Donor-acceptor relationships

1.1. Definitions

Let us give precise definitions, relying on classical plant physiology.

• Source — any organ or tissue that produces organic substances (assimilates) in excess of its own requirements. The source is a net exporter of photoassimilates. In terms of carbon balance, a source has P_net > 0, where P_net is the net photosynthetic productivity minus the costs of its own respiration and growth. A classic example of a source is a mature, fully developed leaf. However, as we will see later, storage organs from which stored substances are mobilised and exported can also act as sources (Schopfer & Brennicke, 2016; Taiz et al., 2023).

• Sink — any organ or tissue that cannot fully supply itself with photosynthetic products and depends on the import of organic substances from other parts of the plant. The sink is a net importer of assimilates. A sink is characterised by P_net < 0; it needs a supply of carbohydrates, amino acids, and other metabolites for its growth, development, respiration, or storage. Typical sinks are roots, young growing leaves, developing fruits and seeds, and underground storage organs (tubers, bulbs) (Hopkins & Hüner, 2009; Medvedev, 2012).

1.2. Dynamics of the System: Role Switching

The most important characteristic of source‑sink relationships is their dynamism. A plant is not a static scheme, but a living, continuously changing organism. The roles of sources and sinks can change depending on several factors: developmental stage, season, and external conditions.

Let us examine this using the example of a leaf, following the concept described in Marschner (2012).

• Young leaf (sink). A leaf that has just unfolded has a small area and little chlorophyll. Its own photosynthesis does not cover the costs of growth and cell‑wall construction. Such a leaf imports sucrose and amino acids from older, mature leaves. It is a sink.

• Mature leaf (source). As the leaf grows and develops, it reaches about 40‑50% of its final area. At this stage, a “sink‑source transition” occurs. The photosynthetic apparatus is formed and operates at full capacity, while the leaf’s own resource requirements decrease. Now the leaf produces more assimilates than it consumes and begins to export them (Giaquinta, 1978; Marschner, 2012). Thus, the leaf becomes a source.

• Senescing leaf (source for mobilisation). At the end of the growing season or under stress, the leaf enters senescence. Photosynthesis drops sharply, but it becomes a source not due to current photosynthesis, but through the breakdown of its own macromolecules – proteins, nucleic acids, chlorophyll. The degradation products, mainly nitrogen and other mineral elements, are mobilised and exported to young, actively growing organs or to seeds (Buchanan et al., 2000; Taiz et al., 2023).

The role switch does not occur simultaneously over the entire leaf area. For example, in many plants the transition from import to export proceeds from the leaf tip to its base (basipetally). That is, the tip may already export sucrose while the base still needs import for its growth. This functional segmentation within a single organ clearly demonstrates the principle of “functional, not anatomical” approach.

Seasonal changes also vividly illustrate the dynamics of the system. For example, in deciduous trees:

• In spring: starch and carbohydrate reserves in roots and wood are mobilised (roots and trunk become sources) and transported to opening buds (sinks).
• In summer: mature leaves (sources) supply the whole plant with photosynthetic products.
• In autumn: leaves are shed, but before that valuable substances are exported to roots and trunk for winter storage, and buds themselves are formed, which enter dormancy and become sinks for reserves.

1.3. Spatial Structure: Three Phloem Sectors

Understanding the dynamics of source‑sink relationships is impossible without briefly mentioning the structure of the transport pathway. This structure is conveniently viewed as three functional sectors of the phloem (van Bel, 2003; Lalonde et al., 2003):

1. Collection phloem. These are the small phloem endings in source leaves. Here, active loading of sucrose and amino acids into sieve tubes against a concentration gradient takes place, generating the driving force for all subsequent transport.

2. Transport phloem. These are the major conducting pathways in stems, petioles, and roots. Their main function is to provide a rapid, low‑resistance flow of assimilates down the hydrostatic pressure gradient from sources to sinks. Although transport phloem is often considered a passive pipe, exchange of substances with surrounding tissues also occurs here – so‑called “transit” (or axial) transport, where part of the assimilates may be temporarily deposited in storage parenchyma cells of the stem and reloaded as needed.

3. Release phloem. These are the phloem endings in sink organs. Here assimilates are unloaded from sieve tubes into consumer cells. The mechanisms of unloading are very diverse: passive efflux down a concentration gradient, symplastic transport through plasmodesmata, or active transport through the apoplast, often with prior cleavage of sucrose by invertase (Taiz et al., 2023; Marschner, 2012).

