Factors of photosynthesis intensity and the influence of the light regime on plant development

The intensity of photosynthesis is determined by two groups of factors.

External factors:

• carbon dioxide concentration in the air;
• inflow of water and nutrients;
• ambient temperature;
• intensity and spectral composition of light;
• air pollution and the presence of toxic substances in it;
• wind regime of the field.

Genetically determined parameters of the plant's architectonics and properties:

• leaf structure;
• amount of chlorophyll in it and the rate of assimilate outflow;
• presence of enzymes involved in photosynthesis;
• number of stomata;
• degree of ecological adaptation to growing conditions.

Light is the energy source for electron transport and carbon dioxide reduction. This is the direct influence of light on photosynthesis, its substrate role. In addition, it indirectly affects the intensity of photosynthesis by regulating the degree of stomatal opening. Therefore, an increase in light intensity should be accompanied by an increase in the photosynthetic activity of plants.

At the same time, the direct dependence of photosynthesis on light intensity is observed only up to the so-called compensation point, i.e., up to the illuminance at which carbon dioxide uptake during photosynthesis and its release during respiration balance each other (Fig. 32; Belikov P.S., Dmitrieva G.A., 1992). The light compensation point for most plants is approximately 10 thousand lux.

With its further increase, the rates of increase in photosynthesis intensity gradually weaken and cease to grow at 30-50 thousand lux. So-called light saturation occurs, i.e., light ceases to be a limiting factor for photosynthesis.

Fig. 32. Dependence of photosynthesis intensity on illuminance

In addition to light intensity, its spectral, i.e., qualitative composition, is also important for photosynthesis. The rate of photosynthesis when illuminated with light of different wavelengths, but equalized in terms of energy quantity, is different. The intensity of photosynthesis is highest in the red part of the spectrum and lowest in the blue and green parts.

It has been established that the quality of light does not affect the primary photochemical reactions, but rather the subsequent conversion of intermediate products and the direction of the photosynthesis process. Short-wave light promotes the formation of amino acids, proteins, and organic acids, while illumination with long-wave rays promotes the synthesis and accumulation of carbohydrates.

Light, by exciting or inactivating photoreactions that regulate individual links of plant metabolism and creating preferential conditions for the formation of certain metabolites, ultimately affects morphogenesis and plant productivity. The most favorable conditions for intensive photosynthesis are created when plants are illuminated with natural light with a broad spectrum.

Carbon dioxide is involved only in the dark phase of photosynthesis, but it plays a dual role. Firstly, it is a source of carbon for the synthesis of organic substances (substrate role), and secondly, it regulates the degree of stomatal opening. The concentration in the air is 0.03%, which corresponds to 0.57 mg of CO2 per 1 L of air, or 300 µl/L. One square meter of leaf surface with average assimilation productivity produces 1 g of carbohydrates per hour and uses about 1.5 g of CO2 for this, i.e., its supply in the nearest 3 m3 of air. The intensity of photosynthesis increases to the greatest extent when the concentration of CO2 in the air increases to 0.1% (Fig. 33 Friedrich G., 1983). With a further increase in the CO2 content in the air, the intensity of photosynthesis continues to increase, but to a lesser extent. Upon reaching carbon dioxide saturation, this factor ceases to be a limiting factor for photosynthesis intensity.

Fig. 33. Dependence of photosynthesis intensity on illuminance and CO2 content in the air

The process of photosynthesis is usually carried out under aerobic conditions at an oxygen concentration of 21%. An increase in content or the absence of oxygen is unfavorable for photosynthesis. Already in the first half of the 20th century, O. Warburg established that O2 release and CO2 uptake decrease as the oxygen content in the atmosphere increases. This phenomenon was named after its discoverer—the "Warburg effect." It is widespread in C3-plants. The nature of this phenomenon is connected with the oxygenase properties of the main enzyme of the Calvin cycle – ribulose-1,5-bisphosphate carboxylase (RuBP carboxylase). At a higher oxygen concentration, photorespiration begins. It has been established that when the oxygen concentration is reduced to 2–3%, phosphoglycolate is not formed, and the Warburg effect disappears. In addition, at a high oxygen concentration in the atmosphere, pseudocyclic phosphorylation can occur during the light phase, which leads to the reduction of atmospheric oxygen to form water instead of the reduction of NADP+. A decrease in the number of reduced molecules of the latter leads to the inhibition of the dark phase of photosynthesis.

Temperature influences the course of all physiological processes occurring in a plant organism without exception, including the intensity of photosynthesis. Temperature mainly affects the dark phase of photosynthesis by regulating enzyme activity and the rate of carbon dioxide diffusion. Its influence on the light phase is less, since light absorption and energy migration do not depend on temperature at those values at which life is possible. When the temperature rises above the optimal value, photosynthetic phosphorylation is disrupted.

Temperature affects photosynthesis indirectly as well, by altering the rate of assimilate outflow from the leaf blade to other organs, whereas the accumulation of assimilates in the leaf blade slows down photosynthesis. The general dependence of photosynthesis on temperature is represented by a unimodal curve, which has three main temperature points:

• maximum;
• optimum;
• minimum.

The temperature optimum for photosynthesis in various plants lies within the range of 25-35 °С. As the temperature gradually increases to these values, the rate of photosynthesis rises, while any further increase causes inhibition of this process. High temperatures enhance transpiration, causing a water deficit, which leads to stomatal closure and reduces the supply of carbon dioxide to the chloroplasts. The low intensity of photosynthesis that occurs at low temperatures is due to insignificant water absorption, which also leads to the closing of stomata.

