Plant stress measurement

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Schematic overview of the classes of stresses in plants Classes of stresses that plants are exposed to.jpg
Schematic overview of the classes of stresses in plants

Plant stress measurement is the quantification of environmental effects on plant health. When plants are subjected to less than ideal growing conditions, they are considered to be under stress. Stress factors can affect growth, survival and crop yields. Plant stress research looks at the response of plants to limitations and excesses of the main abiotic factors (light, temperature, water and nutrients), and of other stress factors that are important in particular situations (e.g. pests, pathogens, or pollutants). Plant stress measurement usually focuses on taking measurements from living plants. It can involve visual assessments of plant vitality, however, more recently the focus has moved to the use of instruments and protocols that reveal the response of particular processes within the plant (especially, photosynthesis, plant cell signalling and plant secondary metabolism)

Contents

Instruments used to measure plant stress

Measurements can be made from living plants using specialised equipment. Among the most commonly used instruments are those that measure parameters related to photosynthesis (chlorophyll content, chlorophyll fluorescence, gas exchange) or water use (porometer, pressure bomb). In addition to these general purpose instruments, researchers often design or adapt other instruments tailored to the specific stress response they are studying.

Photosynthesis systems

Photosynthesis systems use infrared gas analyzers (IRGAS) for measuring photosynthesis. CO2 concentration changes in leaf chambers are measured to provide carbon assimilation values for leaves or whole plants. Research has shown that the rate of photosynthesis is directly related to the amount of carbon assimilated by the plant. Measuring CO2 in the air, before it enters the leaf chamber, 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. [2] 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. [3] CO2 IRGAs are capable of measuring to approximately +/- 1 μmol or 1ppm of CO2.

Because these systems are effective in measuring carbon assimilation and transpiration at low rates, as found in stressed plants, [4] they are often used as the standard to compare to other types of instruments. [5] Photosynthesis instruments come in field portable and laboratory versions. They are also designed to measure ambient environmental conditions, and some systems offer variable microclimate control of the measuring chamber. Microclimate control systems allow adjustment of the measuring chamber temperature, CO2 level, light level, and humidity level for more detailed investigation.

The combination of these systems with fluorometers, can be especially effective for some types of stress, and can be diagnostic, e.g. in the study of cold stress and drought stress. [6] [3] [7]

Chlorophyll fluorometers

Chlorophyll fluorescence emitted from plant leaves gives an insight into the health of the photosynthetic systems within the leaf. Chlorophyll fluorometers are designed to measure variable fluorescence of photosystem II. This variable fluorescence can be used to measure the level of plant stress. The most commonly used protocols include those aimed at measuring the photosynthetic efficiency of photosystem II, both in the light (ΔF/Fm') and in a dark-adapted state (Fv/Fm). Chlorophyll fluorometers are, for the most part, less expensive tools than photosynthesis systems, they also have a faster measurement time and tend to be more portable. For these reasons they have become one of the most important tools for field measurements of plant stress.

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) [6] [8]

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. [3] 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. [9] [10] [3]

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 normalized 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). [11] 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. [9] [3]

Y(II) or ΔF/Fm' and ETR

Y(II) is a measuring protocol that was developed by Bernard Genty with the first publications in 1989 and 1990. [12] [13] 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 and twenty minutes for a plant to reach steady-state photosynthesis at a specific light level. In the field, plants in full sunlight, 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. [12] [13] 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.[ citation needed ]

ETR, or electron transport rate, is also a light-adapted parameter that is directly related to Y(II) by the equation, ETR = Y(II) × PAR × 0.84 × 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, [5] [14] [15] relative ETR measurement is achieved. [16]

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. [3] Leaf absorption characteristics can vary by water content, age, and other factors. [3] If absorption differences are a concern, absorption can be measured with the use of an integrating sphere. [10] 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. [6] [3] [17]

Quenching measurements

Quenching measurements have been traditionally used for light stress, and heat stress measurements.[ citation needed ] In addition, they have been used to study plant photoprotective mechanisms, state transitions, plant photoinhibition, and the distribution of light energy in plants.

