Photosynthesis rate

In plant physiology, two concepts are used: true and observed photosynthesis. This is due to the following considerations. The rate or intensity of photosynthesis is characterized by the amount of CO 2 absorbed by a unit of leaf surface per unit of time. The determination of the intensity of photosynthesis is carried out by the gasometric method by changing (reducing) the amount of CO 2 in a closed chamber with a leaf. However, along with photosynthesis, the process of respiration takes place, during which CO 2 is released. Therefore, the results obtained give an idea of ​​the intensity of the observed photosynthesis. To obtain the value of true photosynthesis, it is necessary to make a correction for respiration. Therefore, before the experiment, the intensity of respiration is determined in the dark, and then the intensity of the observed photosynthesis. Then the amount of CO 2 released during respiration is added to the amount of CO 2 absorbed in the light. Introducing this amendment, consider that the intensity of respiration in the light and in the dark is the same. But these corrections cannot give an estimate of true photosynthesis, because, firstly, when the leaf is darkened, not only true photosynthesis, but also photorespiration is excluded; secondly, the so-called dark breathing is actually dependent on light (see below).

Therefore, in all experimental work on the photosynthetic gas exchange of the leaf, preference is given to data on the observed photosynthesis. A more accurate method for studying the intensity of photosynthesis is the method of labeled atoms (the amount of absorbed 14 CO 2 is measured).

In the case when it is difficult to recalculate the amount of absorbed CO 2 per unit surface (coniferous, seeds, fruits, stem), the data obtained are referred to a mass unit. Considering that the photosynthetic coefficient (the ratio of the volume of released oxygen to the volume of absorbed CO 2 is equal to one), the rate of observed photosynthesis can be estimated by the number of milliliters of oxygen released by a unit of leaf area in 1 hour.

To characterize photosynthesis, other indicators are also used: quantum consumption, quantum yield of photosynthesis, assimilation number.

Quantum consumption is the ratio of the number of absorbed quanta to the number of assimilated CO 2 molecules. The reciprocal is named quantum yield.

Assimilation number- this is the ratio between the amount of CO 2 and the amount of chlorophyll that is contained in the leaf.

Speed ​​(intensity) photosynthesis is one of the most important factors affecting the productivity of agricultural crops, and hence the yield. Therefore, the elucidation of the factors on which photosynthesis depends should lead to the improvement of agrotechnical measures.

Theoretically, the rate of photosynthesis, like the rate of any multistage biochemical process, should be limited by the rate of the slowest reaction. So, for example, dark reactions of photosynthesis require NADPH and ATP, so dark reactions depend on light reactions. In low light, the rate of formation of these substances is too low to provide top speed dark reactions, so light will be the limiting factor.

The principle of limiting factors can be formulated as follows: with the simultaneous influence of several factors, the speed of a chemical process is limited by the factor that is closest to the minimum level (a change in this factor will directly affect this process).

This principle was first established by F. Blackman in 1915. Since then, it has been repeatedly shown that different factors, such as CO 2 concentration and illumination, can interact with each other and limit the process, although often one of them still dominates. Illumination, CO 2 concentration and temperature are the main external factors affecting the rate of photosynthesis. However great importance It also has a water regime, mineral nutrition, etc.

Light. When evaluating the effect of light on a particular process, it is important to distinguish between the influence of its intensity, quality (spectral composition), and exposure time to light.

In low light, the rate of photosynthesis is proportional to the light intensity. Gradually, other factors become limiting, and the increase in speed slows down. On a clear summer day, the illumination is approximately 100,000 lux, and 10,000 lux is enough to saturate photosynthesis with light. Therefore, light can usually be an important limiting factor in shading conditions. At very high light intensity, the discoloration of chlorophyll sometimes begins, and this slows down photosynthesis; however, in nature, plants exposed to such conditions are usually protected from it in one way or another (thick cuticle, drooping leaves, etc.).

The dependence of the intensity of photosynthesis on illumination is described by a curve, which is called the light curve of photosynthesis (Fig. 2.26).

Rice. 2.26. The dependence of the intensity of photosynthesis on illumination (light curve of photosynthesis): 1 is the rate of CO2 release in the dark (respiration rate); 2 – compensation point of photosynthesis; 3 – light saturation position

In low light, more CO 2 is released during respiration than it is bound during photosynthesis, so the beginning of the light curve with the abscissa axis is compensation point photosynthesis, which shows that in this case, photosynthesis uses exactly as much CO 2 as it is released during respiration. In other words, over time there comes a moment when photosynthesis and respiration will exactly balance each other, so that the visible exchange of oxygen and CO 2 will stop. The light compensation point is the light intensity at which the total gas exchange is zero.

Light curves are not the same for all plants. Plants that grow outdoors sunny places, the absorption of CO 2 increases until the light intensity is equal to the total solar lighting. In plants that grow in shaded areas (for example, oxalis), CO 2 uptake increases only at low light intensity.

All plants in relation to the intensity of light are divided into light and shade, or light-loving and shade-tolerant. Most agricultural plants are photophilous.

In shade-tolerant plants, firstly, light saturation occurs at lower illumination, and secondly, in them, the compensation point of photosynthesis occurs earlier, i.e., at lower illumination (Fig. 2.27).

