Plant stress measurement
Encyclopedia
Plant stress measurement is the quantification
of environmental effects on photosynthesis
, and plant health.
When plants are subjected to less than ideal growing conditions, plants are considered to be under stress. Plant stress can affect plant growth, plant survival and crop yields. As a result, financial resources and research have been used to investigate how environmental factors contribute to plant stress. Research into how heat, cold, winter, drought
, floods, lack of nutrients, herbicides, pesticides, chemicals, air pollution
, ph, disease, herbivory, metals, and light level have been both significant and on going.
Measuring plant stress is useful for many reasons including:
content meters, because they are fast, reliable for measuring specific types of plant stress, and relatively inexpensive. However, photosynthesis systems, and combinations of photosynthesis systems with chlorophyll fluorometers, are also used to measure plant stress. The instrument chosen to measure stress is decided by several factors including; the type of plant stress, the instrument’s ability to measure that type or plants stress effectively, the size of the plant population that needs to be measured for reliable results, portability requirements, and the cost of the instrument.
in combination with chlorophyll fluorometers. They are capable of measuring almost all types of plant stress tested, at effective levels. These instruments take longer to make individual measurements due to the requirement for measurement chamber equilibrium to be reached before reliable measurements can be made, As a result, smaller populations are required for study over time. Once a leaf is placed in a measuring chamber, it takes approximately forty-five seconds for the environment inside the chamber to reach equilibrium regardless of manufacturer. After that, measurements are made in close to real time. The combination of these systems with fluorometers, can be especially effective for some types of stress, and can be diagnostic in the study of cold stress and drought stress. (See the Opti-Sciences Plant Stress Guide for more details).
Because these systems are effective in measuring carbon assimilation and transpiration at lower rates, found in stressed plants, and because they measure most types of plant stress, they are often used as the standard to compare other types of instruments. Photosynthesis instruments come in field portable and laboratory versions. They are also available for measuring environmentally ambient conditions, or some systems offer variable microclimate control of the measuring chamber. Microclimate control systems allow adjustment of measuring chamber temperature, CO2 level, light level, and humidiy level for more detailed investigation.
Photosynthesis systems use infrared gas analyzer
s (IRGAS) for measuring photosynthesis. CO2
concentration changes in leaf chambers are measured to provide carbon assimilation values by leaves or a whole plants. Research has shown that the photosynthsis rate is directly related to the amount of carbon assimilated by the plant. Measuring CO2 in the air, before it enters the leaf chanber, and comparing it to air measured for CO2 after it leaves the leaf chamber, provides this value using proven equations. These systems also use IRGAs, or solid state humidity sensors, for measuring H2O changes in leaf chambers. This is done to measure leaf transpiration
, and to correct CO2 measurements. The light absorption spectrum for CO2 and H2O overlap somewhat, therefore, a correction is necessary for reliable CO2 measuring results. The critical measurement for most plant stress measurements is designated by “A” or carbon assimilation rate. When a plant is under stress, less carbon is assimilated. “A” is also known as the photosynthesis rate. CO2 IRGAs are capable of measuring to approximately +/- 1 µmol or 1ppm of CO2 . The range and requirements for H2O measurements are an order of magnitude less sensitive. Here, IRGAs and solid-state sensors measure to about +/- 1 mmols concentration.
There are different fluorescent measuring protocols, and each has their place in plant stress measurement. Research has shown that it is important to match the protocol to the type of plant stress to be measured. In addition, there are some special chlorophyll fluorescence
assays that have been developed to over come some measuring limitations that have been discovered. Most of these assays can also be used with large populations of plants. (See the Opti-Sciences Desk Top Plant Stress Guide for more details)
Chlorophyll fluorometers are designed to measure variable fluorescence of photosystem II
, or PSII. With most types of plant stress, this variable fluorescence can be used to measure the level of plant stress. The most commonly used protocols include: Fv/Fm, a dark adapted protocol, Y(II) or ΔF/Fm’ a light adapted test that is used during steady state photosynthesis, and various OJIP, dark adapted protocols that follow different schools of thought. Longer fluorescence quenching protocols can also be used for plant stress measurement, but because the time required for a measurement is extremely long, only small plant populations can probably be tested. NPQ or non-photochemical quenching is the most popular of these quenching parameters, but other parameters and other quenching protocols are also used. (For more details, refer to the OSI quenching application note)
Some protocols use dark adaptation, a process where the sample is in the dark for a period of time before measurement, to allow various photosynthetic photoprotective mechanisms and state transitions to relax. It also allows PSII to re-oxidize.
