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Amplitude Modulation (AM) Process Overview

Modulation is the process of modifying the characteristic of one signal in accordance with some characteristic of another signal.
In most cases, the information signal, be it voice, video, binary data, or some other information, is normally used to modify a higher-frequency signal known as the carrier.
The information signal is usually called the modulating signal, and the higher-frequency signal which is being modulated is called the carrier or modulated wave.
The carrier is usually a sine wave, while the information signal can be of any shape, permitting both analog and digital signals to be transmitted. In most cases, the carrier frequency is considerably higher than the highest information frequency to be transmitted.
4.2 Amplitude Modulation (AM)
Amplitude modulation is the process of changing the amplitude of a relatively high frequency carrier signal in proportion with the instantaneous value of the modulating signal (information).
The carrier frequency remains constant during the modulation process but that its amplitude varies in accordance with the modulating signal. An increase in the modulating signal amplitude causes the amplitude of the carrier to increase. Both the positive and negative peaks of the carrier wave vary with the modulating signal. An increase or decrease in the amplitude of the modulating signal causes a corresponding increase or decrease in both the positive and negative peaks of the carrier amplitude.
If you interconnect the positive and negative peaks of the carrier waveform with an imaginary line, then you re-create the exact shape of the modulating information signal. This imaginary line on the carrier waveform is known as the envelope, and it is the same as the modulating signal.
Amplitude modulation that results in two sidebands and a carrier is often called double sideband amplitude modulation (DSB-AM).
In its basic form, amplitude modulation produces a signal with power concentrated at the carrier frequency and in two adjacent sidebands. Each sideband is equal in bandwidth to that of the modulating signal and is a mirror image of the other.
Amplitude modulation is inefficient in terms of power usage and much of it is wasted. At least two-thirds of the power is concentrated in the carrier signal, which carries no useful information; the remaining power is split between two identical sidebands, though only one of these is needed since they contain identical information.
4.2.1 Mathematical Representation of AM
Suppose we wish to modulate a simple sine wave on a carrier wave. The equation for the carrier wave of frequency fc, taking its phase to be a reference phase of zero, is
The equation for the simple sine wave of frequency fm (the signal we wish to broadcast) is
Amplitude modulation is performed simply by adding vm(t) to Vc. The amplitude-modulated signal is then
The formula for vam(t) above may be written
The broadcast signal consists of the carrier wave plus two sinusoidal waves each with a frequency slightly different from fc, known as sidebands. For the sinusoidal signals used here, these are at fc fm and fc ? fm. As long as the broadcast (carrier wave) frequencies are sufficiently spaced out so that these side bands do not overlap, stations will not interfere with one another.
4.2.2 Modulation Index of AM
A measure of the degree of modulation is m, the modulation index. This is usually expressed as a percentage called the percent modulation.
Modulation index is simply the ratio of the modulating signal voltage to the carrier voltage.
The modulation index should be a number between 0 and 1.
If the amplitude of the modulating voltage is higher than the carrier voltage, m will be greater than 1. This will cause severe distortion of the modulated waveform. Here, a sine wave information signal modulates a sine wave carrier, but the modulating voltage is much greater than the carrier voltage. This condition is called overmodulation. For overmodulation, the waveform is flattened near the zero line. The received signal will produce an output waveform in the shape of the envelope, which in this case is a sine wave whose negative peaks have been clipped off.
By keeping the amplitude of the modulating signal less than the carrier amplitude, no distortion will occur.
The ideal condition for AM is where Vm = Vc or m = 1, since this will produce the greatest output at the receiver with no distortion.
In the time domain, the degree of modulation for sinusoidal modulation is calculated as follows,
Since the modulation is symmetrical,
From this, it is easy to show that:
4.2.3 Amplitude Modulation Power Calculation
To communicate by radio, the AM signal is amplified by a power amplifier and fed to the antenna with a characteristic impedance, R.