In plants naturally occurring or specifically grown in places with sharply changing temperatures, an adaptation of photosynthesis to the corresponding growth conditions is usually observed in response to temperature changes. It manifests itself both in the varying capacity for photosynthesis within given temperature ranges and in the shifting of the leaf's temperature tolerance limits towards either high or low values. Similar to the physiological response to bright or low light, adaptive reactions can be induced by environmental factors or genetically fixed (Fig. 34, 35; Ort D., Melandri B., Junge W. et al., 1987).

A change in one of the temperature limits is usually accompanied by a weakening of the adaptive reaction to the opposite extreme temperature. Adaptation to high temperature is, at least in part, associated with an increase in the temperature stability of chloroplast membranes, whereas adaptation to low temperature is partially due to an increase in the activity of enzymes that limit the rate of photosynthesis. True, in plants, the quantity of enzymes and the heat resistance of chloroplast membranes can change simultaneously.

Fig. 34. Temperature dependence of CO2 assimilation at CO2 partial pressures of 330 µbar (A, C) and 1000 µbar (B, D) for selected pairs of C3 and C4 species grown at low (A, B) or high (C, D) temperatures. The C4 species Atriplex sabulosa and the C3 species Atriplex glabriuscula were grown at 16°С. The C4 species Tidestromia oblongifolia and the C3 species Larrea divaricata were grown at 45°С.

Fig. 35. Changes in the temperature dependence of CO2 uptake in Geraea canescens grown at temperatures of 20 or 40°С (B) and high CO2 partial pressure (1000 µbar). Chlorophyll a fluorescence curves for the same leaves of these plants (A).

Plants differ in their ability to adapt to changes in ambient temperature. In response to changes in growing temperature, plants from habitats with more stable temperature conditions generally have a lower capacity to change the thermostability of chloroplast membranes. There are differences in the response to changing ambient temperatures between C3 and C4 plants. Most C3 species acclimate and grow at low temperatures without a significant decrease in the rate of photosynthesis. C4 metabolism is partially considered a mechanism for the adaptation of photosynthesis to high temperatures. The basis for this is the high rate of photosynthesis in C4 species at light intensities that are both saturating and growth-limiting, and at temperatures exceeding 30°С. Furthermore, the advantages of C4 plants, which manifest at high temperatures, may be associated with greater water economy compared to C3 plants.

Water is directly involved in photosynthesis as substrate of oxidation and a source of oxygen. In addition, the rate of photosynthesis is determined by the level of leaf hydration, on which the degree of stomatal opening and, consequently, the intake of carbon dioxide into the leaf depends. The dependence of the rate of photosynthesis on the degree of leaf hydration is expressed by the saturation deficit, which reflects the amount of water the plant lacks for full saturation. Photosynthesis reaches its maximum intensity at very low deficit values – on the order of 5-10% of full saturation (Fig. 36; Tarchevsky I.A., 1977).

Fig. 36. Dependence of leaf photosynthesis rate on their water content: on the abscissa axis – the value of leaf dehydration as a percentage of their full water saturation, on the ordinate axis – the rate of photosynthesis in percent.

This fact was discovered by V.A. Brilliant as early as 1925, and subsequently it was named the "Brilliant effect". This phenomenon is adaptive in nature and is explained by the fact that the state of water saturation in leaves is typical for terrestrial plants. Upon full saturation, stomata fill with water, close, and the rate of photosynthesis drops to zero. Under drought conditions, excessive water loss by the plant also causes stomatal closure, which in turn sharply reduces the diffusion of carbon dioxide into the leaf. Furthermore, this causes a reduction in transpiration; as a result, leaf temperature increases. Its rise above 30°С causes a decrease in the rate of photosynthesis. Finally, dehydration affects the conformation and, consequently, the activity of enzymes involved in the dark reactions of photosynthesis.

In the complete absence of wind, depletion of carbon dioxide reserves in the air layer near the ground may occur. Therefore, slight air movement stimulates photosynthesis. Optimal photosynthesis parameters are observed at a wind speed of approximately 100 m/min. At higher wind speeds, a decrease in the intensity of photosynthesis is observed due to increased transpiration, accompanied by the closure of stomata.

Soil salinization directly threatens crop yield, especially under irrigation in semi-arid zones. Most food crops are highly sensitive to high salt concentrations in the root zone. Under such conditions, water exchange is disrupted, forcing plants to spend resources on survival rather than on harvest formation.

• Total area of irrigated agriculture — 160·106 ha
• Share of saline lands of the irrigated area — one third

How salinization blocks leaf growth and disrupts photosynthesis

Most crops are able to regulate internal osmotic pressure to absorb moisture from the saline soil solution. Thanks to this, salt stress does not initially cause severe disruptions in photosynthetic reactions. However, the capabilities of such self-regulation are not fully manifested in all species and plant parts.

Insufficient turgor pressure prevents normal plant cell expansion. This limits the development of vegetative mass and prevents the crop from forming a full-fledged leaf apparatus. In field conditions, a turgor deficit leads to two key consequences:

• limitation of leaf area increase;
• reduction in the size of the plant's photosynthetic apparatus.

As a result, plant growth slows down significantly, even if the salinization itself does not directly affect the intensity of photosynthesis. In parallel, excess salts can significantly change the respiration intensity of the crop. Such changes in the respiration process significantly affect the overall productivity of photosynthesis.