Light that is absorbed by a leaf follows three competitive pathways. It may be used in photochemistry to produce ATP and NADPH used in photosynthesis, it can be re-emitted as fluorescence, or dissipated as heat. The Fv/Fm test is designed to allow the maximum amount of the light energy to take the fluorescence pathway. It compares the dark-adapted leaf pre-photosynthetic fluorescent state, called minimum fluorescence, or Fo, to maximum fluorescence called Fm. In maximum fluorescence, the maximum number of reaction centers have been reduced or closed by a saturating light source. In general, the greater the plant stress, the fewer open reaction centers available, and the Fv/Fm ratio is lowered. Fv/Fm is a measuring protocol that works for many types of plant stress.
In Fv/Fm measurements, after dark adaption, minimum fluorescence is measured, using a modulated light source. This is a measurement of antennae fluorescence using a modulated light intensity that is too low to drive photosynthesis. Next, an intense light flash, or saturation pulse, of a limited duration, is used, to expose the sample, and close all available reaction centers. With all available reaction centers closed, or chemically reduced, maximum fluorescence is measured. The difference between maximum fluorescence and minimum fluorescence is Fv, or variable fluorescence. Fv/Fm is a normalize ratio created by dividing variable fluorescence by maximum fluorescence. It is a measurement ratio that represents the maximum potential quantum efficiency of Photosystem II if all capable reaction centers were open. An Fv/Fm value in the range of 0.79 to 0.84 is the approximate optimal value for many plant species, with lowered values indicating plant stress (Maxwell K., Johnson G. N. 2000), (Kitajima and Butler, 1975). Fv/Fm is a fast test that usually takes a few seconds. It was developed in and around 1975 by Kitajima and Butler. Dark adaptation times vary from about fifteen minutes to overnight. Some researchers will only use pre-dawn values. For a detailed discussion on dark adaptation, refer to the Opti-Sciences Dark adaptation application note.
ETR, or electron transport
rate, is also a light adapted parameter that is directly related to Y(II) by the equation, ETR = Y(II) x PAR x 0.84 x 0.5. By multiplying Y(II) by the irradiation light level in the PAR range (400 nm to 700 nm) in μmols, multiplied by the average ratio of light absorbed by the leaf 0.84, and multiplied by the average ratio of PSII reaction centers to PSI
reaction centers, 0.50 relative ETR measurement is achieved.
Relative ETR values are valuable for stress measurements when comparing one plant to another, as long as the plants to be compared have similar light absorption characteristics. Leaf absorption characteristics can vary by water content, age, and other factors. If absorption differences are a concern, absorption can be measured with the use of an integrating sphere
. For more accurate ETR values, the leaf absorption value and the ratio of PSII reaction centers to PSI reaction centers can be included in the equation. If different leaf absorption ratios are an issue, or they are an unwanted variable, then using Y(II) instead of ETR, may be the best choice. Four electrons must be transported for every CO2 molecule assimilated, or O2 molecule evolved, differences from gas exchange measurements, especially in C3 plants, can occur under conditions that promote photorespiration, cyclic electron transport, and nitrate reduction. For more detailed information concerning the relationship between fluorescence and gas exchange measurements again refer to Opti-Sciences application note #0509 on Yield measurements.
, where the photosynthetic light reaction actually takes place, has changed over the years. It is now understood that a single antennae does not link only to a single reaction center, as was previously described in the puddle model. Current evidence indicates that reaction centers are connected with shared antennae in terrestrial plants.” As a result, the parameters used to provide reliable measurements have changed to represent the newer understanding of this relationship. The model that represents the newer understanding of the antennae - reaction center relationship is called the lake model.
Lake model parameters were provided by Dave Kramer in 2004. Since then, Luke Hendrickson has provided simplified lake model parameters that allow the resurrection of the parameter NPQ, from the puddle model, back into the lake model. This is valuable because there have been so many scientific papers that have used NPQ for plant stress measurement, as compared to papers that have used lake model parameters.
For an in depth overview of quenching, refer to the OSI quenching application note.
It discusses all of the parameters used in lake models by Kramer, Hendrickson, and Klughammer. In addition it also reviews puddle model parameters, and quenching relaxation measurements.