The total transmitted power divides itself between the carrier and the upper and lower sidebands. This is expressed by the following equation:
The power in the sidebands depends upon the value of the modulation index. The greater the percentage of modulation, the higher the sideband power. Of course, maximum power appears in the sidebands when the carrier is 100 percent modulated. The power in each side band Ps is given by the expression:
One way to calculate the total AM power is to use the formula:
A common way to determine modulated power is to measure antenna current. Current in an antenna can be measured because accurate radio-frequency current meters are available.
If the carrier is modulated, the antenna current will be higher because of the additional power in the sidebands. The antenna current IT is:
The total AM power then is:
If the modulated and the unmodulated carrier antenna currents are known, the percentage modulation can be computed by using this formula:
Modulation by Several Sine Waves
In practice, modulation of a carrier by several sine waves simultaneously could happen.
Let V1, V2, V3, etc., be the simultaneous modulation voltages. Then the total modulating voltage Vt is:
The total modulation index would be:
If several sine waves simultaneously modulate the carrier, the carrier power will be unaffected, but the total sideband power will now be the sum of the individual sideband powers.
AM Transmitter Efficiency
AM transmitter efficiency,?:
If m=1, the AM transmitter efficiency is at the maximum.
Example 1
A 400 W carrier is modulated to a depth of 75 percent. Calculate the total power in the modulated wave.
Example 2
A broadcast radio transmitter radiates 10 kW when the modulation percentage is 60. How much of this is carrier power?
Example 3
The antenna current of an AM transmitter is 8 A when only the carrier is sent, but it increases to 8.93 A when the carrier is modulated by a single sine wave.
Find the percentage modulation.
Determine the antenna current when the percent of modulation changes to 0.8.
Example 4
A certain transmitter radiates 9 kW with the carrier unmodulated, and 10.125 kW when the carrier is sinusoidally modulated. Calculate the modulation index, percent of modulation. If another sine wave, corresponding to 40 percent modulation, is transmitted simultaneously, determine the total radiated power.
Example 5
The antenna current of an AM broadcast transmitter, modulated to a depth of 40 percent by an audio sine wave, is 11 A. It increases to 12 A as a result of simultaneous modulation by another audio sine wave. What is the modulation index due to this second wave?
4.2.4 Standard AM Transmitter
An AM transmitter can be divided into two major sections according to the frequencies at which they operate, radio-frequency (RF) and audio-frequency (AF) units.
The RF unit is the section of the transmitter used to generate the RF carrier wave.
The carrier originates in the master oscillator stage is generated as a constant-amplitude, constant-frequency sine wave. The carrier is not of sufficient amplitude and must be amplified in one or more stages before it attains the high power required by the antenna. With the exception of the last stage, the amplifiers between the oscillator and the antenna are called INTERMEDIATE POWER AMPLIFIERS (IPA). The final stage, which connects to the antenna, is called the FINAL POWER AMPLIFIER (FPA).
The second section of the transmitter contains the audio circuitry. This section of the transmitter takes the small signal from the microphone and increases its amplitude to the amount necessary to fully modulate the carrier. The last audio stage is the MODULATOR. It applies its signal to the carrier in the final power amplifier. In this way, intelligence is included in the radiated rf waveform.
4.2.5 Advantages and Disadvantages of Standard AM
The major advantage of the standard AM system is that it uses straightforward and inexpensive transmitting and receiving equipment.
However, it has several disadvantages. The three most important are as follows:
Power is wasted in the transmitted signal.
The transmitted signal requires twice the bandwidth of the transmitted intelligence.
Very precise amplitude and phase relationships between the sidebands and carrier are required.
4.2.10 Types of Radio Receivers
Various types of radio receivers have been proposed, but only two types have survived the test of time; the tuned radio frequency (TRF) receiver and the superheterodyne (superhet) receiver. Today only the superheterodyne is in general use, although the TRF may be found in some fixed-frequency applications. The TRF Receiver
The figure shows a TRF or Tuned Radio Frequency receiver. The TRF receiver offers simplicity and high sensitivity.
