Table of Contents
Transport of organic substances in plants mainly refers to the movement of sugars (especially sucrose), amino acids, and some hormones through the phloem. While water and mineral salts are transported largely upwards in the xylem, organic substances move in multiple directions depending on where they are needed.
Transport Tissues: Focus on Phloem
The general plant transport system and the basic distinction between xylem and phloem are covered elsewhere. Here, we focus on structures specifically relevant for organic transport.
Structure of the Phloem (Overview)
Important components for organic transport are:
- Sieve tube elements
- Elongated living cells arranged end to end.
- End walls form sieve plates with many pores.
- Cytoplasm is present but highly reduced: no nucleus, few organelles.
- Conduct the bulk flow of sugars and other solutes.
- Companion cells
- Nucleated cells closely associated with sieve tubes (via plasmodesmata).
- Rich in mitochondria and ribosomes.
- Perform metabolic functions for sieve elements, including active loading and unloading of solutes.
- Phloem parenchyma
- Storage and lateral transfer of organic substances.
- Sieve cells (in gymnosperms)
- More primitive conducting cells without distinct sieve plates.
- Associated with specialized albuminous cells instead of companion cells.
What Is Transported in the Phloem?
Phloem transport is not limited to sugars:
- Sugars
- Mainly sucrose (a disaccharide) is the principal transport form in most plants.
- In some species, other oligosaccharides (e.g., raffinose) or sugar alcohols may predominate.
- Amino acids and small peptides
- Transport of nitrogen in organic form from roots or storage tissues to growing regions.
- Organic acids
- Such as malate or citrate in some metabolic contexts.
- Hormones and signaling molecules
- Auxins, cytokinins, gibberellins, abscisic acid, ethylene precursors.
- Peptide signals and RNAs (including some mRNAs and small RNAs) can move in phloem.
- Secondary metabolites
- Certain defense compounds, alkaloids, etc., sometimes travel via phloem.
Sources and Sinks
Organic transport in the phloem is organized between sources and sinks.
- Source
- Region where more organic substances (especially sugars) are produced or released than are consumed.
- Typical sources:
- Mature, photosynthesizing leaves.
- Storage organs during mobilization (e.g., germinating seeds, sprouting tubers).
- Sink
- Region where organic substances are consumed, incorporated into biomass, or stored.
- Typical sinks:
- Growing root and shoot tips (meristems).
- Developing leaves, flowers, fruits, and seeds.
- Storage organs during accumulation (e.g., growing tubers, roots, bulbs).
- Source–sink relationships are dynamic
- A young leaf works first as a sink (importing sugars), then later as a source.
- Underground organs can switch from sink (storage phase) to source (germination/sprouting phase).
Phloem Loading
Phloem loading is the process by which sugars (and other solutes) move from photosynthetic (mesophyll) cells into the sieve tubes at the source.
There are two main principles:
Symplastic Loading
- Sugars move from cell to cell via plasmodesmata (cytoplasmic connections).
- Movement is largely by diffusion following a concentration gradient.
- Often involves:
- Conversion of sucrose into larger oligosaccharides in companion cells or phloem parenchyma to maintain a concentration gradient.
- Characteristic of some trees and plants with many plasmodesmata connecting mesophyll and phloem.
Apoplastic Loading
- Sugars first move out of mesophyll cells into the cell wall space (apoplast) near the phloem.
- Then taken actively into sieve tube–companion cell complexes.
Key aspects:
- Sucrose–H\(^+\) symporters
- In the plasma membrane of companion cells or sieve elements.
- Use the proton gradient (more H\(^+\) outside than inside) established by H\(^+\)-ATPases.
- Co-transport H\(^+\) back into the cell together with sucrose, against the sucrose concentration gradient.
- Energy demand
- Active process driven by ATP-consuming proton pumps.
- Allows high sucrose concentrations in the phloem.
Many species use mixed strategies or can switch under different conditions.
Pressure Flow Hypothesis (Mass Flow)
The dominant explanation for long-distance phloem transport is the pressure flow hypothesis (also called mass flow).
Basic Idea
- At the source
- High sucrose concentration is established in sieve elements through loading.
- This decreases the water potential in the phloem sap.
