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برخی اطلاعات پایه در مورد محیط فیزیکی پیرامون گیاه
نویسنده : دکتر پیمان حسیبی تاریخ : پنجشنبه یکم اردیبهشت 1390

                                             PHYSICAL ENVIRONMENT II

 The physical environment of organisms is characterized by light, temperature, moisture and nutrients.

 Solar radiation involves light and thermal energy:

 Visible portion of solar energy is the energy source of photosynthesis.

  1. Infrared radiation is the source of heat and influences the thermal environment.
  2. Solar energy drives the movement of water between the earth and the atmosphere, and the water distribution in the environment.

 LIGHT

 Light is the energy source of photosynthesis, affects the distribution of organisms in land and water, and their daily activities.

 NATURE OF LIGHT

 Visible light is the portion of the electromagnetic spectrum between 400 and 740 nm.

 These wavelengths are known as the photosynthetically active radiation or PAR.

 Near infrared: 740 to 5000 nm.

  • Far infrared or thermal radiation: 5000 to 100,000 nm.
  • Ultraviolet – A: 315 to 380 nm.
  • Ultraviolet – B: 280 to 320 nm.

The ozone layer absorbs most of the UV radiation.

 The UV-B radiation decreases gradually from the tropics where the ozone layer is the thinnest to the poles where the ozone layer is the thickest.

 The UV-B radiation increases with altitude at about 14 –18% for every 1000 m.

 The ozone layer is being destroyed by chlorofluorocarbons (CFCs), mostly over the poles and tropics.

 Most of the UV-B radiation reaching the ground now is of 290-320 nm.

 The intensity of light varies with the time of the day and the season of the year.

 The angle of incidence is the angle at which light strikes a surface. It depends on the height of the sun (altitude of the sun) over the horizon.

 Highest intensity at noon; lowest at dawn and twilight.

 Light intensity varies with the season:

 On June 21, day length at 80ºN is almost 24 hours; at 40ºN, 17 hours; at 0º, 12 hours.

  • On December 21, at 40ºN, the day is about 9 hours
  • At the spring and fall equinoxes in March and September, the day is 12 hours everywhere in the world.
  • As winter approaches, the altitude of the sun drops toward the horizon.

 In mountainous country, the south-facing and west-facing slopes receive the most light.

 FATE OF LIGHT

 Light may be absorbed, reflected or transmitted by an object.

For instance:  When light hits a leaf,

 70% of infrared light is reflected.

  • 6-12% of the visible light is reflected.
  • Leaves with whitish hairs and cuticle reflect more light than deep green leaves.
  • 10-20% of the green light is reflected.
  • 3-10% of the red light is reflected.
  • 3% of the UV light is reflected.

`Most the absorbed visible radiation, 70%, is used in photosynthesis by pigments. The remaining is transmitted through the leaf.

 Transmission depends on the thickness of the leaf; thin leaves transmit more light than thick leaves.

 Plants transmit mostly green and far infrared light, those wavelengths that are reflected most.

 As light penetrates the canopy, different wavelengths as filtered out – the light becomes attenuated.

 The degree of attenuation depends on…

 Density of the canopy.

  • Optical properties of the leaf.
  • Number and sizes of the gaps in the canopy.

 Blue and red are the most absorbed wavelengths, and green and far infrared wavelengths pass through.

 The angle at which light strikes the surface of water, the greater or lesser amount is reflected.

 The higher the sun is from the horizon, the more light will penetrate.

 Water absorbs light quickly, e.g. about 40% lf light reaches a meter deep in clear lake water.

 Wavelengths are absorbed differentially: red is absorbed first, followed by yellow, green, violet and blue.

 In the clearest of seawater, only about 10% of the blue wavelengths reach more than 100 m.

 LEAF AREA INDEX AND EXTINCTION COEFFICIENT

 Light travels in small bundles of energy called photons.

 The rate at which photons strike an area is called the photon flux density.

 Daily and seasonal variations influence the rate of photosynthesis and plant growth.