Puddle model and lake model quenching parameters

Lake model parameters were provided by Dave Kramer in 2004. [18] 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. [19] [20]

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). [3] [21] [22] [23] [24] 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.[ citation needed ]

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. [25] [26] CCI is a linear scale, and SPAD is a logarithmic scale. [25] [26] These instruments and scales have been shown to correlate to chlorophyll chemical tests for chlorophyll content except at very high levels. [25] [26]

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. [27] [28] 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[ clarification needed ] 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. [27] [28]

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 level in combination with nitrogen stress, to measure nitrogen stress at usable levels. [29] In addition, chlorophyll fluorometers will only detect sulfur stress at starvation levels. [10] [3] For best results, chlorophyll content measurements should be made when water deficits are not present.[ citation needed ] Photosynthesis systems will detect both nitrogen and sulfur stress.[ citation needed ]

See also

Related Research Articles

<span class="mw-page-title-main">Chlorophyll</span> Green pigments found in plants, algae and bacteria

Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words χλωρός, khloros and φύλλον, phyllon ("leaf"). Chlorophyll allow plants to absorb energy from light.

<span class="mw-page-title-main">Photosynthesis</span> Biological process to convert light into chemical energy

Photosynthesis is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their activities. Photosynthetic organisms use intracellular organic compounds to store the chemical energy they produce in photosynthesis. Photosynthesis is usually used to refer to oxygenic photosynthesis, a form of photosynthesis where the photosynthetic processes produce oxygen as a byproduct and synthesize carbohydrate molecules like sugars, starches, glycogen, and cellulose to store the chemical energy. To use the chemical energy stored in these organic compounds, the organisms' cells metabolize the organic compounds through another process called cellular respiration. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and it supplies most of the biological energy necessary for complex life on Earth.

<span class="mw-page-title-main">Nitrogen deficiency</span> Nutrient deficiency

Nitrogen deficiency is a deficiency of nitrogen in plants. This can occur when organic matter with high carbon content, such as sawdust, is added to soil. Soil organisms use any nitrogen available to break down carbon sources, making nitrogen unavailable to plants. This is known as "robbing" the soil of nitrogen. All vegetables apart from nitrogen fixing legumes are prone to this disorder.

<span class="mw-page-title-main">Photosystem</span> Structural units of protein involved in photosynthesis

Photosystems are functional and structural units of protein complexes involved in photosynthesis. Together they carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons. Photosystems are found in the thylakoid membranes of plants, algae, and cyanobacteria. These membranes are located inside the chloroplasts of plants and algae, and in the cytoplasmic membrane of photosynthetic bacteria. There are two kinds of photosystems: PSI and PSII.

Chlorophyll <i>a</i> Chemical compound

Chlorophyll a is a specific form of chlorophyll used in oxygenic photosynthesis. It absorbs most energy from wavelengths of violet-blue and orange-red light, and it is a poor absorber of green and near-green portions of the spectrum. Chlorophyll does not reflect light but chlorophyll-containing tissues appear green because green light is diffusively reflected by structures like cell walls. This photosynthetic pigment is essential for photosynthesis in eukaryotes, cyanobacteria and prochlorophytes because of its role as primary electron donor in the electron transport chain. Chlorophyll a also transfers resonance energy in the antenna complex, ending in the reaction center where specific chlorophylls P680 and P700 are located.

<span class="mw-page-title-main">Photosynthetic reaction centre</span> Molecular unit responsible for absorbing light in photosynthesis

A photosynthetic reaction center is a complex of several proteins, pigments, and other co-factors that together execute the primary energy conversion reactions of photosynthesis. Molecular excitations, either originating directly from sunlight or transferred as excitation energy via light-harvesting antenna systems, give rise to electron transfer reactions along the path of a series of protein-bound co-factors. These co-factors are light-absorbing molecules (also named chromophores or pigments) such as chlorophyll and pheophytin, as well as quinones. The energy of the photon is used to excite an electron of a pigment. The free energy created is then used, via a chain of nearby electron acceptors, for a transfer of hydrogen atoms (as protons and electrons) from H2O or hydrogen sulfide towards carbon dioxide, eventually producing glucose. These electron transfer steps ultimately result in the conversion of the energy of photons to chemical energy.

