What environmental factors affect photosynthesis?

(1) Light \ r \ Light is the driving force of photosynthesis and the necessary condition for the formation of chlorophyll, chloroplast and normal leaves. Light also significantly regulates the activity of photosynthetic enzymes and the opening of stomata, so light directly restricts the photosynthetic rate. Light intensity, light quality and light time are part of light factors, which have a profound influence on photosynthesis. \r\n 1, light intensity \r\n (1) In the dark, the leaves do not carry out photosynthesis, but only breathe and release CO2. With the increase of light intensity, the photosynthetic rate increases accordingly. When the light intensity reaches a certain level, the photosynthetic rate of leaves is equal to the respiration rate, that is, CO2 absorption is equal to CO2 release, and apparent photosynthetic rate is zero. The light intensity at this time is called light compensation point. In the weak light region, the photosynthetic rate increases proportionally with the increase of light intensity (proportional stage, straight line A); When it exceeds a certain light intensity, the increase of photosynthetic rate will slow down (curve B); When a certain light intensity is reached, the photosynthetic rate will not increase, but light saturation will occur. The light intensity at the beginning of the maximum photosynthetic rate is called light saturation point, and the stage after this point is called saturation stage. In the proportional stage, the light intensity mainly restricts the photosynthetic rate, while in the saturated stage, the diffusion and fixed rate of CO2 are the main limiting factors. Use the light intensity of the proportional stage? The slope of photosynthetic rate (apparent photosynthetic rate/light intensity) can be used to calculate the apparent photosynthetic quantum yield. \ r \ Light intensity of different plants? The light compensation point and light saturation point of different photosynthetic curves are also very different. Plants with high light compensation points generally have high light saturation points, while herbaceous plants usually have higher light compensation points and light saturation points than woody plants. The light compensation point and light saturation point of sunny plants are higher than those of shade plants. The light saturation point of C4 plants is higher than that of C3 plants. Light compensation point and light saturation point can be used as the main indicators of plant light demand characteristics to measure light demand. Plants with low light compensation point are more tolerant to shade. For example, the light compensation point of soybean is only 0.5klx, which can be intercropped with corn and still grow normally in corn rows. At the light compensation point, photosynthetic accumulation and respiratory consumption cancel each other out. If the respiratory consumption at night is considered, the photosynthetic products are still insufficient, so the minimum light intensity required by plants must be higher than the light compensation point throughout the day. For the population, the light intensity received by the upper leaves often exceeds the light saturation point, while the light intensity of the middle and lower leaves is still lower than the light saturation point. For example, the light saturation point of rice leaves is 40 ~ 50 lx, while that of the population is 60 ~ 80 lx. Therefore, it is an important condition for high yield to improve the light intensity of middle and lower leaves and make them receive more light as much as possible. \ r \ The light compensation point and light saturation point of plants are not fixed values, but will change with the change of external conditions. For example, when the CO2 concentration increases or the temperature decreases, the optical compensation point decreases; When CO2 concentration increases, the light saturation point will increase. In a closed greenhouse, higher temperature and less CO2 will increase the light compensation point, which is not conducive to photosynthesis accumulation. In this case, the room temperature should be properly lowered, ventilated or supplemented with CO2 to ensure the smooth progress of photosynthesis. ? \ r \ C4 plants will not appear light saturation under general light intensity. The reasons are as follows: ①C4 plants are more capable of assimilating CO2 than C3 plants; ②PEPC has a high affinity for CO2, and it has a "CO2 pump", so the CO2 concentration in the air is usually not the limiting factor of photosynthesis of C4 plants. \r\n (2) Strong light injury-photoinhibition: insufficient light energy can be the limiting factor of photosynthesis, and too much light energy can also adversely affect photosynthesis. When the photosynthetic apparatus receives more light energy than it can use, light will cause the decrease of photosynthetic activity, which is called photoinhibition of photosynthesis. ? The