The latter is related to the fact that shade tolerant plants have a low respiration rate. In low light conditions, the intensity of photosynthesis is higher in shade-tolerant plants, and in strong light, on the contrary, in photophilous plants.

The intensity of the light also affects chemical composition end products of photosynthesis. The higher the illumination, the more carbohydrates are formed; in low light - more organic acids.

Experiences in laboratory conditions showed that the quality of photosynthesis products is also affected by a sharp transition "darkness - light" and vice versa. At first, after turning on the light of high intensity, non-carbohydrate products are predominantly formed due to the lack of NADPH and ATP, and only after a while carbohydrates begin to form. Conversely, after the light is turned off, the leaves do not immediately lose their ability to photosynthesize, because for several minutes a supply of ATP and NADP remains in the cells.

After turning off the light, the synthesis of carbohydrates is first inhibited, and only then organic substances and amino acids. The main reason for this phenomenon is due to the fact that the inhibition of the conversion of FHA into PHA (and through it into carbohydrates) occurs earlier than the inhibition of FHA into PEP (and through it into alanine, malate, and aspartate).

The ratio of the forming products of photosynthesis is also affected by the spectral composition of light. Under the influence of blue light in plants, the synthesis of malate, aspartate and other amino acids and proteins increases. This response to blue light was found in both C 3 plants and C 4 plants.

The spectral composition of light also affects the intensity of photosynthesis (Fig. 2.28). Rice. 2.28. The action spectrum of photosynthesis in wheat leaves

Action spectrum is the dependence of the effectiveness of the chemical (biological) action of light on its wavelength. The intensity of photosynthesis in different areas spectrum is not the same. The maximum intensity is observed when plants are illuminated with those rays that are absorbed to the maximum by chlorophylls and other pigments. The intensity of photosynthesis is highest in red rays, because it is proportional not to the amount of energy, but to the number of quanta.

From the overall photosynthesis equation:

6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2

it follows that 686 kcal is needed to form 1 mole of glucose; this means that 686: 6 = 114 kcal is needed to assimilate 1 mole of CO 2. The energy reserve of 1 quantum of red light (700 nm) is 41 kcal/enstein, and blue (400 nm) 65 kcal/enstein. The minimum quantum consumption when illuminated with red light is 114:41 ≈ 3, while in reality 8–10 quanta are spent. Thus, the efficiency of using red light is 114/41 8 = 34%, and blue 114/65 8 = 22%.

CO 2 concentration. Dark reactions require carbon dioxide, which is included in organic compounds. In ordinary field conditions It is CO2 that is the main limiting factor. The concentration of CO 2 in the atmosphere is 0.045%, but if you increase it, you can increase the rate of photosynthesis. With a short-term effect, the optimal concentration of CO 2 is 0.5%, however, with a long-term effect, damage to plants is possible, therefore, the optimum concentration in this case is lower - about 0.1%. Already now, some greenhouse crops, such as tomatoes, have begun to be grown in an atmosphere enriched with CO 2 .

Currently, a group of plants is of great interest, which absorb CO 2 from the atmosphere much more efficiently and therefore provide more high yield- the so-called C 4 -plants.

AT artificial conditions the dependence of photosynthesis on the concentration of CO 2 is described in a carbon dioxide curve, which resembles the light curve of photosynthesis (Fig. 2.29).

At a CO 2 concentration of 0.01%, the rate of photosynthesis is equal to the rate of respiration (compensation point). Carbon dioxide saturation occurs at 0.2–0.3% CO 2, and in some plants, even at these concentrations, a slight increase in photosynthesis is observed.

Rice. 2.29. The dependence of the intensity of photosynthesis of pine needles on the concentration of CO 2 in the air

AT natural conditions the dependence of photosynthesis on CO 2 concentration is described only by the linear part of the curve. It follows that the provision of plants with CO 2 under natural conditions is a factor that limits the yield. Therefore, it is advisable to grow plants indoors with a high content of CO 2 .

Temperature has a noticeable effect on the process of photosynthesis, since the dark, and partly light reactions of photosynthesis are controlled by enzymes. The optimum temperature for temperate plants is usually around 25°C.

The absorption and recovery of CO 2 in all plants increase with increasing temperature until a certain optimal level. In most plants of the temperate zone, a decrease in the intensity of photosynthesis begins after 30 ° C, in some southern views after 40 o C. In high heat (50-60 o C), when enzyme inactivation begins, and the coordination of various reactions is disturbed, photosynthesis quickly stops. As the temperature rises, the rate of respiration increases much faster than the rate of natural photosynthesis. This affects the amount of observed photosynthesis. The dependence of the intensity of the observed photosynthesis on temperature is described by a temperature curve, in which three main points are distinguished: minimum, optimum, and maximum.

The minimum is the temperature at which photosynthesis begins, the optimum is the temperature at which photosynthesis is the most stable and reaches the highest speed, the maximum is the temperature after which photosynthesis stops (Fig. 2.30).

