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Plant Nutrition

PHOTOSYNTHESIS
  • Photosynthesis is the fundamental process by which plants manufacture food molecules (carbohydrates) from raw materials CO2 and H2O, using energy from light.

Process of Photosynthesis

  • Green plants take CO2 through their leaves by diffusion
  • H2O is absorbed through plants’ roots by osmosis, and transported to the leaf through xylem vessels
  • Chlorophyll traps light energy and absorbs it
  • This energy is used to break up H2O molecules, then hydrogen and CO2 bond to form glucose.
  • Glucose is usually changed to sucrose for transport around the plant, or starch for storage.
  • O2 is released as a waste product, or used by the plant for respiration
  • In this process, light energy is converted to chemical energy for the formation of glucose and its subsequent storage.
Investigations on the Necessity for Chlorophyll, Light and Carbon-dioxide for Photosynthesis

CHLOROPHYLL

  • Process
    • Take a potted plant with variegated (green and white) leaves
    • Destarch the plant by keeping it in complete darkness for about 48 hours
    • Expose the plant to sunlight for a few days
    • Test one of the leaves for starch with iodine solution
  • Observations
    • Areas with previously green patches test positive (turn blue black)
    • Areas with previously pale yellow patches test negative (remain brown)
  • Conclusion
    • Photosynthesis takes place only in green patches because of the presence of chlorophyll
    • The pale yellow patches do not perform photosynthesis because of the absence of chlorophyll

LIGHT

  • Process
    • Take a potted plant
    • Destarch the plant by keeping it in complete darkness for about 48 hours
    • Test one of its leaves for starch, to check that it does not contain any
    • Fix a leaf of this plant in between two strips of a thick paper
    • Place the plant in light for a few days
    • Remove the cover from the leaf and test it for starch
  • Observations
    • Positive starch test will be obtained only in the portion of the leaf exposed to light and negative test in the parts with the paper strip
  • Conclusion
    • Light is necessary for photosynthesis

CARBON-DIOXIDE

  • Process
    • Take two destarched potted plants
    • Cover both plants with bell jars and label them as A and B
    • Inside setup A, keep NaHCO3 (sodium bicarbonate). It produces CO2
    • Inside setup B, keep NaOH (sodium hydroxide). It absorbs CO2
    • Keep both setups in the sun for at least 6 hours
    • Perform the starch test on both plants
  • Observations
    • Leaf from the plant in which NaHCO3 has been placed gives positive for starch
    • Leaf from the plant in which NaOH has been kept gives negative for starch
  • Conclusion
    • Plant in setup A gets CO2 whereas the plant in setup B does not.
    • It means CO2 is necessary for photosynthesis
Investigation and the Effects of Varying Light Intensity, Carbon-dioxide and Temperature on the Rate of Photosynthesis

LIGHT INTENSITY

  • Plants need light energy to make chemical energy needed to create carbohydrates.
  • Increasing the light intensity will increase the speed of photosynthesis.
  • However, at high light intensities, the rate becomes constant.
  • Experiment
    • Place a pond weed Elodea upside in a test tube containing water
    • Place the tube in a beaker of fresh water at 25°C. This helps to maintain a constant temperature around the pond weed
    • Place excess sodium bicarbonate in the water to give a constant saturated solution of CO2
    • Place the lamp (the only light source) at a distance from the plant
    • Count the number of oxygen bubbles given off by the plant in a 1-minute period. This is the rate of photosynthesis at that particular light intensity.
    • The gas should be checked to prove that it is indeed oxygen – relights a glowing splint
    • Repeat at different light intensities by moving the lamp to different distances
  • Explanation
    • Light energy absorbed by chlorophyll is converted to ATP and H+
    • At very low light levels, the plant will be respiring only, not photosynthesizing
    • As the light intensity increases, the rate of photosynthesis increases. However, the rate will not increase beyond a certain level of light intensity
    • At high light intensities, the rate becomes constant, even with further increases in light intensity; there are no increases in the rate
    • The plant is unable to harvest the light at these high intensities and the chlorophyll system can be damaged by very intense light levels.

