1. Isotope fractionation in photosynthesis
In the process of photosynthesis, there are usually the following chemical reactions:
6CO2+ 1 1H2O？ →C6H22O 1 1+6O2
The bond of 12CO2 in the atmosphere is easier to break than that of 13CO2, so plant tissues will preferentially absorb 12CO2 in the air during photosynthesis, making the organic matter rich in 12C, while the air is relatively rich in 13C.
In photosynthesis, carbon isotope fractionation in plants can be divided into three stages. The first stage occurs during the diffusion of atmospheric CO2 from the stomata of the leaves to the stomatal cavity, which is mainly affected by atmospheric CO2 concentration, wind speed, humidity and light intensity. The second stage is the process of CO2 entering mesophyll cells from stomata. In these two stages, the morphology of C in plants has not changed, but the dynamic fractionation of light and heavy isotopes is caused by the quality difference. Compared with atmospheric CO2, the δ 13C value of CO2 entering mesophyll decreased by about 4.4‰, and the fractionation effect accounted for about 1/3 of the total effect. The third stage occurs when CO2 enters the photosynthetic cycle and is immobilized by carboxypeptidase, and then starch, polysaccharide, fiber, protein and fat are further synthesized. It is the main body of carbon fixation in plants, and C changes from inorganic form to organic form. Because the bond energy of 13CO2 is greater than that of 12CO2, trees assimilate and utilize 12CO2 more. In the third stage, isotope fractionation is mainly influenced by different ways of carbon assimilation and utilization by plants, and is restricted by environmental factors closely related to the growth of the same plant (Li Rong, 2006). If C3 (Calvin pathway) plants exclude the influence of stomatal conductance, under normal physiological conditions, CO2 is fixed by RuBP carboxylase, and the fractionation is generally around -27 ‰ (arslan et al.,1999); The division of labor between leaf sheath cells and mesophyll cells in C4 (hatch-slack pathway) plants is clear in the process of CO2 assimilation, and the distribution of photosynthesis-related enzymes is different from that in C3 plants, among which the fractionation effect of pyruvate carboxylase (PEPCase) is only 2‰, so it is inferred that carbon isotope fractionation in C4 plants is caused by time-related fractionation (i.e. kinetic fractionation) (Benedict,1978). The δ 13C value of CAM plants is usually between C3 and C4 plants, which is about-34 ‰ ~-1‰.
There are two main carbon assimilating enzymes in plant photosynthesis. In plants with C3 pathway, C is fixed by 1.5-RuBP carboxylase (Rubp), while in plants with C4 pathway, C is fixed by phosphoenolpyruvate carboxylase (PEP). The key difference between these two enzymes is that the former only absorbs CO2, while the latter fixes HCO-3. Due to isotope exchange reaction:
isotope geochemistry
Therefore, C4 plants are richer in 13C than C3 plants, resulting in a higher isotope δ 13C value (-7 ‰ ~- 15 ‰). And C3 plant is relatively poor 13C, ranging from-20 ‰ to 35 ‰. The great difference of carbon stable isotopes makes it very easy to distinguish two types of carbon assimilation pathways in plant photosynthesis (Li Rong, 2006).
CAM (Sedum acid metabolic pathway) plants open their stomata at night to fix CO2 and use it for photosynthesis during the day. Hco-3 (pep carboxylase immobilization) is the main type of fixing C at night. Some CAM plants also fix C and absorb CO2 (fixed by RUDP carboxylase) during the day. Therefore, the difference of δ 13C in CAM plants is related to the enzymes used for carbon fixation, and its values are often very different, which is equivalent to a high δ 13C value in C4 plants and a low δ 13C value in C3 plants. The δ 13C of CAM plants generally varies from-10 ‰ to -22 ‰.
It can be seen that different photosynthetic types of plants have different carbon isotope fractionation modes in the process of absorbing CO2 to synthesize organic matter, which leads to obvious differences in δ 13C values of different types of plants (table 17- 1). It can be seen that C4 and CAM plants contain more 13C. The difference of δ 13C between C3 and C4 plants is-14%. The δ value of CAM plants is between the two, which is mainly caused by the difference of CO2 diffusion resolution and enzyme resolution.