 The amount of PAR (photosynthetically active radiation) is greatly influence by the reflection and absorption of radiation by other leaves.

 The vertical profile of light varies within the forest and within a single plant. The number of leaves above modifies the amount of light that passes to the layers below.

 The quantity of leaves, or foliage density, is expressed in terms of leaf area.

 Leaf area index is the one-sided leaf area per unit of ground area:

 The leaf area index (LAI):     LAI = m2 leaf area/m2 ground area

 E.g.: A leaf area index of 3 means that there are 3 square meters of leaf area over each square meter of ground area.

  • The greater the leaf area over a surface, the lower the quantity of light reaching that surface.

 The amount of sunlight decreases as the light penetrates down the vegetation layers because the amount of leaf area increases.

 Beer’s Law describes the relationship between available light and leaf area index:  AL = e-LAIik

 Moving from the top to the bottom of the canopy, the quantity of light declines exponentially.

  • i = vertical height in the canopy, e.g. i = 20 means 20 meters above the ground
  • AL = light reaching any vertical position i in the canopy.
  • e = base of natural logarithm, 2.718
  • LAIi = the cumulative leaf area above height i.
  • k = light extinction coefficient, e.g. 0.6.
  • The law states that there is an exponential dependence between the transmission of light through a substance and the concentration of the substance and also between the transmission and the length of material that the light travels through.

 Definition of Beer’s Law:

“[for August Beer], physical law stating that the quantity of light absorbed by a substance dissolved in a nonabsorbing solvent is directly proportional to the concentration of the substance and the path length of the light through the solution; the law is sometimes also referred to as the Beer-Lambert law or the Bouguer-Beer law. Beer's law is commonly written in the form A=εcl, where A is the absorbance, c is the concentration in moles per liter, l is the path length in centimeters, and ε is a constant of proportionality known as the molar extinction coefficient. The law is accurate only for dilute solutions; deviations from the law occur in concentrated solutions because of interactions between molecules of the solute, the substance dissolved in the solvent.”

The Columbia Encyclopedia, Sixth Edition. Copyright © 2006 Columbia University Press
http://education.yahoo.com/reference/encyclopedia/entry/Beerslaw

 The extinction coefficient refers to the ability of a substance to absorb light or electromagnetic radiation.

  • Substances with a low extinction coefficient allow light to pass through easily.
  • Opaque substances absorb light (extinct light) and have a high extinction coefficient.

 Leaf angle also influences the amount of light that reaches lower levels.

 In a temperate deciduous forest 1-5% of the light that strikes the canopy in the summer time reaches the ground (LAI = 3-5).

  • In a pine forest, 10-15% reaches the ground (LAI = 2-4).
  • In a tropical rain forest, 0.25-2% reaches the ground (LAI = 6-10).

 Sunflecks allow direct sunlight to reach the forest ground.

 The nature of the forest influences the amount of light that reaches the forest floor.

 Seasonal changes affect the amount of light that reaches the forest floor.

 The rate of light penetration in a body of water depends on the substances dissolved and the turbidity of the water.

 Most of the substances dissolved in lakes are organic acids produced by the decomposition of vegetation.

Turbidity greatly affects the penetration of light in aquatic environments by absorbing and scattering light.

 Clay colloids and fine detritus absorb blue light and reflect yellow wavelength.

 Heavy growth of phytoplankton blocks the light very effectively.

 TEMPERATURE

 The thermal environment of organisms consists of heat and temperature.

 Heat is form of energy that results from the random motion of molecules within the substance. Heat is a flow of energy that results in the movement of atoms and molecules, kinetic energy.

 Other definitions (http://www.thefreedictionary.com/heat):

 A form of energy associated with the motion of atoms or molecules and capable of being transmitted through solid and fluid media by conduction, through fluid media by convection, and through empty space by radiation.

  1. The transfer of energy from one body to another as a result of a difference in temperature or a change in phase.

 Temperature is a measurement of the average kinetic energy of molecules in a substance.