Rice. 2.30. The dependence of the intensity of photosynthesis on the temperature of the leaf: 1 - cotton; 2 – sunflower; 3 - sorghum

Influence of oxygen. More than half a century ago, a seemingly paradoxical phenomenon was noted. Air oxygen, which is a product of photosynthesis, is also its inhibitor: the release of oxygen and the absorption of CO 2 fall as the concentration of O 2 in the air increases. This phenomenon was named after its discoverer - the Warburg effect. This effect is inherent in all C 3 -plants. And only in the leaves of C 4 -plants it could not be detected. It is now firmly established that the nature of the Warburg effect is associated with the oxygenase properties of the main enzyme of the Calvin cycle, RDF-carboxylase. With a high concentration of oxygen, photorespiration begins. It has been established that when the concentration of O 2 is reduced to 2–3%, phosphoglycolate is not formed, and the Warburg effect also disappears. Thus, both of these phenomena, the manifestation of the oxygenase properties of RDF-carboxylase and the formation of glycolate, as well as a decrease in photosynthesis in the presence of O2, are closely related to each other.

A very low content of O 2 or a complete absence, as well as an increase in concentration to 25–30%, inhibits photosynthesis. For most plants, a slight decrease in the natural concentration (21%) of O 2 activates photosynthesis.

Effect of tissue hydration. As already noted, water participates in the light stage of photosynthesis as a hydrogen donor for CO2 reduction. However, the role of the photosynthesis-limiting factor is played not by the minimum amount of water (approximately 1% of the incoming water), but by the water that is part of the cell membranes and is the medium for all biochemical reactions, activates the enzymes of the dark phase. In addition, the degree of opening of the stomata depends on the amount of water in the guard cells, and the turgor state of the whole plant determines the location of the leaves in relation to the sun's rays. The amount of water indirectly affects the change in the rate of starch deposition in the stroma of the chloroplast and even changes in the structure and arrangement of thylakoids in the stroma.

The dependence of the intensity of photosynthesis on the water content of plant tissues, as well as the dependence on temperature, is described by a transition curve that has three main points: minimum, optimum, and maximum.

With dehydration, not only the intensity of photosynthesis changes, but also qualitative composition photosynthesis products: less synthesized malate, sucrose, organic acids; more - glucose, fructose alanine and other amino acids.

In addition, it was found that with a lack of water, ABA, a growth inhibitor, accumulates in the leaves.

Chlorophyll concentration, as a rule, is not a limiting factor, however, the amount of chlorophyll may decrease with various diseases (powdery mildew, rust, viral diseases), lack of minerals and with age (with normal aging). When the leaves turn yellow, they are said to become chlorotic, and the phenomenon itself is called chlorosis. Chlorotic spots on leaves are often a symptom of a disease or mineral deficiency.

Chlorosis can also be caused by a lack of light, since light is needed for the final stage of chlorophyll biosynthesis.

mineral elements. For the synthesis of chlorophyll, mineral elements are also needed: iron, magnesium and nitrogen (the last two elements are included in its structure), therefore they are especially important for photosynthesis. Potassium is also important.

For the normal functioning of the photosynthetic apparatus, the plant must be provided with necessary quantity(optimal) mineral elements. Magnesium, in addition to being a part of chlorophyll, is involved in the action of conjugating proteins in the synthesis of ATP, affects the activity of carboxylation reactions and the reduction of NADP +.

Iron in reduced form is necessary for the processes of biosynthesis of chlorophyll and iron-containing compounds of chloroplasts (cytochromes, ferredoxin). Iron deficiency disrupts cyclic and non-cyclic photophosphorylation, pigment synthesis, and changes in the structure of chloroplasts.

Manganese and chlorine take part in the photooxidation of water.

Copper is part of plastocyanin.

Nitrogen deficiency affects not only the formation of pigment systems and chloroplast structures, but also the amount and activity of RDP carboxylase.

With a lack of phosphorus, photochemical and dark reactions of photosynthesis are disturbed.

Potassium plays a multifunctional role in the ionic regulation of photosynthesis, with its deficiency in chloroplasts, the structure of the grana is destroyed, the stomata open weakly in the light and do not close enough in the dark, the water regime of the leaf worsens, i.e., all photosynthesis processes are disrupted.

Plant age. Only after the creation of phytotrons, where it is possible to grow plants under controlled conditions, was it possible to obtain reliable results. It was found that in all plants only at the very beginning life cycle, when the photosynthetic apparatus is formed, the intensity of photosynthesis increases, reaches a maximum very quickly, then decreases slightly and then changes very little. For example, in cereals, photosynthesis reaches its maximum intensity during the tillering phase. This is explained by the fact that the maximum photosynthetic activity of the leaf coincides with the end of the period of its formation. Then aging begins and photosynthesis decreases.

The intensity of photosynthesis depends primarily on the structure of chloroplasts. As chloroplasts age, thylakoids are destroyed. Prove this using the Hill reaction. It goes worse, the older the chloroplasts. Thus, it was shown that the intensity is determined not by the amount of chlorophyll, but by the structure of the chloroplast.

AT optimal conditions humidity and nitrogen nutrition, the decrease in photosynthesis with age occurs more slowly, since under these conditions chloroplasts age more slowly.

genetic factors. The processes of photosynthesis to a certain extent depend on the heredity of the plant organism. The intensity of photosynthesis is different in plants of different systematic groups and life forms. Herbs have a higher rate of photosynthesis than woody plants(Table 2.5).

The intensity of photosynthesis depends on a number of factors. First, on the wavelength of light. The process proceeds most effectively under the action of the waves of the blue-violet and red parts of the spectrum. In addition, the rate of photosynthesis is affected by the degree of illumination, and up to a certain point the rate of the process increases in proportion to the amount of light, but then the note no longer depends on it.

