Transport of assimilates

3.1. Anatomical Foundation: Sieve Tubes and Companion Cells

The main conducting elements of the phloem are sieve tubes. They are formed by longitudinal rows of cells—sieve‑tube members—connected to one another through specialised regions of the cell wall called sieve plates. These plates contain numerous pores through which the cell sap can pass (Taiz et al., 2023; Medvedev, 2012).

What makes these cells unique? During maturation, a sieve‑tube member loses its nucleus, vacuole, and most organelles—ribosomes, Golgi apparatus, microtubules. The cytoplasm is reduced to a thin parietal layer. This degradation has a profound purpose: it reduces flow resistance and allows the solution to move freely through the tube, encountering almost no intracellular obstacles (Taiz et al., 2023). However, having lost its nucleus and many synthetic systems, such a cell cannot survive on its own. Therefore, each sieve‑tube member is associated with one or more companion cells.

Companion cells are dense, cytoplasm‑rich cells packed with organelles that perform “engineering maintenance” for the sieve tubes. They supply proteins, support energy metabolism, and regulate transport. Between the companion cell and the sieve‑tube member there are numerous plasmodesmatal connections of a special type—pore‑plasmodesma units. These have a high size‑exclusion limit and allow exchange not only of small molecules but also of large proteins and RNA (Lambers & Oliveira, 2019; Taiz et al., 2023).

Thus, the sieve tube and its companion cell form a functional unit—the sieve‑tube/companion‑cell complex. It is here that sucrose is loaded into the phloem, and it is here that transport begins.

3.2. The Mechanism of Movement: The Münch Hypothesis

Now the key question: what force drives sucrose through these living sieve tubes over distances ranging from tens of centimetres in herbs to hundreds of metres in large trees?

The most widely accepted and experimentally supported explanation is the pressure‑flow hypothesis, proposed by the German physiologist Ernst Münch as early as 1930 (Connor et al., 2011; Kuznetsov and Dmitrieva, 2006). The essence of this model is remarkably elegant and easy to understand.

Imagine two chambers connected by a tube. One chamber contains a concentrated sugar solution (the donor leaf), the other a dilute solution (the root or another acceptor organ). Both chambers are immersed in water. Water, as we know, moves along a water‑potential gradient from a region of higher water potential (water) to a region of lower potential (concentrated solution).

In the actual plant, this principle works as follows:

1. In the donor leaf, sucrose is actively loaded into the sieve tubes. This creates a high osmotic pressure (low water potential) inside the sieve tube.

2. Water from neighbouring xylem vessels, where the solute concentration is lower, enters the sieve tubes through aquaporins, following the osmotic gradient. As a result, the hydrostatic (turgor) pressure inside the sieve tube of the leaf increases.

3. In the acceptor organ (e.g., the root), sucrose is actively unloaded from the sieve tubes—either directly into cells or after hydrolysis by invertase. The sugar concentration in the sieve tube drops, osmotic pressure decreases, and water leaves the sieve tube, returning to the xylem or surrounding tissues. Hydrostatic pressure in this part of the tube falls.

4. A gradient of hydrostatic pressure is established from the leaf to the root. It is this pressure difference, not simple diffusion, that drives the entire fluid—the phloem sap carrying sucrose—to flow from source to sink (Taiz et al., 2023; Connor et al., 2011).

This model explains the remarkable transport velocities, which are orders of magnitude faster than free diffusion: up to 1–1.5 m/h (Taiz et al., 2023). The mass flow ensures simultaneous movement of both water and solutes at the same speed.

It is important to emphasise that the sieve tube itself does not require energy expenditure along the pathway. Energy is spent only on two processes: active loading (in the donor) and active unloading (in the acceptor), as well as on maintaining ion gradients, e.g., for potassium. Between these, the solution moves passively—like water through a hose under pressure.

Of course, reality is more complex: there is resistance from sieve plates, leakage and re‑capture of sugars along the path, and adjustments in sieve‑tube diameter among different plant species. Nevertheless, despite many refinements and debates (e.g., whether the model works equally well in large trees), the Münch hypothesis remains the foundation of our understanding of long‑distance assimilate transport (Lambers & Oliveira, 2019).