The TRF receiver started with an antenna, usually a long wire strung outdoors.
Then came two or more RF tuned circuits, separated by RF amplifiers. These were called RF because they all amplified the actual radio frequency (RF) signal.
Eventually came a detector, which was simply a rectifier diode and capacitor.
This was followed by an AF amplifier, because it now amplified the audio frequency signal. The audio signal then went to a speaker.
One difficulty of the TRF was that, each time you wanted to change stations, you had to retune all the tuned circuits.
A second problem had to do with the actual physical construction of the radio. If two tuned circuits were too close to each other, the two inductors would act as a transformer. Some of the amplified signal from one of the later stages would get back into an earlier stage, only to be amplified again and again. The more the tuned circuits there were, the worse the problem became. The Superheterodyne Receiver
The amplification in the superheterodyne circuit is provided in three separate sections: the RF section (extends from the antenna to the mixer), the IF section (goes from the mixer to the detector), and the AF section (extends from the detector to the speaker). The Superheterodyne Receiver
In an AM superheterodyne radio receiver, the AM signal that operates in the 535 – 1605 kHz range is received by the antenna and coupled into a tunable-circuit RF section, which must be capable of tuning over the entire broadcast band.
The frequency-conversion section, more commonly called the mixer stage, where mixing (heterodyning) of the received RF signal and the LO signal occurs. Note that the RF, mixer and LO stages are ganged (interconnected) for simultaneous tuning.
The mixer circuit is tunable over the entire broadcast band, and it is tuned to the same frequency as the RF stage for any setting on the selector dial.
The LO is also a variable-frequency stage, the frequency of which is always fixed amount higher than the RF frequency of the other two ganged stages.
The output of the mixer stage is the difference frequency is a constant value because of the relation between RF and LO tuning.
For AM, the standard difference frequency is 455 kHz. It is still a radio frequency, but to distinguish it from the received RF signal, and because it lies between the original RF carrier and AF modulating frequencies, it is termed the intermediate frequency (IF). In the process, the modulating signal contained in the original carrier signal is converted from a higher region in the RF spectrum to a lower IF region.
The IF section is designed for optimum results at the single, fixed frequency of 455 kHz. For this reason, there is no tracking problem. It can contain any number of amplifier circuits. The IF section primarily determines the sensitivity and selectivity characteristics of the superheterodyne receiver.
The amplified IF signal is coupled to the detector where the original modulating information is recovered. The detected audio signal is coupled to suitable voltage and power amplifiers, and finally to the loudspeaker load.
4.2.11 SSB Transmitter
The figure below is the block diagram of a single-sideband transmitter.
4.2.11 SSB Transmitter
The audio amplifier increases the amplitude of the incoming signal to a level adequate to operate the SSB generator. Usually the audio amplifier is just a voltage amplifier.
The SSB generator (modulator) combines its audio input and its carrier input to produce the two sidebands. The two sidebands are then fed to a filter that selects the desired sideband and suppresses the other one. By eliminating the carrier and one of the sidebands, intelligence is transmitted at a savings in power and frequency bandwidth.
In most cases SSB generators operate at very low frequencies when compared with the normally transmitted frequencies. For that reason, we must convert (or translate) the filter output to the desired frequency. This is the purpose of the mixer stage. A second output is obtained from the frequency generator and fed to a frequency multiplier to obtain a higher carrier frequency for the mixer stage. The output from the mixer is fed to a linear power amplifier to build up the level of the signal for transmission.
In ssb the carrier is suppressed (or eliminated) at the transmitter, and the sideband frequencies produced by the carrier are reduced to a minimum. You will probably find this reduction (or elimination) is the most difficult aspect in the understanding of ssb. In a single-sideband suppressed carrier, no carrier is present in the transmitted signal. It is eliminated after modulation is accomplished and is reinserted at the receiver during the demodulation process.
