Optimization of crop photosynthesis for increasing yield

How to increase the efficiency of light utilization in crops

Crop yield directly depends on how efficiently plants absorb solar energy. Any agricultural practice that increases the harvest ultimately increases the photosynthetic efficiency of the crops. Theoretically, plant productivity is limited only by the influx of solar radiation and its assimilation by chloroplasts.

• Spectrum of photosynthetically active radiation (PAR) — 380–720 nm
• Share of PAR in solar radiation — about 50%
• Solar energy utilization in standard fields — 2–3%
• Solar energy utilization in high-yield fields — about 5%

For plants to make the most efficient use of available light, an agronomist must optimize growing conditions while taking into account the biological characteristics of the crops. The light regime can be regulated both at the stage of planning the planting and during the growing season. A complex of organizational and agrotechnical practices is used for this purpose:

• Row orientation: for uniform illumination, crops are arranged in a north-south direction.
• Site topography: sun-loving crops are placed on southern slopes and elevations, while shade-tolerant ones are placed on northern slopes and in lowlands.
• Sowing density: light penetration within the plant community is regulated by stand density, timely weeding, thinning of seedlings, and the use of cover crops in mixed cropping.
• Shelterbelts: shade-tolerant crops can be shaded by taller light-loving plants (for example, planting corn between cucumbers).
• Plant architecture: in cereal crops, photosynthesis is more efficient if the upper leaves are angled sharply toward the stem. In sugar beet, a funnel-shaped rosette of leaves is more productive than a prostrate one, as it accelerates the outflow of sugars into the root crop.

C3 and C4 crops: what is the difference for productivity

Agricultural crops are divided into two groups based on their type of photosynthesis, which determines their requirements for light and moisture. Most traditional cereals belong to the C3 type. The first products of photosynthesis in these plants are three-carbon compounds (trioses). The net productivity of such crops is strongly dependent on external conditions.

C4-type crops include corn, millet, sorghum, sugarcane, and rice. In these crops, four-carbon metabolites are the first to be synthesized. The net photosynthesis productivity in C4 plants is higher than in C3 plants, especially when the leaf area is well-developed. Currently, breeders are working to "improve" C3 crops using the genetic and structural features of C4 plants.

• C3 plants (wheat, rye, triticale, barley, oats): have a loose arrangement of chlorenchyma cells in the leaf mesophyll and relatively small chloroplasts. Photosynthesis increases with a rise in carbon dioxide concentration under saturating light intensity.
• C4 plants: have a bundle sheath of specialized chlorenchyma cells containing large chloroplasts. They do not exhibit light saturation or intense photorespiration, and their compensation point for carbon dioxide is extremely low.

How to form an optimal leaf apparatus

Yield directly depends on the sink strength of plants — their ability to redirect organic substances accumulated in the leaves into the grain, tubers, or fruits. Primary photosynthesis occurs in the leaves, although stems, awns, and green fruits also participate in this process. The agronomist's task is to create conditions under which the leaf apparatus functions with maximum efficiency, and the products of photosynthesis quickly move from vegetative organs to storage organs.

The development of the leaf surface occurs unevenly. During the first month of the growing season, the crops capture only a small fraction of solar radiation. Then the leaf area grows rapidly, reaching its maximum in cereal grains during the milk stage, in grain legumes during full seed filling, and in perennial grasses during flowering. In cereals, the flag leaf and green tissues of the ear play a decisive role in harvest formation.

• Leaf area in the first 20–30 days — 0.3–0.7 m²
• Maximum leaf area of forage grasses — 6–8 m²
• Total biomass of a grain agrocenosis — 12 t/ha
• Grain harvest within biomass structure — 5–6 t/ha
• Proportion of dry mass in stems and roots at maturity — 50–60%

Modern breeding is focused on creating cultivars that direct maximum resources to the formation of the economically valuable part of the harvest. After passing the peak of the growing season, the leaf area decreases rapidly, and active nutrient outflow begins. At maturity, most of the dry mass (mainly fiber) remains in the stems and roots. Our goal is to optimize plant stand density to prevent excessive growth of vegetative mass to the detriment of fruiting.