The photosynthetic efficiency is the fraction of light energy converted into chemical energy during photosynthesis in green plants and algae. Photosynthesis can be described by the simplified chemical reaction

In biophysics, the Kautsky effect is a phenomenon consisting of a typical variation in the behavior of a plant fluorescence when exposed to light. It was discovered in 1931 by H. Kautsky and A. Hirsch.

<span class="mw-page-title-main">Photoinhibition</span>

Photoinhibition is light-induced reduction in the photosynthetic capacity of a plant, alga, or cyanobacterium. Photosystem II (PSII) is more sensitive to light than the rest of the photosynthetic machinery, and most researchers define the term as light-induced damage to PSII. In living organisms, photoinhibited PSII centres are continuously repaired via degradation and synthesis of the D1 protein of the photosynthetic reaction center of PSII. Photoinhibition is also used in a wider sense, as dynamic photoinhibition, to describe all reactions that decrease the efficiency of photosynthesis when plants are exposed to light.

<span class="mw-page-title-main">Fluorometer</span>

A fluorometer, fluorimeter or fluormeter is a device used to measure parameters of visible spectrum fluorescence: its intensity and wavelength distribution of emission spectrum after excitation by a certain spectrum of light. These parameters are used to identify the presence and the amount of specific molecules in a medium. Modern fluorometers are capable of detecting fluorescent molecule concentrations as low as 1 part per trillion.

Non-photochemical quenching (NPQ) is a mechanism employed by plants and algae to protect themselves from the adverse effects of high light intensity. It involves the quenching of singlet excited state chlorophylls (Chl) via enhanced internal conversion to the ground state, thus harmlessly dissipating excess excitation energy as heat through molecular vibrations. NPQ occurs in almost all photosynthetic eukaryotes, and helps to regulate and protect photosynthesis in environments where light energy absorption exceeds the capacity for light utilization in photosynthesis.

<span class="mw-page-title-main">Light-dependent reactions</span> Photosynthetic reactions

Light-dependent reactions are certain photochemical reactions involved in photosynthesis, the main process by which plants acquire energy. There are two light dependent reactions: the first occurs at photosystem II (PSII) and the second occurs at photosystem I (PSI).

<span class="mw-page-title-main">Photosynthesis system</span> Instruments measuring photosynthetic rates

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.

<span class="mw-page-title-main">Chlorophyll fluorescence</span> Light re-emitted by chlorophyll molecules during return from excited to non-excited states

Chlorophyll fluorescence is light re-emitted by chlorophyll molecules during return from excited to non-excited states. It is used as an indicator of photosynthetic energy conversion in plants, algae and bacteria. Excited chlorophyll dissipates the absorbed light energy by driving photosynthesis, as heat in non-photochemical quenching or by emission as fluorescence radiation. As these processes are complementary processes, the analysis of chlorophyll fluorescence is an important tool in plant research with a wide spectrum of applications.

The nutrient content of a plant can be assessed by testing a sample of tissue from that plant. These tests are important in agriculture since fertilizer application can be fine-tuned if the plants nutrient status is known. Nitrogen most commonly limits plant growth and is the most managed nutrient.

Plant breeding is process of development of new cultivars. Plant breeding involves development of varieties for different environmental conditions – some of them are not favorable. Among them, heat stress is one of such factor that reduces the production and quality significantly. So breeding against heat is a very important criterion for breeding for current as well as future environments produced by global climate change.

<span class="mw-page-title-main">Chlororespiration</span>

Chlororespiration is a respiratory process that takes place within plants. Inside plant cells there is an organelle called the chloroplast which is surrounded by the thylakoid membrane. This membrane contains an enzyme called NAD(P)H dehydrogenase which transfers electrons in a linear chain to oxygen molecules. This electron transport chain (ETC) within the chloroplast also interacts with those in the mitochondria where respiration takes place. Photosynthesis is also a process that Chlororespiration interacts with. If photosynthesis is inhibited by environmental stressors like water deficit, increased heat, and/or increased/decreased light exposure, or even chilling stress then chlororespiration is one of the crucial ways that plants use to compensate for chemical energy synthesis.