4.3 Frequency Modulation (FM) Principles
Frequency modulation is the process of changing the frequency of the carrier signal as the amplitude of the modulating (information) signal varies. In FM, the carrier amplitude remains constant. Frequency modulation produces pairs of sidebands spaced from the carrier in multiples of the modulating frequency.
As the modulating signal amplitude varies, the carrier frequency varies above and below its normal center frequency with no modulation. The amount of change in carrier frequency produced by the modulating signal is known as the frequency deviation. Maximum frequency deviation occurs at the maximum amplitude of the modulating signal.
4.3.1 Phase Modulation (PM)
Phase modulation produces frequency modulation. Since the amount of phase shift is varying, the effect is changing as the carrier frequency is changed. Since FM is produced by phase modulation, the latter is often referred to as indirect FM. (FM is only produced as long as the phase shift is being varied.)
4.3.3 FM Sidebands and the Modulation Index
In FM and PM, sum and difference sideband frequencies are produced. In addition, a theoretically infinite number of pairs of upper and lower sidebands are generated. As a result, the spectrum of an FM/PM signal is usually wider than an equivalent AM signal.
From the spectrum of a typical FM signal, the sidebands are spaced from the carrier fc and are spaced from one another by a frequency equal to the modulating frequency fm.
As the amplitude of the modulating signal varies, the frequency deviation will change. The number of sidebands produced, their amplitude, and their spacing depend upon the frequency deviation and modulating frequency.
Although the FM process produces an infinite number of upper and lower sidebands, only those with the largest amplitudes are significant in carrying the information. Typically any sideband whose amplitude is less than 1 percent of the unmodulated carrier is considered insignificant. As a result, this markedly narrows the bandwidth of an FM signal.
4.3.3 FM Sidebands and the Modulation Index
4.3.3 FM Sidebands and the Modulation Index
The ratio of the frequency deviation to the modulating frequency is known as the modulation index mf.
Whenever the maximum allowable frequency deviation and the maximum modulating frequency are used in computing the modulation index, mf is known as the deviation index.
Knowing the modulation index, you can compute the number and amplitudes of the significant sidebands. This is done through a complex mathematical process known as the Bessel functions.
As you can see, the spectrum of an FM signal varies considerably in bandwidth depending upon the modulation index. The higher the modulation index, the wider the bandwidth of the FM signal.
The unmodulated carrier has a relative amplitude of 1.0. With modulation, the carrier amplitude decreases while the amplitudes of the various sidebands increase. With some values of modulation index, the carrier can disappear completely.
A Graph of the Bessel Coefficients
Bessel Functions Table
4.3.3 FM Sidebands and the Modulation Index
The total bandwidth of an FM signal can be determined by knowing the modulation index and the Bessel functions. The bandwidth can then be determined with the sample formula:
Narrowband FM (NBFM) is defined as the condition where mf is small enough to make all terms after the first two in the series expansion of the FM equation negligible. Narrowband Approximation: mf = fd/fm < 0.2.
An alternative way to calculate the bandwidth of an FM signal is to use Carson’s rule. This rule takes into consideration only the power in the most significant sidebands whose amplitudes are greater than 2 percent of the carrier. Carson’s rule is given by the expression:
4.3.3 FM Sidebands and the Modulation Index
In FM and PM, increasing the amplitude or the frequency of the modulating signal will not cause overmodulation or distortion.
Increasing the modulating signal amplitude simply increases the frequency deviation. This, in turn, increases the modulation index which simply produces more significant sidebands and a wider bandwidth.
For practice reasons of spectrum conservation and receiver performance, there is usually some limit put on the upper frequency deviation and the upper modulating frequency.