Plants in nature are mainly C3 plants and C4 plants, almost all tree species and temperate plants belong to C3 plants (wheat and cotton), and C4 plants mainly include (sub) tropical Chenopodiaceae and Salicaceae plants (corn and sorghum). CAM plants are relatively rare, only distributed in cactus family and bromeliad family, mostly succulent plants, such as cactus, yucca, agave and Sedum.
Table 17- 1 Variation range of δ 13C values of leaves of different functional groups
(Quoted from Li Rong, 2006)
2. The influence of external environmental factors on carbon isotope fractionation of plants.
The carbon isotope composition of plants is not only related to its photosynthetic carbon metabolism pathway, but also influenced by external environmental factors. According to previous studies, the climatic and environmental factors affecting carbon isotope fractionation in plants include precipitation, temperature, pressure, light, atmospheric pressure and carbon isotope composition of CO2 in the atmosphere.
Carbon isotope fractionation of plants under (1) water stress
Water deficit will restrict stomatal or non-stomatal factors, thus affecting the photosynthetic efficiency and transpiration efficiency of plants. δδ 13C value can comprehensively reflect this point. When the soil moisture content is low, the air humidity is low, and the rainfall is insufficient, plants will close their stomata to reduce the evaporation of water, resulting in the decrease of stomatal conductance, which will lead to the decrease of intercellular CO2 concentration in plant leaves and the increase of the value of photosynthetic product δ 13C, that is, the δ 13C value tends to increase with the decrease of humidity.
At present, there are three main types of plant responses to water stress (Chen Yinghua et al., 2004):
1) Stomatal limiting factor: Early studies suggested that stomatal closure was the most sensitive response of plants under mild and moderate water stress (Chave et al., 2002). At this time, the intercellular CO2 concentration (Ci) may decrease, and plants can selectively absorb 12CO2, resulting in the increase of δ 13C. However, some plants have increased stomatal conductance with the aggravation of water stress. This may be because epidermal cells lose water faster than guard cells in stomatal turnover, which leads to the passive opening of stomata and reduces the limitation of stomata. At this time, the intercellular CO2 concentration increased and the δ 13C value of plants decreased. Therefore, an indispensable criterion of stomatal limitation is the decrease of Ci (intercellular CO2 concentration).
2) Non-stomatal limiting factors: under severe water stress, photosynthetic efficiency and stomatal conductance decreased, but intercellular CO2 concentration increased or remained unchanged (raschke &: Resemann,1986; Bei Shi Lage et al., 1987). Some people infer that this is caused by non-stomatal factors, such as the decrease of photosynthetic activity (ATP, Rubisco content and activity, etc.) of mesophyll cells. ), rather than the result of decreased stomatal conductance (Ebukanson,1987; Gimenez et al., 1992). Farquhal & Sharkey (1982) also thought that only when the intercellular CO2 concentration decreased and the stomatal limit value increased, it could be concluded that the decrease of photosynthetic rate was mainly caused by the decrease of stomatal conductance. On the contrary, if the decrease of photosynthetic rate of leaves is accompanied by the increase of Ci, then the main limiting factor of photosynthesis is the non-stomatal factor, that is, the decrease of photosynthetic activity of mesophyll cells. At this time, the activity of photosynthesis-related enzymes decreased, and the resolution of 13C weakened, resulting in the increase of δ 13C value (Chen Yinghua et al., 2004).
3) Uneven stomatal closure: In addition to the above two situations, plants also have a phenomenon of "uneven stomatal closure", that is, some stomata remain open, while others are completely closed. Because stomatal closure can fully explain the previously suspected non-stomatal limitation of photosynthesis (Downton, 1988). Therefore, only under the premise of determining whether uneven stomatal closure occurs through experiments can we correctly judge whether the main reasons for the changes of δ 13C value and photosynthetic efficiency are non-stomatal factors under environmental stress, especially under water deficit.