The relationship between carbon assimilation measurements made by photosynthesis systems of the dark Calvin cycle, and measurements of variable fluorescence of photosystem II (PSII), made by chlorophyll fluorometers of the light reaction, are not always straightforward. For this reason, choosing the correct chlorophyll fluorescence protocol can also be different for C3
and C4
plants. It has been found, for example, that Y(II) and ETR are good tests for drought stress in C4
plants, but a special assay is required to measure drought stress in most C3 plants at usable levels. In C3 plants, photorespiration, and the Mehler reaction, are thought to be a principle cause. (Flexas 2000) (For more information, refer the Opti-Sciences plant stress guide.)
There are volumes of research papers available for measuring most types of plant stress using chlorophyll fluorometers, and the various protocols available.
Chlorophyll content meters are commonly used for nutrient plant stress measurement, that includes nitrogen stress, and sulfur stress. Because research has shown, that if used correctly, chlorophyll content meters are reliable for nitrogen management work, these meters are often the instruments of choice for crop fertilizer management because they are relatively inexpensive. Research has demonstrated that by comparing well fertilized plants to test plants, the ratio of the chlorophyll content index of test plants, divided by the chlorophyll content index of well fertilized plants, will provide a ratio that is an indication of when fertilization should occur, and how much should be used. It is common to use a well fertilized stand of crops in a specific field and sometimes in different areas of the same field, as the fertilization reference, due to differences from field to field and within a field. The research done to date uses either ten and thirty measurements on test and well fertilized crops, to provide average values. Studies have been done for corn and wheat. One paper suggests that when the ratio drops below 95% it is time to fertigate. The amounts of fertilizer are also recommended.
Crop consultants also use these tools for fertilizer recommendations. However, because strict scientific protocols are more time consuming and more expensive, consultants sometimes use well fertilized plants located in low-lying areas as the standard well-fertilized plants. They typically also use fewer measurements. The evidence for this approach involves anecdotal discussions with crop consultants.
Chlorophyll content meters are sensitive to both nitrogen and sulfur stress at usable levels. Chlorophyll fluorometers require a special assay, involving a high actinic light levels in combination with nitrogen stress, to measure nitrogen stress at usable levels. In addition, Chlorophyll fluorometers will only detect sulfur stress at starvation levels. For best results, chlorophyll content measurements should be made when water deficits are not present. Photosynthesis systems will detect both nitrogen and sulfur stress (OSI plant stress guide.)
Quantification
Quantification has several distinct senses. In mathematics and empirical science, it is the act of counting and measuring that maps human sense observations and experiences into members of some set of numbers. Quantification in this sense is fundamental to the scientific method.In logic,...
of environmental effects on photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
, and plant health.
When plants are subjected to less than ideal growing conditions, plants are considered to be under stress. Plant stress can affect plant growth, plant survival and crop yields. As a result, financial resources and research have been used to investigate how environmental factors contribute to plant stress. Research into how heat, cold, winter, drought
Drought
A drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region...
, floods, lack of nutrients, herbicides, pesticides, chemicals, air pollution
Air pollution
Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or cause damage to the natural environment or built environment, into the atmosphere....
, ph, disease, herbivory, metals, and light level have been both significant and on going.
Measuring plant stress is useful for many reasons including:
- Finding plants that are resistant to plant stress for breeding.
- Developing optimal plant growth protocols. Knowing what types of nutrients to add to the soil and how much should be used, can save money, and enhance crop yields.
- Determining the growing characteristics and limitations of plants under different stress conditions. For example, the effects of different amounts of herbicides and pesticides on plant health, and growth, are very valuable and can be used to reduce pollution.
- Climate range, for specific types of plants, can be studied. The effect of heat, cold, winter, drought and light level can be studied for all types of plants.
- Determination of optimal and the most economical use of water resources.
Instruments used to measure plant stress
Plant stress can be measured by different types of instrument with varying results. The most common methods of measuring plant stress involve chlorophyll fluorometers and chlorophyllChlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
content meters, because they are fast, reliable for measuring specific types of plant stress, and relatively inexpensive. However, photosynthesis systems, and combinations of photosynthesis systems with chlorophyll fluorometers, are also used to measure plant stress. The instrument chosen to measure stress is decided by several factors including; the type of plant stress, the instrument’s ability to measure that type or plants stress effectively, the size of the plant population that needs to be measured for reliable results, portability requirements, and the cost of the instrument.
