Physiological framework of crop yield

1.1. From Light Energy to Organic Matter

At the foundation of everything lies photosynthesis. Photosynthesis is the only mechanism on our planet capable of converting electromagnetic radiation (light) energy into chemical energy.

PAR (Photosynthetically Active Radiation) — is the portion of the solar spectrum (with wavelengths roughly 400–700 nm) that plant pigments can capture. According to various estimates, of all the solar energy reaching the Earth's surface, only about 0.1% is ultimately converted into biomass (Schopfer & Brennicke, 2016; Taiz et al., 2023). This shows how "inefficient" this primary step is from an energy standpoint, and how great the potential for growth is when conditions become more favourable.

Photosynthesis — is the process by which PAR energy is captured by chlorophyll and triggers a chain of redox reactions. As a result, simple inorganic molecules (carbon dioxide and water) are synthesised into complex organic compounds—carbohydrates—which become the foundation for the growth and development of the entire plant.

Photosynthetic efficiency. It is important to understand that photosynthesis is not simply "light in, sugar out." Its efficiency varies greatly and depends on many factors. When considering an individual leaf, its photosynthetic activity has limits.

Light response curve of photosynthesis. As light intensity increases, the rate of photosynthesis first rises linearly (the light-limited phase) and then plateaus (the CO₂- or enzyme-limited phase). Light saturation for individual leaves of many crops occurs at intensities well below full sunlight (Taiz et al., 2023).

"The light-response curve of photosynthesis in most leaves saturates between 500 and 1000 μmol m⁻² s⁻¹, which is considerably lower than full sunlight (about 2000 μmol m⁻² s⁻¹)" (Taiz et al., 2023).

Difference between C₃ and C₄ plants. The key difference from a yield-physiology perspective is the type of photosynthesis. In C₃ plants (wheat, rice, soybean, potato), the key enzyme Rubisco exhibits not only carboxylase but also oxygenase activity, leading to photorespiration—a process that "steals" up to 25–30% of fixed carbon, especially under hot conditions. In C₄ plants (maize, sugarcane, sorghum), a specialised concentrating mechanism almost completely suppresses photorespiration.

"The quantum yield in C₃ plants decreases with temperature, reflecting stimulation of photorespiration, whereas in C₄ plants it remains constant" (Taiz et al., 2023).

Consequently, C₄ plants have higher water and nitrogen use efficiency, particularly in hot, sunny climates, making them more productive.

1.2. From Leaf Photosynthesis to Crop Biomass: Introducing RUE

So far, we have discussed individual leaves. But for the agronomist, what matters is not the photosynthesis of a single leaf, but the total productivity of the entire crop. Here we move to the next key concept—Radiation Use Efficiency (RUE).

RUE — is the amount of dry biomass (in grams or kilograms) that a crop accumulates per megajoule (MJ) of intercepted PAR. This coefficient has allowed physiologists to move from studying single leaves to entire fields. And here, an important paradox awaited us.

The photosynthesis paradox. Although photosynthesis of individual leaves is well described by a hyperbolic curve, for a whole canopy, a surprising linearity is observed. Numerous measurements of entire plant communities have shown that biomass accumulation is linearly dependent on the amount of intercepted radiation. RUE remains remarkably stable for a given species under non-stress conditions (Sinclair, 1994; Taiz et al., 2023).

"Unlike the nonlinear photosynthesis curves of individual leaves, the stability of RUE and the nearly linear CO₂ assimilation responses of whole canopies are explained by the pattern of light distribution within the leaf canopy" (Sinclair, 1994).

This is explained by the fact that in a canopy, upper leaves receive excess light and operate on the plateau, while lower leaves work in the linear zone because they receive diffuse light. Summed together, this yields a linear response.

RUE as a limiting factor. Knowledge of RUE and its limits is very important. For example, research has shown that for C₃ crops, RUE is close to its theoretical maximum, and further increases in single-leaf photosynthesis yield only a very small increase in RUE (Sinclair, 1994).

