Relationship between photosynthetic parameters and fluorescence parameters
Photosynthetic parameters and fluorescence parameters often appear in the relevant literature of photosynthetic research. In general, the photosynthesis experiments must have photosynthesis parameters and fluorescence parameters, and the data appears to be complete. No investigation into the photosynthetic performance of plants under field conditions see complete without some fluorescence data.—Giles N.Johnson. In recent years, with the introduction of high-precision, high-time-resolution fluorometers (such as the Hanstech's FMS-2 fluorometer, Handy PEA plant efficiency meter), the fluorometer is easy to measure, fast, and has a large amount of data. The advantages are quickly favored by the majority of researchers. There is also a growing body of literature on fluorescence parameters. Even some people who are not very clear can even say that the fluorescence parameters can completely replace the photosynthetic parameters, and it is not necessary to directly measure the fluorescence parameters by measuring photosynthesis. But what is the relationship between photosynthesis and fluorescence? How do we choose when we experiment? When do we use fluorescence parameters when using photosynthetic parameters?
The basic principle of chlorophyll fluorescence production:
Figure 1 Schematic diagram of the basic principle of chlorophyll fluorescence production
From the figure, we can see that after the chlorophyll molecule absorbs light energy (excitation energy), it transitions from the ground state to the excited state (the first singlet state and the second singlet state in the figure). The excited state is unstable and will return again. In the ground state, in the process of returning the excited state to the ground state, most of the energy is transferred to the reaction center to promote the photochemical reaction and the subsequent electron transfer, photosynthetic phosphorylation, immobilization, reduction of CO2 and finally storage of energy in the organic matter; The form of heat is dissipated; a small portion of the energy is emitted in the form of fluorescence. The relationship between the three is a competition between each other. Therefore, we can use chlorophyll fluorescence to study changes in photosynthesis.
Relationship between photosynthesis and fluorescence
From the principle of fluorescence generation above, we know that fluorescence and energy used in photochemical reactions are competitive, so fluorescence can be used to reflect photosynthesis or accurately reflect photochemical reactions. Therefore, the use of fluorescence to completely replace photosynthetic is wrong, at least very irresponsible. The most common fluorescence parameters associated with photosynthesis are Fv/Fm, ΦPSII, NPQ, qP, and so on.
Fv/Fm: The maximum photochemical efficiency of PSII under dark adaptation, reflecting the potential photochemical ability of plants, commonly used as an indicator for analyzing light inhibition. It is far from the parameters such as photosynthetic rate and cannot be used instead of photosynthetic rate.
NPQ: Non-optical quenching refers to the decrease in fluorescence yield due to non-photochemical reactions, generally used to indicate the amount of heat dissipation;
qP: photochemical quenching refers to the decrease of fluorescence yield caused by photochemical reaction, which is commonly used to reflect the openness of the reaction center. Combined with NPQ, it can reflect the excess excitation energy and whether the plant is damaged by excess excitation energy;
ΦPSII: The actual photochemical efficiency of PSII under light, ΦPSII*PFD (light intensity)*0.5*α= ETR is used to indicate the electron transfer rate. In some literatures, Yield is also used to represent ΦPSII.
From the meaning of each fluorescence parameter, ΦPSII and electron transport rate ETR are most correlated with photosynthetic rate. The ETR of plants grown in a non-adversive environment is directly proportional to the photosynthetic rate. Therefore, many graduate students use ETR or even ΦPSII instead of photosynthetic rate under the misdirection of relevant instrument dealers. Of course, such an application can sometimes explain the problem. However, when it is further analyzed, it is easy to cause problems and affect the publication of the article, especially the publication of the English publication (SCI).
After the chlorophyll molecule is excited, most of the energy in the process of returning to the ground state is used for photochemical reaction and subsequent electron transfer. The electron is finally transferred to NADP+ to generate NADPH for CO2 fixation. The reduced CO2 is the photosynthetic rate measured by photosynthetic apparatus. basis. In addition to carbon metabolism (CO2 fixation), electrons from the photosynthetic electron transport chain are used for nitrogen metabolism, sulfur metabolism, Miller reaction, water-water circulation, and photorespiration. In non-adversity conditions ETR and photosynthetic rate are directly proportional. However, fluorescence parameters are mainly used in adversity research. Under adverse conditions, electrons from the optical system are more distributed to the light respiration, Miller reaction and other processes. At this time, ETR is not very good. The reaction photosynthetic rate, ΦPSII is not much convincing. Therefore, fluorescence is used to analyze and explain the reasons for photosynthesis and photosynthetic rate reduction under adverse conditions, and cannot be directly used instead of photosynthetic rate.
Fluorescence parameters can also reflect a wealth of information about photosynthetic apparatus, and can also be directly applied in the literature. The application in the literature mainly focuses on the following aspects:
1. Fv/Fm, indicator of photoinhibition: Under adverse conditions (high temperature, low temperature, drought, etc.), plants are often accompanied by strong light, which is prone to photoinhibition. When photoinhibition, Fv/Fm value decreases significantly, so in the literature Among them, the decline of Fv/Fm is often used as a sign of light suppression.
Fv/Fm is also commonly used to analyze whether plants undergo photoinhibition or photodamage under adverse conditions. Plants that undergo photoinhibition can return to the level of control under non-stress conditions, indicating that only photoinhibition is not photodamage, otherwise it indicates Light damage occurred.
2. qP: indicates the degree of closure of the reaction center, and 1-qP indicates the degree of openness of the reaction center to analyze the activity of the reaction center under adverse conditions.
3. NPQ: The heat dissipation level of the reacting plants is often combined with the xanthophyll cycle. At the same time, parameters such as qI, qE, qT are also common in the literature, indicating the problem of heat dissipation.
4. ETR: Combine the measurement of photosynthetic parameters such as photorespiration to study the distribution of light energy.
5. PI (from Handy PEA) performance index: widely used in the breeding of excellent varieties or stress-resistant varieties, and the effect is very good, on the one hand, it can screen a large number of offspring population; on the other hand, it can be excellent for identification. Further in-depth analysis of varieties or stress-resistant varieties provides a deeper theoretical basis.
6. ΦPSII is commonly used to analyze whether it is the cause of photosynthetic decline, and it is also commonly used to analyze whether the decrease of ΦPSII is the cause or result of photosynthetic decrease. In some cases, the stress damages the PSII in the photosynthetic apparatus, which leads to the decrease of ΦPSII and further affects the photosynthetic rate. Sometimes, some adverse conditions (such as drought) cause the photosynthetic rate to decrease due to the influence of the pores, making the function of PSII reversible. Down-regulation to accommodate the reduction in photosynthetic rate.
7. WUE = net photosynthetic rate Pn / transpiration rate E, water use efficiency can also reflect drought resistance.
Photosynthetic parameters and fluorescence parameters are not equivalent, so we should pay special attention when using them, and select the appropriate instruments to determine useful photosynthesis or fluorescence data according to their own experimental design and the final problem. Remember to blindly measure many fluorescence parameters, and more often the fluorescence parameters are to further explain and explain photosynthesis.
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