Study on the Photosynthesis of Plants in Crassulaceae Acid Metabolism
Background information:
Crassulacean acid metabolism pathway (CAM pathway) refers to an additional pathway for photosynthetic fixed carbon dioxide that some succulent plants (earlier found in Crassulaceae plants) grow in tropical and subtropical arid and semi-arid regions . Plants with this pathway are called CAM plants. Under its natural conditions, the stomata are closed during the day and open at night. They have this approach, which not only maintains water balance, but also assimilate carbon dioxide. The characteristics of the pathway are: Phosphoenolpyruvate (PEP) acts as a carbon dioxide acceptor in the cells at night, and is catalyzed by PEP carboxylase to form oxaloacetic acid, which is then reduced to malic acid and stored in vacuoles; The acid is transferred from the vacuole into the chloroplast for decarboxylation to release carbon dioxide, and then converted into sugar through the Calvin cycle. Therefore, organic acids, especially malic acid, in the green part of these plants change day and night, accumulate at night, and decrease during the day. Starch decreases at night (due to the conversion to carbon dioxide acceptor PEP) and accumulates during the day (due to photosynthesis). It has been found that many families of plants such as Agaveaceae, Cactaceae, Euphorbiaceae, Liliaceae, Cucurbitaceae, Echinaceae, and Bromeliaceae have this approach. Generally speaking, CAM plants are succulent, but some are not succulent. Not all succulent plants are CAM plants. This type of plant adapts to the environment by changing its metabolic type. Due to the characteristics of this pathway, the photosynthetic rate is very low (3-10 mg CO2 · dm-2 · hr-1), so it grows slowly, but it can be difficult for other plants. Survive and grow under the ecological conditions of survival. If the C4 plant is spatially staggered by the carbon dioxide fixation and the Calvin cycle, then the Crassulaceae cycle is staggered by time.
Figure 1 Cactaceae and Bromeliaceae
Study on the plant photosynthesis with sedative acid metabolism pathway:
CAM plants have this special photosynthetic pathway, which makes their photosynthesis measurements very different from other plants.
1 CAM plant leaves often have different shapes, fleshy, and thick leaves, such as the cactus above. (At present, the photosynthetic apparatus leaf room is mainly designed for crops such as corn, wheat, rice, soybean, etc., so it is difficult to use it for the leaves of CAM plants)
2 The principle of photosynthesis instrument (portable photosynthesis meter) is based on the total reaction formula of photosynthesis:
CO2 + 2H2O * + 4.69kJ → (CH2O) + O * 2 + H2O
The photosynthetic rate was calculated by measuring the change of CO2 concentration by infrared CO2 gas analyzer. The white weather hole of the CAM plant is closed and does not exchange gas with the outside world. Therefore, the photosynthetic rate of the CAM plant cannot be measured with a photosynthesis instrument. This is also the fundamental reason why many users use photosynthesis apparatus to measure the photosynthesis of some plants around 0.
3 The photosynthetic rate of CAM plants is very low, and the measurement must be performed with more precise instruments than other crops.
Based on the above characteristics, in order to determine the photosynthetic rate of CAM plants, we must open up new paths. As mentioned in the previous chapters, the plant material with irregular leaves that cannot be measured with a photosynthesis instrument can be measured with an oxygen electrode.
Method 1: Determine the photosynthetic rate of CAM plants with an oxygen electrode,
According to the total reaction formula of photosynthesis: CO2 + 2H2O * + 4.69kJ → (CH2O) + O * 2 + H2O
The photosynthetic rate measured by the oxygen electrode is different from the photosynthetic rate measured by the photosynthesis apparatus (see "Differences and Advantages and Disadvantages of the Photosynthesis Apparatus and the Oxygen Electrode to Measure the Photosynthesis Rate" for details). The oxygen electrode measures the photosynthesis rate through the release of oxygen. In the system, the effect of gas holes on gas release is eliminated. Moreover, the accuracy of the oxygen electrode is very high, and it is not easily affected by the low photosynthetic rate of the CAM plant.
Oxygen electrodes that can be used to determine the photosynthetic rate of CAM plants are: Chlorolab-2 and Chlorolab-3 (Hansatech, UK)
Method 2: Use a CO2 analyzer to measure the CO2 absorption of the population at multiple points at night to estimate the net photosynthetic accumulation
The CAM plant separates the fixation of carbon dioxide and the Calvin cycle by time, and opens the stomata at night to absorb CO2, so we can estimate it with a CO2 analyzer at night. Because the photosynthetic rate of CAM plants is generally low, even if the absorption of CO2 at night is relatively small, we can measure the absorption of CO2 by the group, so that it can be measured by the group assimilation room equipped with the photosynthesis apparatus. The group assimilation room adopts a closed type The air path can be used (the CAM photosynthetic rate is low). At night, multi-point measurement, like the day-to-day measurement of daytime changes, measures CO2 absorption at several points, so that the photosynthetic accumulation of CAM plants can be estimated.
Photosynthesis apparatuses are generally equipped with group assimilation rooms, such as CIRAS-2 with CPY-2 and CPY-3 assimilation rooms (PP Systems, USA)
In summary, we must pay attention to when using photosynthesis apparatus to determine the photosynthetic rate, if the experimental materials we study are agave, cactaceae, euphorbiaceae, liliaceae, cucurbitaceae, bromeliaceae, bromeliaceae In one of them, you must pay attention to whether the material is a CAM plant. If it is a CAM plant, it is generally necessary to select an oxygen electrode to measure the photosynthetic rate. Oxygen electrode is a material that can be measured by photosynthesis equipment, such as algae and special parts of plants.
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