The maximum deviation permitted can be used in a ratio with the actual carrier deviation to produce a percentage of modulation for FM. The FM percentage of modulation is:
When maximum deviations are specified, it is important that the percentage of modulation be held to less than 100 percent. The reason for this is that FM stations operate in assigned frequency channels. These are adjacent to other channels containing other stations. If the deviation is allowed to exceed the maximum, a greater number of pairs of sidebands will be produced and the signal bandwidth may be excessive. This can cause undesirable adjacent channel interference.
4.3.3 Advantages and Disadvantages of FM
4.3.4 FM Transmitter
The figure below is a block diagram of an fm transmitter showing waveforms found at various test points. In high-power applications you often find one or more intermediate amplifiers added between the second doubler and the final power amplifier.
4.3.4 FM Transmitter
The following shows the block diagram of a frequency-modulated transmitter. The modulating signal applied to a varicap causes the reactance to vary. The varicap is connected across the tank circuit of the oscillator. With no modulation, the oscillator generates a steady center frequency. With modulation applied, the varicap causes the frequency of the oscillator to vary around the center frequency in accordance with the modulating signal. The oscillator output is then fed to a frequency multiplier to increase the frequency and then to a power amplifier to increase the amplitude to the desired level for transmission.
4.3.5 FM Receiver
The figure below is a block diagram showing waveforms of a typical fm superheterodyne receiver.
4.3.5 FM Receiver
The amplitude of the incoming signal is increased in the RF stages.
The mixer combines the incoming RF with the local oscillator signal to produce the intermediate frequency, which is then amplified by one or more IF amplifier stages.
The FM receiver has a wide-band IF amplifier. The bandwidth for any type of modulation must be wide enough to receive and pass all the side-frequency components of the modulated signal without distortion. The IF amplifier in an FM receiver must have a broader bandpass than an AM receiver.
There are two fundamental sections of the FM receiver that are electrically different from the AM receiver. These are the discriminator (detector) and the limiter.
In FM receivers, a DISCRIMINATOR is a circuit designed to respond to frequency shift variations. A discriminator is preceded by a LIMITER circuit, which limits all signals to the same amplitude level to minimize noise interference. The audio frequency component is then extracted by the discriminator, amplified in the AF amplifier, and used to drive the speaker.
4.3.6 Phase-Locked Loop Demodulator
The development of ICs has made the phase-locked loop (PLL) increasingly popular as an FM demodulator. The PLL offers many advantages over the other types of demodulator.
It requires no costly inductors or transformers, eliminating the need for intricate and time-consuming coil adjustments.
It provides excellent performance at low cost with a minimum of external components.
A basic PLL consists of a phase detector, a dc amplifier, an LP filter, and a voltage-controlled oscillator (VCO).
The VCO operates at the input frequency. The phase detector compares the input and VCO frequencies. The phase detector then develops an error voltage proportional to the amount and direction of the frequency difference. The dc amplifier increases the error voltage to a level needed to drive the VCO. The error signal is then coupled to the LP filter.
The filter sets many of the dynamic characteristics of the PLL. It determines the frequency range over which the loop will capture and hold its phase lock, and it determines the speed with which the loop will respond to variations of the input frequency.
The error voltage from the filter is used to control the VCO. For example, if the input frequency swings above fs (source frequency), an error voltage generated by the phase detector is amplified, fed to the filter, and applied to the VCO. The error voltage will cause the VCO frequency to increase in an exact lock with the input frequency. When the input signal is frequency modulated, the VCO tracks the FM deviation exactly, and the resulting error voltage is an exact reproduction of the intelligence signal.

Effect of Light Intensity on Photosynthesis

Photosynthesis is the process by which a plant cell converts carbon dioxide (CO2) to food sugars via light energy. The aim of the experiment was to determine whether different light intensities have different effects on the rate of photosynthesis of two plants, one grown in normal light conditions and one grown in shade conditions. Each plant was placed in a sealed plastic box and was allowed to acclimatise. A Vernier CO2 electrode was used to measure the concentration of CO2 at one minute time intervals for each plant over a period of five minutes. This was repeated for different light intensities which were measured using a light sensor. Overall the rate of change in CO2 concentrations decreased for both plants. At certain light intensities the rate of change in CO2 levels was a negative value (approximately 45 arbitrary units for shade grown plants and 80 arbitrary units for light grown plants). This implies that from this point at greater light intensities the net rate of photosynthesis became greater than the net rate of respiration. The rate of change in CO2 levels for the light grown plant is greater than that of the shade grown plant for light intensities greater than 50 arbitrary units. This implies that the light grown plant was able to achieve a higher rate of photosynthesis than the shade grown plant.