 ·         The temperature of a gas is equal to the translational kinetic energy of the average molecule in that sample.

·          The term, translation, means motion of the molecule as a unit.

·         The heat content of that sample of gas is the sum of its energies, internal and external, except nuclear energy, of all its molecules.

·         The internal energy of a molecule consists of the motions of the nuclei and electrons of the atoms of the molecule. The motions of the nuclei are not to be confused with nuclear energy, which is inside a nucleus.

 The amount of solar radiation that reaches the ground depends on the time of the year, slope, cloud cover, time of day, etc.

 Low temperatures slow metabolism.

 Very high temperatures denature proteins and cause death.

 Organisms capable of withstanding wide temperature variations are called eurytherms; those with a narrow range are called stenotherms.

 THERMAL ENERGY EXCHANGE

 Organisms must maintain a balance between the energy gained and the energy lost

 Energy absorbed + metabolic energy = energy lost from the body + energy stored

 Sources of energy:

 1.     Radiation: direct sunlight; diffuse sunlight scattered by water vapor and atmospheric dust; reflected sunlight from objects; long-wave infrared radiation (thermal energy) emitted by organisms, rocks, and all objects in the environment; metabolic energy emitted by organisms as infrared or long-wave energy.

 2.     Conduction: the direct transfer of heat from one object to another. It requires direct contact between objects. The amount of heat lost or gained depends on the surface area, the temperature difference between the two objects, the thickness of the insulation, and the conductivity of the materials.

 3.     Convection: the transfer of heat by the circulation of fluids either liquid or gas. The amount of heat transferred depends on the shape of the organism, the velocity of the fluid and the physical properties of the fluid.

 4.     Evaporation requires heat to change from liquid to gas. It cools the surface where evaporation is taking place.

 There is a boundary layer of still air adhering to the surface. The resistance to heat transfer offered by this layer depends on the shape and orientation of the organism, wind velocity, and difference in temperature between the surface and the air.

 Heat constantly produced by organisms is passively lost to the environment.

WATER

 Water is essential to life on Earth.

 Water is the only substance that is found naturally in all three states at the temperatures found on earth.

 STRUCTURE OF WATER

 An atom of oxygen is covalently bonded to two atoms of oxygen at a 105º angle.

 The outer energy layer of oxygen has 6 electrons. Two of these electrons are each bonded to a hydrogen atom. The other two pairs of electrons are positioned opposite to the H – O bond.

 The side of the molecule where the hydrogen atoms are located is electropositive, and the side with unshared electrons tends to be electronegative. Water is a polar molecule.

 The negative and positive sides of adjacent molecules attract each other and form hydrogen bonds between the molecules.

 Each water molecule is bound to four other molecules.

 The shape of the molecules and the intermolecular attraction produce an open tetrahedral arrangement of molecules.

 In the solid state, this arrangement of molecules makes a lattice-like that occupies more space than in the liquid form.

 The lattice partially collapses in the liquid form due to the motion of the molecules. In the liquid form, the molecules can be packed more closely together increasing the density of water.

 Water is most dense at 3.98ºC. At higher temperature, the molecules moves faster and tend to expand, to push each other apart resulting in lower density: less molecules per unit of volume.

 As energy is added to ice or liquid, the molecules change to the gas form:

 Sublimation: change from solid to gas.

  • Evaporation: change from liquid to gas.

 Seawater behaves differently from fresh water.

 Seawater is defined as water with a minimum salinity of 24.7 parts per thousand.

 The specific gravity relative to pure water is correlated with salinity.

 There is no definite freezing point for seawater. The freezing point varies with salinity.

 As water freezes, the remaining water becomes saltier which further lowers the freezing point.

 PHYSICAL PROPERTIES

 1. High specific heat

 Water stores large amount of heat with a relative small rise in temperature.

 Energy goes in to break the hydrogen bonds and not into raising the kinetic energy of the molecules.

 Therefore, water has a high specific heat, the number of calories necessary to raise 1 g of a substance 1ºC.