light intensity at noon on sunny days often exceeds the light saturation point of plants. Many C3 plants, such as rice, wheat, cotton, soybean, bamboo and camellia, etc. , will suffer from photoinhibition, which will temporarily reduce the photosynthetic rate of plants, and when it is serious, the leaves will turn yellow and the photosynthetic activity will be lost. When strong light coexists with other environmental stresses such as high temperature, low temperature and drought, the phenomenon of photoinhibition is particularly serious. Generally speaking, shade plants with low light saturation point are more susceptible to light inhibition. If the ginseng seedlings are moved to the open field for cultivation, under direct light, the leaves will turn green quickly, and reddish-brown burning spots will appear, which will make the ginseng seedlings unable to grow normally. The yield of field crops with reduced photoinhibition can reach more than 15%. Therefore, the causes of photoinhibition and its defense system have attracted people's attention. \r\n2, light quality \ r \ nIn solar radiation, only the visible part can be utilized by photosynthesis. When the leaves of plants are irradiated by visible light with different wavelengths, the measured photosynthetic rate (compared with quantum yield) is different. In the red region of 600 ~ 680 nm, the photosynthetic rate has a big peak and a small peak in the blue region of about 435nm. It can be seen that the action spectrum of photosynthesis is roughly consistent with the absorption spectrum of chloroplast pigment. ? \ r \ Figure 4-28 shows the relationship between light quality and photosynthetic rate under low light in the proportional period. In this case, the influence of light quality on photosynthesis actually works through photochemical reaction. In recent years, people used strong monochromatic light to study the effect of light quality on photosynthetic rate of plant leaves. It was found that the photosynthetic rate under blue light was higher than that under red light, which may be related to the promotion of stomatal opening by blue light. It is reported that plants growing under blue light have high PEPC activity. \ r \ Under natural conditions, plants are more or less exposed to light with different wavelengths. For example, on cloudy days, not only the light intensity decreases, but also the ratio of blue light to green light increases. The leaves of trees absorb more red and blue light, so there is more green light in the light passing through the canopy. Because green light is the inefficient light of photosynthesis, it will reduce the utilization efficiency of light energy for plants growing under the canopy with insufficient light. This is a fact. \ r \ The water layer will also change the intensity and quality of light. The deeper the water layer, the weaker the light. For example, the light intensity at a depth of 20 meters is one twentieth of the light intensity on the water surface. If the water quality is not good, the light intensity at this depth will be weaker. The water layer absorbs more red and orange parts of light waves than blue and green parts, and there are relatively more light with shorter wavelength in the deep water layer. Therefore, green algae containing chlorophyll and absorbing more red light are distributed on the surface of seawater; Red algae containing phycoerythrin and absorbing more green and blue light are distributed in deep seawater, which is a manifestation of algae adapting to light. \r\n3, illumination time \ r \ nAfter a period of illumination, the photosynthetic rate of substances (leaves or cells) is very low or negative at first, and will gradually increase and tend to be stable after a period of illumination. The period from the beginning of illumination to the stable photosynthetic rate is called "the lag period of photosynthesis" or "the photosynthetic induction period". Generally, the photosynthetic lag time of the whole leaf is about 30 ~ 60 min, while the lag time of photosynthetic tissues such as leaves, cells and protoplasts without stomata is about 10 min. A similar situation will occur when plants shift from weak light to strong light. In addition, the photorespiration of plants is also lagging behind. During the photorespiration lag period, the photorespiration rate and photosynthetic rate will increase in proportion. The reason for the lag period is that the light-induced enzyme activity and the proliferation of intermediate products of photosynthetic carbon cycle need a preparation process, and the time required for light-induced stomatal opening is the main factor to prolong the lag period of leaves. \ r \ Because the exposure time has a great influence on the photosynthetic rate of plant leaves, the leaves should be fully pre-illuminated when measuring the photosynthetic rate. \ r \ n (ii) CO2 \ r \ nThe photosynthetic curve of CO2 is similar to that of light and light, and there are proportional