Photosynthesis is the process by which a green plant converts light energy to chemical energy thereby creating food sugars (Johnson G, 2006). Plants are the main energy source of every food chain, making photosynthesis one of the most important biological processes (Raven P. et al, 1986). It is also believed that many forests, due to photosynthesis, can act as carbon sinks by net absorption of carbon dioxide and may be able to help prevent climate change (Johnson G, 2006).
During C3 photosynthesis, carbon dioxide (CO2) is taken in from the atmosphere, via stomata, to provide carbon for the carbohydrate food source produced (Raven P. et al, 1986). Water is also required for photosynthesis. A molecule of water needs to be split to release an electron for an electron transport chain which reduces NADP and ADP to NADPH and ATP (Raven P. et al, 1986). This is known as the light dependent stage of photosynthesis since light energy is required to hydrolyse the molecule of water. The next stage of photosynthesis is known as the light independent stage (Johnson G, 2006). This where CO2 is taken in and fixed via the Calvin cycle to create organic compounds (Raven P. et al, 1986). The coenzymes that are reduced by the electron transport chain in the light dependent reaction are needed for these reactions. Oxygen is produced as a by-product of the hydrolysis of water.
Various species of plant adapted to warmer climates have slightly alternative methods of photosynthesising; The C4 and CAM pathways (Johnson G, 2006). These pathways are essentially the same as the C3 pathway in that CO2 is converted to food sugars via light. However, the CAM pathway is separated in time and the C4 pathway is separated spatially (Raven P. et al, 1986). A plant species that photosynthesises using the C3 pathway was chosen for this experiment since this is deemed the standard, non-adapted form of photosynthesis (Johnson G, 2006, Raven P. et al, 1986).
Many plants can adapt to both shade conditions (low light conditions) and full light conditions (Salisbury and Ross, 1992). These adaptations are designed to optimise photosynthesis under these different light conditions. Plants adapted to shade may not be able to photosynthesis at the same rate as light grown plants at high light intensities and vice versa. The lack of light in shade environments can lead to stunted and slower growth (Johnson G, 2006).
This study aims to determine whether the amount of light, and therefore light energy available, affects the rate of photosynthesis by comparing shade and light grown plants. This can be achieved by measuring the change in CO2 concentration of each plant in a sealed environment over a given time period at different light intensities. CO2 is absorbed during photosynthesis, therefore a decrease in CO2 concentrations would demonstrate that photosynthesis was occurring. The study will also determine whether plants grown in light or shade conditions have the ability to photosynthesize equally at different light intensities.
Materials and Methods
Two plants of the same species were used. One was grown under shade (low light) conditions. The other was grown under direct light (high light) conditions. The leaves of each plant were placed in a sealed plastic box and were allowed to acclimatise. A Vernier CO2 electrode was used to measure the concentration of CO2 at one minute time intervals for five minutes for each plant. This was repeated for different light intensities (0, 10, 50, 150, 200, 300 and 400 arbitrary units) which were measured using a light sensor. To ensure the temperature remained constant a temperature sensor was used to monitor the temperature. The concentration of CO2 was logged onto a computer. To calculate the rate of change in CO2 concentration, the difference in CO2 concentration was plotted against time graphically and a tangent taken.
The rate of change in CO2 concentration was calculated for the different light intensities for each plant. This was then displayed graphically to allow comparison of the results (Figure 1).