 The small calorie or gram calorie approximates the energy needed to increase the temperature of 1 g of water by 1 °C. This is about 4.185 J.

  • The large calorie or kilogram calorie approximates the energy needed to increase the temperature of 1 kg of water by 1 °C. This is about 4.185 kJ, and exactly 1000 small calories.

 The specific heat of pure water is by definition 1.

 Very few other substances have a specific heat higher than water, e.g. ammonia.

 Water also has the highest heat of fusion and heat of evaporation of all known substances that are liquid between 0ºC and 100ºC.

 Heat of fusion of water: 80 cal.

  • Heat of evaporation of water: 536 cal.

 2. Polarity

 Cohesion is the result of the hydrogen bonds that hold the molecules of water together. Cohesion occurs between the molecules of the substance.

 Adhesion occurs between the molecules of different substances. The polarity of water allows the formation of hydrogen bonds and other intermolecular attraction with other substances.

 Adhesion is responsible for capillarity, the ability to move through the pores of soil, conducting ducts of plants, etc.

 Non-polar molecules like oil repel water.

 3. Surface tension

 Surface tension is the result of the cohesion of water molecules. Below the surface, there are the strong attracting forces of other molecules, and above, the weak attraction of air molecules. This difference causes the water to become taut causing the spherical shape of water drops for instance.

 4. Viscosity

 Viscosity is the internal resistance of a fluid to flow.

 Layers of a liquid move at different velocities and this create what is called shear stress.

 Water has great viscosity compared to other liquids due to the hydrogen bonds.

 Water flows in concentric parallel layers. The rate of flow is greatest at the center and decreases toward the sides. This is called lateral or laminar viscosity.

 Eddy viscosity or turbulence is caused by the movement of water from one layer to another, horizontally and vertically.

Viscosity causes resistance to object moving through water.

 Viscosity has been a strong selective force in the evolution of streamlined aquatic organisms and rod-shaped features of plankton.

 A summary of the properties of water can be found here: http://www.ozh2o.com/h2phys.html

Water CYCLE

 Water is found in six forms in the biosphere:

 Vapor

  1. Fresh water (streams, lakes)
  2. Groundwater
  3. Snow and ice
  4. Salt water (oceans)
  5. Water in the body of organisms

 Distribution of water:

1.     Oceans: 97.25%

2.     Ice: 2%

3.     Groundwater: 0.7%

4.     Fresh water: <0.05%

 Fresh water:

1.     Soil moisture: 33%

2.     Lakes: 60%

3.     Atmosphere: 6%

4.     Rivers: 1%

 Groundwater fills the pores and spaces within the Earth.

 A portion of the groundwater lies below 1000 m and saline. This water is not part of the water cycle.

 LOCAL WATER CYCLE

 Precipitation: rising air cools and water vapor condenses; coalescence of droplets causes rain drops.

 Interception: water is intercepted by vegetation and never reaches the soil. Throughfall and stemflow allow the water to reach the soil.

 Infiltration: water that reaches the soil moves into the ground by infiltration. The infiltration capacity of the soil is determined by the soil porosity.

 Overlandflow occurs when the soil cannot absorb more water.

 Percolation is the movement of water deeper into the soil and accumulates as groundwater.

 Transpiration occurs through plants. Evapotranspiration is the total amount of evaporating water from the surface of the soil and vegetation.

 ELEMENTAL NUTRIENTS

 Organisms need chemical elements to live.

 Those needed in large amounts are called macronutrients.

 O, C, H, N, P, Ca, K, Mg, Na, S, Cl.

 Elements needed in small quantities are called micronutrients.

 Some of the most important micronutrients are: Fe, Mn, Co, Cu, Mo, Zn, I, Se.

 Some micronutrients are needed only by some organisms and not by others.

 Weathering of rocks is a major source of nutrients.

 Acidity of the soil affects the availability of nutrients.

 The pH scale is used to measure the acidity of the soil.

 Reference: www.cbu.edu/~esalgado/BIOL412/Ch03.doc




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