CARBON-DIOXIDE CONCENTRATION

  • When the concentration of CO2 is low, the rate of photosynthesis is also low.
  • The plant has to spend time waiting for more CO2 to arrive.
  • Increasing the concentration of CO2 increases the rate of photosynthesis
  • Experiment
    • Place a pond weed Elodea upside in a test tube containing water at 25°
    • Place the tube in a beaker of fresh water
    • Place excess sodium bicarbonate in the water to give a constant saturated solution of CO2
    • Place the lamp (the only light source) at a fixed distance from the plant
    • Maintain the room temperature at 20°C
    • Count the number of oxygen bubbles given off by the plant in a 1-minute period. This is the rate of photosynthesis at that particular concentration of CO2
    • The gas should be checked to prove that it is indeed oxygen – relight a glowing splint
    • Repeat at different lower CO2 concentrations by using different dilutions of a saturated solution
    • Plot the results, placing CO2 concentration on the x-axis
  • Explanation
    • The rate of photosynthesis increases linearly with increasing CO2 concentration.
    • The rate falls gradually, and at a certain CO2 concentration, it stays constant.
    • Here, a rise in CO2 levels has no effect as the other factors such as light intensity become limiting

TEMPERATURE

  • As the temperature increases, the rate of photosynthesis increase.
  • When it reaches the optimum temperature possible, the rate of photosynthesis is at its maximum.
  • Beyond this temperature, the rate of photosynthesis falls and stops
  • Experiment
    • Place a pond weed Elodea upside in a test tube containing water at 25°
    • Place the tube in a beaker of fresh water
    • Place excess sodium bicarbonate in the water to give a constant saturated solution of CO2
    • Place the lamp (the only light source) at a fixed distance from the plant
    • Maintain the room temperature at 20°C
    • Count the number of oxygen bubbles given off by the plant in a 1-minute period. This is the rate of photosynthesis at that particular concentration of CO2
    • The gas should be checked to prove that it is indeed oxygen – relight a glowing splint
    • Repeat at different temperatures: 0°C – surround the beaker with an ice jacket; greater than room temperature (25°C, 30°C, 35°C, 40°C, 45°C, etc.) bu using a hot plate
  • Explanation
    • At low temperatures, the enzyme does not have enough energy to meet many substrate molecules, so the reaction is slowed.
    • When the temperature rises, the particles in the reaction move quicker and collide more, so the rate of photosynthesis rises also
    • At the optimum temperature, the enzyme is most efficient and the rate is maximum
    • At temperatures above 40°C, the rate slows down. This is because the enzymes involved in the chemical reactions of photosynthesis are temperature sensitive and destroyed (denatured) at higher temperatures.
Limiting Factors in Photosynthesis
  • A limiting factor is something present in the environment in such short supply that it restricts life processes.
  • Three factors can limit the speed of photosynthesis:
    • light intensity,
    • carbon-dioxide concentration, and
    • temperature
  • Sunlight
    • Light energy is vital to the process of photosynthesis
    • It is severely limiting at times of partial light conditions, e.g. dawn or dusk
    • As light intensity increases, the rate of photosynthesis will increase, until the plant is photosynthesizing as fast as it can
    • At this point, even if light becomes brighter, the plant cannot photosynthesize any faster
    • Over the first part of the curve (between A and B), light is a limiting factor. The plant is limited in how fast it can photosynthesize because it does not have enough light.
    • Between B and C, light is not a limiting factor. Even if more light is shone on the plant, it still cannot photosynthesize any faster.
  • Carbon-dioxide
    • In photosynthesis, CO2 is a key limiting factor
    • The usual atmospheric level of CO2 is 0.03%.
    • In perfect conditions of water availability, light and temperature, this CO2 level holds back the photosynthesis potential.
    • The more CO2 a plant is given, the faster it can photosynthesize up to a point, but then a maximum is reached.
Optimum Conditions for Photosynthesis in Green House
  • When plants are growing outside, we cannot much about changing the conditions that they need for photosynthesis.
  • But if crops are grown in green houses, then it is possible to control conditions so that they are photosynthesizing as fast as possible
  • CO2 Enrichment
    • CO2 concentration can be controlled.
    • CO2 is a limiting factor for photosynthesis because its natural concentration in the air is so very low (0.04%).