"Due to the plateauing of RUE in response to increases in the rate of CO₂ assimilation of individual leaves, only a small increase in RUE, and consequently in yield, can be expected from increasing, even substantially, leaf photosynthetic activity" (Sinclair, 1994).

This explains why decades of breeding for increased single-leaf photosynthesis did not produce a major breakthrough in yield. It is crucial that solar energy is not just absorbed, but efficiently distributed throughout the canopy.

1.3. From Biomass to Grain: Harvest Efficiency and the Search for "Free" Reserves

But yield is not the entire biomass. It is only the part we harvest. For this, we have the concept of harvest efficiency (or Harvest Index, HI) . It is the coefficient that shows what proportion of the total dry biomass is allocated to the economically valuable part (grain, fruits, tubers).

For a long time, yield increases in the 20th century were achieved largely through increasing HI. The classic example is the "Green Revolution," with the creation of semi-dwarf wheat and rice varieties. Less assimilates were spent on stems, and a greater share of biomass was redistributed to grain. In modern cereal varieties, HI approaches 0.5–0.6, and further increases are limited by physiological constraints (Boote & Tollenaar, 1994; Foulkes & Reynolds, 2015).

For the agronomist, this means that future yield growth will increasingly depend on increasing total biomass, not just on redistribution. And to increase biomass, we need to maximise both the amount of PAR intercepted and its efficiency of use. The amount of intercepted PAR depends on the length of the growing season, leaf area, and canopy architecture. That is, we come back to the first step of our path.

Link to fertilisers. This energy pathway is closely linked to mineral nutrition. Fertilisers, especially nitrogen, are the "fuel" that allows the plant to build a powerful photosynthetic apparatus.

• Nitrogen is a key component of chlorophyll molecules, enzymes (including Rubisco), and nucleic acids required for growth. Without sufficient nitrogen, the leaf apparatus develops poorly, and photosynthesis itself becomes inefficient.
• For example, with increasing maize yields, nitrogen use efficiency (NUE) also improved. Nitrogen fertiliser application allowed a significant increase in leaf area index (LAI) and, consequently, PAR interception (Grassini et al., 2015; Marschner et al., 2012).

"The relationship between crop productivity and resource consumption follows linear (solar radiation and evapotranspiration) or curvilinear functions (nitrogen)" (Grassini et al., 2015).

Summary of Chapter 1

So, the first and most important conclusion: yield is a physical quantity based on the storage of solar energy. It cannot arise from nothing.

1. The path to yield begins with the absorption of PAR energy. The efficiency of this absorption depends on the size and architecture of the leaf canopy.

2. This energy is then converted into biomass through photosynthesis. The efficiency of this conversion (RUE) has its physiological limits, but it can be enhanced by improving growing conditions (C₄ pathway, fertilisation, irrigation).

3. Finally, biomass is redistributed into yield (grain, fruits). The efficiency of this redistribution (HI) in modern varieties is already close to its biological maximum.

Understanding this path is the foundation that will help us in subsequent chapters to understand why disruptions at any of these stages (light deficit, stress, nitrogen deficiency) lead to irreversible loss of potential yield. We will also understand why it is so important for the plant to effectively "manage" its resources.

With this, we conclude the first chapter. In the next part of our lecture, we will discuss critical periods in plant development and why the full physiological picture of yield cannot be reduced solely to photosynthesis.

2.1. Definition and Meaning of Critical Periods

A critical period is a stage of ontogenesis during which the plant is most sensitive to deficiency or excess of environmental factors, and during which processes determining the main elements of future productivity (number and potential size of economically valuable organs) take place (Tretyakov et al., 2000; Foulkes & Reynolds, 2015).

Simply put: this is the "moment of truth" when it is decided how many grains will be in the ear, how many flowers will set fruit, and how large the fruit will be. Once this stage is passed, the main quantitative parameters of the yield are already set, and subsequent improvements can only partially adjust the situation (e.g., increase grain weight, but not their number).

2.2. Physiological Basis of Criticality

Why is the plant so vulnerable at specific moments?