Thus, we see that the source‑sink system represents a continuously changing functional network. The role of each organ is determined not by its origin, but by its current physiological state: its ability to produce, store, or consume assimilates. This dynamic equilibrium underlies the entire integrative activity of the plant.

In the next section, we will move on to the next logical question: what makes one sink more attractive for resources than another, and how exactly does this “competition” occur? We will discuss sink strength.

2.1. What Is Sink Strength?

Imagine a water supply system. One city (sink) may consume more water than another for two reasons: either it is larger in size (more population), or its consumption intensity is higher (e.g., an industrial plant that requires a lot of water). The same happens in a plant.

Sink strength is a measure of its competitive ability in the struggle for resources. It is defined as the product of two factors: sink size and sink activity (Taiz et al., 2023; Marschner, 2012).

Let us consider these components in more detail.

2.2. Examples from Developmental Physiology

Let us see how this concept works in practice, using examples from plant physiology.

Example 1: Competition among grains in a cereal ear.

In wheat or barley ears, grains located closer to the base are often larger than those at the tip. From the perspective of our concept, this is explained by differences in sink strength: grains in the middle part of the ear are set earlier and have an advantage in size (larger sink size) and activity (faster cell cycle). They more quickly “intercept” assimilates arriving via the phloem, thereby creating a local resource deficit for later and weaker sinks (such as upper grains) (Sadras & Calderini, 2015).

Example 2: Fruit development.

In fruit crops such as tomato, earlier and well‑pollinated ovaries dominate over later ones. This phenomenon is described in the work of Bangerth (1989). Early fruits, thanks to their developing endosperm, become powerful sources of auxin (IAA). Auxin, in turn, is a potent regulator that enhances attracting ability (sink strength). This hormonal signal from the dominant fruit makes it an even stronger sink compared to its neighbours, and it diverts the lion’s share of resources, suppressing the development of the others. We will return to the role of hormones in the fourth section of our lecture.

Example 3: Role switching depending on hormones.

It is noteworthy that sink activity is closely linked to hormonal status. In the book by Tretyakov (2000), it is indicated that auxins activate H⁺-pumps, stimulating substance transport, while cytokinins, together with auxins, activate cell division and growth, which directly increases both sink size and activity.

Thus, we see that sink strength is not an innate property but a dynamic parameter that depends on the physiological state of the organ. An organ that grows faster (divides and elongates), respires more actively, and converts sucrose into starch or other substances more quickly will always have an advantage in competition.

Conclusion and transition to the next section.

So, we have found that resource distribution in a plant is not a chaotic process but a fierce yet regulated competition between sinks. The winner is the one with higher sink strength – that is, larger size and/or more active utilisation of incoming substances.

Now the next logical question arises: if resources are limited, how does competition between sinks affect the whole plant and, importantly for us, the formation of yield? This is what we will discuss in the third section of our lecture.

3.1. Resource Limitation: The Foundation of Competition

For a plant, the main “transport resource” for which competition occurs is free carbon – the assimilates transported in the phloem (mainly sucrose). The pool of these assimilates at any given moment is limited for several reasons:

1. Limited “productivity” of the source. The capacity of the photosynthetic apparatus is not infinite. It is limited by light intensity, water availability, CO₂ concentration, and nutrient content (especially nitrogen). Even under optimal conditions, the rate of photosynthesis at peak sunlight may not keep up with the “demands” of all sinks (Medvedev, 2012; Lambers & Oliveira, 2019).

2. Transport capacity of the phloem. The throughput of sieve tubes is also limited. If several powerful sinks require a lot of sucrose, the flow rate in the phloem can become a limiting factor.

3. Law of diminishing returns. The relationship between light and photosynthesis is not linear – it approaches saturation. Increasing light energy does not lead to a proportional increase in sugar production. Adding limitations in water and mineral nutrition, we arrive at a situation where assimilate supply is constrained, while demand from roots, tips, developing fruits, and storage organs is always high and growing (Boote & Tollenaar, 1994; Marschner, 2012).

It is this mismatch between potentially high demand and limited supply that creates competition among sinks for available assimilates.

3.2. How Does Competition Manifest? Principles of Redistribution

Competition does not mean that the plant “fights” itself. It is a physiological process that obeys certain hierarchical laws. Our sources show that distribution follows two main principles (Taiz et al., 2023; Marschner, 2012):

Principle 1: Proximity to the source.

The phloem, like any transport system, works along the path of least resistance. Assimilates from a given leaf first go to sinks that have the most direct and close conductive connections with it (Hopkins & Hüner, 2009). This is often called the “proximity principle” or “positional dominance”.