Thus, the phloem is not just a set of passive tubes; it is a dynamic system in which active transport at the ends generates pressure, and passive flow along the pressure gradient delivers sugars to their destination.

Now that we understand how the transport highway works, we can turn to the most intriguing question: why do some organs receive more sugar than others? How does the plant distribute this flow among many competing consumers?

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4.1. What Is Sink Strength?

Sink strength is the ability of an organ or tissue to attract assimilates from the transport system. It is not merely a “desire” to get sugar; it is a measurable physiological characteristic determined by two factors (Taiz et al., 2023; Connor et al., 2011):

Let us consider these two components.

Sink size is the biomass or number of cells in the organ. The larger the organ, the more “consumers” it has and the more sugar it can potentially take up. For example, a large fruit or a massive root system is a large sink. However, size alone does not guarantee priority. One can imagine a large but “lazy” organ.

Sink activity is the rate of sucrose uptake and utilisation per unit mass (or per cell). This is a much more dynamic and controllable parameter. Sink activity is determined by:

• the rate of sucrose unloading from the phloem,
• the intensity of its metabolism (hydrolysis, starch synthesis, respiration, growth),
• the concentration of transporters in the membranes of acceptor cells.

Thus, sink strength is an integrative characteristic that reflects the competitive ability of the organ. The plant does not allocate resources “fairly” or “according to needs” in the human sense. It allocates them in accordance with the sink strength of each organ. Organs with greater sink strength win the competition and obtain a larger share of the phloem stream (Connor et al., 2011; Taiz et al., 2023).

4.2. What Determines Sink Activity?

Let us examine in more detail the specific processes that make up sink activity, because these are the processes we can observe and, potentially, manipulate.

First is phloem unloading. In the acceptor organ, sucrose must leave the sieve tube. This can occur by two routes:

• Symplastically—through plasmodesmata, down a concentration gradient, without energy expenditure. This is typical of rapidly growing meristematic tissues (root tips, young leaves), where sucrose is quickly consumed for growth and its concentration in cells remains low, ensuring a continuous inflow (Lambers & Oliveira, 2019; Taiz et al., 2023).
• Apoplastically—with sucrose exiting into the cell walls (apoplast) and then being actively taken up by acceptor cells via membrane transporters. This route requires energy and allows the creation of steep concentration gradients. It is characteristic of seeds, fruits, and storage tissues, where the final sugar concentration must be very high (Marschner, 2012; Taiz et al., 2023).

Second is sucrose utilisation. Once inside the cell, sucrose must not accumulate in free form; otherwise, it would create a reverse osmotic gradient and stop the flow. It must be rapidly channelled into metabolism:

• Hydrolysis by invertase to glucose and fructose, which then go into respiration or the synthesis of structural components.
• Cleavage by sucrose synthase to form UDP‑glucose and fructose, which triggers the synthesis of starch or cellulose.
• Synthesis of storage compounds—starch in amyloplasts, proteins, oils.

The higher the rate of utilisation, the lower the sucrose concentration in the acceptor cell, the greater the concentration gradient between the sieve tube and the cell, and the more sucrose will be imported. Therefore, sink activity often correlates with respiratory activity or the rate of storage compound synthesis (Connor et al., 2011; Marschner, 2012).

Third is hormonal regulation. Sink activity is not constant. It is modulated by phytohormones. For example:

• Auxin (IAA), produced in seeds and fruits, enhances assimilate import, acting as an “attractor”. Removing seeds from a strawberry fruit stops fruit growth, while auxin treatment restores it (Connor et al., 2011).
• Cytokinins, coming from the roots, stimulate cell division and the formation of sinks in the shoots.
• Abscisic acid (ABA) can enhance phloem unloading under certain conditions, e.g., during drought, promoting resource redistribution to the roots.
• Gibberellins and other hormones also participate in regulation (Marschner, 2012; Sadras & Calderini, 2015).

Thus, sink strength is not a static trait but a dynamic characteristic that changes with developmental stage, environmental conditions, and the hormonal status of the plant.