- Water enters from neighboring xylem or surrounding cells by osmosis.
- The influx of water raises turgor pressure in the phloem at the source.
- Along the phloem pathway
- Sieve tubes provide a low-resistance path.
- The pressure difference between source (high pressure) and sink (lower pressure) drives bulk flow of the solution.
- Solutes move together with water; individual molecules are not actively “pumped” over long distances.
- At the sink
- Sugars are removed from the sieve elements by phloem unloading (into sink cells).
- This increases water potential in the phloem at the sink.
- Water leaves the phloem (often back into xylem), lowering turgor pressure there.
- The pressure gradient from source to sink is maintained.
Flow direction is determined by relative positions of active sources and sinks, not by a fixed “up” or “down” as in xylem.
Phloem Unloading
Phloem unloading is the transfer of solutes from sieve tubes to sink tissues.
Mechanisms depend on the nature of the sink:
Growing Tissues (Meristems, Young Leaves, Developing Fruits)
- Often involve symplastic unloading:
- Sugars move via plasmodesmata directly into sink cells.
- Once there, they may be:
- Metabolized (e.g., converted to starch or structural carbohydrates).
- Used in respiration for ATP production.
- Rapid use or conversion maintains a low sugar concentration in sink cells, promoting continued import.
Storage Organs (Tubers, Roots, Seeds)
- May show apoplastic unloading:
- Sucrose is released from phloem into cell wall space.
- Then transported into storage parenchyma cells via specific transporters.
- In some seeds, sucrose is hydrolyzed to glucose and fructose outside cells and then taken up.
Regulation
- Unloading is tightly coupled to development and sink strength:
- High metabolic activity or active storage processes increase sink strength.
- Hormones and local signals modulate plasmodesmata permeability and transporter activity.
Direction and Speed of Phloem Transport
- Bidirectionality at plant scale
- A single plant can simultaneously transport substances:
- Upwards from leaves to growing shoot tips.
- Downwards from leaves to roots.
- However, in a single sieve tube, flow is generally unidirectional at a given time.
- Typical flow rates
- Around 1 m per hour is common; in some plants faster rates have been measured.
- Much faster than simple diffusion, which would be too slow for whole-plant supply.
Experimental Evidence for Phloem Transport
Only the key types of evidence specific to organic transport are outlined here.
- Girdling experiments (ringing)
- Removing a ring of bark (including phloem but leaving xylem) from a tree stem:
- Roots eventually starve and may die (loss of sugar supply).
- Sugars accumulate above the ring, often causing swelling.
- Shows that organic substances move in the phloem, not xylem.
- Radioisotope tracing
- Exposing a leaf to CO\(_2\) containing radioactive carbon (\(^{14}\)C).
- Tracking the movement of labeled sugars reveals transport routes and velocities.
- Aphid stylet techniques
- Aphids insert stylets directly into sieve tubes.
- Stylets can be severed to collect pure phloem sap.
- Analysis shows high sucrose concentrations and presence of amino acids and hormones.
Phloem and Plant Health
- Damage to phloem
- Interrupts supply of energy-rich compounds to sinks.
- Can lead to:
- Root dieback.
- Poor fruit development.
- Reduced growth of new shoots.
- Phloem-feeding pests and pathogens
- Aphids, whiteflies, and some mites feed on phloem sap.
- Certain viruses move specifically in phloem, using it as a highway through the plant.
- Practical relevance
- Crop yield depends heavily on efficient source–sink transport.
- Breeding and management practices can influence:
- Leaf area (sources).
- Root and fruit development (sinks).
- Vascular connectivity between them.
Summary of Key Points
- Organic substances (primarily sucrose, amino acids, and hormones) are transported mainly in the phloem.
- Transport occurs from sources (net exporters, such as mature leaves) to sinks (net importers, such as roots, fruits, and young leaves).
- Phloem loading concentrates sugars in sieve tubes, often using active transport via sucrose–H\(^+\) symporters.
- The pressure flow hypothesis explains long-distance movement as bulk flow driven by turgor pressure gradients between sources and sinks.
- Phloem unloading and subsequent use or storage of solutes maintain sink activity and influence overall plant growth and yield.