Transport in Plants

• Turgidity is the point at which the cell membrane pushes against the cell wall.
• This makes the cell swollen and firm, usually due to being full of liquid.
• Mass flow/Bulk flow is the bulk transport of materials from one point to another.
• It is a result of a pressure difference (a pressure potential gradient) between two points.
• Materials are carried at similar speeds unlike in diffusion due to a change from high to low turgor pressure.

Levels of Transport in Plants

• The Cellular Level involves the uptake and release of water and solutes by individual cells, like water and mineral absorption by root cells.
• The Tissue Level involves short distance transport from cell to cell at the level of tissues and organs.
• An exmaple is loading sugar from mature leaf cells to the phloem sieve tube elements, which usually occurs along the plant's radial axis.
• The Whole Plant Level involves long distance transport of sap within xylem and phloem over the whole plant.
• Mature plant cells have three compartments: the cell wall, the cytoplasm, and a large central vacuole.
• Special specific proteins are embedded within the membranes in order to control passage of substances.

Lateral Transport Routes

• Symplastic route
• Transmembrane route
• Apoplastic route

Water Absorption

• Water is mostly absorbed by the younger parts of the roots in the root hair regions.
• This is because they are more permeable than older areas due to the presence of suberin deposits in the outer cells of the cortex.
• Root hairs are tubular extensions of epidermal cells and greatly increase the surface area for uptake of water and salts.
• Most water absorption occurs near the root tip where the epidermis is permeable, and through mycorrhizae where hyphae absorb water/minerals and transfer them to host plant roots.
• Soil particles are usually coated with water and dissolved minerals, while epidermal cell walls are hydrophilic and attract water.
• Water moves into cells by osmosis via two pathways: apoplast and symplast.
• Apoplast pathway flows into the walls and offers the least resistance.
• Symplast pathway flows across the membrane of the epidermal cells to the parenchyma cells of the cortex where selective absorption of ions occurs.

Mineral Ion Uptake

• Roots take up water only if their water potential is more negative than that of the soil solution.
• Water potential of roots depends on tension generated by transpirational water loss, and negative solute potential due to ion accumulation.
• Active transport is required for cation uptake by epidermal root hair cells in which the cells actively pump out H+ via a proton pump.
• This makes the extracellular environment positively charged compared to the intracellular environment, resulting in a proton concentration difference across the cell wall.
• Cations, e.g. Ca2+, K+ and Mg2+, are adsorbed to soil particles, which are not free in the soil.
• H+ ions displace positively charged mineral ions, allowing them to diffuse into the root along a gradient.
• The ions enter through the specific ion channels on the membrane.
• Anions, such as nitrate (NO3-), phosphate (PO43-) and sulfate (SO42-), are not bound to soil particles so they can be leached out by water.
• Cells accumulate anions (i.e. NO3-) by coupling their transport to the inward diffusion of H+ ions through a cotransporter.

Water and Mineral Transport to the Endodermis

• Water and mineral ions move across the root cortex up to the endodermis via the symplast and apoplast routes.
• Symplast = water crosses one cell wall and one plasma membrane and then runs through the plasmodesmata in the symplast.
• Apoplast = formed by the continuum of cell walls of adjacent cells and doesn't cross any plasma membranes.
• A switch between these pathways can occur.

Water and Mineral Transport to the Xylem

• Upon reaching the endodermis, water and minerals encounter the Casparian strip, which is a belt of impermeable suberin around the endodermal cell.
• The apoplast pathway is blocked, and all water and minerals are transported via the symplast pathway, which slows water flow and controls entry of toxic substances/pathogens.
• Parenchyma cells in the pericycle help minerals move back to the apoplast; transfer cells in the pericycle use ATP to move ions from the symplast to cell walls (the apoplast).
• After passing the endodermal barrier, water and minerals leave the symplast, enter the apoplast of the stele (parenchyma cells), and the lumen of the xylem vessels.
• High solute concentration in the xylem sap causes root pressure.
• This negative potential draws water into the stele which exerts pressure along a water potential gradient.
• Root pressure can raise water to about 60cm.

Guttation

• Guttation is the forcing out of liquid water through special openings (hydathodes) in the leaves.
• It's typical when there is high humidity, when soil is water laden, and early morning after a night where transpiration rate is low.

Long Distance Transport

• Diffusion is too slow over long distances, so bulk flow is essential.
• Vertical movement from the root to the stem along the xylem has to occur against gravity at 15m/hr or even faster due to root pressure which pushes up xylem and transpiration.
• Cohesion-tension mechanism is used to pull up xylem.

Transpiration-Adhesion-Cohesion-Tension (TACT) mechanism

• Stomata lead into the airspaces amidst leaf mesophyll tissue, which is saturated with water vapour.
• The leaf air spaces contain a higher water content so water vapour diffuses out of the stoma.
• A thin film of moisture then evaporates coating the mesophyll cells to re-saturate the air spaces and create a negative pressure.
• This results in tension and a pulling force that draws water out of the leaf's xylem, through the symplast and apoplast pathways and into the mesophyll air spaces near the stomata.
• As water leaves the xylem veins, it creates a tension on the whole water column leading it to be drawn upwards from the roots.
• The above-mentioned tension generated at the leaves is transmitted from leaves to root tips via cohesion and adhesion.
• All processes are passive.
• Tree diameter shrinks during the day, when the transpiration rate is the fastest.
• Sap flow starts early in the upper branches moving onto the lower branches later.

Cavitation

• The filling of the tracheid or xylem vessel with air is cavitation, also known as an airlock.
• In terms of transpiration, a water column is pulled out of plants by evaporation at the surface of the leaf cell.
• Cavitation rarely occurs unless the plant has a lot of radiation or when the temperature is below zero or high.