1. Active growth and high resource demands. During critical periods, certain plant organs grow at maximum rates. This requires huge amounts of assimilates (photosynthesis products), water, and minerals. If any resource is limited at this time, intense competition arises between organs, and the most sensitive—those on which future yield depends (e.g., flowers or young ovaries)—may fail to develop or die (Sadras & Calderini, 2015).

2. Differentiation and organogenesis. During critical periods, key morphogenetic processes occur: the initiation and differentiation of organs. If a deficit of assimilates or phytohormones occurs at this time, it leads to irreversible changes—fewer primordia on the growing cone (in cereals) or fewer flowers (in legumes). In essence, the developmental programme is disrupted, and the number of rudimentary organs is permanently reduced.

"Both agricultural plants and weeds are most sensitive at the youngest age, during emergence, because at this time the metabolic links providing hormones for active growth and the initiation of reproductive organs are disrupted, and the growing points are damaged. However, the period of gamete formation (stages VII–VIII) is critical. At this time, plants are very sensitive to stress, sharply reducing productivity, especially seed productivity" (Tretyakov et al., 2000, Chapter 8).

3. Irreversibility of processes. Many organogenesis processes are irreversible. For example, the number of spikelets per wheat ear is determined during the stem elongation phase. If drought at this period reduces this number, no subsequent improvement in conditions can increase the number of spikelets. The plant can only, to some extent, increase the number of grains per spikelet or their weight, but the overall potential is already lowered.

2.3. Critical Periods for Major Crops

Each crop has its most vulnerable phases, but for most cereals and grain legumes, these are well studied.

Cereal crops (wheat, barley, rye, oats): The critical period is stem elongation – heading (for paniculate types, panicle emergence) (Tretyakov et al., 2000; Foulkes & Reynolds, 2015). During this time, spikelets and flowers are initiated, determining the potential number of grains per ear. Stress at this stage leads to "gappy ears" or "blank heads."

"Cereals are most sensitive to moisture during the stem elongation – heading phases. Consequently, during the critical period, generative organs are formed, and flowering and fertilisation occur" (Tretyakov et al., 2000, Chapter 8).

It is important to note that the period from the beginning of stem elongation to flowering is also a time of active stem growth, creating strong competition for assimilates between the stem and the developing ear (Foulkes & Reynolds, 2015).

Maize: The critical period is from 2 weeks before tassel emergence to the end of fertilisation (silking) . During this time, the number of rows and the number of kernels per ear are determined. Of particular importance is the so-called anthesis-silking interval (ASI) —the period between pollen shed and silk emergence. If stress lengthens this interval, some silks remain unpollinated, and the ear develops with missing kernels ("blank" areas). Drought or nitrogen deficiency at this stage sharply reduce yield (Grassini et al., 2015; Boote & Tollenaar, 1994).

Grain legumes (soybean, common bean, pea): The critical period is flowering – beginning of pod set. During this time, flowers and ovaries are initiated. Lack of moisture, high temperatures, or nutrient deficiency lead to massive flower and bud abortion. This is one of the plant's most effective self-regulation mechanisms, but for the agronomist, it is a loss of potential yield. As shown for soybean, during flowering, the plant is particularly sensitive to stress (Boote & Tollenaar, 1994).

2.4. What Happens Under Stress During a Critical Period?

The consequences can vary, but all lead to reduced productivity:

• Reduction in organ number: decreased number of spikelets, flowers, ovaries. This is the most "costly" consequence, as it is practically irrecoverable.
• Flower sterility: production of non-viable pollen or unreceptive stigmas, leading to empty kernels (gappy ears). This is common under high temperatures.
• Reduction in potential organ size: for example, reduced ovary size, which, under normal filling, gives smaller grain. This occurs if stress affected cell division in the endosperm.
• Accelerated senescence: stress can trigger premature death of the most active tissues, including reproductive organs.