Another factor is the concentration carbon dioxide. The higher it is, the more intense the process of photosynthesis. AT normal conditions the lack of carbon dioxide is the main limiting factor, since in atmospheric air contains a small percentage. However, in greenhouse conditions this deficiency can be eliminated, which will favorably affect the rate of photosynthesis and the growth rate of plants.

An important factor in the intensity of photosynthesis is temperature. All photosynthesis reactions are catalyzed by enzymes. optimal temperature is an interval of 25-30 ° C. At more low temperatures the rate of action of enzymes is sharply reduced.

Water is an important factor influencing photosynthesis. However, it is impossible to quantify this factor, since water is involved in many other metabolic processes occurring in the plant cell.

The Importance of Photosynthesis. Photosynthesis is a fundamental process in living nature. Thanks to him from inorganic substances– carbon dioxide and water – with the participation of energy sunlight green plants synthesize organic substances necessary for the life of all life on Earth. The primary synthesis of these substances ensures the implementation of the processes of assimilation and dissimilation in all organisms.

The products of photosynthesis - organic substances - are used by organisms:

• to build cells;
• as a source of energy for life processes.

Man uses substances created by plants:

• as food (fruits, seeds, etc.);
• as an energy source (coal, peat, wood);
• as a building material.

Mankind owes its existence to photosynthesis. All fuels on Earth are products of photosynthesis. Using fossil fuels, we get the energy stored as a result of photosynthesis by ancient plants that existed in past geological epochs.

Simultaneously with the synthesis of organic substances, a by-product of photosynthesis, oxygen, is released into the Earth's atmosphere, which is necessary for the respiration of organisms. Without oxygen, life on our planet is impossible. Its reserves are constantly spent on products of combustion, oxidation, respiration occurring in nature. According to scientists, without photosynthesis, the entire supply of oxygen would be used up within 3,000 years. Therefore, photosynthesis is of the greatest importance for life on Earth.

For many centuries, biologists have tried to unravel the mystery of the green leaf. Long time it was believed that plants create nutrients from water and minerals. This belief is connected with the experiment of the Dutch researcher Anna van Helmont, conducted back in the 17th century. He planted a willow tree in a tub, accurately measuring the mass of the plant (2.3 kg) and dry soil (90.8 kg). For five years, he only watered the plant, adding nothing to the soil. After five years, the mass of the tree increased by 74 kg, while the mass of the soil decreased by only 0.06 kg. The scientist concluded that the plant forms all substances from water. Thus, one substance was established that the plant absorbs during photosynthesis.

The first attempt to scientifically determine the function of a green leaf was made in 1667 by the Italian naturalist Marcello Malpighi. He noticed that if the first germinal leaves are torn off from pumpkin seedlings, then the plant stops developing. Studying the structure of plants, he made an assumption: under the action of sun rays some transformations take place in the leaves of the plant and water evaporates. However, these assumptions were ignored at the time.

After 100 years, the Swiss scientist Charles Bonnet conducted several experiments by placing a leaf of a plant in water and lighting it with sunlight. Only he made an incorrect conclusion, believing that the plant does not participate in the formation of bubbles.

The discovery of the role of the green leaf belongs to the English chemist Joseph Priestley. In 1772, while studying the importance of air for the combustion of substances and breathing, he set up an experiment and found out that plants improve the air and make it suitable for breathing and burning. After a series of experiments, Priestley noticed that plants improve the air in the light. He was the first to suggest the role of light in the life of plants.

In 1800, the Swiss scientist Jean Senebier scientifically explained the essence of this process (by that time Lavoisier had already discovered oxygen and studied its properties): plant leaves decompose carbon dioxide and release oxygen only under the action of sunlight.

In the second half of the 19th century, an alcohol extract was obtained from the leaves of green plants. This substance is called chlorophyll.

The German naturalist Robert Mayer discovered the absorption of sunlight by a plant and its transformation into the energy of chemical bonds of organic substances (the amount of carbon stored in a plant in the form of organic substances directly depends on the amount of light falling on the plant).

Kliment Arkadievich Timiryazev, Russian scientist investigated the influence various sites spectrum of sunlight on the process of photosynthesis. He managed to establish that it is in the red rays that photosynthesis proceeds most efficiently, and to prove that the intensity of this process corresponds to the absorption of light by chlorophyll.

K.A. Timiryazev emphasized that by assimilating carbon, the plant also assimilates sunlight, converting its energy into the energy of organic substances.

	The rate of photosynthesis depends on factors such as light,
		concentration of carbon dioxide, water, temperature. Why are these factors
		are limiting for photosynthesis reactions?
			(Other formulations of the answer are allowed that do not distort its meaning)
		Response elements:
			Light is the source of energy for the light reactions of photosynthesis.
			its deficiency reduces the intensity of photosynthesis;
			carbon dioxide and water are necessary for the synthesis of glucose, when they
			deficiency reduces the intensity of photosynthesis;
		3) all photosynthesis reactions are carried out with the participation
			enzymes whose activity depends on temperature
		biological errors
		Wrong answer
				Maximum score

C5 diploid set chromosomes. Determine the chromosome set (n) and the number of DNA molecules (c) in the cell at the end of meiosis telophase I and meiosis anaphase II. Explain the results in each case.

