, ya que  necesitan germinar en un suelo seco y cerca de la superficie para que las plántulas aseguren su emergencia.
	Los primeros investigadores, al estudiar los requerimientos de luz para que las semillas de lechuga germinaran, demostraron que la luz roja estimulaba la germinación, y que la luz de una longitud de onda ligeramente superior (rojo lejano) la inhibía aún de forma más efectiva que la ausencia total de iluminación.

Effect of the light:
http://es.slideshare.net/KarinaPz/informe4morfo

http://www.euita.upv.es/varios/biologia/Temas/tema_15.htm

 Effect of metals:

https://www.wwoa.org/public-education/lettuce-seed-bioassay.php


 Effect of chemicals:
 https://www.google.es/url?sa=t&rct=j&q=toxicology%20experiment%3A%20seed%20bioassay&source=web&cd=1&ved=0CCQQFjAA&url=https%3A%2F%2Fwww.toxicology.org%2FISOT%2FRC%2FNLSOT%2Fdocs%2FSeedBioassay.pdf&ei=n1i9VP_SGMS0UY61grAI&usg=AFQjCNHU7IrvKTOYioJidaPFZRl6oemIcA&sig2=SGTMpS5pAjHDQlqJuxZqIQ&bvm=bv.83829542,d.d24