stages and saturation stages. When CO2 concentration is zero under light, leaves only breathe and release CO2 under light and darkness. The OA part in the figure is the rate at which leaves release CO2 to CO2-free gas under illumination (essentially the balance value of photorespiration, dark respiration and photosynthesis), which is usually used to represent photorespiration rate. In the proportional period, the photosynthetic rate increased with the increase of CO2 concentration. When the photosynthetic rate is equal to the respiration rate, the CO2 concentration in the environment is the CO2 compensation point. When it reaches a certain concentration (S), the photosynthetic rate reaches the maximum value (Pm), and the CO2 concentration at the beginning of reaching the maximum photosynthetic rate is called the CO2 saturation point. In the proportional stage of CO2 photosynthetic curve, CO2 concentration is the limiting factor of photosynthesis, and the slope (CE) of the straight line is limited by Rubisco activity and the amount of activated Rubisco, so CE is called carboxylation efficiency. It can be inferred from the change of CE that the quantity and activity of Rubisco are higher, that is, the photosynthetic rate is higher when the concentration of CO2 is lower, that is, the carboxylation efficiency of Rubisco is higher. In the saturation stage, CO2 is no longer the limiting factor of photosynthesis, but the amount of CO2 receptor, that is, the regeneration rate of RuBP, becomes the factor affecting photosynthesis. Because the regeneration of RuBP is affected by ATP supply, the photosynthetic rate in saturation period reflects the photosynthetic electron transfer and photosynthetic phosphorylation activity, so Pm is called photosynthetic capacity. \ r \ Comparing the CO2 photosynthetic curves of C3 plants and C4 plants, it can be seen that (1)C4 plants have low CO2 compensation points, and the photosynthetic rate increases faster than C3 plants at low CO2 concentration, and the CO2 utilization rate is high; (2) The CO2 saturation point of 2)C4 plants is lower than that of C3 plants, and it can reach saturation at atmospheric CO2 concentration; However, the CO2 saturation point of C3 plants is not obvious, and the photosynthetic rate will increase with the increase of CO2 concentration. The low CO2 saturation point of C4 plants may be related to the sensitivity of C4 plants' stomata to CO2 concentration, that is, when CO2 concentration exceeds the air level, the stomatal opening of C4 plants becomes smaller. In addition, C4 plants have low PEPC Km, high affinity for CO2 and CO2 concentration mechanism, which are also the reasons for the low CO2 saturation point of C4 plants. \ r \ n \ r \ Under normal physiological conditions, the CO2 compensation point of plants is relatively stable, such as 52 2 μ l l-1 for wheat 100, 55 2 μ l l-1for barley 125. Thousands of oat seedlings and 50,000 wheat seedlings were measured, but no seedlings with low CO2 compensation points similar to C4 plants were found. Under the conditions of temperature rising, light intensity weakening, water shortage and oxygen concentration increasing, the CO2 compensation point also rises. \r\n 2。 CO2 is the carbon source of photosynthesis, and the CO2 needed by terrestrial plants is mainly obtained from the atmosphere. Path and resistance of CO2 from atmosphere to carboxylase site. \r\nCO2 diffuses from the atmosphere to mesophyll cell gap in gas phase and from mesophyll cell gap to chloroplast matrix in liquid phase, and the driving force of diffusion is. CO2 concentration difference; Any factor that can increase the concentration difference and reduce the resistance can promote the circulation of CO2 and increase the photosynthetic rate. \ r \ The concentration of CO2 in the air is low, about 350μ L L- 1? (0.035%), and the partial pressure is 3.5× 10-5? The CO2 saturation point of C3 plants is1000 ~1500μ l l-1? It is about 3 ~ 5 times that of air. The concentration of. CO2 can be reduced to 200μ l L-1? About. Due to the consumption of. Photosynthesis and the existence of carbon dioxide. Diffusion resistance and concentration of CO2. The CO2 in chloroplast matrix is very low, which is close to the compensation point of CO2. Carbon dioxide Therefore, strengthening ventilation or trying to increase the application amount of CO2 can significantly improve the photosynthetic rate of crops, especially C3 plants. \r\n (3) Temperature \r\Dark reaction in photosynthesis is a chemical reaction catalyzed by enzyme, so it is affected by temperature. Under strong light and strong light. The influence of CO2 concentration and temperature on photosynthetic rate is greater than that under low light. CO2 concentration, because in strong light and high. CO2 concentration and temperature can be the main limiting factors of photosynthesis. \ r \ Photosynthesis has a certain temperature range and three basic points. The lowest temperature (cold limit) and the