Example: The upper leaves of wheat (flag leaf) send their assimilates primarily to the developing ear. Lower, older leaves, which are closer to the roots, predominantly supply the root system. This is clearly demonstrated in experiments with radioactive labelling ^{14}CO₂: if the flag leaf is labelled, radioactivity is found in the grains; if a lower leaf is labelled, it is found in the roots (Schopfer & Brennicke, 2016; Taiz et al., 2023).

Principle 2: Sink strength.

This is the decisive factor. If a nearby sink has low strength (e.g., it has slowed growth or almost reached maturity) while a distant sink has very high strength (e.g., fruits are actively forming), then transport may reorient. It is sink strength that “draws” assimilates towards itself, creating a steeper sucrose concentration gradient in the phloem, as we discussed in the previous section. Thus, competition can be viewed as a “war” of gradients, in which the strongest one wins (Medvedev, 2012; Lambers & Oliveira, 2019).

3.3. Intrinsic and Interspecific Competition

Competition in a plant is of two types:

1. Intrinsic (between organs of the same plant). This is the main subject of our discussion. It is competition between roots and shoots, between vegetative and reproductive organs, between individual fruits or grains within the same inflorescence.

Example of “rivalry”: A well‑known phenomenon in crop production is the suppression of reproductive organs under strong vegetative growth. Excess nitrogen fertiliser stimulates vigorous shoot and leaf growth (strong sinks). As a result, they “intercept” assimilates and reduce the supply of sugars to flowers and fruits, which may lead to abscission, reduced fruit set, and ultimately lower yield (Marschner, 2012). This is a classic example of competition between vegetative and generative sinks.

Example of “outsourcing”: Conversely, under heavy fruit load (e.g., in apple or tomato), shoot growth is weakened because photoassimilates are predominantly directed to the fruits. This is the basis of the agronomic practice of fruit thinning (removing excess fruits) so that the remaining ones become larger and of better quality (Tretyakov, 2000).

2. Interspecific competition (between different plants). This is what we call competition in an agrophytocenosis. Plants compete for light, water, nutrients, and space. From the viewpoint of source‑sink relationships, this is competition for resources at the ecosystem level, which shades the leaves of competitors (weakening the source) and depletes soil resources.

3.4. Competition and Yield Formation: Where Do Losses Come From?

Competition is not just a physiological phenomenon; it is the main physiological limitation of yield. During evolution, the plant has “learned” to produce significantly more potential sinks (flowers, ovaries) than it can support. This gives it a safety margin in case some are lost to pests or weather.

It is this competition, arising from resource limitation, that underlies phenomena that reduce yield:

• Abortion (die‑off) of organs. A significant proportion of flowers, ovaries, and even developing fruits may drop or wither. This is not accidental but a result of competitive struggle, where the strongest sinks win. For example, in legumes (soybean, bean), flower and young pod abscission is a classic way to adjust fruit number according to assimilate supply (Marschner, 2012).
• Reduced grain filling. Even if the fruit does not abscise, fierce competition occurs between seeds inside it. In cereals, grains within a panicle or ear can differ greatly in weight. Upper and peripheral grains, being less active sinks, receive fewer resources and become shrivelled. This is a huge loss of potential yield (Sadras & Calderini, 2015).
• Restricted growth of “secondary” organs. Under resource scarcity, roots suffer first. Therefore, excessive above‑ground growth under water deficiency may lead to roots being unable to supply the plant with water, causing even greater inhibition and reduced productivity.

Thus, we arrive at an important conclusion. Competition between organs is an inevitable consequence of resource limitation. It is a powerful regulator of ontogeny, and it ultimately determines how much and what quality of yield will be produced. The task of the physiologist and agronomist is to understand the nature of this competition and to manage it, directing resources to the desired organs (mainly seeds and fruits) rather than to vegetative mass that is useless for humans. But who is the main “arbiter” in this fierce competitive struggle? Who decides whose request will be satisfied first? We move on to the next section, where we will discuss the key role of hormonal regulation.

4.1. Hormones as Integrators of Signals

A plant, like any complex system, requires coordination. Roots experiencing water deficit must “inform” the leaves so that they close stomata. Young, actively growing fruits must “announce” their need for sugars to enhance assimilate inflow from leaves. It is hormones that serve as such “messengers” and “regulators”, shaping the attracting capacity of tissues.