1) at the end of the telophase of meiosis I, the set of chromosomes is n; DNA number, 2s;
2) in the anaphase of meiosis II, the set of chromosomes is 2n; DNA number, 2s;
3) at the end of telophase I	reduction division has occurred, the number
chromosomes and DNA decreased by 2 times, chromosomes
dichromatid;
4) in the anaphase of meiosis	II to the poles diverge sister
chromatids (chromosomes), so the number of chromosomes is equal to the number
The answer includes all the above elements, does not contain
biological errors
The answer includes 2-3 of the above elements and does not contain
biological errors, OR the answer includes the 4 above
element, but contains non-gross biological errors
The answer includes 1 of the above items and does not contain
biological errors, OR response includes 2-3 of the above
higher than the elements, but contains non-gross biological errors
Wrong answer
	Maximum score

© 2014 federal Service for Supervision in the Sphere of Education and Science of the Russian Federation

C6 In humans, the gene for normal hearing (B) dominates the gene for deafness and is located in the autosome; the gene for color blindness (color blindness - d) is recessive and linked to the X chromosome. In a family where the mother suffered from deafness, but had normal color vision, and the father had normal hearing (homozygous), color blind, a girl was born with normal hearing, but color blind. Make a scheme for solving the problem. Determine the genotypes of parents, daughters, possible genotypes of children and their ratio. What patterns of heredity are manifested in this case?

	(correct answer must contain the following items)
The scheme for solving the problem includes:
1) genotypes of parents:
	♀ bbXD Xd		♂ ВВXd Y
	bXD , bXd		ВXd , ВY
2) possible genotypes of children:
	ВbXD Xd – a girl with normal hearing and vision 25%;
	ВbXd Xd – girl with normal hearing, colorblind 25%;
	ВbXD Y – a boy with normal hearing and vision 25%;
	BbXd Y is a boy with normal hearing and vision 25%.
3) the law of independent			trait inheritance and
sex-linked inheritance trait
The answer includes all the above elements, does not contain
biological errors
The answer includes 2 of the above elements and does not contain
biological errors OR response includes 3 of the above
element, but contains non-gross biological errors
The answer includes 1 of the above items and does not contain
biological errors, OR response includes 2 of the above
elements, but contains minor biological errors
Wrong answer
			Maximum score

© 2014 Federal Service for Supervision of Education and Science of the Russian Federation

The intensity of the photosynthesis process can be expressed in the following units: in milligrams of CO 2, assimilated by 1 dm 2 of a leaf in 1 hour; in milliliters O 2 allocated 1 dm 2 sheets for 1 hour; in milligrams of dry matter accumulated by 1 dm 2 leaves in 1 hour.

When interpreting data obtained by any method, it should be borne in mind that in the light, plants not only photosynthesize, but also respire. In this regard, all indicators measured by one method or another are the result of two directly opposite processes, or the difference between the indicators of photosynthesis and respiration. This is visible photosynthesis. So, for example, the observed change in the content of CO 2 is the difference between the amount that is absorbed during photosynthesis and that released during respiration. In order to go to true the value of photosynthesis, in all cases it is necessary to make a correction that takes into account the intensity of the respiration process.

The influence of external conditions on the intensity of the photosynthesis process

In a natural setting, all factors interact with each other, that is, the effect of one factor depends on the intensity of all the others. In general terms, this can be formulated as follows: a change in the intensity of one factor, while the others remain unchanged, affects photosynthesis, starting from the minimum level at which the process begins, and ending with the optimum, upon reaching which the process ceases to change (the curve reaches a plateau). In many cases, an increase in the intensity of the factor after a certain level even leads to a slowdown in the process. However, if you start to change any other factor, then the optimal value of the intensity of the first factor changes upwards. In other words, the plateau is reached at a higher tension value. The rate of the process, in particular the rate of photosynthesis, depends primarily on the intensity of the factor that is at a minimum (limiting factor). An example is the interaction of factors such as light intensity and CO 2 content. The higher the content of carbon dioxide (within certain limits), the higher the illumination, the indicators of photosynthesis go to a plateau.

Influence of light

Increasing the intensity of illumination affects the process of photosynthesis, the difference depending on the type of plant and the intensity of other factors. Plants in the process of historical development have adapted to grow in various lighting conditions. On this basis, plants are divided into groups: light-loving, shade-tolerant and shade-loving. These ecological groups are characterized by a number of anatomical and physiological features. They differ in the content and composition of pigments.

Light-loving plants are characterized by a lighter leaf color, a lower total chlorophyll content compared to shade-tolerant ones. The leaves of shade-tolerant plants have a relatively high content of xanthophyll and chlorophyll compared to light-loving ones. b. This feature in the composition of the pigments allows the leaves of shade-tolerant plants to use the "waste light" that has already passed through the leaves of light-loving plants. Light-loving plants are plants in open habitats that are more likely to lack water supply. In this regard, their leaves, compared with shade-tolerant ones, have a more xeromorphic anatomical structure, are thicker, and have a more strongly developed palisade parenchyma. Some light-loving plants the palisade parenchyma is located not only on the upper, but also on the lower side of the leaf. The leaves of light-loving plants, compared with shade-tolerant ones, are also characterized by smaller cells, smaller chloroplasts, smaller stomata with a larger number of them per unit leaf surface, and a denser network of veins.