Figure 1. The rate of change in CO2 concentrations for each light intensity. The two plants are shown (shade grown and light grown) for comparison. The compensation points for both plants are labelled. The compensation for the shade grown plant is further to the left than the light grown plant.
The compensation point for the light grown plant.
The compensation point for the shade grown plant.
Overall the rate of change in CO2 concentrations decreased for both plants. At a light intensity of 50 arbitrary units for the shade grown plant, and 150 arbitrary units for the light grown plant, the rate of change in CO2 levels was a negative value (-1.5, -10.4). This implies that the net rate of photosynthesis was greater than the net rate of respiration. The rate of change in CO2 levels for the light grown plant is greater than that of the shade grown plant for light intensities greater than 50 arbitrary units. The rate of change in CO2 levels increased at 400 arbitrary units for both plants.
A reduction in the concentration of CO2 over time implies that photosynthesis is occurring. This would give a negative rate of change in CO2 levels. As such, the shade grown plant has a higher rate of photosynthesis at lower light intensities than the light grown plant (Figure 1). At higher light intensities this changes. The light grown plant has a higher rate of photosynthesis at higher light intensities than the shade grown plant (Figure 1). At approximately 300 arbitrary units the light grown plant reaches its light saturation point (Figure 1). This is the point where another factor becomes limiting, for example the amount of CO2 available (Raven P. et al, 1986). The shade grown plant reaches its saturation point at approximately 200 arbitrary units (Figure 1). The rate of change in CO2 concentration is less for the shade grown plant than the light grown plant.
The shade grown plant appears to be better adapted to photosynthesising at low light intensities than the light grown plant. One of these adaptations may be having thinner, larger leaves (Salisbury and Ross, 1992). A larger surface area would allow the chloroplasts to be arranged, by phototaxis, into patterns that would maximise light absorption (Salisbury and Ross, 1992). Leaves from the plant grown at high light intensities might be thicker since they are more likely to have a waxy protective coating to prevent solarization of the chlorophyll pigments (Salisbury and Ross, 1992).
Plants grown in the shade are more likely to invest resources in light harvesting equipment such as granna and thylakoids than enzymes that are required for the Calvin Cycle (Salisbury and Ross, 1992). This is because in the shade light will be the main limiting factor to the rate of photosynthesis as opposed to CO2 levels. As such when a shade grown plant is exposed to high light intensities the rate of photosynthesis cannot increase dramatically since the rate of the light independent reactions cannot increase. This is another reason why the rate of photosynthesis in the shade grown plant doesn’t increase as much as the light grown plant (Figure 1).
The rate of photosynthesis decreases at 400 arbitrary units for both plants (Figure1). This is a very high light intensity and as such denatures chlorophyll and the photosynthetic apparatus by solarization. The light grown plant would be more likely to have certain carotenoid pigments which could convert the excess light energy into heat energy and thus prevent the solarization of chlorophylls (Salisbury and Ross, 1992). This would explain why the light grown plant has a higher rate of photosynthesis at higher light intensities than the shade grown plant (Figure 1).
The compensation point is the point at which the rate of photosynthesis and the rate of respiration are equal (Salisbury and Ross, 1992). As such there is no change in the concentration of CO2. Only when the light intensity is higher than the compensation point can photosynthesis occur (Salisbury and Ross, 1992). The compensation point for the shade grown plant is much lower than the compensation point for the light grown plant. This would be expected, since under shade conditions it is more beneficial to have a low compensation point, because it allows photosynthesis to occur in low light levels (Salisbury and Ross, 1992).
Future studies may benefit from testing a wider variety of C3 plants to see if the conclusions found here are specific to this species of plant or to C3 plants in general. Also it may be benifitial to use the whole plant rather than just the leaves. Repeat cycles of the experiment would confirm that the results are consistent and not linked to a failure in the control environment or equipment on a one off basis. The results discussed here were based on just the leaves rather than the plant as a whole. As such firm conclusions cannot be drawn.