4.3. Connection with Previous Sections

Now let us link this to what we have already discussed.

We began by noting that the donor leaf produces sucrose in excess. But the leaf itself is also a sink at early stages of development, until it reaches maturity and begins to export. The transition of a leaf from sink to donor (role reversal) is a classic example of changing sink strength and loading activity (Taiz et al., 2023; Tretyakov, 2000).

Next, we discussed the phloem as a transport highway. Movement along this highway is driven by a hydrostatic pressure gradient created by the difference in osmotic pressure between donor and sink. But what determines the magnitude of this gradient? Precisely sink strength! If the sink is active, it rapidly unloads sucrose, lowering osmotic pressure in the sieve tubes on its side; water leaves the tubes, and pressure drops. The lower the pressure on the sink side, the greater the pressure difference with the donor, and the faster the phloem sap flows (Taiz et al., 2023; Connor et al., 2011). Thus, it is sink strength that creates “pull” and determines the distribution of flow among competing organs.

Consequently, assimilate distribution is not a passive process. It is an active competition among organs for the transport stream, where the winners are those with greater sink strength. This is exactly what we will discuss in the next section—who wins this competition and how the final yield is formed.

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5.1. Hierarchy of Sinks: Who Is in Charge?

A plant is not a democratic society where resources are distributed evenly. It is a strictly hierarchical system in which priorities are determined by the strategies of survival and reproduction.

In most cases, generative organs (flowers, fruits, seeds) have the highest sink strength. The evolutionary logic is obvious: the plant’s primary task is to leave offspring. As soon as fruits set, they become the most powerful acceptors, capturing the bulk of assimilates. This is why, during the fruiting period, vegetative growth often slows down, and older leaves turn yellow and senesce earlier—they give up their resources to the seeds (Connor et al., 2011; Sadras & Calderini, 2015).

The second most important category consists of meristematic tissues and growing organs—apical buds, young leaves, root tips. They have high sink activity due to intense cell division, synthesis of new membranes, and cell walls. This ensures plant growth and the colonisation of new territories (soil, space).

The third category is storage organs (taproots, tubers, bulbs, seeds). They can increase their sink strength as they develop and, once they reach a certain size, become dominant. For example, in potato, after tuber initiation, the tubers become the main sink, and it is there that the main assimilate flow is directed (Connor et al., 2011; Tretyakov, 2000).

It is important to realise that this hierarchy is not rigid. It changes with developmental phase. Early in the growing season, the main sinks are growing leaves and roots. In the middle—flowers and ovaries. At the end—filling seeds or fruits. The plant sequentially “switches” priorities, directing resources where they are most needed at the moment to fulfil its primary biological objective (Connor et al., 2011; Marschner, 2012).

5.2. What Makes One Sink Stronger Than Another?

We have already discussed the mathematical formula: sink strength = size × activity. Now let us look at the specific factors that determine the outcome of this competition in a real plant.

Factor 1: Proximity to the source (anatomical factor).

A sink located closer to the source has an advantage. Recall the Münch model: the phloem flow travels along the sieve tubes, and along the way it can be partially “intercepted” by intermediate sinks. The farther the sink is from the leaf, the greater the chance that part of the sucrose will be unloaded en route. This phenomenon is known as the distance effect (Connor et al., 2011). This is why, on the same plant, the upper fruits (closer to the leaves) often turn out larger than the lower ones, even though their potential is similar.

Factor 2: Hormonal status (signalling factor).

Hormones produced within the sink itself can significantly boost its strength. Auxin from seeds is a classic example. The more seeds in a fruit, the more auxin, the higher the sink activity, and the more sucrose flows into the fruit (Connor et al., 2011). Cytokinins from the roots stimulate shoot growth, while gibberellins can enhance stem growth at the expense of roots. This hormonal “language” allows sinks to send signals—“I am here, I need more!”—and thus influence resource allocation (Marschner, 2012; Sadras & Calderini, 2015).

Factor 3: Metabolic activity (utilisation rate).

