Air Nutrition of Plants (Photosynthesis)

Air nutrition of plants, or photosynthesis, is the process of formation of nitrogen-free organic substances (carbohydrates) by plants from atmospheric carbon dioxide and water under the action of sunlight:

Terrestrial plants annually absorb from the atmosphere about 20 billion t of carbon in the form of carbon dioxide, or on average 1300 kg per 1 ha; the entire set of plants, including marine algae, about 150 billion t. Land plants process 4217 kJ of cosmic solar energy into assimilation products annually.

However, photosynthetically active radiation (PAR), that is, sunlight with a wavelength from 380 to 720 nm, constitutes 47–49% of integral solar radiation. In crops, PAR utilization coefficients do not exceed 0.5–3%. The maximum possible PAR efficiency for photosynthesis is considered to be 28%. The most intensive biomass accumulation—up to 700 kg/ha per day—occurs under good conditions of illumination, temperature, and water supply, and a high level of nutrient availability, and amounts to up to 14% of the total PAR input per day.

Simple carbohydrates formed during photosynthesis serve as the starting material for the synthesis of complex carbohydrates: sucrose C₁₂H₂₂O₁₁, starch (C₆H₁₀O₅)_n, cellulose (C₆H₁₀O₅)_n.

Photosynthetic activity depends on the species characteristics of the plant, the age of individual leaves and the whole plant, the intensity and wavelength of light, and the level of nitrogen nutrition.

Only 2–4% of solar energy incident on the surface of vegetating plants is used for the synthesis of organic substances. The rest is spent on transpiration and reflection. The plant evaporates water for cooling. The evaporation process itself involves a large expenditure of energy. More than 25% of solar energy is spent on evaporation by leaves; in southern regions, up to 70–95%, which is approximately 10–45 times more than is stored in the harvest.

One task of modern science is to find ways to increase the coefficient of solar energy utilization.

If the consequences of predatory management, involuntarily removing nutrients from the soil, are still correctable in one way or another, by fertilizing the land, then only the wasteful, unskilled use of the main source of national wealth—sunlight—is ultimately irreparable.

K. A. Timiryazev

For the formation of complex organic substances from the primary products of photosynthesis, energy generated in the plant as a result of respiratory processes, that is, the oxidation of carbohydrates by oxygen, is expended. This process is opposite to photosynthesis:

The energy released during respiration is used for:

• synthesis of other organic compounds;
• absorption by roots of salts and water from the soil and their movement through plant parts;
• performance by roots of work in the soil during their growth.

The energy of respiration is also used to overcome soil resistance by sprouts during germination.

The energy released in the process of respiratory oxidation of substances passes into a specific form of energy storage—macroergic phosphate bonds of adenosine triphosphate (ATP).

Macroergic compounds can be divided into two groups:

• glycerophosphate, 3-phosphoglyceric acid, glucose-6-phosphate, fructose-6-phosphate. Compounds of this group store from 0.8 to 3.0 kcal per 1 mol of substance;
• adenosine triphosphate (ATP), adenosine diphosphate (ADP), 1,3-diphosphoglyceric acid, phosphoenolpyruvic acid. Compounds of this group store from 6 to 16 kcal per 1 mol.

In all metabolic reactions, energy is used in coupled processes of energy release and utilization, and energy transfer from one reaction to another can occur only when two reactions proceed sequentially and have common intermediate products. Thus, sucrose formation can proceed coupled with ATP hydrolysis:

Total:

Processes of starch formation from glucose and proteins from amino acids proceed similarly.

In dry hot years with dry winds, photosynthesis in plants is possible only in the early morning and evening hours. During the rest of the time, there is a loss of plastic substances and energy to resistance and protective reactions against unfavorable environmental conditions (moisture deficit and elevated temperature). At the same time, the balance between formation and expenditure of macroergic phosphorus compounds is disturbed, energy potential decreases, and oxidative potential in the cell increases, which leads to oxidative destruction of carbohydrates and proteins; as a result, ammonia accumulates in the tissues of the plant organism and poisoning occurs.

A positive effect of phosphorus and potassium on the water content of protoplasm colloids was noted, which leads to a decrease in moisture expenditure on transpiration. Plant tissues supplied with phosphorus are characterized by high water-holding capacity. Such plants have more stable water exchange due to an increase in the content of osmotically and colloidally bound water and increased hydration of protoplasm components. The effect of phosphorus is especially pronounced under conditions of insufficient water supply in the early periods of plant development.

At the current stage of development of agricultural science, possibilities for regulating photosynthesis processes are limited. The assimilation surface of leaves in crops can vary from 5–6 to 40–50 thousand m² per 1 ha. Sparse crops absorb only 20–25% of incident PAR and use only 1–2% of absorbed for photosynthesis. With sufficient crop density during the growing season, plants can absorb 50–60% of incident PAR and accumulate in organic substances of the harvest up to 2–3% of absorbed energy. Theoretically, this indicator can be increased to 20–25%. If the coefficient of utilization of absorbed energy for photosynthesis is increased to 6–8%, this will lead to a reduction in water consumption for creating 1 t of dry matter from 400–500 to 75–100 t.