(2) Effect of temperature on carbon isotope fractionation of plants.
Temperature is an important climatic factor affecting carbon isotope fractionation of plants, but its influence on δ 13C value of plants is complicated. Recently, some scholars think that there is a nonlinear relationship between δ 13C and temperature, which has the characteristics of parabola. The inflection point of parabola corresponds to the optimal growth temperature point of plants. If the temperature is lower than the optimal growth temperature, δ 13C-T is positively correlated, and vice versa. Plants have the largest assimilation rate and carbon isotope fractionation at the optimum growth temperature.
(3) Effect of light on carbon isotope fractionation in plants
The change of light conditions can affect the stomatal conductance coefficient, CO2 absorption rate and partial pressure of leaves, thus affecting the carbon isotope composition of plants. The δ 13C of plants increased with the increase of light.
(4) Influence of atmospheric pressure on carbon isotope fractionation of plants.
Changes in atmospheric pressure will lead to changes in the partial pressure of CO2 (pi/pa) inside and outside plant leaves, thus affecting carbon isotope fractionation of plants. Many research results show that the carbon isotope composition of plants becomes heavier with the increase of altitude, which is mainly due to the decrease of atmospheric pressure, the decrease of plant pi/pa value and the decrease of plant carbon isotope fractionation.
(5) Effect of 5)CO2 concentration on carbon isotope fractionation in plants.
The carbon isotope composition of carbon dioxide in the atmosphere is not a constant. Just like the concentration of the atmosphere, it changes with time and space. Generally, the atmosphere with higher CO2 concentration has lower δ 13C value (Wang Guoan, 200 1).
(6) Effects of carbon sources on carbon isotope fractionation in plants.
The source of carbon is also an external factor affecting the carbon isotope composition of plants. Terrestrial plants mainly absorb carbon from the atmosphere, δ13co2 =-7 ‰; Marine organisms absorb dissolved carbon, δ 13 chco3-= 0 ‰. On the other hand, the water in mainland China is generally rich in 12C, so the freshwater plants are worse than the marine plants in 13C, especially the water in the reduction basin with strong bacterial activity and poor mixing with the outside world, and the dissolved carbon is very negative, so the δ 13C values of plants in different lakes are also different, but the water in the inland evaporation basin is rich in 13C (.
3. Variation of carbon isotope composition in plant tissues
Many scholars have studied the carbon isotope composition of different biochemical components in plants (Parker,1964; Degens et al.,1968a; Smith and Epstein,1970; De Niro and Epstein, 1977), it was found that the contents of 13C in different chemical components in plants were significantly different (Dennis,1980; Bowden, 1996). For the same plant, the δ 13C of aboveground biomass and underground biomass are also different, because the roots and branches contain different proportions of organic compounds. In fact, the difference of isotopic composition in different parts of plants reflects the difference of different organic compounds in plants (Zhu Calligraphy, 2006). Compared with the whole plant tissue, pectin, hemicellulose, amino acids and sugars are rich in 13C to 1 ‰ ~ 4 ‰, while cellulose, lignin and lipids are deficient in 13C to 1 ‰ ~ 5 ‰, which may lead to the existence of1‰ between the most abundant and the least abundant organic matter in plants. Wedin( 1995) and others found that the δ 13C value of grasses and grasses was always lower than 3.6‰ of the whole plant tissue on average, and the δ 13C value of lignin contained in aboveground parts of plants was lower than that contained in underground parts.
Although there are significant differences in stable carbon isotopic compositions of different organic compounds in the same plant, there is little difference in δ 13C values among plant tissues (Deines,1980; Bowden,1996), Tieszen (1990) and others found that the difference of δ 13C values among different tissues of six C4 grasses in East Africa was less than 2‰, and the loss of 13C of roots and living leaves was less than that of dead leaves and stems. Balesdent( 1987) et al. measured the average δ 13C of different tissues of maize. The results showed that the leaves were-13.5 ‰, the roots were-1 1.8 ‰, and the leaf sheath and stem were in between.