Plant Physiology - Musienko M. M. 2001

Photosynthesis: Physiological, Biochemical, and Ecological Aspects
Photosynthesis and Biological Productivity

Over billions of years, plant organisms have shaped our planet's modern atmosphere and created the conditions for The Development of contemporary life forms, including humans, by providing food, energy, and numerous other biological resources essential for survival. Photosynthesis converts light energy into chemical energy, which is then stored in biomass. Plant biomass refers to the dry mass of living above-ground and below-ground plant parts per unit of soil surface area at a given time. Production, on the other hand, is defined as the biomass or mass of organic matter assimilated by a specific plant community or individual species per unit area over a unit of time.

Primary production, in turn, is subdivided into gross primary production and net primary production. Gross primary production represents the total amount of assimilated organic matter, including Respiration losses, whereas net primary production is the fraction of organic matter (minus respiration losses) available for human use or remaining as accumulated chemical energy. The average terrestrial plant productivity (including freshwater bodies) is estimated at 669 g/m² per year, compared to 155 g/m² per year in seas and oceans. Total terrestrial productivity amounts to 100.2 × 10⁹ t of biomass per year, marine productivity to 55 × 10⁹ t of biomass per year, and planetary productivity as a whole reaches 155.2 × 10⁹ t of biomass per year. Refined assessment Methods have updated these figures, and as of 2000, the total planetary biomass production is estimated at 1797 × 10⁹ t per year. This corresponds to 10¹⁵ kJ of energy. Calculations indicate that global net photosynthetic productivity amounts to 78 × 10⁹ t of carbon per year, 7% of which is utilized directly or via animal organisms by humans as food, fuel, and raw Materials.

The accumulation of living biomass over the course of a year constitutes the net ecosystem productivity. Among terrestrial phototrophs, tropical forests make the largest contribution to gross production, accounting for up to 29%. Overall, THE CONTRIBUTION OF all forest types reaches 68%.

Although the area of the oceans is 2.5 times larger than that of the land, their primary production accounts for only 1/2 to 1/3 of terrestrial production. Total net primary production is calculated to be equivalent to 10–13% of the carbon content in the atmosphere. It has been established that the turnover time of atmospheric carbon driven by the biosphere is 7–10 years. Taking respiration and Photorespiration into account, this timeframe is halved to 3–5 years. Carbon exchange between the atmosphere and the ocean surface also takes 7–8 years. The assimilation of CO₂ by cultivated crops determines humanity's reliance on photosynthesis, a dependency that becomes especially prominent today given the sharp discrepancy between production growth and population increase.

Biomass can be utilized not only as a food source but also as a raw material and for biofuel production. It has been estimated that annual photosynthetic production contains nearly 10 times more energy than is currently required to meet the energy needs of the entire global population. Rational utilization of biomass as an energy source requires substantial scientific and technological advancements.

Consequently, while retaining its role as the primary source of food and raw materials, photosynthesis will increasingly be harnessed as a source of novel, unconventional fuels such as alcohol, biogas, and photohydrogen. The photosynthetic process operates against the thermodynamic potential gradient and overall Entropy increase. Nevertheless, owing to its perfection, it proceeds quite readily and with high efficiency under normal environmental conditions.

A characteristic feature of modern phytocenoses is their multi-species composition, which was shaped through divergent and adaptive evolution. The evolution of plant organisms likely occurred not by establishing a few ecologically universal genotypes, but rather through The formation of diverse, often highly specialized bio- and ecoforms that combine in various ways to create today's highly productive phytocenoses. By executing genetic programs within ontogenetic cycles, such plant forms express them across varying intervals of adaptive phenotypic Variability inherent to a specific genotype.

To ensure high productivity, phytocenoses must maintain optimal parameters of optical and diffusion density, permeability, along with specific sizes and quantities of functional units of Photosynthetic Systems across various Levels of Organization—ranging from the total surface area of photosynthetic organisms to METABOLISM/14.html">Chloroplasts, RCs, and Photosystems.

Given that the maximum energy efficiency of photosynthesis occurs at a quantum requirement of eight quanta per CO₂ molecule, and considering that respiration in phytocenoses accounts for a 40–60% loss of assimilated CO₂ and energy, researchers have established the potential levels of organic productivity for phytocenoses with an Energy Conversion Efficiency of 4–6% for utilized PAR. Achieving this level of productivity, particularly in agrocenoses, is the ultimate goal of scientific research in agrotechnology and efforts directed at the conservation and utilization of natural photosynthetic systems within numerous international and national programs.

The potential photosynthetic capacity of a crop stand is characterized by the plant photosynthetic potential, which represents the sum of daily leaf area index values throughout the entire growing season or a specific segment thereof.

Quite frequently, There is a need to determine the efficiency of the photosynthetic apparatus directed toward the formation of economically valuable Organs, such as grains, ROOT crops, etc. Therefore, a distinction is made between biological yield and economic yield.

Biological yield is the sum of all daily increments over the growing season:

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where Y_biol is the biological yield, C represents daily dry mass increments (in kg/ha) per day, and n is the number of days.

To convert the mass of assimilated CO₂ into dry matter, an efficiency coefficient of 0.64 must be applied (1 g of assimilated CO₂ corresponds to 0.64 g of CARBOHYDRATES). However, not all dry matter accumulates. A fraction of it (25–30%) is expended on respiration, and some is lost through exosmosis or organ shedding. At the same time, a small portion (5–10%) of substances is absorbed via The Root System. Taking all these factors into account, the coefficient is approximately 0.50.

Calculations indicate that the maximum photosynthetic rate can reach 100 mg CO₂/dm² per hour, whereas the most frequently observed value for this parameter is 10–15 mg CO₂/dm² per hour.

The size of the leaf surface in a crop stand is expressed using the leaf area index. The leaf area index is The ratio of the total leaf surface area to the area of soil occupied by the crop stand. In wheat, for instance, this index is 7, meaning that for every hectare of crop, the leaf area amounts to 70,000 m².

Thus, total plant dry mass accumulation depends on the photosynthetic rate, efficiency coefficient, leaf surface area, and the total number of days in the growing season.

Economic yield constitutes only a specific fraction of the biological yield, and moreover, the harvest index varies significantly among different crops. For instance, in cereal grains, the most

valuable part is the grain; in potatoes, it is the tubers; in many other plants, it is the root crops, and so on. Therefore, the economically valuable yield is determined by taking into account the harvest index (K_econ), that is:

Consequently, by understanding the potentials and regularities of photosynthetic systems across all levels of organization (from Reaction Centers and chloroplasts to phytocenoses) and aligning them with other vital plant Functions (energetics, growth, morphogenesis), it is possible to establish production systems that operate with the minimum necessary expenditure of substrates and energy. While highlighting the crucial role of photosynthesis in the production process, one must also account for its dependence on GROWTH AND DEVELOPMENT processes, respiration, Water regime, and mineral Nutrition.