Light-loving and shade-tolerant plants also differ in physiological characteristics. The high pigment content allows shade-tolerant plants to make better use of small amounts of light. In light-loving plants, the intensity of photosynthesis increases with increasing light intensity over a wider range. An important feature that determines the ability of plants to grow under greater or lesser illumination is the position of the compensation point. Under compensation point is understood as the illumination at which the processes of photosynthesis and respiration balance each other. In other words, this is the illumination at which the plant forms so much per unit of time in the process of photosynthesis organic matter how much it spends in the process of breathing. Naturally, the growth of a green plant can proceed only when the illumination is above the compensation point. The lower the respiration intensity, the lower the compensation point and the lower the illumination of the plants grow. Shade-tolerant plants are characterized by a lower respiration rate, which allows them to grow at lower light levels. The compensation point rises markedly with increasing temperature, since an increase in temperature increases respiration more than photosynthesis. That is why, in low light, an increase in temperature can reduce the growth rate of plants.

For photosynthesis, as for any process that includes photochemical reactions, the presence of a lower threshold of illumination is characteristic, at which it only begins (about one candle at a distance of 1 m). In general, the dependence of photosynthesis on light intensity can be expressed as a logarithmic curve. Initially, an increase in light intensity leads to a proportional increase in photosynthesis (zone of maximum effect). With a further increase in light intensity, photosynthesis continues to increase, but more slowly (zone of weakened effect) and, finally, light intensity increases, and photosynthesis does not change (zone of no effect - plateau). The slope of the curves expressing the dependence of the intensity of photosynthesis on illumination is different for different plants. There are plants in which photosynthesis increases until they are illuminated by direct sunlight. At the same time, for many plants, an increase in the intensity of illumination above 50% of direct sunlight is already redundant. This circumstance is due to the fact that the final yield of photosynthesis products depends on the speed of not so much light as tempo reactions. Meanwhile, the intensity of illumination affects the rate of only light reactions. Therefore, in order for the intensity of light to have an effect after reaching a certain level, it is necessary to increase the rate of dark reactions. In turn, the rate of dark reactions of photosynthesis to a large extent depends on temperature and carbon dioxide content. With an increase in temperature or with an increase in the content of carbon dioxide, the optimal illumination changes in the direction of increase.

Under natural conditions, due to mutual shading, only a small fraction of solar energy falls on the lower leaves. Thus, in a dense sowing of vetch plants in the flowering stage, the light intensity in the surface layer is only 3% of the total daylight. Often the lower leaves are illuminated with light close to the "compensation point". Thus, in crops, the total intensity of photosynthesis of all plant leaves can increase up to a level corresponding to the full intensity of sunlight.

At a very high intensity of light directly falling on the leaf, a depression of photosynthesis can be observed. In the initial stages of depression caused by high light intensity, chloroplasts move to the side walls of the cell (phototaxis). With a further increase in illumination, the intensity of photosynthesis can sharply decrease. The reason for the depression of photosynthesis by bright light can be overheating and a violation of the water balance. Perhaps in bright light there is an excess of excited chlorophyll molecules, the energy of which is spent on the oxidation of some enzymes necessary for the normal course of the photosynthesis process.

Utilization factor solar energy

On a clear sunny day, about 30,168 kJ falls on 1 dm 2 of the leaf surface in 1 hour. Of this amount, approximately 75%, or 22,626 kJ, is absorbed, 25% of the incident energy passes through the sheet and is reflected from it. Based on the amount of dry matter accumulated by the leaf over a certain period of time, the amount of stored energy was calculated and compared with the amount that the leaf receives. According to the data obtained, the efficiency of photosynthesis was 2.6%. We can even more simply approach the calculation of the quantity of interest to us. Thus, one corn plant accumulates an average of 18.3 g of dry matter per day. It can be assumed that all this substance is starch. The heat of combustion of 1 g of starch will be 17.6 kJ. Therefore, the daily energy gain will be (18.3X17.6) 322 kJ. With a density of 15 thousand plants per 1 ha, a field of 1 ha accumulates 4,830,651 kJ per day, and receives 209,500,000 kJ per day. Thus, the energy use is 2.3%.

Consequently, calculations show that the efficiency of the photosynthesis process under natural conditions is negligible. The task of increasing the efficiency of the use of solar energy is one of the most important in plant physiology. This task is quite realistic, since theoretically the efficiency of the photosynthesis process can reach a much higher value.

Temperature effect

The influence of temperature on photosynthesis depends on the intensity of illumination. At low illumination, photosynthesis does not depend on temperature (Q 10 \u003d 1). This is due to the fact that under low illumination, the intensity of photosynthesis is limited by the rate of light photochemical reactions. On the contrary, at high illumination, the rate of photosynthesis is determined by the course of dark reactions, and in this case the effect of temperature is very pronounced. The temperature coefficient Q 10 can be about two. So, for sunflower, an increase in temperature in the range from 9 to 19 ° C increases the intensity of photosynthesis by 2.5 times. The temperature limits within which the processes of photosynthesis are possible are different for different plants. The minimum temperature for photosynthesis of plants in the middle zone is about 0 ° C, for tropical plants 5-10 ° C. There is evidence that polar plants can carry out photosynthesis at temperatures below 0°C. The optimal temperature for photosynthesis for most plants is around 30-33°C. At temperatures above 30-33°C, the intensity of photosynthesis drops sharply. This is due to the fact that the dependence of the process of photosynthesis on temperature is the resultant of opposite processes. Thus, an increase in temperature increases the rate of dark reactions of photosynthesis. At the same time, at a temperature of 25-30°C, the process of inactivation of chloroplasts occurs. An increase in temperature can also cause the stomatal openings to close.