Xylem

• It is fundamental for transportation of water and mineral ions, and support.
• Composed of xylem vessels, tracheids, parenchyma, and sclerenchyma fibres.
• Xylem Vessels = mainly responsible for water transport in flowering plants.
• They are long, tubular structures formed by the fusion of several vessel elements, end to end in a row, and dead at maturity.

Transpiration and Stomata

• Transpiration occurs at stomata (90% mostly from the lower leaf surface), lenticels, and the cuticle.
• A stoma is a pore found between a pair of guard cells.
• It leads into air spaces which increases the surface area for carbon dioxide diffusion and water evaporation.
• Guard cells are surrounded by subsidiary cells.
• Guard cells are firmly joined together at each end.
• The inner wall is thicker (closer to the stoma) due to more cellulose being deposited, making it less elastic.
• Unlike epidermal cells, guard cells possess chloroplasts.

Stomata

• They allow mineral transport due to water uptake, secrete a waxy cuticle that is impermeable to water and carbon dioxide, and perform gas exchange between leaf and atmosphere.
• They enable efficient carbon dioxide uptake but reduce water loss.
• The transpiration to photosynthesis ratio evaluates how efficiently a plant uses water.
• Water loss [cm³] : CO2 assimilation [g] (most plants = 600:1; corn = 300:1)
• Stomata open during the day for carbon dioxide diffusion so photosynthesis can happen and close if too much water is lost.
• Stomata close at night because photosynthesis cannot happen in the dark so water loss is limited.
• The accumulation of soluble products can also cause them to open since guard cells synthesize sugars which contribute to a more negative water potential.

Stomata Movement Mechanism

• Blue light causes H+ ions to be actively pumped out of the guard cells through proton pumps.
• This makes the cell interior negatively charged.
• Voltage-gated potassium channels open causing K+ to enter, which causes Cl- to enter to balance the charge and malate anions derived from starch are also used.
• Water then enters via osmosis rendering Guard cells turgid, causing an increase in pressure which results in a gap.
• In the absence of blue light, proton pumps become less active, K+ diffuses passively, water follows, and cells sag together.
• If mesophyll cells dehydrate then water potential becomes negative, abscisic acid is released, and causes K+ ions to leave guard cells and lead to their closure.
• External factors influencing transpiration include atmospheric humidity, temperature, atmospheric pressure, light, and wind speed.
• Sunlight activates the blue-light receptor and initates the stomata opening.
• Water vapour escaping through a stoma spreads out into diffusion shells and in still air creates a boundary layer, air currents reduce the boundary layer and increase transpiration.
• Factors include water, leaf area, carbon dioxide concentration and circadian rhythms (internal clock).
• Pore size and stomatal density can affect the diffusion rate to influence the rate of transpiration.
• When stomata are close, then the diffusion shells overlap.

Adaptations to particular environments

• Mesophytes: They are adapted to living in ample water supply and in well-drained soils.
• Usually have broad, flat and green leaves with a great number of stomata on the lower surface to be controlled by closure.
• Roots are well developed, branched and provided with a root cap, to compensate water losses.
• Hydrophytes: Require a large supply of water and grow completely or partly submerged in water.
• They have long, slender, spongy and flexible stems, possess lacunae for buoyancy, and exhibit broad aerial floating leaves/dissected submerged leaves.
• Examples include plants whose leaves float on the surface, (e.g. water lily) where stomata tend to remain open.
• A specific type occurs locally which is totally submerged and is called Elodea and Egeria which have thin leaves with aerenchyma and minimal xylem.

Xerophytes

• They are adapted to living in dry regions and able to survive long periods of drought.
• They are an epidermis multilayered, dense covering of epidermal hairs, have leaves that shed during the dry season, stomata concentrated on lower leaf, thick small leaves and/or thick cuticles.
• Some plants, like Ammophila arenaria, roll up their leaves trapping the humid air around them.
• Halophytes - They are adapted to a saline habitat in which they accumulate sodium/chloride ions to lower the plant's water potential.
• Such plants possess salt glands, succulent leaves/stem, thick cutinised epidermis, and have long roots.

Phloem

• Translocation is the mass flow movement of organic products (mainly sucrose) from a source to a sink, driven via positive pressure.
• It occurs along the vertical and radial axis.
• Sieve tubes carry food from a sugar source (organ of photosynthesis or sucrose produced by hydrolysis of starch) to a sugar sink (organ where sugars are stored/used).
• In these tissues, the cortex of roots can be either sources or sinks.
• If a ring of bark is removed externally to the xylem, it can be observed that the movement of organic solutes stops which accumulates the region above the bark.
• Aphids can be used to extract phloem if anaesthetized, where it can be determined that 90% of of the transport is sucrose.

Phloem Structure

• The phloem consists of sieve tube elements (separated into lateral areas) as well as companion cells.
• Sieve tube elements are living cells that are interconnected by perforations from modified plasmodesmata
• They also have thicker lateral walls due to increased cellulose deposited, while having plasma membranes.
• Elements are dependent on adjacent companion cells that have organelles along the lateral wall.
• The elements facilitate longitudinal flow of material.

Phloem Loading/Unloading

• Loading refers to the translocation of sugars from sources to sieve tubes and unloading refers to the removal to sinks.
• Both processes are heavily energy consuming since elements need to be produced via symplastic and apoplastic pathways.
• Passage through the symplast selects substances to be accumulated for translocation due to membrane selectivity.
• Proton pumps move out of the companion cells to create a concentration gradient where sucrose is passively dragged down an electrochemical gradient.
• Unloading serves to maintain the osmotic gradient as well as build up sugars in storage.
• The aphids closer to the source will have greater sugar flow if embedded in the pholem.
• Slow movement - maximum flow rate of 1m/hr
• Fast - maximum flow rate of 45m/hr