Photosynthesis systems
The most expensive plant stress measuring instrument is the photosynthesis systemPhotosynthesis system
Photosynthesis systems are electronic scientific instruments designed for non-destructive measurement of photosynthetic rates in the field. Photosynthesis systems are commonly used in agronomic and environmental research, as well as studies of the global carbon cycle.- How photosynthesis systems...
in combination with chlorophyll fluorometers. They are capable of measuring almost all types of plant stress tested, at effective levels. These instruments take longer to make individual measurements due to the requirement for measurement chamber equilibrium to be reached before reliable measurements can be made, As a result, smaller populations are required for study over time. Once a leaf is placed in a measuring chamber, it takes approximately forty-five seconds for the environment inside the chamber to reach equilibrium regardless of manufacturer. After that, measurements are made in close to real time. The combination of these systems with fluorometers, can be especially effective for some types of stress, and can be diagnostic in the study of cold stress and drought stress. (See the Opti-Sciences Plant Stress Guide for more details).
Because these systems are effective in measuring carbon assimilation and transpiration at lower rates, found in stressed plants, and because they measure most types of plant stress, they are often used as the standard to compare other types of instruments. Photosynthesis instruments come in field portable and laboratory versions. They are also available for measuring environmentally ambient conditions, or some systems offer variable microclimate control of the measuring chamber. Microclimate control systems allow adjustment of measuring chamber temperature, CO2 level, light level, and humidiy level for more detailed investigation.
Photosynthesis systems use infrared gas analyzer
Infrared gas analyzer
]An infrared gas analyzer measures trace gases by determining the absorption of an emitted infrared light source through a certain air sample. Trace gases found in the Earth's atmosphere get excited under specific wavelengths found in the infrared range. The concept behind the technology can be...
s (IRGAS) for measuring photosynthesis. CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
concentration changes in leaf chambers are measured to provide carbon assimilation values by leaves or a whole plants. Research has shown that the photosynthsis rate is directly related to the amount of carbon assimilated by the plant. Measuring CO2 in the air, before it enters the leaf chanber, and comparing it to air measured for CO2 after it leaves the leaf chamber, provides this value using proven equations. These systems also use IRGAs, or solid state humidity sensors, for measuring H2O changes in leaf chambers. This is done to measure leaf transpiration
Transpiration
Transpiration is a process similar to evaporation. It is a part of the water cycle, and it is the loss of water vapor from parts of plants , especially in leaves but also in stems, flowers and roots. Leaf surfaces are dotted with openings which are collectively called stomata, and in most plants...
, and to correct CO2 measurements. The light absorption spectrum for CO2 and H2O overlap somewhat, therefore, a correction is necessary for reliable CO2 measuring results. The critical measurement for most plant stress measurements is designated by “A” or carbon assimilation rate. When a plant is under stress, less carbon is assimilated. “A” is also known as the photosynthesis rate. CO2 IRGAs are capable of measuring to approximately +/- 1 µmol or 1ppm of CO2 . The range and requirements for H2O measurements are an order of magnitude less sensitive. Here, IRGAs and solid-state sensors measure to about +/- 1 mmols concentration.
Chlorophyll fluorometers
Chlorophyll fluorometers are, for the most part, less expensive tools than photosynthesis systems, and they can be used with large populations of plants, due to their fast testing protocols. For these reasons, chlorophyll fluorometers are the most used tools for plant stress measurement, except when measuring nitrogen, and sulfur stress.There are different fluorescent measuring protocols, and each has their place in plant stress measurement. Research has shown that it is important to match the protocol to the type of plant stress to be measured. In addition, there are some special chlorophyll fluorescence
Chlorophyll fluorescence
Chlorophyll fluorescence is light that has been re-emitted after being absorbed by chlorophyll molecules of plant leaves. By measuring the intensity and nature of this fluorescence, plant ecophysiology can be investigated....
assays that have been developed to over come some measuring limitations that have been discovered. Most of these assays can also be used with large populations of plants. (See the Opti-Sciences Desk Top Plant Stress Guide for more details)
Chlorophyll fluorometers are designed to measure variable fluorescence of photosystem II
Photosystem II
Photosystem II is the first protein complex in the Light-dependent reactions. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. The enzyme uses photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce...