Effect of CO content 2 in the air

The source of carbon for the process of photosynthesis is carbon dioxide. Attempts to replace carbon dioxide with carbon monoxide (CO) have been unsuccessful. Mostly in the process of photosynthesis, CO 2 of the atmosphere is used. The content of CO 2 in the air is only 0.03%. The process of photosynthesis is carried out when the content of CO 2 is not less than 0.008%. Increasing the content of CO 2 to 1.5% causes a directly proportional increase in the intensity of photosynthesis. With an increase in the content of CO 2 above 1.5%, photosynthesis continues to increase, but much more slowly. With an increase in the content of CO 2 to 15-20%, the process of photosynthesis reaches a plateau. When the content of CO 2 is above 70%, depression of photosynthesis occurs. There are plants that are more sensitive to an increase in CO 2 concentration, in which the inhibition of photosynthesis begins to manifest itself already at a CO 2 content of 5%. An increase in CO 2 concentration has an inhibitory effect for various reasons. First of all, an increase in CO 2 causes the stomata to close. At the same time, high CO 2 concentrations have a particularly unfavorable effect at high illumination. The latter suggests that CO 2 at certain concentrations inhibits dark enzymatic reactions.

Under natural conditions, the content of CO 2 is so low that it can limit the increase in the process of photosynthesis. It should also be taken into account that during the daytime the CO 2 content in the air around the plants decreases.

In connection with the above, an increase in the content of CO 2 in the air is one of the important ways to increase the intensity of photosynthesis and, as a result, the accumulation of dry matter by a plant. However, in the field, the regulation of CO 2 content is difficult. Partially this can be achieved by surface application of manure or other organic fertilizers (mulching). It is easier to achieve an increase in the content of CO 2 in closed ground. In this case CO 2 supplements give good results and should be widely used. Different plants use the same concentrations of CO 2 differently. Plants in which photosynthesis follows the “C-4” pathway (corn) have a higher ability to fix CO 2 due to the high activity of the enzyme phosphoenolpyruvate carboxylase.

Impact of water supply

A slight water deficit (5-15%) in leaf cells has a beneficial effect on the intensity of photosynthesis. When leaf cells are completely saturated with water, photosynthesis decreases. This may be partly due to the fact that when the mesophyll cells are fully saturated, the guard stomatal cells are somewhat compressed, and the stomatal fissures cannot open (hydropassive movements). However, this is not the only thing. A slight dehydration_of the leaves has a favorable effect on the process of photosynthesis, regardless of the degree of opening of the stomata. An increase in water deficit over 15-20% leads to a noticeable decrease in the intensity of photosynthesis. This is primarily due to the closure of stomata (hydroactive movements), which sharply reduces the diffusion of CO2 into the leaf. In addition, it causes a reduction in transpiration, as a result, the temperature of the leaves increases. Meanwhile, an increase in temperature above 30°C causes a decrease in photosynthesis. Finally, dehydration affects the conformation and, consequently, the activity of the enzymes involved in the tempo phase of photosynthesis.

Oxygen supply and intensityphotosynthesis

Despite the fact that oxygen is one of the products of the photosynthesis process, under conditions of complete anaerobiosis, the photosynthesis process stops. It can be assumed that the influence of anaerobiosis is indirect, associated with the inhibition of the respiration process and the accumulation of products of incomplete oxidation, in particular organic acids. This assumption is confirmed by the fact that the harmful effect of anaerobiosis is more pronounced in an acidic environment. An increase in oxygen concentration (up to 25%) also inhibits photosynthesis (the Warburg effect).

The inhibitory effect of high oxygen concentrations on photosynthesis is especially pronounced at increased light intensity. These observations forced attention to the peculiarities of the process of respiration in the presence of light (photorespiration). The chemistry of this process is different from ordinary dark respiration. photorespiration- this is the absorption of oxygen and the release of CO 2 in the light using the intermediate products of the Calvin cycle as a substrate. Apparently, phosphoglyceric acid formed in the Calvin cycle during photorespiration is oxidized and decarboxylated to glycolic acid, and glycolic acid is oxidized to glyoxylic acid. The formation of glycolic acid occurs in chloroplasts, but it does not accumulate there, but is transported to special peroxisome organelles. Peroxisomes convert glycolic acid to glyoxylic acid. Glyoxylic acid, in turn, undergoes amination and then decarboxylation, which releases carbon dioxide.

The release of CO 2 during photorespiration can reach 50% of the total CO 2 absorbed during photosynthesis. In this regard, it can be assumed that a decrease in the intensity of photorespiration should lead to an increase in plant productivity. Thus, mutant forms of tobacco that do not have the ability to form glycolic acid are characterized by an increased accumulation of dry mass. There is evidence that a slight decrease in the oxygen content in the atmosphere favorably affects the rate of accumulation of dry matter by seedlings. In corn and other plants that carry out photosynthesis along the "C-4" path, photorespiration does not go. It is possible that this type of exchange contributes to greater productivity of these plants.

