Plant Transport – Transpiration and Phloem Movement

Brown algae – Macrocystis and Laminaria - California

Giant Sequoia

Water transport in plants

Plant Transport Plant transport occurs at three levels: 1. The uptake and loss of water and solutes by individual cells, such as absorption of water and minerals from the soil by root cells. 2. Short-distance transport of substances from cell to cell at the level of tissues and organs, such as moving sugar from photosynthetic cells of leaf to the phloem sieve tubes 3. Long distance transport of sap within xylem and phloem at the level of the entire plant.

Xylem Transport

Water Potential The movement of water in and out of plant cells is driven by water potential. The net uptake or loss of water by a cell occurs by osmosis, the passive transport of water across a membrane. Water usually moves from hypotonic (low solute concentration) to hypertonic (high solute concentration) – this is what happens in animal cells. But plants have a rigid cell wall that provides physical pressure. So in plants the movement of water depends upon a combination of solute concentration and physical pressure known as water potential symbolized by the Greek letter psi Ψ

More Water Potential Plant biologists measure water potential in units of pressure called megapascals MPa – 1 MPa equals 10 atmospheres of pressure – an atmosphere of pressure is the pressure of a column of air at sea level A car tire is usually inflated to about 0.2 MPa (or two atmospheres); water pressure in home plumbing is 0.25 MPa Adding solutes to water lowers its water potential. This is because the water molecules form shells around the solute and have less freedom to move than they do in pure water. Pure water has a water potential of 0 MPa – so adding solutes results in a solution with a negative measure of MPa.

Water Potential Water potential = physical pressure potential + solute potential (AKA osmotic potential) Ψ = Ψ p + Ψ s where Ψ = total water potential; Ψ p = physical pressure potential; Ψ s = solute pressure potential If we add physical pressure to a solution – which can occur via a partially elastic cell wall or by pushing on the solution as by the plunger of a syringe (compression – a positive pressure) – we raise its water potential Tension is a negative pressure – as if we are pulling the plunger of a syringe out of the syringe to draw in liquid – tension lowers water potential

Aquaporins

Water can move through transport proteins known as aquaporins. Aquaporins do not effect the water potential gradient or direction of water flow but they effect the rate at which water diffuses down its water potential gradient

Vacuolated Plant Cells Plant cells have three basic compartments. 1. Outside the cell is a thick cell wall that helps maintain the plant cells shape. It does not regulate the movement of material in and out of the cell – that is done by the plasma membrane. 2. The plasma membrane serves as the barrier between the cell wall and the cytosol – the cytoplasm inside the cell but outside of the organelles 3. Most mature plant cells have a large vacuole that contains cell sap. It may occupy 90% of the cell volume. It is surrounded by a membrane called the tonoplast that regulates traffic between the cytosol and the cell sap.

Vacuolated Plant Cells Most plant cells have openings in the cell walls called plasmodesmata. The plasmodesmata connect the cytosol compartments of neighboring cells allowing easy movement of substances between cells. The connected cytoplasms of many cells is known as the symplast. The cell walls form a continuum of spaces between cells. This is known as the apoplast.

Lateral Transport – localized short distance movement in plant organs

Root with mycorrhizae

Bulk Flow and Xylem Sap Once water and minerals reach the xylem via lateral transport they move up the xylem vessels via bulk flow - the movement of a fluid driven by pressure. Xylem sap flows upward to the veins of leaves due to the pressure of transpiration – the loss of water vapor from the leaves and other aerial parts of the plant. An early botanical question was whether xylem sap was pushed up or pulled up the plant. Pushing of the xylem sap occurs via root pressure – root cells expend energy to pump minerals into the xylem. Minerals accumulate in the xylem sap lowering water potential there. Thus water flows into the xylem. It can cause guttation, where water is extruded from pores in leaves.

Guttation

Transpirational Pull Xylem sap is pulled up the plant via transpirational pull. Leaves actually generate the negative pressure necessary to bring water to them. The transpirational pull on xylem sap is transmitted all the way down the plant – from the leaves, through the stem (shoot) to the roots. The cohesion of water due to hydrogen bonding makes it possible to pull a column of sap from above without the water separating. Transpirational pull can only work through an unbroken chain of water molecules. Cavitation, the formation of a water vapor pocket in a xylem vessel such as when xylem sap freezes in winter, breaks the chain.

Water transport potentials Dry air can generate a negative water potential of -100 MPa. Typical water potential of a transpiring leaf is -1 to -1.5 MPa (negative 10 to 15 atmospheres) and that potential is transmitted all the way down to the roots to pull water up Note – water will cavitate (develop air bubbles) at pressures of -0.2 MPa – so how do plants prevent this from happening? Living phloem and parenchyma cells near the xylem act as water reservoirs and actively push water into the xylem to keep interior xylem pressure around 0 MPa and to repair any breaks in the water column

Translocation of phloem sap The transport of food throughout a plant is known as translocation. Sugar from mesophyll cells in the leaves and other sources must be loaded before it can be moved. In some species, sugar moves all the way from mesophyll cells to sieve tube members via the symplast. In other species, sugars moves by a combination of symplast and apoplast. Often sieve tube members accumulate very high sucrose concentrations – 2 to 3 times higher than concentrations in the mesophyll – so phloem requires active transport using proton pumps. At the sink end of a sieve tube, the phloem unloads its sugar. Phloem unloading is a highly variable process.

Phloem bulk flow Phloem moves at up to 1 m/hour – too fast to be by diffusion. So phloem also moves via bulk flow – pressure drives it. Pressure flow in phloem comes about because water flows into the phloem from the xylem due to the high sugar concentration in the phloem sap. This sets up a pressure gradient that drives phloem sap downhill. When the sugar is unloaded, the water flows out by osmosis and is recycled via the xylem.

Phloem Movement Phloem movement is still not well understood. We know that is not the rate of photosynthesis that limits plant growth and crop yields, rather it is the ability to transport sugars away from the leaf that limits yield. If we could somehow increase the rate of sugar movement, we could probably increase crop yields.