Mass Transport in Plants

Mass transport in plants is the movement of substances in a single direction and speed. This is seen in the xylem and phloem, transport vessels in plants. The xylem is responsible for transporting water and minerals up the plant via the transpiration stream. The phloem transports amino acids and sugars in both directions: up and down the plant.

Substances can move in four main ways:

• Diffusion: the movement of substances down their concentration gradient.

• Facilitated diffusion: the movement of substances down their concentration gradient through membrane proteins.

• Osmosis: water movement down a water potential gradient (from high to low water concentration) through a partially permeable membrane.

• Active transport: the movement of substances against their concentration gradient, using energy in the form of ATP.

Mass transport in plants

Xylem is the vascular plant tissue that transports water in the stem and leaves of plants. Related to the xylem transport in plants is the cohesion theory of water transport. To transport organic substances, plants use phloem.

To understand water transport in plants, you need to study key concepts such as water potential and the different water movement types.

The role of water, solute, and pressure potentials

Water potential is the concentration of water molecules in a solution. Water potential is affected by solute concentration and pressure. Water will move towards low water potential (high concentration of solutes). Water potential is the key factor in driving the transpiration stream up the plant.

Water potential is higher in the soil than the plant, which allows it to diffuse into the root cells. The plant will manipulate the concentration of solutes (solute potential) to aid osmosis from the soil.

Pressure (turgor) potential can be either positive (compression) or negative (vacuum) in the plant cells. When the cell is at a maximum pressure potential, it becomes turgid. Plants can manipulate the pressure by opening and closing stomata and altering solute concentrations.

Water potential in the plant cells can be calculated using the following equation:

\(\psi = \psi_s + \psi_P\)

• \(\psi\) is the total water potential in megapascals
• \(\psi_s\) is the solute potential
• \(\psi_P\) is the pressure potential

Diffusion and mass transport in plants

Before entering the xylem, water in the roots will move via apoplast and symplast pathways through diffusion. This type of diffusion is osmosis; it involves water moving down a water potential gradient through a partially permeable membrane. There are two main pathways for water movement: the apoplast pathway and the symplast pathway.

Apoplast pathway

• Water moves through the spaces of the cell walls, dead cells (xylem) and xylem tubes.

• Due to cohesive forces, more water is pulled up into the xylem.

• The water will eventually reach a Casparian strip made of waxy suberin and is impermeable to water. Casparian strips will direct water in this pathway to enter the cytoplasm, where it becomes a part of the symplast pathway.

Symplast pathway

• Water will travel via the cytoplasm, vacuoles and plasmodesmata of the cells.

• Water moves by osmosis; the neighbouring cell has a lower water potential, so water moves into it.

Cohesion-tension theory

Cohesion (water molecules clinging to each other) and tension (water molecules clinging to the walls of the xylem) are the main drivers of the transpiration stream. Evaporation through the leaf stomata creates a negative water potential which forces water to move upwards towards the leaves.

Amino acids and sugars, such as sucrose, are transported in another type of vascular tissue called the phloem. They are transported in a bi-directional movement from the leaves (source) to the growing parts of the plant (e.g., shoots and roots), roots (sinks), flowers and fruits.

Fig. 3. Xylem and phloem

Mass Flow Hypothesis

Mass flow describes the movement of fluids from an area of high to low hydrostatic pressure, and it explains the transportation of food from sources to sinks. The mass flow hypothesis states that:

1. Sucrose is actively co-transported into sieve tube elements from the companion cells via diffusion, reducing the sieve tube's water potential.

2. Water moves from the xylem (high water potential) into the phloem (low water potential), which increases the hydrostatic pressure in the phloem.

3. Sources have a higher hydrostatic pressure, while the sinks have lower hydrostatic pressure. This allows solutes to move to the sinks down the concentration gradient.

4. Sinks will use or store the solutes, increasing the water potential in the phloem. Water will move out of the phloem, down the hydrostatic pressure gradient.

As part of the mass-flow hypotheses, the pressure-flow hypothesis proposes that osmotic pressure in sieve tubes rises when flow into source regions (locations of photosynthesis or mobilization and exportation of storage products) occurs.

For and against the mass flow hypothesis

This hypothesis is still under investigation. Below, you will find supporting and conflicting evidence for mass flow.

Active loading

Active loading can also be referred to as apoplastic loading. Sucrose is actively loaded (requires ATP) into the sieve tube elements from companion cells.

1. Hydrogen ions (protons) are pumped from companion cells to surrounding cells via proton pumps.

2. The proton concentration becomes much higher in the surrounding cells than in the companion cells.

3. Hydrogen ions will diffuse back into the companion cells (down the gradient) through co-transporter proteins.

4. When hydrogen ions move down the gradient, they take a sucrose molecule (against the concentration gradient). This also happens from the companion cell to the sieve tube element.

Tracer and ringing experiments

Tracer and ringing experiments investigate the translocation of sugars in the plant.

The ringing experiment

• A ring containing phloem bark and cortex is removed to leave the xylem in the centre.

• Since the xylem is intact, water transport will not be affected, but sugar transport will stop at the ring as the phloem has been removed. This causes a swelling in the tissue, thus supporting the concept of phloem translocation.

The tracer experiment

• Plants are grown in a laboratory containing radioactively labelled carbon dioxide (C¹⁴).

• Through photosynthesis, the radioactive carbon is incorporated into the sugars.

• Autoradiography (a technique used to detect radioactive material in the sample) is used to observe the movement of sugars in the plant.

• The results show that the radioactive carbon is only present in the phloem and absent in the xylem, thus supporting the concept of phloem translocation.

Mass transport in non-vascular plants

Non-vascular plants (bryophytes), including mosses, liverworts and hornworts, lack a vascular bundle and do not have xylem or phloem to transport water, minerals and food. Instead, non-vascular plants contain much simpler tissue.

• Bryophytes will obtain water by osmosis, and nutrients will diffuse into the plant. Only the plant parts that are close to water and nutrient sources will take them up. They do not have a transport system (vascular tissue) to distribute it within the plant.

• Bryophytes do not have roots like vascular plants do; instead, they have rhizoids. Rhizoids resemble seeds; however, they function differently. They will anchor the plant but are not able to take up water. They will absorb water by osmosis, such as the other parts of bryophytes.