Recall the definition from Medvedev (2012): phytohormones are chemical signals that, at very low concentrations (10^-6 – 10^-12 M), cause profound physiological changes without directly participating in the biochemical processes they trigger. They are ideal regulators.

Let us consider how the main groups of phytohormones manage competition between sinks.

4.2. Main Hormonal Regulators and Their Effects on Source‑Sink Relationships

1. Auxins (indole‑3‑acetic acid, IAA): “conductors” of the flow

Source: Auxins are synthesised in young, actively growing organs – shoot apical meristems, young leaves, developing seeds and fruits (Medvedev, 2012; Taiz et al., 2023).

Mechanism of action on sink strength: Auxin is the main regulator of attracting ability. Its action is multifaceted:

1. Activation of H⁺-pumps. Auxin activates the proton pumps (H⁺-ATPases) in the plasma membrane of cells. This leads to acidification of the apoplast and creation of an electrochemical gradient necessary for active transport of sucrose and amino acids from the apoplast into sink cells. In simpler terms, auxin “switches on” the mechanism of substance unloading from the phloem (Medvedev, 2012; Tretyakov, 2000).

2. Stimulation of elongation and division growth. Auxin stimulates cell growth. Rapidly growing tissue is a powerful sink because it actively consumes assimilates for building new cell walls and increasing vacuole volume.

3. Formation of the conducting system. Auxin, moving polarly through the phloem (top‑down), induces differentiation of new conducting elements (xylem and phloem), creating a more efficient path for resource transport towards itself.

Classic example: Developing seeds and fruits are powerful “factories” of auxin. The concentration gradient of IAA in seeds ensures their dominant position as sinks. Hence, fruits with more seeds are often larger. This also explains the fruit dominance phenomenon described by Bangerth (1989) and mentioned earlier (Taiz et al., 2023).

2. Cytokinins: “activators” of shoots and antagonists of auxin

Source: The main site of cytokinin synthesis is roots. From there they are transported via xylem to the shoots, which is a crucial mechanism of root‑shoot correlations (Marschner, 2012; Taiz et al., 2023).

Mechanism of action: Cytokinins act as antagonists to auxin in regulating source‑sink relationships:

1. Stimulation of shoot growth. Cytokinins, coming from the roots, stimulate cell division and development of axillary buds, relieving apical dominance caused by auxin. Thereby they make lateral shoots stronger sinks.

2. Delay of leaf senescence. Cytokinins maintain photosynthetic activity of leaves, prolonging their period of work as sources. They also promote attraction of nutrients to the treated leaf area, enhancing the attracting capacity of tissues (Medvedev, 2012).

Example from physiology: Under nitrogen deficiency, cytokinin synthesis in roots drops sharply. This leads to reduced supply to shoots, accelerating leaf senescence and redistributing resources in favour of roots (enlargement of the root system to search for nutrients) at the expense of the above‑ground part. Conversely, under good nitrogen nutrition, cytokinin synthesis increases, stimulating shoot growth (Marschner, 2012).

3. Gibberellins: “stimulators” of growth and mobilisation

Source: Synthesised mainly in young, growing leaves, as well as in developing fruits and seeds (Medvedev, 2012).

Mechanism of action: The main role of gibberellins is stimulation of growth processes. They cause strong stem elongation through cell elongation growth, creating a powerful sink demand in growth zones. Gibberellins also play a critical role in mobilisation of stored substances in seeds. During germination, gibberellins synthesised by the embryo stimulate secretion of α‑amylase by aleurone layer cells, leading to hydrolysis of endosperm starch and providing nourishment to the seedling (Hopkins & Hüner, 2009; Taiz et al., 2023). Here gibberellin converts the endosperm from a sink (storage) into a source (mobilising).

4. Abscisic acid (ABA): “strategist” under stress

Source: Synthesised in response to stresses (drought, salinity, low temperatures) in leaves, but especially actively in roots under water deficit (Medvedev, 2012; Marschner, 2012).

Mechanism of action: ABA is the main stress hormone. It acts as a powerful inhibitor of growth processes. It not only slows growth but also switches priorities in resource allocation in favour of roots.

1. Stomatal closure. A rapid response to water deficit is ABA synthesis in roots, its transport via xylem to leaves, and stomatal closure. This reduces photosynthesis and consequently decreases the total pool of available assimilates.

2. Suppression of shoot growth and stimulation of root growth. ABA inhibits cell division and elongation in shoots, making them weaker sinks. At the same time, under moderate stress, ABA may help maintain root growth. As a result, the root‑to‑shoot ratio shifts in favour of roots, which is an adaptive strategy for water seeking (Taiz et al., 2023; Tretyakov, 2000).