highest temperature (hot limit) of photosynthesis mean that the photosynthetic rate is zero at this temperature, and the temperature that can make the photosynthetic rate reach the highest is called the photosynthetic optimum temperature. The temperature of photosynthesis varies greatly with different plant species. For example, the photosynthetic rate of lettuce with low temperature tolerance can be obviously measured at 5℃, while that of cucumber with high temperature tolerance can only be measured at 20℃; The cold limit of photosynthesis of cold-tolerant plants is close to the freezing temperature of cells; When the temperature of tropical plants, such as corn, sorghum and rubber tree, drops to 10 ~ 5℃, photosynthesis is inhibited. The main reason for inhibiting photosynthesis at low temperature is that membrane lipids are in gel phase and chloroplast ultrastructure is destroyed at low temperature. In addition, the slow enzymatic reaction and the imbalance of stomatal opening and closing at low temperature are the reasons for the inhibition of photosynthesis. \r\n C4 plants have a high thermal limit, which can reach 50 ~ 60℃, while C3 plants have a low thermal limit, which is generally 40 ~ 50℃. Wheat suffered from continuous high temperature during milk ripening, although its appearance was still green, but its photosynthetic function was seriously damaged. The reasons for the thermal limit of photosynthesis are as follows: first, the thermal denaturation of membrane lipids and enzyme proteins leads to the damage of photosynthetic organs and the inactivation of enzymes in chloroplasts; Second, because the high temperature stimulated the light and dark breathing, the apparent photosynthetic rate dropped rapidly. The temperature difference between day and night has great influence on the net assimilation rate of photosynthesis. During the day, the temperature is high and the sunshine is abundant, which is beneficial to photosynthesis; The low temperature at night reduces respiratory consumption, so in a certain temperature range, the large temperature difference between day and night is beneficial to photosynthesis accumulation. \ r \ Environmental temperature should be controlled in agricultural practice to avoid the adverse effects of high and low temperature on photosynthesis. Glass greenhouse and plastic greenhouse have the functions of heat preservation and temperature increase, which can improve photosynthetic productivity and have been widely used in vegetable cultivation in winter and spring. \r\n\r\n (4) Water \r\nWater has direct and indirect effects on photosynthesis. The direct reason is that water is the raw material of photosynthesis, and photosynthesis cannot be carried out without water. However, the water used for photosynthesis is less than 1% of transpiration loss, so water shortage mainly indirectly affects photosynthesis. \ r \ Lack of water will reduce the photosynthetic rate. When there is slight water shortage, the photosynthetic capacity can be restored after water supply. If the water shortage is serious, the leaf water potential can be restored to the original level after water supply, but the photosynthetic rate is difficult to restore to the original level. Therefore, in the rice baking field, when cotton and peanuts squat, we should control the degree of baking or squatting, and don't overdo it. \r\nThe main reasons why water deficit reduces photosynthesis are: \r\n( 1) stomatal conductance decreases, and the photosynthetic rate of leaves is positively correlated with stomatal conductance. When water is deficient, the abscisic acid content in leaves increases, which leads to the closure of stomata, the decrease of conductance and the decrease of CO2 entering leaves. The leaf water potential that initially caused the decrease of stomatal conductance and photosynthetic rate varied greatly with different plant species, with rice being-0.2 ~-0.3 MPa; Corn is -0.3 ~-0.4 MPa; While soybean and sunflower are between -0.6 ~- 1.2 MPa. \r\n(2) The output of photosynthetic products is slow. Water deficit will slow down the output of photosynthetic products. In addition, when water is scarce, starch hydrolysis in leaves will be strengthened and sugar will accumulate, which will lead to the decrease of photosynthetic rate. \r\n(3) The photosynthetic mechanism is destroyed. When water is lacking, the electron transfer rate of chloroplasts decreases, which is decoupled from photosynthetic phosphorylation, thus affecting the formation of assimilation ability. Severe water shortage will also deform chloroplasts and destroy the lamellar structure, which will not only reduce the photosynthetic rate, but also make the photosynthetic capacity unable to recover. \r\n(4) The enlargement of photosynthetic area was inhibited. Under the condition of water shortage, the growth is inhibited and the leaf area expansion is limited. Some leaves are covered with salt crystals and fluff or wax, which reduces the consumption