Temperature regime and critical phases of water consumption

Sowing in the field functions as a complex self-regulating system (agrocenosis), the parameters of which change over time under the influence of the external environment. The main factors for managing this system remain heat and moisture. Each crop has its own cardinal temperature points: minimum (when biological processes are triggered), optimum (reaction rate is maximum), and maximum (process suppression occurs).

Water accounts for 75 to 90% of plant mass, ensuring cell turgor, enzyme activity, and protection against overheating in hot weather. Most cultivated crops are mesophytes and occupy an intermediate position between drought-tolerant xerophytes and moisture-loving hydrophytes (such as rice). An excess of moisture is just as harmful to plants as a deficit, as it impairs soil aeration and suppresses beneficial microflora.

To prevent harvest losses, it is critically important to control the moisture availability of crops during key phases of their development. Water deficit during these periods leads to an irreversible decrease in productivity.

Water balance of crops and transpiration coefficient

Water acts as the main limiting factor for yield formation. The bulk of the absorbed moisture is not spent on building plant tissues, but on transpiration, which protects the crops from overheating. In the conditions of Belarus, where an average of 700 mm of precipitation falls per year, it is important to retain as much moisture as possible in the root zone of the soil.

• Proportion of moisture for photosynthesis — about 0.2%
• 1 mm of precipitation per 1 ha — 10 tons of water
• Upward water flow velocity in woody plants — 14 m/h
• Downward assimilates flow velocity in woody plants — 0.7–1.5 m/h

A portion of the falling water quickly flows into ravines, causing erosion and washing away small particles of the upper fertile horizons. The remaining moisture is retained in the soil and spent on creating dry matter. Water costs per unit of dry mass are determined by the transpiration coefficient and depend on the biological characteristics of the crop.

Mineral nutrition and critical phases of consumption

Mineral nutrition of plants is closely related to photosynthesis and begins in ontogenesis almost simultaneously with it. Carbon, oxygen, and hydrogen make up 93–95% of the dry mass of plants and are absorbed mainly through the leaves from the air and water. All other nutrients enter through the root system from the soil, which transfers them to plants depending on its type and the biological characteristics of the crop.

For the vital activity of crops, 10 macronutrients (C, O, N, H, P, S, K, Ca, Mg, Fe) and 6 micronutrients (B, Zn, Cu, Co, Mn, Mo) are necessary. The first four elements are called organogenic, the others are ash elements. Nitrogen is absorbed by the roots both in the form of the ammonium cation (NH4+) and in the form of the nitrate anion (NO3-), and is also fixed by nodule bacteria in the rhizosphere of grain legumes.

Each element performs a strictly defined function in the plant organism:

• Phosphorus is necessary in the early stages for the synthesis of DNA, RNA, and ATP; it accelerates ripening and improves the development of generative organs.
• Potassium is responsible for the movement of ions through the cell plasmalemma and the formation of carbohydrates; it increases winter hardiness and resistance to disease.
• Calcium neutralizes the harmful effects of hydrogen and aluminum ions on cells.
• Magnesium, iron, and sulfur are part of plant compounds and catalyze redox processes.
• Micronutrients act as cofactors of enzymes and are part of pigments, hormones, and vitamins.

Nutrient consumption fluctuates according to the phases of the growing season. Each crop has its own peculiarities of nutrient absorption. However, the maximum uptake always occurs during specific critical periods of plant development:

• for cereals — stem elongation and heading;
• for grain legumes — flowering and fruit set.

Since all ash elements and nitrogen enter the plant through the root system, such replenishment of nutrients is called root-zone. The intake of nutrients through leaves is called foliar. It is carried out in the form of spraying or dusting, mainly to provide top dressing for growing crops with micronutrients.