3. Induction of seed dormancy. ABA is the main inducer of seed dormancy, blocking germination until favourable conditions arrive.

4.3. Hormonal Balance as the Basis of Regulation

It is important to understand the most essential principle: phytohormones never act alone. The effect on source‑sink relationships is determined not by the concentration of a single hormone, but by their ratios. Classic examples are:

• Auxin/cytokinin balance: determines whether a callus will develop into a shoot (high cytokinin ratio) or a root (high auxin ratio) (Medvedev, 2012). In general, this balance governs plant architecture, setting priorities for growth of apices or lateral shoots.
• Gibberellin/ABA balance: determines seed release from dormancy. High ABA content maintains dormancy, while an increase in gibberellin level triggers germination, meaning the endosperm shifts from a “sleeping” sink to an active source of nutrition for the seedling (Tretyakov, 2000).

Conclusion and transition to the next section.

Thus, we see that hormones are the language of management of competition. They not only determine sink strength (stimulating or suppressing growth, transport, metabolism) but also ensure integration of the whole plant, linking signals from the roots (water and mineral status) with shoot responses (growth, photosynthesis, fruit development). The plant as a single organism governs its development through a complex network of hormonal interactions, constantly reallocating resources depending on priorities dictated by external and internal conditions.

Now that we understand that hormones are the “microscopes” and “buttons” of control, we can move on to the most practical and interesting question: how does mineral nutrition affect this intricate process? In other words, why can the addition of certain fertilisers drastically change the distribution of yield, turning it either into a “vegetative monster” or a “storehouse of seeds”? This – the link between nutrition and hormonal regulation – is what we will discuss in the final, fifth section of our lecture.

5.1. Nitrogen (N) – The Main Regulator of Distribution

Nitrogen is the element at the centre of source‑sink regulation because it is a key component of both the photosynthetic apparatus (Rubisco, chlorophyll) and the hormonal system (cytokinins).

5.2. Potassium (K) – Regulator of Phloem Transport

Potassium plays a special role in source‑sink relationships because it directly participates in assimilate transport in the phloem.

• Role in phloem loading and unloading. As noted in several sources (Marschner, 2012; Tretyakov, 2000), potassium is the main inorganic cation of phloem sap. A high concentration of K⁺ in sieve tubes creates an osmotic gradient necessary for water influx and maintenance of hydrostatic pressure driving mass flow. In addition, potassium activates H⁺-ATPases, which provide the energy for sucrose loading into sieve tubes.
• Effect on yield quality. Adequate potassium nutrition improves the transport of sugars from leaves to storage organs (root crops, tubers, grain). For example, in sugar beet, potassium promotes sugar accumulation in the root. In cereals, potassium improves grain filling. Under potassium deficiency, assimilate transport is impaired, and sugars and starch accumulate in leaves, which may lead to photo‑oxidative damage and premature senescence (Marschner, 2012).

5.3. Phosphorus (P) and Other Elements

• Phosphorus – key element of energy metabolism (ATP). Its deficiency reduces the overall energy supply of the plant, limiting both photosynthesis and active transport. Moreover, as Marschner (2012) points out, phosphorus participates in the regulation of flowering and fruiting. Phosphorus deficiency reduces seed and fruit set, weakening the potential sink strength of reproductive organs.
• Micronutrients (e.g., zinc, copper, boron) are often cofactors of enzymes involved in hormone synthesis (e.g., tryptophan → IAA) or in carbohydrate metabolism. Their deficiency can disturb hormonal balance and lead to developmental abnormalities that affect assimilate distribution.

5.4. Synthesis: Nutrition as a Lever of Management

Thus, we see that mineral elements are not merely passive nutritional components. They are active regulators that, through the hormonal system and transport processes, govern the direction of assimilate flows.

• Nitrogen manages the competitive strength of vegetative organs (via cytokinins) and therefore determines the balance shift: in favour of shoots or roots/reproductive organs.
• Potassium ensures the efficiency of the transport conveyor itself – the phloem.
• Other elements provide the energy and structural basis for all these processes.

Understanding these links gives the agronomist the key to managing yield. Timely top‑dressing, balanced fertiliser rates, and consideration of plant developmental phases – all allow purposeful influence on hormonal status and, consequently, on where the plant directs its produced resources: into leaves, roots, seeds, or fruits. This is precisely the practical significance of knowledge about source‑sink relationships for agriculture.