, or PSII. With most types of plant stress, this variable fluorescence can be used to measure the level of plant stress. The most commonly used protocols include: Fv/Fm, a dark adapted protocol, Y(II) or ΔF/Fm’ a light adapted test that is used during steady state photosynthesis, and various OJIP, dark adapted protocols that follow different schools of thought. Longer fluorescence quenching protocols can also be used for plant stress measurement, but because the time required for a measurement is extremely long, only small plant populations can probably be tested. NPQ or non-photochemical quenching is the most popular of these quenching parameters, but other parameters and other quenching protocols are also used. (For more details, refer to the OSI quenching application note)
Some protocols use dark adaptation, a process where the sample is in the dark for a period of time before measurement, to allow various photosynthetic photoprotective mechanisms and state transitions to relax. It also allows PSII to re-oxidize.
Fv/Fm
Fv/Fm tests whether or not plant stress affects photosystem II in a dark adapted state. Fv/Fm is the most used chlorophyll fluorescence measuring parameter in the world. “The majority of fluorescence measurements are now made using modulated fluorometers with the leaf poised in a known state.” (Neil Baker 2004)Light that is absorbed by a leaf follows three competitive pathways. It may be used in photochemistry to produce ATP and NADPH used in photosynthesis, it can be re-emitted as fluorescence, or dissipated as heat. The Fv/Fm test is designed to allow the maximum amount of the light energy to take the fluorescence pathway. It compares the dark-adapted leaf pre-photosynthetic fluorescent state, called minimum fluorescence, or Fo, to maximum fluorescence called Fm. In maximum fluorescence, the maximum number of reaction centers have been reduced or closed by a saturating light source. In general, the greater the plant stress, the fewer open reaction centers available, and the Fv/Fm ratio is lowered. Fv/Fm is a measuring protocol that works for many types of plant stress.
In Fv/Fm measurements, after dark adaption, minimum fluorescence is measured, using a modulated light source. This is a measurement of antennae fluorescence using a modulated light intensity that is too low to drive photosynthesis. Next, an intense light flash, or saturation pulse, of a limited duration, is used, to expose the sample, and close all available reaction centers. With all available reaction centers closed, or chemically reduced, maximum fluorescence is measured. The difference between maximum fluorescence and minimum fluorescence is Fv, or variable fluorescence. Fv/Fm is a normalize ratio created by dividing variable fluorescence by maximum fluorescence. It is a measurement ratio that represents the maximum potential quantum efficiency of Photosystem II if all capable reaction centers were open. An Fv/Fm value in the range of 0.79 to 0.84 is the approximate optimal value for many plant species, with lowered values indicating plant stress (Maxwell K., Johnson G. N. 2000), (Kitajima and Butler, 1975). Fv/Fm is a fast test that usually takes a few seconds. It was developed in and around 1975 by Kitajima and Butler. Dark adaptation times vary from about fifteen minutes to overnight. Some researchers will only use pre-dawn values. For a detailed discussion on dark adaptation, refer to the Opti-Sciences Dark adaptation application note.
Y(II) or ΔF/Fm’ and ETR
Y(II) is a measuring protocal that was developed by Bernard Genty with the first publications in 1989, and 1990. It is a light adapted test that allows one to measure plant stress while the plant is undergoing the photosynthetic process at steady state photosynthesis lighting conditions. Like FvFm, Y(II) represents a measurement ratio of plant efficiency, but in this case, it is an indication of the amount of energy used in photochemistry by photosystem II under steady-state photosynthetic lighting conditions. For most types of plant stress, Y(II) correlates to plant carbon assimilation in a linear fashion in C4 plants. In C3 plants, most types of plant stress correlate to carbon assimilation in a curve-linear fashion. According to Maxwell and Johnson, it takes between fifteen to twenty minutes for a plant to reach steady state photosynthesis at a specific light level. In the field, plants in full sun light, and not under canopy, or partly cloudy conditions, are considered to be at steady state. In this test, light irradiation levels and leaf temperature must be controlled or measured, because while the Y(II) parameter levels vary with most types of plant stress, it also varies with light level and temperature. Y(II) values will be higher at lower light levels than at higher light levels. Y(II) has the advantage that it is more sensitive to a larger number of plant stress types than Fv/Fm. (OSI Desk Top Plant Stress Guide.)ETR, or electron transport
Electron transport chain
An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...
rate, is also a light adapted parameter that is directly related to Y(II) by the equation, ETR = Y(II) x PAR x 0.84 x 0.5. By multiplying Y(II) by the irradiation light level in the PAR range (400 nm to 700 nm) in μmols, multiplied by the average ratio of light absorbed by the leaf 0.84, and multiplied by the average ratio of PSII reaction centers to PSI
Photosystem I
Photosystem I is the second photosystem in the photosynthetic light reactions of algae, plants, and some bacteria. Photosystem I is so named because it was discovered before photosystem II. Aspects of PS I were discovered in the 1950s, but the significances of these discoveries was not yet known...
reaction centers, 0.50 relative ETR measurement is achieved.