    • In a closed green house, it is possible to provide extra CO2 for the plants, e.g. by burning fossils fuels or releasing pure CO2 from a gas cylinder
  • Optimum Light
    • Light can also be controlled.
    • In cloudy or dark conditions, extra artificial lighting can be provided, so that light is not limiting the rate of photosynthesis.
    • The kind of lights that are used can be chosen carefully so that they provide just the right wavelengths that the plants need
  • Optimum Temperature
    • In some countries, where it is too cold for good growth of some crop plants, the heated green houses can be used.
    • This is done, for example, with tomatoes. The temperature in the green house can be kept at the optimum level to encourage the tomatoes to grow fast and strongly, and to produce a large yield of fruit that ripens quickly.
    • The temperature can be raised bu using a heating system.
    • If fossil fuels are burned, there is also a benefit from the CO2 produced
LEAF STRUCTURE
  • The leaf consists of a broad, flat part called the lamina, which is joined to the rest of the plant by a leaf stalk or petiole.
  • Running through the petiole are vascular bundles, which then form the veins in the leaf.
  • Although a leaf looks thin, it is made up of several layers of cells
  • You can see these if you look at a transverse section (cross-section) of a leaf under a microscope
Parts of Leaf
  • Cuticle
    • It is made of wax and secreted by cells of the upper epidermis.
    • It waterproofs the leaf
  • Upper Epidermis
    • These cells are thin and transparent to allow light to pass through the leaf and acts ad a barrier to disease organism.
    • They do not contain chloroplast.
  • Palisade Mesophyll
    • The main region for photosynthesis.
    • Cells are columnar (quite long) and packed with chloroplasts to trap light energy.
    • They receive carbon-dioxide by diffusion from air spaces in the spongy mesophyll
  • Spongy Mesophyll
    • These cells are more spherical and loosely packed
    • They contain chloroplasts, but not as many as in palisade cells
    • Air spaces between cells allow gaseous exchange – carbon-dioxide to the cells, oxygen from the cells during photosynthesis
  • Vascular Bundle
    • This is a leaf vein, made up of xylem, and phloem
    • Xylem vessels bring water and minerals to the leaf
    • Phloem vessels transport sugars and amino acids away (this is called translocation)
  • Lower Epidermis
    • This acts as a protective layer
    • Stomata are present to regulate the loss of water vapor (this is called transpiration)
    • It is the site of gaseous exchange into and out of the leaf
  • Stomata
    • Each stoma is surrounded by a pair of guard cells
    • These can control whether the stoma is open or closed
    • Water vapor passes out during transpiration
    • Carbon-dioxide diffuses in and oxygen diffuses out during photosynthesis
Adaptation of Leaves for Photosynthesis
  • Their broad, flat shape offers a large surface area for absorption of sunlight and carbon-dioxide
  • Most leaves are thin and the carbon-dioxide only has to diffuse across short distances to reach inner cells
  • The large spaces between cells inside the leaf provide an easy passage through which carbon-dioxide can diffuse
  • There are many stomata (pores) in the lower surface of the leaf. These allow the exchange of carbon-dioxide and oxygen with the air outside
  • There are more chloroplasts in the upper (palisade) cells than in the lower (spongy mesophyll) cells. The palisade cells, being on the upper surface, will receive most sunlight and this will reach the chloroplasts without being absorbed by too many cell walls
  • The branching network of veins provides a good water supply to the photosynthesizing cells. No cell is very far from a water-conducting vessel in one of these veins
Mineral Requirements
  • Plants need mineral ions to control chemical activities, grow, and produce materials.
  • The most important minerals are magnesium ions and nitrate ions.
  • Nitrate Ions
    • Plants absorb nitrate ions from the soil, through their root hairs.
    • Nitrate ions combine with glucose to form amino acids which make up proteins
    • Deficiency of nitrate ions causes poor growth, especially of leaves. The stem becomes weak, lower leaves become yellow and die, while upper leaves turn pale green
  • Magnesium Ions
    • Plants absorb magnesium ions from the soil solution.
    • It is used for the manufacture of chlorophyll.
    • Every chlorophyll pigment contains one magnesium atom
    • Deficiency of magnesium ions makes leaves turn yellow from the bottom of the stem upwards and eventually stops photosynthesis
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