2.5. Agronomic Takeaway

Understanding critical periods is key to effective yield management. It means that:

1. Major agronomic practices (irrigation, nitrogen top-dressing, pest control) should be timed specifically to these phases. You cannot be late with irrigation if the critical period has already passed and the number of grains has been lost.

2. Crop monitoring should be most intensive during these periods. Visual assessment and instrumental diagnostics (e.g., SPAD method for nitrogen status) allow timely detection of problems.

3. Breeding for stress tolerance is often aimed precisely at "smoothing out" the sharpness of critical periods. For example, creating varieties with a shorter anthesis-silking interval (maize) or with higher flower tolerance to drought.

Thus, the second physiological framework tells us: yield is not only total biomass, but also its correct distribution over time. There are narrow "gates" through which future productivity passes, and our task is to ensure that the plant experiences minimal stress at these moments.

Now that we know when the number of yield components is set, a natural question arises: how does their growth and filling occur, and where does the plant allocate resources if some organs were damaged? It turns out that the plant does not simply endure losses—it redistributes what remains. This is the subject of the next section, on remobilisation.

3.1. What is Senescence and How is it Different from Death?

The process of leaf ageing is called senescence. It is important to emphasise: this is not passive degradation, but an active, regulated, and energy-requiring process. It is a programme aimed at the most efficient recycling of resources before the leaf becomes unnecessary for the plant as a whole.

Key differences between senescence and simple death (necrosis):

• Programmed: it is part of the ontogenetic programme. It is triggered at a specific time or in response to signals (e.g., senescence of the mother plant after flowering, or signals of nitrogen deficiency).
• Orderly: the process follows a strict plan. First, chloroplasts change (they become gerontoplasts), then chlorophyll, proteins, and RNA are broken down, and only at the very end, after all valuable components have been extracted, does the cell die.
• Recycling: the main point of senescence is not death, but resource salvage. Metabolites released during breakdown are transported to other parts of the plant (Thomas & Ougham, 2015).

"Senescence is the means by which resources are recycled from obsolete parts of the body to new developing structures" (Thomas & Ougham, 2015).

3.2. What Exactly is Remobilised and How?

Primarily, nitrogen and phosphorus are remobilised—the two most growth-limiting elements. The main sources are:

• Proteins. The main "bank" of nitrogen in the leaf is the enzyme Rubisco. It can constitute up to 50% of all leaf protein, and during senescence it is actively broken down. The amino acids released are exported from the leaf and then reused to synthesise storage proteins in seeds.
• Chlorophyll. The green pigment is not only the photosynthetic apparatus but also a "storehouse" of bound nitrogen. Upon chlorophyll breakdown, nitrogen is released and also remobilised (Thomas & Ougham, 2015).
• Nucleic acids (RNA). They are broken down to nucleotides, which can be reused for synthesising new RNA in growing tissues.

The process proceeds in an orderly fashion: first, macromolecules in chloroplasts are broken down, then those in the cytoplasm. Breakdown products (amino acids, sugars, ions) are loaded into the phloem and transported to active growth points—young leaves, developing ears, fruits, seeds.

3.3. How is Senescence Linked to Yield?

This link is direct and vital, especially for annual cereals and grain legumes.

• Grain as a powerful sink. After flowering, seeds become the dominant sink (consumer) for all incoming assimilates and nutrients. If the plant cannot take up enough nitrogen from the soil during grain filling (e.g., due to drought or soil depletion), it compensates by remobilisation from leaves and stems.
• Importance for late-season crops. In this context, old leaves are a genuine "reserve fund" for yield formation. As shown in research, the proportion of grain nitrogen derived from remobilisation can be very significant. For example, for maize and wheat, nitrogen remobilised from vegetative organs can supply a substantial part of grain needs, especially under late-season stress.

"Internal nitrogen redistribution between sources and sinks partly explains the variations in senescence patterns... Remobilisation of nitrogen from Rubisco and other leaf proteins... is a major nitrogen source for the developing grain" (Thomas & Ougham, 2015).