Relative ETR values are valuable for stress measurements when comparing one plant to another, as long as the plants to be compared have similar light absorption characteristics. Leaf absorption characteristics can vary by water content, age, and other factors. If absorption differences are a concern, absorption can be measured with the use of an integrating sphere
Integrating sphere
An Integrating sphere is an optical component consisting of a hollow cavity with its interior coated for high diffuse reflectivity , having relatively small holes as needed for entrance and exit ports....
. For more accurate ETR values, the leaf absorption value and the ratio of PSII reaction centers to PSI reaction centers can be included in the equation. If different leaf absorption ratios are an issue, or they are an unwanted variable, then using Y(II) instead of ETR, may be the best choice. Four electrons must be transported for every CO2 molecule assimilated, or O2 molecule evolved, differences from gas exchange measurements, especially in C3 plants, can occur under conditions that promote photorespiration, cyclic electron transport, and nitrate reduction. For more detailed information concerning the relationship between fluorescence and gas exchange measurements again refer to Opti-Sciences application note #0509 on Yield measurements.
Quenching measurements
Quenching measurements have been traditionally used for light stress, and heat stress measurements. In addition, they have been used to study plant photoprotective mechanisms, state transitions, plant photoinhibition, and the distribution of light energy in plants. While they can be used for many types of plant stress measurement, the time required, and the additional expense required for this capability, limit their use. These tests commonly require overnight dark adaptation, and fifteen to twenty minutes in lighted conditions to reach steady state photosynthesis before measurement.Puddle model and lake model quenching parameters
“Understanding of the organization of plant antennae, or plant light collection structures, and reaction centersPhotosynthetic reaction centre
A photosynthetic reaction center is a complex of several proteins, pigments and other co-factors assembled together to execute the primary energy conversion reactions of photosynthesis...
, where the photosynthetic light reaction actually takes place, has changed over the years. It is now understood that a single antennae does not link only to a single reaction center, as was previously described in the puddle model. Current evidence indicates that reaction centers are connected with shared antennae in terrestrial plants.” As a result, the parameters used to provide reliable measurements have changed to represent the newer understanding of this relationship. The model that represents the newer understanding of the antennae - reaction center relationship is called the lake model.
Lake model parameters were provided by Dave Kramer in 2004. Since then, Luke Hendrickson has provided simplified lake model parameters that allow the resurrection of the parameter NPQ, from the puddle model, back into the lake model. This is valuable because there have been so many scientific papers that have used NPQ for plant stress measurement, as compared to papers that have used lake model parameters.
For an in depth overview of quenching, refer to the OSI quenching application note.
It discusses all of the parameters used in lake models by Kramer, Hendrickson, and Klughammer. In addition it also reviews puddle model parameters, and quenching relaxation measurements.
OJIP or OJIDP
OJIP or OJIDP is a dark adapted chlorophyll fluorescence technique that is used for plant stress measurement. It has been found that by using a high time resolution scale, the rise to maximum fluorescence from minimum fluorescence has intermediate peaks and dips, designated by the OJID and P nomenclature. Over the years, there have been multiple theories of what the rise, time scale, peaks and dips mean. In addition, there is more than one school as to how this information should be used for plant stress testing (Strasser 2004), (Vredenburg 2004, 2009, 2011). Like Fv/Fm, and the other protocols, the research shows that OJIP works better for some types of plant stress than it does for others. (OSI Desk Top Plant Stress Guide).Choosing the best chlorophyll fluorescence protocol and parameter
When choosing the correct protocol, and measuring parameter, for a specific type of plant stress, it is important to understand the limitations of the instrument, and the protocol used. For example, it was found that when measuring Oak leaves, a photosynthesis system could detect heat stress at 30oC and above, Y(II) could detect heat stress at 35oC and above, NPQ could detect heat stress at 35oC and above, and Fv/Fm could only detect heat stress at 45oC and above. (Haldiman P, & Feller U. 2004) OJIP was found to detect heat stress at 44oC and above on samples tested. (Strasser 2004)The relationship between carbon assimilation measurements made by photosynthesis systems of the dark Calvin cycle, and measurements of variable fluorescence of photosystem II (PSII), made by chlorophyll fluorometers of the light reaction, are not always straightforward. For this reason, choosing the correct chlorophyll fluorescence protocol can also be different for C3
C3 carbon fixation
carbon fixation is a metabolic pathway for carbon fixation in photosynthesis. This process converts carbon dioxide and ribulose bisphosphate into 3-phosphoglycerate through the following reaction:...