<span class="mw-page-title-main">Govindjee</span> Indian-American biochemist (born 1932)

Govindjee is an Indian-American scientist and educator. He is Professor Emeritus of Biochemistry, Biophysics and Plant Biology at the University of Illinois at Urbana-Champaign, where he taught from 1961 until 1999. As Professor Emeritus since 1999, Govindjee has continued to be active in the field of photosynthesis through teaching and publishing. He is recognized internationally as a leading expert on photosynthesis.

<span class="mw-page-title-main">Dualex</span>

The Dualex is an optical sensor developed by Force-A for the assessment of flavonol, anthocyanin, and chlorophyll contents in leaves. The sensor is a result of technology transfer from the CNRS and University of Paris-Sud Orsay. It allows to perform real-time and non-destructive measurements. The main applications are plant science and agriculture research.

A light-induced fluorescence transient (LIFT) is a device to remotely measure chlorophyll fluorescence in plants in a fast and non-destructive way. By using a series of excitation light pulses, LIFT combines chlorophyll fluorescence data with spectral and RGB information to provide insights into various photosynthetic traits and vegetation indices. LIFT combines the pump-probe method with the principle of laser-induced fluorescence.

References

  1. ↑ Shulaev, Vladimir; Cortes, Diego; Miller, Gad; Mittler, Ron (2008-01-09). "Metabolomics for plant stress response". Physiologia Plantarum. 132 (2): 199–208. doi:10.1111/j.1399-3054.2007.01025.x. ISSN   0031-9317. PMID   18251861.
  2. ↑ Long S.P., Farage P.K., Garcia R.L., (1996) Measurement of leaf and canopy photosynthetic CO2 exchange in the field, Journal of Experimental Botany, Vol. 47, No. 304, pp. 1629-1642
  3. 1 2 3 4 5 6 7 8 9 10 Baker N.R. (2008) Chlorophyll Fluorescence: A Probe of Photosynthesis In Vivo, Annu. Rev. Plant Biol.2008. 59:89–113
  4. ↑ Long S.P., and Bernacchi C.J. (2003) Gas exchange measurements, what can they tell us about the underlying limitations to photosynthesis? Procedures and sources of error Journal of Experimental Botany, Page 1 of 9
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  7. ↑ Fryer M. J., Andrews J.R., Oxborough K., Blowers D. A., Baker N.E. (1998) Relationship between CO2 Assimilation, Photosynthetic Electron Transport, and Active O2 Metabolism in Leaves of Maize in the Field during Periods of Low Temperature. Plant Physiol. (1998) 116:571–580
  8. ↑ Rosyara, U. R., S. Subdedi, R. C. Sharma and E. Duveiller. 2010. Photochemical Efficiency and SPAD Value as Indirect Selection Criteria for Combined Selection of Spot Blotch and Terminal Heat Stress in Wheat. Journal of Phytopathology Volume 158, Issue 11-12, pages 813–821
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  11. ↑ Kitajima M, Butler WL (1975) Quenching of chlorophyll fluorescence and primary photochemistry in chloroplasts by dibromothymoquinone. Biochim Biophys Acta 376:105-115
  12. 1 2 Genty B., Briantais J. M. & Baker N. R. (1989) The relationship between the quantum yield of photosynthetic electron transport and quenching of chlorophyll fluorescence, Biochimica et Biophysica Acta 990, 87-92
  13. 1 2 Genty B., Harbinson J., Baker N.R. (1990) Relative quantum efficiencies of the two photosystems of leaves in photo respiratory and non-photo respiratory conditions. Plant Physiol Biochem 28: 1-10
  14. ↑ Laisk A and Loreto F (1996) Determining photosynthetic parameters from leaf CO2 exchange and chlorophyll fluorescence. Ribulose-1,5-bisphosphate carboxylase / oxygenase specificity factor, dark respiration in the light, excitation distribution between photosystems, alternative electron transport rate, and mesophyll diffusion resistance. Plant Physiol110: 903–912.