Link to critical periods. If stress occurred during the critical period and few grains were set, active remobilisation and delayed leaf senescence can help maintain or even increase average grain weight. Conversely, if sink strength (grain number) is low, old leaves may remain "overfilled" with assimilates, which can, paradoxically, accelerate their senescence (Thomas & Ougham, 2015).

3.4. Why is This Agronomically Important?

Understanding remobilisation gives the agronomist two powerful management levers:

1. Late nitrogen top-dressing. Applying nitrogen at heading–early grain filling can not only increase photosynthesis but also "keep" leaves from senescing prematurely, prolonging the period of active photosynthesis and increasing assimilate supply to the grain. This is particularly effective for high‑nitrogen‑demand crops (e.g., maize and wheat).

2. The "stay-green" trait. This is a genetic trait meaning delayed senescence. In some crops (maize, sorghum), it is associated with higher yields, especially under stress. However, in grain legumes such as soybean, "stay‑green" can be disadvantageous because nitrogen continues to be fixed in leaves and is not redistributed to grain, reducing the product output per unit area (Thomas & Ougham, 2015; Boote & Tollenaar, 1994).

"In maize, rice, and probably sorghum, selection for delayed senescence may already have reached its limit... In modern high-yielding varieties, senescence traits are only weakly related to yield" (Thomas & Ougham, 2015).

Summary of Chapter 3

Thus, the third physiological framework tells us: the old leaf is not ballast, but a strategic reserve. Senescence is not death, but a carefully organised recycling process aimed at maximising support for the developing yield. The plant "sacrifices" old leaves to "feed" the seeds.

This explains why:

• Nitrogen top-dressing late in the season can be very effective, as it prevents leaves from senescing too quickly.
• The "stay-green" trait can be beneficial for some crops but not for others.
• Yield depends not only on photosynthesis but also on the plant's ability to efficiently manage its internal resources, distributing them from old, dying tissues to young, human‑valued organs.

Now we have three frameworks: the energy pathway, critical periods, and resource remobilisation. But what happens when things do not go according to plan? How does the plant respond to drought, salinity, heat, or nutrient deficiency? We will explore this in the next chapter.

4.1. Homeostasis and Stress: A System of Countermeasures

To understand compensation, we must first define stress. Stress is a general, non‑specific adaptive response of an organism to the action of any adverse factor, causing it to deviate from its optimal state (Tretyakov et al., 2000, Chapter 8). It is a state of tension in which physiological processes exceed normal limits.

But the plant, like any living system, possesses homeostasis—the ability to maintain relative constancy of its internal environment (pH, ion concentration, water potential) under changing external conditions. When the stressor is weak or short‑lived, the plant's regulatory systems can restore normal functioning. If the stress is strong and prolonged, deeper adaptive mechanisms are activated.

4.2. Main Pathways of Compensation: How the Plant "Saves" Yield

The plant uses several strategies to survive stress and minimise productivity losses.

4.3. The Cost of Compensation: Why is it Not "Free"?

It is important to understand: compensation is never free. Every defence mechanism has its own physiological "price."

• Stomatal closure saves from overheating and desiccation, but sharply reduces photosynthesis (Sinclair, 1994).
• Synthesis of stress proteins and osmolytes requires energy (ATP) and nutrients.
• Increased root growth comes at the expense of reduced shoot development.
• Remobilisation and senescence lead to loss of photosynthetic surface.

The agronomist must understand this balance. Sometimes helping the plant (irrigation, fertilisation) is not just "feeding," but a way to prevent the activation of defence mechanisms that, while allowing survival, reduce final yield. For example, early drought during tillering in wheat may trigger compensatory mechanisms, but it can also reduce the number of productive tillers and spikelets—and this is no longer compensable.

4.4. Examples of Compensation in Agronomic Practice

1. "Stay‑green" (delayed leaf senescence). This trait in sorghum and maize allows leaves to retain photosynthetic activity during drought. The plant does not initiate early senescence, so its productivity drops less. However, in soybean, delayed senescence can be disadvantageous, as nitrogen that could have been directed to grain remains in leaves (Thomas & Ougham, 2015).