and C4
C4 carbon fixation
C4 carbon fixation is one of three biochemical mechanisms, along with and CAM photosynthesis, used in carbon fixation. It is named for the 4-carbon molecule present in the first product of carbon fixation in these plants, in contrast to the 3-carbon molecule products in plants. fixation is an...
plants. It has been found, for example, that Y(II) and ETR are good tests for drought stress in C4
C4 carbon fixation
C4 carbon fixation is one of three biochemical mechanisms, along with and CAM photosynthesis, used in carbon fixation. It is named for the 4-carbon molecule present in the first product of carbon fixation in these plants, in contrast to the 3-carbon molecule products in plants. fixation is an...
plants, but a special assay is required to measure drought stress in most C3 plants at usable levels. In C3 plants, photorespiration, and the Mehler reaction, are thought to be a principle cause. (Flexas 2000) (For more information, refer the Opti-Sciences plant stress guide.)
There are volumes of research papers available for measuring most types of plant stress using chlorophyll fluorometers, and the various protocols available.
Chlorophyll content meters
These are instruments that use light transmission through a leaf, at two wavelengths, to determine the greenness and thickness of leaves. Transmission in the infrared range provides a measurement related to leaf thickness, and a wavelength in the red light range, is used to determine greenness. The ratio of the transmission of the two wavelengths provides a chlorophyll content index that is referred to as CCI or alternatively as a SPAD index. CCI is a linear scale, and SPAD is a logarithmic scale. These instruments and scales have been shown correlate to chlorophyll chemical tests for chlorophyll content except at very high levels.Chlorophyll content meters are commonly used for nutrient plant stress measurement, that includes nitrogen stress, and sulfur stress. Because research has shown, that if used correctly, chlorophyll content meters are reliable for nitrogen management work, these meters are often the instruments of choice for crop fertilizer management because they are relatively inexpensive. Research has demonstrated that by comparing well fertilized plants to test plants, the ratio of the chlorophyll content index of test plants, divided by the chlorophyll content index of well fertilized plants, will provide a ratio that is an indication of when fertilization should occur, and how much should be used. It is common to use a well fertilized stand of crops in a specific field and sometimes in different areas of the same field, as the fertilization reference, due to differences from field to field and within a field. The research done to date uses either ten and thirty measurements on test and well fertilized crops, to provide average values. Studies have been done for corn and wheat. One paper suggests that when the ratio drops below 95% it is time to fertigate. The amounts of fertilizer are also recommended.
Crop consultants also use these tools for fertilizer recommendations. However, because strict scientific protocols are more time consuming and more expensive, consultants sometimes use well fertilized plants located in low-lying areas as the standard well-fertilized plants. They typically also use fewer measurements. The evidence for this approach involves anecdotal discussions with crop consultants.
Chlorophyll content meters are sensitive to both nitrogen and sulfur stress at usable levels. Chlorophyll fluorometers require a special assay, involving a high actinic light levels in combination with nitrogen stress, to measure nitrogen stress at usable levels. In addition, Chlorophyll fluorometers will only detect sulfur stress at starvation levels. For best results, chlorophyll content measurements should be made when water deficits are not present. Photosynthesis systems will detect both nitrogen and sulfur stress (OSI plant stress guide.)
External links
- http://www.optisci.com/index.htm Main page
- http://www.optisci.com/adc_co2.htm Photosynthesis page instrument manual request
- http://www.optisci.com/ccm200.htm Chlorophyll content page
- http://www.optisci.com/requestpage.htm Stress guide page
- http://www.optisci.com/requestapps.htm Application notes
- http://envsupport.licor.com/?m=Current&menu=Photosynthesis_Systems&spec=LI-6400,Manuals Instrument manual request