Photosynthetic activity of crops as the basis for harvest formation

How canopy and photosynthesis determine yield

The basic principle of yield formation lies in the photosynthetic activity of crops. To forecast biomass accumulation, an agronomist must manage two interrelated processes: the development of leaf area and the efficiency of photosynthesis itself. These indicators are assessed via photosynthetic potential (PP) and net assimilation rate (NAR).

• Growing season of grain crops — 100–110 days
• PP of well-developed crops during the growing season — 2.0–2.5 million m² · days/ha
• Average NAR for wheat and barley — 5–7 g/(m² · days)

Photosynthetic potential (PP) reflects the sum of leaf areas for each day over a specific period. For example, at the beginning of a 10-day interval, the leaf area was 20,000 m²/ha, and at the end — 28,000 m²/ha. Thus, the PP for this period is calculated as follows: ((20 + 28) / 2) × 10 = 240,000 m² · days/ha.

Net assimilation rate (NAR) shows the intensity of crop activity. It is the amount of dry organic mass in grams synthesized by 1 m² of leaf area per day. The indicator is calculated using the formula: NAR = (W2 – W1) / PP, where W2 and W1 are the dry mass of plants per unit area at the end and beginning of the period.

The total increase in dry biomass (Y) depends directly on both factors and is calculated as their product: Y = PP × NAR. For example, over a 100-day growing season, the average NAR was 6 g/(m² · days), and the PP was 2 million m² · days/ha. In this case, the amount of dry biomass will be 12 t/ha.

Photosynthetic productivity changes throughout the season. In the first month of the growing season, the NAR is at its maximum, as plants do not shade each other and all leaves are well illuminated. As leaf area increases, the lower canopy layers begin to experience light deficiency, and the average NAR decreases. Biomass accumulation is slow at the beginning and end of the growing season, reaching its peak in the middle of the cycle when accumulated assimilates are actively redistributed from vegetative organs to reproductive ones.

A crop functions most efficiently during the period when the leaf area is close to optimal. During this time, plants form the main part of the future harvest. Key parameters of crop performance during this peak period are shown in the table below.

Agricultural practices help regulate photosynthesis indicators. Controlling weeds, pests, and diseases allows for direct influence on leaf efficiency. In practice, the following measures are applied:

• Weed control — in clean fields, light penetration is improved, and the crop's NAR is always significantly higher.
• Protection against pests and diseases — plant health promotion contributes to an increase in NAR.
• Genetic modification and breeding — a fundamental path to increasing yield through improved NAR.

It is also important to consider the biological type of the cultivated crop. C4-type plants possess a higher NAR compared to C3-plants, especially when leaf area is increased. This group includes the following crops:

• sugarcane;
• corn;
• sorghum;
• millet.

Fundamental laws of yield: minimum and indispensability of factors

Yield formation is subject to the fundamental laws of plant science. The first is the law of indispensability (or equivalence) of life factors. According to this law, a plant cannot develop normally in the absence of any life factor.

The second fundamental law is the law of minimum, maximum, and optimum. The first quantitative patterns of factor effects state that yield depends on the factor that is at its minimum. In specific zonal conditions, the leading or critical significance is acquired by the factor that is lacking first.