2. Irrigation and nitrogen top‑dressing during critical periods. If we supply water and nitrogen exactly when the plant begins to experience stress, we can "switch off" its defence mechanisms, preventing photosynthesis decline and retaining more assimilates for grain. This underpins many precision agriculture technologies.

Summary of Chapter 4

Thus, the fourth physiological framework states: the plant is not a passive object, but an active, adaptive system capable of compensating for disturbances.

It can:

• Protect its cells at the molecular level (antioxidants, stress proteins, osmolytes).
• Regulate water loss (stomata, osmotic adjustment, root modification).
• Reorganise metabolism (switch to energy‑saving pathways).
• Change its shape and structure (morphological plasticity, regeneration).

However, every compensation comes at a cost: it is achieved at the expense of reduced productivity or resource expenditure. The agronomist's task is to use agronomic measures to help the plant avoid triggering these costly mechanisms, especially during critical periods.

Now that we know about compensation at the individual plant level, we move to the fifth and final framework: why is yield not just the sum of yields of individual plants, but the result of their interactions within the agrocenosis?

5.1. The Plant in a Crop: From Individual to Population

When we consider a single plant, we see an autonomous system fighting for survival. But in a crop, thousands of plants are in close contact, and their physiology changes dramatically. This manifests in several key aspects.

5.2. Why Does Individual Physiology Not Always Match Canopy Physiology?

This is the key paradox that every agronomist must understand. What is good for a single plant may be bad for the whole field.

• The productivity paradox. A plant that is the best competitor (fast‑growing, with a strong stem, deep roots) may produce less yield per hectare than a plant with a more "altruistic" architecture that distributes resources more evenly among all organs. That is why the ideal variety for a crop is not the ideal plant for solitary growth. It is a "social" plant that gets along well with neighbours.
• The role of plant density. One of the main agronomic tools is choosing the optimal plant density. If sown too sparsely, the area is not fully used (reduced PAR use efficiency). If too densely, competition intensifies, plants elongate, may lodge, and yield also drops. The optimal density is a balance between individual plant productivity and the productivity of the entire canopy.

"The relationship between crop productivity and resource consumption follows linear (solar radiation and evapotranspiration) or curvilinear functions (nitrogen)... At high yields, efficiency limits become more stringent" (Grassini et al., 2015).

Shifting to the "crop as a system" concept. In agronomic physiology, we increasingly move from studying individual leaves or plants to studying the whole canopy. The main parameters we assess are not single‑leaf photosynthesis, but the overall productivity, LAI (Leaf Area Index), RUE (Radiation Use Efficiency), and NUE (Nitrogen Use Efficiency) of the entire crop. These integral indicators determine yield.

5.3. The Agrocenosis as an Integrated Physiological System

Thus, we conclude that a crop is not just a sum of plants, but a self‑regulating physiological system with its own laws.

1. Competition is the engine of evolution in the agrocenosis. It "selects" genotypes that better adapt to density, forming a more uniform stand and more rationally distributed leaf surface.

2. Optimising plant density is the main agronomic tool. It allows us to tune the balance between individual productivity and the productivity of the whole system.

3. Fertilisers and irrigation are resources for the system. They must be supplied not just to "feed" each plant, but in a way that maximises resource‑use efficiency of the whole system. For example, split nitrogen application helps avoid excessive vegetative growth and directs resources towards grain formation (Grassini et al., 2015; Marschner et al., 2012).

Final Summary of Chapter 5

So, the fifth physiological framework states: yield is a property of the system (the agrocenosis), not simply the sum of properties of individual plants.

This means that:

• A variety's success is determined not only by its individual productivity, but also by its ability to perform well in a dense community.
• Agronomic practices (choosing density, timing and methods of fertilisation, tillage, irrigation) should be aimed at optimising the performance of the whole system, not each plant individually.
• Crop physiology has its own laws that differ from the physiology of an individual plant. We must account for competition, self‑regulation, and plant‑to‑plant interactions to properly manage yield formation.