  15. ↑ Eichelman H, Oja V., Rasulov B., Padu E., Bichele I., Pettai H., Niinemets O., Laisk A. (2004) Development of Leaf Photosynthetic Parameters in Betual pendula Roth Leaves: Correlation with Photosystem I Density, Plant Biology 6 (2004): 307-318
  16. ↑ Schreiber U, (2004) Pulse-Amplitude-Modulation (PAM) Fluorometry and Saturation Pulse Method: An Overview From Chapter 11, "Chlorophyll a Fluorescence a Signature of Photosynthesis", edited by George Papaqeorgiou and Govindjee, published by Springer 2004,PO Box 17, 3300 AA Dordrecht, The Netherlands, page 279–319
  17. ↑ Flexas (2000)– "Steady-State and Maximum Chlorophyll Fluorescence Responses to Water StressIn Grape Vine Leaves: A New Remote Sensing System", J. Flexas, MJ Briantais, Z Cerovic, H Medrano, I Moya, Remote Sensing of Environment 73:283-270
  18. ↑ Kramer D. M., Johnson G., Kiirats O., Edwards G. (2004) New fluorescence parameters for determination of QA redox state and excitation energy fluxes. Photosynthesis Research 79: 209-218
  19. ↑ Klughammer C., and Schreiber U. (2008) PAM Application notes 2008 1:27 -35
  20. ↑ Hendrickson L., Furbank R., & Chow (2004) A simple alternative approach to assessing the fate of absorbed Light energy using chlorophyll fluorescence. Photosynthesis Research 82: 73-81
  21. ↑ , Strasser R.J. Tsimilli-Michael M., and Srivastava A. (2004) - Analysis of Chlorophyll a Fluorescence Transient. From Chapter 12, "Chlorophyll a Fluorescence a Signature of Photosynthesis", edited by George Papaqeorgiou and Govindjee, published by Springer 2004, PO Box 17, 3300 AA Dordrecht, The Netherlands, page 340
  22. ↑ . Vredenberg, W.J., (2004) System analysis of photoelectrochemical control of chlorophyll fluorescence in terms of trapping models of Photosystem II: a challenging view. In: Papageorgiou, G.C., Govindjee(Eds.), Chlorophyll a Fluorescence: A Signature of Photosynthesis, Advances in Photosynthesis and Respiration, vol. 19. Springer, Dordrecht, pp. 133–172.
  23. ↑ . Vredenberg, W.J., Prasil, O., (2009) Modeling of chlorophyll a fluorescence kinetics in plant cells: derivation of a descriptive algorithm. In: Laisk, A., Nedbal, L., Govindjee(Eds.), Photosynthesis in Silico: Understanding Complexity from Molecules to Ecosystems. Springer, Dordrecht (the Netherlands), pp. 125–149.
  24. ↑ Vredenburg, W.J (2011) Kinetic analyses and mathematical modeling of primary photochemical and photoelectrochemical processes in plant photosystems, BioSystems 103 (2011) 138–151
  25. 1 2 3 Knighton N., Bugbee B., (2004)– A comparison of Opti-Sciences CCM-200 chlorophyll meter and the Minolta SPAD 502 chlorophyll meter, Crop Physiology Laboratory - Utah State University
  26. 1 2 3 Richardson A. D., Duigan S.P., Berlyn G.P., (2002) An evaluation of noninvasive methods to estimate foliar chlorophyll content New Phytologist (2002)153 : 185–194
  27. 1 2 Shapiro C., Schepers J., Francis D., Shanahan J., (2006) Using a Chlorophyll Meter to improve N Management. NebGuide # 1621 University of Nebraska – Lincoln Extension
  28. 1 2 Francis D.D., Piekielek D.D. Francis and W.P. Piekielek. SSMG-12. Introduction. Determining when to apply additional N ... Assessing Crop Nitrogen Needs with. Chlorophyll Meters
  29. ↑ Cheng L., Fuchigami L., Breen P., (2001) "The relationship between photosystem II efficiency and quantum yield for CO2 assimilation is not affected by nitrogen content in apple leaves."Journal of Experimental Botany, 52(362):1865-1872
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