Photosynthesis is the process of the formation of nitrogen-free organic substances (carbohydrates) by plants from atmospheric carbon dioxide and water under the influence of sunlight:
6 CO2 + 6 H2O + 674 kcal → C6H12O6 + 6 O2.
Plants growing on land absorb about 20 billion tons of carbon from the atmosphere annually in the form of carbon dioxide, or on average 1,300 kg per hectare; the entire plant community, including seaweeds, absorbs about 150 billion tons. Terrestrial plants convert 4,217 kJ of cosmic solar energy into assimilation products annually.
However, the coefficients of photosynthetically active radiation (PAR), i.e. sunlight with a wavelength from 380 to 720 nm, for the creation of organic matter amounts to 47-49% of the integral solar radiation. In crops, the rates of use of PAR do not exceed 0.5-3%. The maximum possible for photosynthesis is considered to be the efficiency of PAR of 28%. The most intensive accumulation of biomass – up to 700 kg/ha per day – occurs under good conditions of light, temperature and water supply, high level of nutrient supply and amounts to 14% of the total PAR input per day.
Simple carbohydrates formed during photosynthesis serve as a starting material for the synthesis of complex carbohydrates: sucrose C12H22O11, starch (C6H10O5)n, fiber (C6H10O5)n.
Photosynthetic activity depends on plant species, 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 the solar energy that reaches the surface of vegetating plants is used to synthesize organic matter. The rest is used for transpiration and reflection. The plant evaporates water for cooling. The evaporation process itself involves a large expenditure of energy. Over 25% of solar energy is used by leaves for evaporation, and in southern regions it can reach 70-95%, which is about 10-45 times more than is stored in the harvest.
One of the challenges of modern science is finding ways to increase the solar energy rate.
“If the consequences of a predatory farming, involuntarily removing nutrients from the soil, are correctable in one way or another, by fertilizing the land, only the wasteful, inept use of the main source of the people’s wealth – sunlight – is irreparable.”
To form complex organic substances from the primary products of photosynthesis, the energy generated in the plant as a result of respiratory processes, i.e. the oxidation of carbohydrates by oxygen, is expended. This process is the opposite of photosynthesis:
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + 674 kcal.
The energy released during respiration is used for:
- synthesis of other organic compounds;
- roots absorb salts and water from the soil and move them through the plant;
- roots doing work in the soil as they grow.
Breathing energy is also used to overcome the soil resistance of the sprouts as they germinate.
Energy released in the process of respiratory oxidation of substances is converted into a specific form of energy storage – macroergic phosphate bonds of adenosine triphosphoric acid (ATP).
Macroergic compounds can be divided into two groups:
- glycerophosphate, 3-phosphoglyceric acid, glucose-6-phosphate, and fructose-6-phosphate. Compounds of this group accumulate from 0.8 to 3.0 kcal per 1 mole of substance;
- adenosine triphosphoric acid (ATP), adenosine diphosphoric acid (ADP), 1,3-diphosphoglyceric acid, phosphoenolpyruvic acid. Compounds of this group accumulate from 6 to 16 kcal per 1 mole.
In all metabolic reactions, energy is used in conjugated processes of energy release and utilization, and energy transfer from one reaction to another can only occur when two reactions go in series and have common intermediate products. Thus, the formation of sucrose can proceed in conjunction with the hydrolysis of ATP:
ATP + glucose → glucosephosphate + ADP (ΔF = -7000);
glucosephosphate + fructose → sucrose + H3PO4.
ATP + glucose + fructose → sucrose + ADP + H3PO4 (ΔF = -7000).
The processes of formation of starch from glucose and proteins from amino acids proceed similarly.
During dry, hot years with dry winds, photosynthesis in plants is possible only in the early morning and evening hours. The rest of the time there is a loss of plastic substances and energy for resistance and defense reactions to unfavorable environmental conditions (deficit of moisture and increased temperature). The balance between formation and expenditure of macro-ergic phosphorus compounds is disturbed, energy potential is reduced, oxidative potential in the cell is increased, which leads to oxidative destruction of carbohydrates, proteins, due to which ammonia is accumulated in tissues of plant organism and their poisoning occurs.
A positive effect of phosphorus and potassium on the water content of protoplasm colloids has been noted, which leads to a reduction of moisture consumption for transpiration. Plant tissues supplied with phosphorus are characterized by high water retention capacity. In such plants, water exchange is more stable due to increased content of osmotically and colloidally bound water and increased hydration of protoplasm components. The effect of phosphorus is particularly manifested under conditions of insufficient water supply in the early periods of plant development.
At the current stage of development of agricultural science, the ability to regulate photosynthetic processes is limited. Assimilative surface of leaves in crops can vary from 5-6 to 40-50 thousand m2 per 1 ha. Thinned crops absorb only 20-25% of the incident PAR and use for photosynthesis only 1-2% of the absorbed. With sufficient density of crops during the vegetation period plants can absorb 50-60% of the incident PAR and accumulate in the organic matter of the crop up to 2-3% of the absorbed energy. Theoretically, this indicator can be increased up to 20-25%. If the coefficient of utilization of absorbed energy for photosynthesis is increased up to 6-8%, it will lead to reduction of water consumption for creation of 1 ton of dry matter from 400-500 to 75-100 tons.
Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.
Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. – Moscow: Kolos, 2002. – 584 p.: ill.