The influence of mineral nutrition

Influence potassium on photosynthesis in many ways. With a lack of potassium, the intensity of photosynthesis decreases after a short period of time. Potassium can influence photosynthesis indirectly, through an increase in the hydration of the cytoplasm, an acceleration of the outflow of assimilates from leaves, and an increase in the degree of opening of stomata. At the same time, there is also a direct influence of potassium, since it activates the processes of phosphorylation.

Very great value phosphorus for photosynthesis. Phosphorylated compounds take part in all stages of photosynthesis. The energy of light is accumulated in phosphorus bonds.

Recently, much attention has been paid to elucidating the role manganese. When studying the photosynthesis of a strain of chlorella, which can grow both in the dark due to ready-made organic matter, and in the light, it was shown that manganese is necessary only in the latter case. For those microorganisms that carry out the process of photoreduction, manganese is not needed. At the same time, the absence of manganese sharply inhibits the Hill reaction and the process of non-cyclic photophosphorylation. All this proves that the role of manganese is determined by its participation in water photooxidation reactions.

Many compounds that function as carriers contain iron(cytochromes, ferredoxin) or copper(plastocyanin). Naturally, with a lack of these elements, the intensity of photosynthesis decreases.

Factors affecting the efficiency of photosynthesis

The intensity or speed of the photosynthesis process in a plant depends on a number of internal and external factors. From internal factors highest value have the structure of the leaf and the content of chlorophyll in it, the accumulation of photosynthesis products in chloroplasts, the influence of enzymes, as well as the presence of small amounts of essential inorganic substances. External factors- these are the parameters of the radiation falling on the leaves, the ambient temperature, the concentration of carbon dioxide and oxygen in the atmosphere near the plant. Let's take a closer look at some of these factors.

The influence of physical and chemical factors on the process of photosynthesis

When studying the impact of microwave radiation on wheat, such “indirect” signs were the germination rate, germination, intensity (speed) of sprout development, which are the result of processes that have not been fully studied in the biosystem under microwave exposure. Even in cases where it is possible to model changes in cellular level, correlation studies are carried out after irradiation and growing plants. Thus, in most cases, the response of a biological object to an impact is assessed by "remote" effects. One of these "remote" effects for green plants may be the intensity of photosynthetic reactions.

The effect of light intensity on photosynthetic activity is shown in fig. 2. At low light intensities, the rate of photosynthesis, as measured by the release of oxygen, increases in direct proportion to the increase in light intensity. The corresponding section on the graph, indicated by the letter X, is called the initial site, or the area in which the rate of photosynthesis is limited by light. As the light intensity increases further, the increase in photosynthesis becomes less and less pronounced, and finally, when the illumination reaches a certain level (about 10,000 lux), a further increase in light intensity no longer affects the rate of photosynthesis. In the figure, this corresponds to the horizontal sections of the curves, or plateaus. The plateau region, denoted by the letter Y, is called the light saturation region. If you want to increase the rate of photosynthesis in this area, you should not change the light intensity, but some other factors. The intensity of sunlight falling on the surface of the earth on a clear summer day in many places on our planet is about 105 lux, or about 1000 W/m2.

Besides important role temperature also plays a role in photosynthesis (the second factor). In the case of low light intensities, the rate of photosynthesis at 15°C and 25°C is the same. Reactions proceeding at light intensities that correspond to the light-limiting region, like true photochemical reactions, are not sensitive to temperature. However, at higher intensities, the rate of photosynthesis at 25°C is much higher than at 15°C. Most plants in temperate climates function well in the temperature range from 10°C to 35°C, the most favorable conditions are temperatures around 25°C.

The third factor affecting the rate of photosynthesis is the change in the frequency of the light quantum (wave color). Radiant energy is emitted and propagated in the form of discrete units - quanta, or photons. A quantum of light has energy E = h·n= h·c /l where h is Planck's constant. It is clear from this formula that the value of the photon energy for different parts of the spectrum is different: the shorter the wavelength, the longer it is.

The energy of quanta corresponding to the extreme parts of the visible range - violet (about 400 nm) and far red differs only by a factor of two, and all photons in this range are in principle capable of triggering photosynthesis, although, as we will see below, leaf pigments selectively absorb light of certain wavelengths.

Comparative characteristics of different parts of the spectrum are given in Table 1.

Table 1.

In the region of light limitation, the rate of photosynthesis does not change with a decrease in the concentration of CO2 in environment(fourth factor). But at higher illumination intensities, which lie outside the light-limiting region, photosynthesis increases significantly with increasing CO2 concentration. In some crops, photosynthesis increased linearly with increasing CO2 concentration up to 0.5% (these measurements were carried out in short-term experiments, since long-term exposure to such high concentrations of CO2 damages the leaves). The rate of photosynthesis reaches very high values ​​at a CO2 content of about 0.1%. The average concentration of carbon dioxide in the atmosphere ranges from 0.03 to 0.04%. Therefore, under normal conditions, plants do not have enough CO2 in order to maximum efficiency use the sunlight that hits them.

Influence of internal factors

Also, the rate of photosynthesis is influenced by internal factors, such as the amount of chlorophyll in the plant, the area of ​​the green surface of the plant, etc. In our work, we study the influence of external factors.