of water and the inhibition of light, but at the same time, the photosynthetic rate decreases because of the reduction of light absorption. \ r \ Too much water will also affect photosynthesis. Excessive soil moisture and poor ventilation hinder root activity and indirectly affect photosynthesis; Rain falls on leaves, on the one hand, it blocks stomata and affects gas exchange, on the other hand, it makes mesophyll cells in a low permeability state, which will reduce photosynthetic rate. \r\n (5) Mineral nutrition \r\nMineral nutrition has a wide range of functions in photosynthesis, which can be summarized as follows: \r\n 1. Components of chloroplast structure, such as nitrogen, phosphorus, sulfur and magnesium, are indispensable for chlorophyll, protein, nucleic acid and lamellar membrane. \r\n2。 And Cl- \r\n3. The important role of phosphate group is to form ATP and NADPH with assimilative power, all the intermediate products in the cycle of photosynthetic carbon reduction, the precursor of starch synthesis ADPG and the precursor of sucrose synthesis UDPG. These compounds all contain phosphate groups. \r\n4。 Do enzymes such as Rubisco and FBPase need Mg2+ for activation? ; Iron, copper, manganese and zinc participate in the synthesis of chlorophyll; K+？ And Ca2+? Adjust the opening and closing of pores; Potassium and phosphorus promote the transformation and transportation of photosynthetic products. \ r \ Among the three elements of fertilizer, N has the greatest influence on photosynthesis. In a certain range, the contents of nitrogen, chlorophyll and Rubisco in leaves are positively correlated with photosynthetic rate. 80% nitrogen content in leaves exists in chloroplasts. Nitrogen application can not only increase chlorophyll content and accelerate light reaction, but also increase the content and activity of photosynthetic enzymes and accelerate dark reaction. Rubisco extracted from leaves with good nitrogen nutrition is not only rich in content, but also highly active. However, some experiments have pointed out that when Rubisco content exceeds a certain value, the amount of enzyme is not proportional to photosynthetic rate. \ r \ Heavy metals such as thallium, cadmium, nickel and lead are harmful to photosynthesis, and most of them affect stomatal function. In addition, cadmium can also inhibit the activity of PS ⅱ. \r\n (6) Diurnal variation of photosynthetic rate \r\nDuring a day, external light intensity, temperature, soil and air humidity, air CO2 concentration, water content of plants and photosynthetic intermediates, stomatal opening, etc. Are constantly changing, and these changes will cause the diurnal variation of photosynthetic rate, in which the diurnal variation of light intensity has the greatest influence on the diurnal variation of photosynthetic rate. Under the condition of warm and sufficient water supply, the diurnal variation of photosynthetic rate with light intensity showed a single peak curve. After sunrise, the photosynthetic rate gradually increased, reached the peak before noon, and then gradually decreased, and the photosynthetic rate tended to be negative (respiratory rate) after sunset. If the cloud cover changes during the day, the photosynthetic rate will change with the change of light intensity. \ r \ nIn addition, the photosynthetic rate also corresponds to the change of stomatal conductance. Under the same light intensity, the photosynthetic rate in the afternoon is usually lower than that in the morning, which is due to the accumulation of photosynthetic products and feedback inhibition in the leaves after photosynthesis in the morning. When the light is strong and the temperature is too high, the diurnal variation of photosynthetic rate shows a double-peak curve, with a big peak in the morning and a small peak in the afternoon. The photosynthetic rate decreases around noon, showing a "day-to-day depression" phenomenon, which is aggravated with the decrease of soil water content. The main factors causing photosynthetic "nap" are atmospheric drought and soil drought. At dry and hot noon, the transpiration loss of leaves is aggravated, so the soil moisture is also deficient. Then the water loss of plants is greater than the water absorption, which will cause wilting and decrease the stomatal conductance, thus reducing the absorption. Carbon dioxide In addition, strong light, high temperature and low CO2 concentration at noon and afternoon will lead to a sharp increase in photorespiration and photoinhibition, which will also reduce the photosynthetic rate at noon or afternoon. \ r \ Photosynthetic "siesta" is a common phenomenon when plants encounter drought, and it is also a way for plants to adapt to environmental water shortage. The loss caused by "siesta" can reach 30% of photosynthetic yield, or even more. Therefore, timely irrigation or drought-resistant varieties should be used in production to enhance photosynthetic capacity to alleviate the degree of "siesta".