We have already mentioned that the rate of sucrose utilisation determines the concentration gradient. If a sink actively respires, synthesises proteins, and deposits starch, it constantly lowers the free sucrose concentration in its cells, thereby creating a powerful “pull” along the osmotic gradient. Therefore, sinks with high metabolic activity outcompete “lazy” ones (Connor et al., 2011; Lambers & Oliveira, 2019).

5.3. When the Donor Cannot Keep Up: Feedback

Now an important addition. Competition is not one‑sided. Sink strength determines where the flow goes, but the flow itself depends on donor productivity. If the leaves experience stress (drought, lack of light or nitrogen), they reduce photosynthetic intensity. Then the total amount of sucrose in the system drops, and competition intensifies—sinks begin to fight over a diminished resource.

But there is also feedback. If a sink is very strong and actively “pumps out” sucrose from the phloem, this lowers the sugar concentration in the donor leaf, relieving inhibition of photosynthesis and even stimulating it. This phenomenon is known as sink‑demand regulation of photosynthesis (Connor et al., 2011; Sadras & Calderini, 2015). If part of the fruit is removed (reducing the sink), the sugar concentration in the leaves rises, and photosynthesis may slow down or even stop due to feedback inhibition. This is why fruit thinning (removing excess flowers or young fruits) can paradoxically increase the final yield of the remaining fruits, but does not necessarily raise the overall productivity of the whole canopy.

5.4. From Theory to Practice: Where Is Yield Born?

Now—the main agronomic take‑home message. Yield is not simply the result of photosynthesis. Yield is the result of economically valuable organs winning the competition for assimilates.

Understanding this fundamentally changes our approach to breeding and agronomic practices.

1. Breeding for increased sink strength. All modern high‑yielding varieties differ from their wild ancestors not necessarily by more intense photosynthesis (often it has not even changed!). The key difference is that their generative sinks (ears, fruits) have significantly greater sink strength (Connor et al., 2011). They literally “pull” resources towards themselves, leaving a minimum to the vegetative mass. This is reflected in a high harvest index—the share of economically valuable part in the total biomass (Sadras & Calderini, 2015; Tretyakov, 2000).

2. Managing sinks in agronomic practice. We can influence assimilate distribution by creating or weakening sinks. For example:

• Pinching (topping) of shoot tips—removing the apical sink (growing point)—redirects resources to lateral shoots and fruits.
• Fruit load regulation—removing excess ovaries—strengthens the remaining fruits, making them larger.
• Adjusting plant density—the denser the stand, the greater the competition for light, and the plant directs more resources to stems and leaves at the expense of roots, which is critically important for root crops.
• Management of the root system (e.g., fertilisation that promotes better root development) strengthens the root sink, which is important for cereals in dry years when roots must extract water and nutrients, but may compete with grain filling (Sadras & Calderini, 2015; Marschner, 2012).

3. Forecasting and modelling. Knowledge of sink strength and its dynamics allows us to build models of the production process. We can predict how changes in the agronomic background (e.g., nitrogen application, which enhances vegetative growth) will affect assimilate redistribution and final yield. This is the basis for precision agriculture and yield programming (Tretyakov, 2000; Connor et al., 2011).

5.5. Concluding Remarks

Thus, we have completed the full journey. Photosynthesis creates the resources. The phloem transports them along a pressure gradient. Sink strength determines who receives these resources. Competition among sinks, their hierarchy, and their ability to draw the flow towards themselves—these are what ultimately shape the plant’s architecture and the magnitude of the yield.

A plant is not a passive factory but an active, self‑regulating system constantly solving the problem of optimal allocation of limited resources. Our task as physiologists and agronomists is to understand these rules and use them to direct as many assimilates as possible to the organs we want as our harvest.

In the next lecture, we will examine in more detail that part of the system related to the “expenditure item”—respiration—and consider how much energy is spent on maintenance and growth, and why this, too, is critically important for productivity.

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Key concepts we have covered today:

• Donor (source) and acceptor (sink) of assimilates.
• Transport form—sucrose.
• Phloem, sieve tubes, companion cells.
• Pressure‑flow hypothesis (Münch).
• Sink strength as the product of size and activity.
• Competition among sinks, organ hierarchy.
• Management of assimilate distribution as the basis for yield enhancement.